Severe Thunderstorms and Tornadoes in the
United States
Peter Folger
Specialist in Energy and Natural Resources Policy
February 2, 2010
Congressional Research Service
7-5700
www.crs.gov
R40097
CRS Report for Congress
P
repared for Members and Committees of Congress
Severe Thunderstorms and Tornadoes in the United States
Summary
Severe thunderstorms and tornadoes affect communities across the United States every year,
causing fatalities, destroying property and crops, and disrupting businesses. Tornadoes are the
most destructive products of severe thunderstorms, and second only to flash flooding as the cause
for most thunderstorm-related fatalities. Damages from violent tornadoes seem to be increasing,
similar to the trend for other natural hazards—in part due to changing population demographics
and more weather-sensitive infrastructure—and some analysts indicate that losses of $1 billion or
more from single tornado events are becoming more frequent. Insurance industry analysts state
that tornadoes, severe thunderstorms, and related weather events have caused nearly 57%, on
average, of all insured catastrophe losses in the United States in any given year since 1953.
Policies that could reduce U.S. vulnerability to severe thunderstorms and tornadoes include
improvements in the capability to accurately detect storms and to effectively warn those in harm’s
way. The National Weather Service (NWS) has the statutory authority to forecast weather and
issue warnings. Some researchers suggest that there are limits to the effectiveness of
improvements in forecasting ability and warning systems for reducing losses and saving lives
from severe weather. The research suggests that, for example, social, behavioral, and
demographic factors now play an increasingly important role in tornado-related fatalities.
At issue for Congress is its role in mitigating damages, injuries, and fatalities from severe
thunderstorms and tornadoes. The National Science and Technology Council has recommended
the implementation of hazard mitigation strategies and technologies, including some—such as
conducting weather-related research and development and disseminating results—that Congress
has supported through annual appropriations for the National Oceanic and Atmospheric
Administration, the National Science Foundation, the Federal Emergency Management Agency,
the National Aeronautics and Space Administration, and other federal agencies. Other
recommended strategies include land use and zoning changes, which are typically not in the
purview of Congress.
Congress attempted to clarify the federal role in mitigating damages from windstorms (including
tornadoes and thunderstorms) by passing the National Windstorm Impact Reduction Act of 2004
(P.L. 108-360). It is not evident whether the program made progress toward its objective:
achievement of major measurable reductions in the losses of life and property from windstorms.
Authorization for the program expired at the end of FY2008. Stand-alone legislation (H.R. 2627)
to reauthorize the program was introduced on May 21, 2009. On October 15, 2009, H.R. 3820,
the Natural Hazards Risk Reduction Act of 2009, was introduced to reauthorize the National
Earthquakes Hazards Reduction Program (Title I) and the wind hazards program (Title II)
through FY2014. H.R. 3820 was ordered to be reported by the House Science and Technology
Committee on October 21, 2009.
It is not clear whether changes to climate over the past half-century have increased the frequency
or intensity of thunderstorms and tornadoes. An issue for Congress is whether future climate
change linked to increases in greenhouse gas emissions will lead to more frequent and more
intense thunderstorms and tornadoes, and whether ongoing efforts by Congress to mitigate long-
term global warming will reduce potential future losses from thunderstorms and tornadoes.
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Severe Thunderstorms and Tornadoes in the United States
Contents
Overview .................................................................................................................................... 1
Issues for Congress ..................................................................................................................... 1
Focus on Short-Term Detection and Warning......................................................................... 2
Mitigation ............................................................................................................................. 4
The National Windstorm Impact Reduction Program....................................................... 4
Reauthorizing the National Windstorm Impact Reduction Program: H.R. 3820................ 5
Other Legislation ............................................................................................................ 6
Climate Change and Severe Weather ..................................................................................... 7
Forecasting and Warning: The Role of the National Weather Service ..................................... 8
Forecasting ..................................................................................................................... 9
Communicating the Severe Weather Risk ...................................................................... 10
Summary and Conclusions ........................................................................................................ 11
Figures
Figure A-1. Map Showing the Number of Recorded Tornadoes Greater than F3 in the
United States Between 1950 and 1998.................................................................................... 19
Tables
Table 1. LW&F, NWS and Total NOAA Funding from FY2004 to FY2010 ................................. 3
Table A-1. F-Scale and Enhanced F-Scale for Tornado Damage................................................. 20
Appendixes
Appendix. Risk from Severe Thunderstorms and Tornadoes ...................................................... 13
Contacts
Author Contact Information ...................................................................................................... 21
Acknowledgments .................................................................................................................... 21
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Severe Thunderstorms and Tornadoes in the United States
Overview
Severe thunderstorms and tornadoes affect communities across the United States every year,
causing fatalities, destroying property and crops, and disrupting businesses. State and local
governments are typically the first to respond to the consequences of extreme weather events, but
the federal government has responsibilities for forecasting and issuing warnings to citizens and
communities lying in harm’s way. When severe weather catastrophes overwhelm the resources of
state and local governments, the Stafford Act authorizes the President to issue major disaster or
emergency declarations, resulting in the distribution of a wide range of federal aid to those
affected.1 Also, U.S. Department of Agriculture programs, such as federal crop insurance and
emergency disaster loans, can help farmers recover financially from severe weather disasters even
without a presidential disaster declaration.2
Many observers note that although the number of lives lost each year to natural hazards in the
United States has decreased, the costs of major disasters continues to rise.3 According to the
National Science and Technology Council: “Due to changes in population demographics and
more complex weather-sensitive infrastructure, Americans today are more vulnerable than ever to
severe weather events caused by tornadoes, hurricanes, severe storms, heat waves, and winter
weather.”4
This report discusses issues that may be of interest to Congress in three general categories: (1)
forecasting and issuing warnings for severe thunderstorms and tornadoes; (2) the role of
mitigation; and (3) the effect of climate change. It also describes the role of the National Weather
Service in forecasting severe weather and communicating the risk to communities and
individuals. The Appendix describes in more detail the risk these hazards pose to communities
and individuals; where, when, how, and why they occur in the United States; and what damage
they may cause.
Issues for Congress
This report focuses on the risk from severe thunderstorms and tornadoes to U.S. citizens and
infrastructure, the federally sponsored forecast and warning systems, federally backed efforts to
improve the scientific understanding of severe weather phenomena, and efforts to mitigate the
risk of catastrophe. Congress has oversight and funding responsibilities for the federal agencies
charged with these tasks. At issue is whether those programs are effective at reducing damage,
injuries, and loss of life from severe thunderstorms and tornadoes.
1 For more information about the Robert T. Stafford Disaster Relief and Emergency Assistance Act (P.L. 100-707,
which amended and renamed the Disaster and Relief Act of 1974), see CRS Report RL33053, Federal Stafford Act
Disaster Assistance: Presidential Declarations, Eligible Activities, and Funding, by Keith Bea.
2 For more information on federal agricultural assistance, see CRS Report RS21212, Agricultural Disaster Assistance,
by Dennis A. Shields and Ralph M. Chite.
3 National Science and Technology Council, Committee on Environment and Natural Resources, Subcommittee on
Disaster Reduction, “Grand Challenges for Disaster Reduction” (Washington, DC, June 2005), p. 1. Hereafter referred
to as Grand Challenges for Disaster Reduction (2005).
4 Ibid., p. 4.
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Also at issue is the concept of disaster resilience; namely, those precautions and strategies—such
as improved building materials and structural systems—that decrease the vulnerability of
communities and individuals to severe thunderstorms and tornadoes. The federal role in
supporting programs of hazard mitigation, such as those included in the National Windstorm
Impact Reduction Act of 2004 (P.L. 108-360), is a concern for Congress. Authorization for the
Windstorm Impact Reduction Program expired at the end of FY2008, and it is not clear whether
the program achieved any of the goals specified in the legislation.
Projections of a changing climate for the United States and the possibility of a more intense
hydrologic cycle (e.g., more intense storms, rainfall, heat waves, and other phenomena) have
raised questions about whether the costs of severe weather disasters will continue to rise in the
future. Observers note that extreme events, more than shifts in average climate conditions, drive
changes in natural and human systems.5 According to the U.S. Climate Change Research
Program:
In the future, with continued global warming, heat waves and heavy downpours are very likely to
further increase in frequency and intensity. Substantial areas of North America are likely to have
more frequent droughts of greater severity. Hurricane wind speeds, rainfall intensity, and storm
surge levels are likely to increase. The strongest cold season storms are likely to become more
frequent, with stronger winds and more extreme wave heights.6
Distinguishing between increased frequency and intensity of extreme weather events due to
global warming and other factors that contribute to higher costs from disasters—such as changing
demographics in hazard-prone regions—also may be an important issue for Congress.7
Focus on Short-Term Detection and Warning
Policies that reduce U.S. vulnerability to severe thunderstorms and tornadoes will likely include
improvements in the capability to accurately detect storms and to effectively warn those in harm’s
way. The National Weather Service (NWS) is authorized by law to forecast weather and issue
warnings. How Congress chooses to fund NWS and conduct oversight of its activities bears
directly on the agency’s effectiveness.
In its strategic plan for 2005-2010, the NWS cited several preferred outcomes for the nation,
including (1) reduced loss of life, injury, and damage to the economy; and (2) better, quicker, and
more valuable weather and water information to support improved decisions.8 To achieve these
outcomes, NWS would employ a strategy to “improve the reliability, lead-time, and
understanding of weather and water information and services that predict changes in
environmental conditions.”9 Improving the lead-time and reliability of severe thunderstorm and
5 C. S. Parmesan et al., “Impacts of extreme weather and climate on terrestrial biota,” Bulletin of the American
Meteorological Society, vol. 83 (2000), pp. 443-450.
6 U.S. Climate Change Science Program, Synthesis and Assessment Product 3.3, “Weather and Climate Extremes in a
Changing Climate” (Washington, DC, June 2008), p. VII. Hereafter referred to as CCSP, Weather and Climate
Extremes in a Changing Climate (2008).
7 For example, see the paper by Roger A. Pielke, Jr., et al., “Normalized Hurricane Damages in the United States:
1900-2005,” Natural Hazards Review, vol. 9, no. 1 (2008), pp. 29-42, for a trend analysis of damage caused by
hurricanes normalized for changes in demographics, including wealth and population density.
8 NOAA, National Weather Service Strategic Plan for 2005-2010 (Jan. 3, 2005), p. 13; at http://www.nws.noaa.gov/sp/.
9 Ibid.
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tornado forecasts could provide communities and individuals with information to help them better
prepare and thus reduce their vulnerability.
NWS receives the most funding of any agency or program within NOAA’s budget, and the local
warnings and forecasts (LW&F) program receives approximately 70% of the NWS funding each
year. Table 1 shows appropriations for the LW&F program, NWS, and overall NOAA
appropriations since FY2004.10
Table 1. LW&F, NWS and Total NOAA Funding from FY2004 to FY2010
(millions of dollars, not adjusted for inflation)
Change
FY2004 FY2005 FY2006 FY2007a FY2008 FY2009 FY2010 FY04-FY10
LW&F $576.1 $560.5 $591.4 $616.5 $659.3 $682.3 $710.6 23.3%
NWS $824.9 $782.3 $848.2 $884.4 $911.4 $958.9 $999.8 21.2%
NOAAb $3,731.7 $3,918.7 $3,911.5 $3,684.1 $3,907.3 $4,373.9 $4,747.5
27.2%
Source: NOAA, Blue Book (Budget Summary) for FY2004 through FY2009. FY2010 values from personal
communication, Julie Hite, Chief, Budget Outreach and Communications Division, NOAA, January 26, 2010.
Notes: LW&F is the local warnings and forecasts program within NWS. Funding values used as reported in the
Control Tables chapter of the budget summaries (not adjusted for inflation) for FY2004-FY2009. Table does not
include $830 million provided to NOAA in P.L. 111-5, the American Recovery and Reinvestment Act (ARRA).
N/A is not available.
a. These values are from the FY2007 budget request.
b. FY2004-FY2009 values reflect the total enacted amounts, as reported in the Control Tables chapter of the
budget summaries in the NOAA Blue Book.
As Table 1 shows, spending on LW&F has increased at a higher percentage than total NWS
spending over the seven-year period The LW&F program averages nearly three-quarters of the
NWS budget each year, indicating that short-term weather prediction and warning is a high
priority for NWS and for NOAA, in accord with its statutory authority. Reports from RAND and
other research organizations suggest, however, that reorienting research and development funding
toward longer-term loss reduction efforts—particularly for weather-related hazards—might
provide the nation with more long-lasting solutions to reducing natural disaster losses.11 These
recommendations favor increased focus on mitigation techniques and R&D to reduce loss of life
and property from severe weather hazards.
10 These values do not reflect spending on NOAA programs included in P.L. 111-5, the American Recovery and
Reinvestment Act. The act provided $230 million for activities under the Operations, Research, and Facilities account;
and $600 million under the Procurement, Acquisitions, and Construction account. For more information on FY2010
NOAA funding, see CRS Report R40644, Commerce, Justice, Science, and Related Agencies: FY2010 Appropriations,
coordinated by Nathan James, Oscar R. Gonzales, and Jennifer D. Williams.
11 Charles Meade and Megan Abbott, “Assessing federal research and development for hazard loss reduction,” prepared
for the Office of Science and Technology Policy (RAND, Arlington, VA.), 2003, 65 p. See also “Advice to the New
Administration and Congress: Actions to Make our Nation Resilient to Severe Weather and Climate Change,” a
document produced by the University Corporation for Atmospheric Research and seven other stakeholder
organizations; at http://www.ucar.edu/td/.
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Mitigation
The National Science and Technology Council (NSTC) noted that the nation’s primary focus on
disaster response and recovery is “an impractical and inefficient strategy for dealing with these
ongoing threats. Instead, communities must break the cycle of destruction and recovery by
enhancing their disaster resilience” (italics added).12 Among the six “Grand Challenges”
identified in its report, the NSTC recommended that communities implement hazard mitigation
strategies and technologies, such as development of advanced construction materials and
structural systems that allow facilities to “remain robust in the face of all potential hazards.”13 The
report also recommended nonstructural mitigation measures, such as land use and zoning
regulations that recognize the risk of natural hazards. Land use measures, such as zoning laws and
building codes, are typically prerogatives of state and local governments, and thus the federal role
in that aspect of hazard mitigation is limited.14
The National Windstorm Impact Reduction Program
The federal role in developing disaster-resilient materials and structures, and evaluating their
relative effectiveness in mitigating damages from severe thunderstorms and tornadoes, has been
unclear. Congress attempted to clarify the federal role in mitigating damages from windstorms
(including tornadoes and thunderstorms) by passing the National Windstorm Impact Reduction
Act of 2004 (P.L. 108-360). The legislation identified three primary mitigation components: (1)
improved understanding of windstorms; (2) windstorm impact assessment; and (3) windstorm
impact reduction.
Authorization for the National Windstorm Impact Reduction Program expired on September 30,
2008. It is not clear whether the program made progress toward its objective: achievement of
major measurable reductions in the losses of life and property from windstorms. That goal may
have been difficult to achieve within the three-year authorization, even if appropriations for the
program matched authorization levels, and the program was well implemented. On these points,
the House Science and Technology Committee found that spending on the program by the four
agencies responsible for program implementation—the National Science Foundation (NSF), the
National Institutes of Standards and Technology (NIST), NOAA, and the Federal Emergency
Management Agency (FEMA)—was far below authorized levels, and the committee found that
the program had not been well implemented.15
12 Grand Challenges for Disaster Reduction (2005), p. 1.
13 Ibid., p. 8. For example, the report observes that the cumulative impacts on the hydrology of flooding, including flash
floods, must be incorporated into land use measures.
14 See also testimony given by Sharon Hays, Associate Director and Deputy Director for Science, Office of Science and
Technology Policy, to the Subcommittee on Technology and Innovation, House Science and Technology Committee,
Hearing: The National Windstorm Impact Reduction Program: Strengthening Windstorm Hazard Mitigation (July 24,
2008).
15 Subcommittee on Technology and Innovation, House Science and Technology Committee, Hearing: The National
Windstorm Impact Reduction Program: Strengthening Windstorm Hazard Mitigation (July 24, 2008), Hearing Charter.
The subcommittee estimated that NOAA, NIST, and FEMA spent approximately $7.45 million between FY2004 and
FY2008, approximately 17% of authorized amounts for the three agencies. Spending for NSF under the program is
more difficult to estimate, although NSF reportedly spent approximately $6.7 million on research related to the
windstorm reduction program in FY2008.
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The George W. Bush Administration cited many research activities and other programs underway
within each of the four agencies that contribute to goals of the windstorm program and to the
need to incorporate wind hazards reduction measures into an “all-hazards view.”16 A biennial
progress report discussed the importance of aspects of the program—specifically the goal of
increasing the effectiveness of existing program efforts through better coordination among
agencies.17 The report indicated that interagency coordination is improving through the work of
the Interagency Working Group, established under §204 of P.L. 108-360.18 However, it is difficult
to assess the effect of improved interagency coordination on the broader goals of the program—to
achieve measurable reductions in loss of life and property—or whether the law has compelled the
agencies to do more than what they had already been trying to achieve without passage of the
National Windstorm Impact Reduction Act. The Bush Administration further concluded that the
benefits of an improved understanding of severe wind hazards will not be fully realized until that
understanding is incorporated more completely into actions at the state and local level.19
Reauthorizing the National Windstorm Impact Reduction Program: H.R. 3820
In the 111th Congress, H.R. 2627 was introduced on May 21, 2009, as a stand-alone bill to
reauthorize the National Windstorm Impact Reduction Program. On October 15, 2009, H.R. 3820
was introduced to reauthorize both the national wind program (Title II) and the National
Earthquake Hazards Reduction Program (Title I). H.R. 3820 is entitled the “Natural Hazards Risk
Reduction Act of 2009,” and it finds that a major goal of federal natural hazards-related research
and development should be to reduce loss of life and damage to infrastructure through “increasing
the adoption of hazard mitigation measures.” Further, the bill finds that social science research is
needed to better understand the institutional, social, behavioral, and economic factors affecting
perceived risk, preparation, and response, so that risk mitigation measures can be increasingly
implemented. By calling for social science research, the bill appears to take an “all-hazards” view
toward understanding factors common across earthquake hazards and windstorm hazards, so that
households, businesses, and communities may adopt hazard mitigation measures for different
types of natural hazards.
In H.R. 3820, the Director of NIST would be responsible for planning and coordinating the
program, and NIST would be the lead agency,20 similar to its role as lead agency for the National
Earthquake Hazards Reduction Program.21 The other three agencies given responsibilities for the
program would be FEMA, NOAA, and NSF.
16 Sharon Hays, Associate Director and Deputy Director for Science, Office of Science and Technology Policy, to the
Subcommittee on Technology and Innovation, House Science and Technology Committee, hearing: The National
Windstorm Impact Reduction Program: Strengthening Windstorm Hazard Mitigation (July 24, 2008).
17 Windstorm Impact Reduction Program Biennial Progress Report for Fiscal Years 2005-2006.
18 Ibid.
19 Sharon Hays testimony (July 24, 2008).
20 Under the P.L. 108-360, the Director of the Office of Science and Technology Policy was responsible for
establishing an interagency working group and for selecting which agency would serve as chair of the working group.
21 Also known as NEHRP, a multiagency program reauthorized under P.L. 108-360 to reduce the national risk from
earthquake disasters. For more information, see CRS Report RL33861, Earthquakes: Risk, Detection, Warning, and
Research, by Peter Folger.
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H.R. 3820 would authorize appropriations for the four program agencies over a five-year period
through FY2014 for the following amounts:
• FEMA—$51,403,100;
• NSF—$51,402,100;
• NIST—$21,873,600; and
• NOAA—$12,030,500.
The total authorized amount for the program agencies from FY2009 through FY2014 would be
$136,710,300, which is more than the $72.5 million authorized under P.L. 108-360. (P.L. 108-360
authorized appropriations over a three-year period.) However, it is slightly less than the $150
million that would be authorized for a five-year period by H.R. 2627, the stand-alone bill
reauthorizing the wind hazards program. H.R. 2627 would distribute authorized appropriations
among more agencies than H.R. 3820, and would include the Department of Transportation, the
National Aeronautics and Space Administration, and the U.S. Army Corps of Engineers as wind
hazards program agencies along with NIST, FEMA, NOAA, and NSF.
In addition to reauthorizing the earthquake and wind programs, H.R. 3820 would establish an
interagency coordinating committee (under Title III of the bill) that would oversee planning and
coordination of both the earthquake and wind hazards programs and would make
recommendations for planning and coordination of any other federal research program for natural
hazard mitigation. Title III appears to reflect an “all-hazards” approach to planning and
coordinating, as mentioned above, and would add more agencies to the coordinating committee,
including the U.S. Geological Survey, the Office of Science and Technology Policy, and the
Office of Management and Budget, as well as the head of any other agency the coordinating
committee considers appropriate. The interagency coordinating committee would be responsible
for developing strategic plans for the earthquake and wind hazards programs, issuing progress
reports on both programs, and establishing outside advisory committees. Federal employees
would be precluded from serving on the outside advisory committees. The committees would
perform assessments on the earthquake and wind hazards programs, and would submit reports on
their assessments every two years to the chair of the interagency coordinating committee.
Other Legislation
Members of Congress have recognized the increased vulnerability to tornadoes of citizens living
in manufactured housing. On December 2, 2009, the House passed H.R. 320, CJ’s Home
Protection Act of 2009, which awaits further action by the Senate Committee on Banking,
Housing, and Urban Affairs. The bill would amend § 604 of the National Manufactured Housing
Construction and Safety Standards Act of 2004 (42 U.S.C. 5403) to require that all manufactured
homes be equipped with a NOAA Weather Radio.22 NOAA Weather Radios would presumably
give the residents of manufactured homes a better chance of receiving tornado warnings and
allow them to take precautionary measures. In the 110th Congress, the House passed similar
legislation, H.R. 2787, but the Senate did not act on its version of the bill, S. 2724.
22 NOAA Weather Radio broadcasts official NWS warnings, watches, forecasts, and other hazard information 24 hours
a day, seven days a week, to all 50 states, adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands, and the U.S.
Pacific Territories. See http://www.nws.noaa.gov/nwr/.
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In 2003, the 108th Congress passed the Tornado Shelters Act (P.L. 108-146), which amended the
Housing and Community Development Act of 1974 (42 U.S.C. 5305(a)) to authorize
communities to use Community Development Block Grant (CDBG) funds for construction of
tornado-safe shelters in manufactured home parks. The law was aimed at communities of at least
20 manufactured homes that consist predominately of low- and moderate-income residents. To be
eligible, the community has to be located in a state that was struck by a tornado during the fiscal
year when funds were made available or during the previous three fiscal years.23
Climate Change and Severe Weather
According to several reports that synthesize recent findings in the scientific literature, there will
likely be changes in the intensity, duration, frequency, and geographic extent of weather and
climate extremes as the Earth continues to warm.24 These changes may continue upward trends
that have already been observed, such as the frequency of unusually warm nights, frequency and
intensity of extreme precipitation events, and the length of the frost-free season.25 However, the
evidence is unclear as to whether the frequency and intensity of severe thunderstorms and
tornadoes has increased or will increase in the future due to climate change.
Part of the difficulty in sorting out trends in frequency and intensity of severe thunderstorms and
tornadoes lies in the way they have been observed and reported. For example, the number of
annual reported tornado occurrences has doubled between 1954 and 2003.26 Some studies indicate
that the doubling reflects changes in observing and reporting. When the apparent trend produced
by these changes is removed, the adjusted data show little or no trend in the number of reported
tornadoes since the 1950s.27
Reports of severe thunderstorms without associated tornadoes increased by a factor of 20 between
1955 and 2004.28 However, researchers indicate that the increase is mainly in reports of
marginally severe thunderstorms, and suggest that the evidence for a change in the long-term
trend of severe thunderstorms is lacking.29 In studies that re-analyze environmental conditions
that could have produced severe thunderstorms, changes in the frequency of those environmental
conditions are observed. However, the record of observations may not be long enough to
determine the range of natural variability.30 Given the uncertainty, it is not yet possible to
determine if these changes are due to natural variability or changing climatic conditions from
greenhouse warming.
Excessive rainfall from severe thunderstorms may trigger flash floods, which are typically
responsible for most flood-related deaths in the United States each year. According to researchers,
23 For more information on the CDBG program, see CRS Report RL33330, Community Development Block Grant
Funds in Disaster Relief and Recovery, by Eugene Boyd and Oscar R. Gonzales.
24 CCSP, Weather and Climate Extremes in a Changing Climate (2008); IPCC, Climate Change 2007: Synthesis
Report. Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel
on Climate Change (Geneva, Switzerland), 2007, 104 p.
25 CCSP, Weather and Climate Extremes in a Changing Climate (2008), p. 35.
26 Ibid., p. 76.
27 Ibid., Figure 2.25.
28 Ibid., p. 77.
29 Ibid.
30 Ibid.
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one of the clearest trends over the last 30 years has been an increase in the frequency and
intensity of heavy precipitation events.31 Despite this clear trend, the relationship between these
heavy precipitation events and the frequency and intensity of severe thunderstorms is not clear,
nor do scientists agree on the relationship between increased precipitation and streamflow
extremes, such as flooding.32 As a further complication, studies of trends in streamflow using
similar data have produced different results.33 Also, human influences on streamflow, such as
building dams and creating large reservoirs, may mask climatic changes.34
Climate model predictions suggest that with continued global warming due to increasing
concentrations of atmospheric greenhouse gases, precipitation intensity is projected to increase.35
One set of research findings, for example, indicates that global precipitation increased by 7.4%
per degree Celsius rise during 1987-2006 (plus or minus 2.6%).36 Other researchers suggest that a
20-year period is too short to infer any long-term changes, but that the relationship between
warming and more precipitation likely holds true.37 However, it is unclear whether this
intensification of precipitation is or will be linked in the future to more severe thunderstorms, and
to more severe or more frequent flash floods.
Forecasting and Warning: The Role of the National Weather Service
The NWS, at the discretion of the Secretary of Commerce, has statutory authority for weather
forecasting and for issuing storm warnings (15 U.S.C. §313). The NWS provides weather, water,
and climate forecasts and warnings for the United States, its territories, adjacent waters, and
ocean areas. The NWS employs 4,700 employees in 122 weather forecast offices, 13 river
forecast centers, national centers, and other supporting agencies around the country.38
Each of the 122 weather forecast offices is equipped with technologies for observing, forecasting,
and warning of severe thunderstorms and tornadoes.39 These technologies include Weather
Surveillance Radar (WSR-88D, also known as NEXRAD, a network of 161 radars), Automated
Surface Observing Systems (ASOS)40 at over 1,200 sites, access to data from two Geostationary
31 Ibid., pp. 46-47.
32 Personal communication, Kenneth Kunkel, Executive Director of the Division of Atmospheric Science, Desert
Research Institute, Reno, NV, Sept. 22, 2009.
33 See, for example, H. F. Lins and J. R. Slack, “Streamflow trends in the United States,” Geophysical Research
Letters, vol. 26 (1999), pp. 227-230; and P. Ya. Groisman et al., “Heavy precipitation and high streamflow in the
contiguous United States: trends in the 20th century,” Bulletin of the American Meteorological Society, vol. 82 (2001),
pp. 219-246.
34 CCSP, Weather and Climate Extremes in a Changing Climate (2008), p. 53.
35 Ibid., p. 102.
36 F. J. Wentz et al., “How much more rain will global warming bring?,” Science, vol. 317, no. 5835 (2007), pp. 233-
235.
37 Michael Previdi and Beate G. Liepert, “Interdecadal variability of rainfall on a warming planet,” Eos, vol. 89, no. 21
(May 20, 2008), pp. 193-195.
38 The nine centers of the NWS include the Aviation Weather Center, Climate Prediction Center, Environmental
Modeling Center, Hydrometeorological Prediction Center, National Center for Environmental Protection, Ocean
Prediction Center, Space Weather Prediction Center, Storm Prediction Center, and Tropical Prediction Center. See
http://www.nws.noaa.gov/organization.php.
39 Joseph H. Golden and Christopher R. Adams, “The Tornado Problem: Forecast, Warning, and Response,” Natural
Hazards Review (May 2000), p. 107.
40 ASOS is a climatological observing network that generates weather reports at hourly intervals, except when weather
(continued...)
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Operational Environmental Satellites (GOES 8 and 10), and the Automated Weather Interactive
Processing System (AWIPS).41
Forecasting
Severe thunderstorm and tornado forecasts are made by the Storm Prediction Center (SPC) and
local weather forecast offices. Forecasters at the SPC use numerical weather prediction models to
determine if atmospheric conditions, temperature, and wind flow patterns may lead to formation
of severe weather. The SPC uses its suite of products to relay forecasts of organized severe
weather as much as three days ahead of time, and continually refines the forecast up until the
event has concluded.42 The severe weather forecast process typically follows the following
pattern: (1) convective outlook; (2) mesoscale discussion; (3) watch; and (4) warning.
Convective Outlook
The severe weather forecast process typically begins with a forecast issued one to two days in
advance of where both severe and non-severe thunderstorms are expected to occur around the
country.43 This is known as a convective outlook. Areas of possible severe thunderstorms are
labeled slight risk, moderate risk, or high risk, depending upon the coverage and intensity of
expected severe thunderstorms in a region. The outlooks are the first severe weather threat
notifications that the local NWS offices and local emergency officials receive.
Mesoscale Discussion
As a convective outlook area becomes more defined, a next step in the forecast process is often
needed to describe an evolving severe weather threat. This is known as a mesoscale discussion.44
Mesoscale discussions contain information that helps forecasters at local NWS offices understand
the causes and prepare for the types of severe weather expected.
Watch
If development of severe thunderstorms or tornadoes is imminent, or likely to occur in the next
several hours, the next step is a severe storm watch. Such watches alert the public, aviators, and
local NWS offices that environmental conditions have become favorable for the development of
severe storms or tornadoes. Following the issuance of a severe storm watch, local networks of
(...continued)
conditions are changing rapidly.
41 AWIPS is an interactive computer system that integrates meteorological and hydrological data from an array of
meteorological sensors—radar, satellites, surface instruments—and enables the forecaster to prepare and issue more
accurate and timely forecasts and warnings.
42 The SPC suite of products includes satellite imagery, radars, surface weather stations, weather balloon soundings,
wind profilers, lightning detection network, and information from local NWS offices. See http://www.spc.noaa.gov/
misc/aboutus.html.
43NOAA Storm Prediction Center, the Severe Storms Forecast Process: Outlook to Mesoscale Discussion to Watch to
Warning, at http://www.spc.noaa.gov/misc/aboutus.html.
44 Ibid.
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storm spotters45 are activated, and forecasters in the threat area closely monitor radar imagery and
spotter reports to issue the appropriate severe thunderstorm and tornado warnings.46
Warning
As the severe weather threat continues to develop, the local NWS offices and the storm spotters
try to detect severe thunderstorms and tornadoes using radar or other detection technology and
visual evidence. When severe hail, damaging winds, or a tornado appears imminent from radar or
visual evidence, local NWS offices will issue a severe thunderstorm or tornado warning as
appropriate. The warning contains specific language about areas at risk, time frames, specific
hazards, recommended protective behavior for those at risk, and the office issuing the warning.47
Communicating the Severe Weather Risk
Several methods exist to communicate alerts and warnings to the public. The NWS maintains and
operates NOAA Weather Radio (NWR). NWR is a nationwide network of radio stations
broadcasting continuous weather information directly from the nearest NWS office. The NWR
works with the Emergency Alert System (EAS). The EAS is an automated simultaneous
retransmission system that allows NWS warnings to be disseminated over most radio and
television networks, and over cable and satellite TV systems.48 NOAA Weather Radio broadcasts
official NWS warnings, watches, forecasts, and other hazard information 24 hours a day, 7 days a
week, to all 50 states, adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands, and the U.S.
Pacific Territories.49
Issuing severe weather warnings to the public has evolved into what some observers term a
weather warning partnership: a roughly triangular exchange of information between the NWS,
private forecasters and the news media, and local emergency managers. The objective of the
weather warning partnership is to provide a consistent warning message to the public at risk.50
The NWS depends on weather warning partnerships with the electronic news media and local and
state emergency management officials to ensure that communities are prepared for severe weather
outbreaks and to further communicate the outlooks, watches, and warnings to the public.51 Many
emergency management officials and news media monitor NWS outlooks so that they have
enough lead time for activating preparedness capabilities such as storm spotters, increasing
response levels, and preparing to activate the warning communication systems. The partnership is
essential in guaranteeing that there is a shared understanding of the weather threats and that
45 Storm spotters report critical weather information in real time to the NWS from a specific location.
46 NOAA Storm Prediction Center, the Severe Storms Forecast Process: Outlook to Mesoscale Discussion to Watch to
Warning, at http://www.spc.noaa.gov/misc/aboutus.html.
47 Joseph H. Golden and Christopher R. Adams, “The Tornado Problem: Forecast, Warning, and Response,” Natural
Hazards Review (May 2000), p. 111-112.
48 EAS is administered by FEMA, in cooperation with the Federal Communication Commission and NWS. For more
information on EAS, see CRS Report RL32527, The Emergency Alert System (EAS) and All-Hazard Warnings, by
Linda K. Moore.
49 NOAA Weather Radio, at http://www.nws.noaa.gov/nwr/.
50 R.A. Maddux, “The Weather Warning Partnership,” Proceedings from the Hazards Research and Applications
Workshop, Natural Hazards Research and Applications Information Center, University of Colorado, Boulder (1991).
51 Golden and Adams (2000), p. 112.
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accurate warning information is communicated to the public at risk. Observers have noted that
this shared understanding helps prevent conflicting warnings—which could lead to delays in
seeking shelter—from being communicated to the public.52
Summary and Conclusions
Congress may consider several options for potentially reducing the costs of severe thunderstorms
and tornadoes: improving detection and warning systems; fostering efforts to build more resilient
buildings and infrastructure; and supporting research and development to better understand why
and where severe thunderstorms and tornadoes occur, as well as other measures. Whether and
how climate change is influencing or could affect the frequency and intensity of thunderstorms
and tornadoes is not yet evident. Thus it is not clear whether long-term efforts to mitigate
greenhouse gas-induced global warming—such as by reducing emissions of carbon dioxide and
other gases—will also mitigate damage to property and reduce injuries and losses of life from
severe thunderstorms and tornadoes.
Enhancing the scientific understanding of how and why severe thunderstorms and tornadoes
form, and improving the accuracy and timeliness of forecasting and warning systems, will likely
provide individuals and communities in the United States better information to help them avoid
damage and injury from severe weather events. The role of the federal government in weather and
climate research, thunderstorm and tornado forecasting, and issuing warnings is substantial.
Spending on weather forecasts and warnings comprises the bulk of the NWS budget, which is
itself the largest component of NOAA’s annual budget. Several other federal agencies contribute
to the weather and climate enterprise, including NSF, NASA, the U.S. Geological Survey, and
others. The federal investment in weather-related response and recovery, including programs at
the Department of Agriculture and FEMA, is also substantial.
Many observers and stakeholders call for increased funding for improving the understanding of
physical processes that produce extreme events, such as severe thunderstorms and tornadoes, and
how these processes change with climate.53 Observers and stakeholders are broadly in agreement
about the types of R&D needed, such as integrated data and observation systems, improved
remote sensing capabilities, better modeling capability, and others.54 Even if funding increased
substantially, however, it may not necessarily lead to significant decreases in damages, injuries, or
deaths from severe thunderstorms and tornadoes. Shifting populations, changes in wealth density,
and construction of dense infrastructure in areas prone to severe weather could offset
improvements in forecasting and warning systems:
...the potential for considerable loss of life and property due to tornadoes continues to exist,
especially in highly vulnerable regions of the country. Further, the increasing population and
migration patterns of this population suggest that the overall vulnerability and risk to humans and
52 Golden and Adams (2000).
53 CCSP, Weather and Climate Extremes in a Changing Climate (2008), p. 122. See, for example, “Advice to the New
Administration and Congress: Actions to Make our Nation Resilient to Severe Weather and Climate Change,” a
document produced by the University Corporation for Atmospheric Research and seven other stakeholder
organizations; at http://www.ucar.edu/td/. The organizations call for $9 billion in funding for weather and climate-
related federal expenditures in addition to currently estimated spending over the next five years.
54 Grand Challenges for Disaster Reduction (2005), p. 14.
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their property may amplify in the future despite improvements in forecasting, detection, and
warning dissemination.55 (References omitted.)
The Super Tuesday Tornado Outbreak of 2008 seems to support research results indicating that
demographics and other socioeconomic and behavioral factors combine to make the mid-South
regions particularly vulnerable to tornado fatalities. In a report following the Super Tuesday
Tornado Outbreak, NOAA observed several factors consistent with research pointing to the
importance of social and demographic factors in determining risk from tornadoes. 56 These were:
• 63% of the fatalities were in manufactured houses;
• most fatalities occurred at night;
• most areas affected by the tornadoes were heavily forested;
• many reported that February was not a month in what they perceived as “tornado
season.”
In addition, implementing hazard mitigation strategies may include developing and enforcing
land-use planning and zoning laws, which are traditionally state and local issues and not
Congressional concerns per se.57 How federal agencies disseminate the results of federally
sponsored R&D, from activities such as those authorized in the National Windstorm Impact
Reduction Program, to states and local communities, may be more squarely in Congress’s
purview, and more directly addressed through oversight of the programs and annual
appropriations for the participating agencies.
55 Walker S. Ashley, “Spatial and Temporal Analysis of Tornado Fatalities in the United States: 1880 - 2005,” Weather
and Forecasting, vol. 22 (December 2007), p. 1226.
56 U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Super
Tuesday Tornado Outbreak of February 5-6, 2008, March 2009, p. 20.
57 See, for example, Grand Challenge #3—develop hazard mitigation strategies and technologies; one of six grand
challenges developed by the National Science and Technology Council, Grand Challenges for Disaster Reduction
(2005). Arguably, the National Flood Insurance Program (NFIP)—administered by FEMA—is an example of federal
involvement in local community development. The NFIP makes flood insurance available to communities that agree to
adopt and enforce floodplain management ordinances. For more information on the NFIP, see CRS Report RL34610,
Midwest Flooding Disaster: Rethinking Federal Flood Insurance?, by Rawle O. King.
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Appendix. Risk from Severe Thunderstorms and
Tornadoes
This appendix discusses why severe thunderstorms and tornadoes are threats to some areas of the
country and not others, and why they occur during some parts of the year and not others. It also
describes the role of the National Weather Service (NWS) in forecasting and issuing warnings,
and its relationship to private forecasters and the news media in providing clear and consistent
messages to the public at risk.
Thunderstorms and tornadoes affect U.S. citizens and communities every year, albeit rarely with
the same level of widespread destruction as a major hurricane or flood. Although floods are one
consequence of severe thunderstorms, floods that cause widespread and prolonged destruction are
typically not annual events in the United States. Major floods, such as those that affected parts of
the Mississippi River region in the Midwest in 2008 and in 1993, are the result of many factors
and are not solely caused by heavy rains from severe thunderstorms.58
Severe Thunderstorms
Compared to tropical storms such as hurricanes, thunderstorms are small and short-lived, but can
still be dangerous. An average thunderstorm is 15 miles in diameter and lasts an average of 30
minutes. Thunderstorms occur much more frequently than large tropical storms. There are an
estimated 100,000 thunderstorms in the world each year, of which 10% are severe.59 A severe
thunderstorm is defined by the NWS as one that produces hail at least three-quarters of an inch in
diameter, has winds of 58 miles per hour or higher, or produces a tornado.60
Severe thunderstorms may produce lightning, high winds, hail, flash floods, and tornadoes, any of
which may be a hazard to people and property. Strong, straight-line winds can knock down trees
and power lines, and can sometimes cause damage equal to that caused by many tornadoes.61
Downbursts—outward bursts of damaging winds on or near the ground—can cause wind shear
and lead to aircraft accidents. Tornadoes (discussed separately below), the most destructive
phenomenon associated with thunderstorms, can destroy structures and cause fatalities.62
Risks from Severe Thunderstorms
Severe thunderstorms can produce lightning, high winds, hail, and heavy rainfall that may lead to
flash flooding. All of these phenomena may pose a risk to people and property depending on their
location and the storm’s intensity.
58 For a list of CRS experts on the Midwest flooding of 2008, see http://www.crs.gov/experts/WE04010.shtml.
59 National Severe Storms Laboratory, “Severe Weather Primer: Thunderstorms,” at http://www.nssl.noaa.gov/primer/
tstorm/tst_basics.html.
60 NWS, “A Comprehensive Glossary of Weather Terms for Storm Spotters,” at http://www.srh.noaa.gov/oun/
severewx/glossary4.php.
61 Straight-line winds are strong winds produced by a thunderstorm that are not associated with rotation, as
distinguished from tornadoes, which are narrow, violently rotating columns of air extending from the base of a
thunderstorm to the ground.
62 National Severe Storms Laboratory, “Severe Weather Primer: Thunderstorms,” at http://www.nssl.noaa.gov/primer/
tstorm/tst_damage.html.
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Lightning
Lightning is commonly considered the most dangerous and most frequently encountered weather
hazard. Between 1977 and 2006, an average of 62 people were killed each year by lightning in
the United States. Lightning-caused fatalities are often highest each year in Florida.63 Lightning is
also the primary cause of wildfires, which threaten natural resources, homes, businesses, and
lives, particularly in the West. NOAA’s National Severe Storms Laboratory estimates that lighting
causes approximately $4-$5 billion in damage each year, affecting buildings, communications
systems, power lines, and electrical systems.64
High Winds
Damage caused by severe straight-line winds during thunderstorms is more common than damage
caused from tornadoes.65 During severe thunderstorms, straight-line wind speeds may reach up to
100 miles per hour (damaging winds are classified as those exceeding 50-60 mph).66 Estimates of
the annual amount of damage caused by high winds are not provided in this report because wind
damage from tropical storms, thunderstorms, and tornadoes are often reported together.
Damaging winds can develop with little or no advanced warning. Microbursts—one category of
damaging winds—are dangerous to aviation and can occur in an isolated rain shower or
thunderstorm.67 Downbursts or microbursts may produce wind shear—a variation in wind speed
and/or direction over a short distance—which can slow airspeed and cause an aircraft to lose
altitude when a plane is taking off or landing and is near the ground.
Hail
Although Florida typically experiences the most thunderstorms in the United States each year,
Nebraska, Colorado, and Wyoming normally experience the most hail storms.68 Crops are
particularly vulnerable to hail damage; even relatively small hail can severely damage plants in
minutes. Hail greater than three-quarters of an inch in diameter is considered severe and
potentially damaging to aircraft. Hail also damages vehicles, roofs of buildings and homes, and
landscaping. Damage from hail approaches $1 billion in the United States each year.69 Hail has
been known to cause injury to humans, and occasionally has been fatal.
63 Florida has more lightning strikes than any other state and has the fourth-highest population in the United States. On
average, 10 people die each year in Florida from lightning. National Weather Service, “Natural Hazard Statistics,” at
http://www.weather.gov/os/hazstats.shtml#.
64 National Severe Storms Laboratory, “Severe Weather Primer: Lightning,” at http://www.nssl.noaa.gov/primer/
lightning/ltg_damage.html. The National Lightning Safety Institute indicates that losses from lightning may be $5-$6
billion per year. See http://www.lightningsafety.com/nlsi_lls/nlsi_annual_usa_losses.htm.
65 National Severe Storms Laboratory, “Severe Weather Primer: Damaging Winds,” at http://www.nssl.noaa.gov/
primer/wind/wind_basics.html.
66 Ibid.
67 Microbursts are small, concentrated downbursts from a thunderstorm, usually less than 4 kilometers across, that
produce an outward burst of damaging winds at the ground surface. Downbursts are similar to microbursts but larger,
usually greater than 4 kilometers across. See http://www.nssl.noaa.gov/primer/wind/wind_basics.html.
68 In the high plains of these three states the freezing levels are much closer to the ground than at sea level. At sea level
the hail has more time to melt before reaching the ground. See National Severe Storms Laboratory, “Severe Weather
Primer: Hail,” at http://www.nssl.noaa.gov/primer/hail/hail_damage.html.
69 Ibid.
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Flash Floods
Floods are a common and widespread natural hazard in the United States.70 As discussed in this
report, flash floods can cause significant damage and fatalities, but they result from short-lived
thunderstorms, and not from a prolonged weather pattern that produces higher than normal
amounts of precipitation over several days or weeks.71
Flash floods are short in duration.72 They are most commonly associated with thunderstorms,
severe weather, and melting snow or ice. Flash floods can occur within minutes or a few hours of
excessive rainfall, such as that from a severe thunderstorm or a series of thunderstorms occurring
over the same location. Because flash floods can occur suddenly and with little warning, they are
the most dangerous types of floods; typically most flood-related deaths each year in the United
States are caused by flash floods.73 It is difficult to assess the costs of actual damage from flash
floods each year; cost estimates may vary widely, and the actual costs may not consistently
correlate to preliminary estimates.74
Where and When Severe Thunderstorms Form
Thunderstorms occur most frequently over the Florida peninsula and in other parts of the
Southeast, although the most severe weather threat from thunderstorms extends from Texas to
southern Minnesota along the Great Plains and midwestern United States.75 Thunderstorms are
most likely to occur in the spring and summer and during the afternoon and evening. In the Great
Plains, most thunderstorms occur in the afternoon and at night; and along the Gulf Coast,
southeastern United States, and western states they occur most frequently in the afternoon.76
The greatest potential for severe weather develops in geographical regions that are subject to
warm, humid air at low levels, while dry, conditionally unstable air prevails aloft. Thunderstorms
form during the summer in the southern Great Plains when a southerly flow of warm, very moist
air from the Gulf of Mexico meets with a dry, westerly current aloft. The thunderstorms that form
in Colorado, Arizona, and New Mexico are due to orographic lifting—ascending airflow caused
by the Rocky Mountains. Few thunderstorms occur along the west coast of the United States
because this region is frequently influenced by cooler, maritime air masses that suppress
convectional uplift over land.77
70 Slowly developing and widespread floods, such as the 2008 and 1993 floods along the Mississippi River, can cause
billions of dollars in flood-related damages, although the number of deaths from floods is small in the United States
relative to some other countries. For example, in 1998 floods resulting from Hurricane Mitch resulted in over 9,000
deaths in Central America, although some sources estimate as many as 18,000 deaths from Hurricane Mitch. See Roger
A. Pielke, Jr. and Mary W. Downton, “Precipitation and damaging floods: Trends in the United States, 1932-97,”
Journal of Climate, vol. 13 (Oct. 15, 2000), pp. 3625-3637.
71 Significant flooding along the main stems of large river basins, such as the Mississippi River, results from excessive
precipitation over weeks or months. See CCSP, Weather and Climate Extremes in a Changing Climate (2008), p. 50.
72 A National Weather Service definition of flash flood is a flood caused by heavy or excessive rainfall in a short period
of time, generally less than six hours.
73 Deaths due to flooding created by Hurricane Katrina in 2005 far exceeded the annual average number of flash flood-
related deaths; however, many complicating factors make comparison between hurricane-induced flooding and flash
floods difficult, such as the role of levees, storm surge, tides, and other factors.
74 Mary W. Downton and Roger A. Pielke, Jr., “How accurate are disaster loss data? The case of U.S. flood damage,”
Natural Hazards, vol. 35 (2005), pp. 211-228.
75 NOAA National Severe Storms Laboratory, at http://www.nssl.noaa.gov/primer/tstorm/tst_climatology.html.
76 Ibid.
77 PhysicalGeography.net, “Thunderstorms and Tornadoes,” at http://www.physicalgeography.net/fundamentals/
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How and Why Thunderstorms Form
A thunderstorm forms when moist, unstable air is vertically lifted in the area by unequal warming
of the Earth’s surface, orographic lifting due to a topographic obstruction (such as a mountain or
mountain range), or the presence of a weather front. Three types of thunderstorms can produce
severe weather: a squall line, a multicell storm, and a supercell storm.78
Squall Line
A squall line is a line of storms with a continuous, well developed gust front—a boundary that
separates a cold downdraft of a thunderstorm from warm, humid surface air—at the leading edge
of the line. Severe weather frequently occurs near the updraft/downdraft interface at the storm’s
leading edge. Downburst winds are the main threat. Hail as large as golf balls along with
gustnadoes—weak and short lived tornadoes—can occur. Flash flooding can occur when the
squall line slows down or even becomes stationary, with thunderstorms forming parallel to the
line and repeatedly moving across the same area.
Multicell Storm
A multicell storm consists of a group of cells moving as a single unit, with each cell in a different
stage of the thunderstorm life cycle.79 As the multicell storm evolves, individual cells take turns at
being the most dominant. New cells tend to form along the upwind (typically western or
southwestern) edge of the cluster, with mature cells located at the center and dissipating cells
found along the downwind (eastern or northeastern) portion of the cluster. Multicell storms come
in a variety of shapes, sizes, and intensities.80 They are stronger than single cell thunderstorms,
but less severe than supercell storms. Each cell in a multicell storm lasts about 20 minutes;
however, the multicell cluster may persist for several hours. Most flash floods occur during
multicell storm events.81
Supercell Storm
A supercell storm is defined as a storm with a persistent rotating updraft in which the entire storm
behaves as a single entity, rather than as a group of cells.82 These supercell storms are the most
dangerous and rarest of the thunderstorms; they produce strong downbursts of 80 mph or more
and damaging hail, and they can last for hours. Some are very prolific precipitation producers,
whereas others produce very little precipitation that reaches the ground. The leading edge of the
(...continued)
7t.html.
78 John M. Wallace and Peter V. Hobbs, Atmospheric Science: An Introductory Survey (San Diego, CA: Academic
Press, 1977), p. 240.
79 A cell in meteorology refers to an updraft or downdraft or combination of both. Updrafts and downdrafts, or their
combination represent types of convection in an unstable atmosphere. The terms convection and thunderstorm are often
used interchangeably, although thunderstorms are only one form of convection. For more information on
meteorological terminology, see http://www.srh.noaa.gov/oun/severewx/glossary.php.
80 Wallace and Hobbs, pp. 244 - 245.
81 The Weather World 2010 Project, University of Illinois, Multicell Cluster Storms, at http://ww2010.atmos.uiuc.edu/
(Gh)/guides/mtr/svr/type/clstr/home.rxml.
82 Wallace and Hobbs, pp. 245-248.
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precipitation from a supercell is usually light rain. Heavier rain falls closer to the updraft with
torrential rain and/or large hail immediately north and east of the main updraft. Severe weather
tends to form near the main updraft, typically towards the rear of the storm.83 Most large and
violent tornadoes come from supercell storms.
Tornadoes
Tornadoes—the most violent storms on Earth—can sometimes produce winds that exceed 300
mph. They are the destructive products of severe thunderstorms, and second only to flash
flooding as the cause for convective storm related fatalities.
Risks from Tornadoes
Damages from violent tornadoes seem to be increasing, similar to the trend for other natural
hazards. According to some insurance industry analysts, losses of $1 billion or more from single
tornado events are becoming more frequent.84 For example, the tornado outbreak of February 5-6,
2008 (dubbed the Super Tuesday Tornado Outbreak), in several southern states (including
Arkansas, Kentucky, Alabama, and Tennessee) caused approximately $850 million in insured
losses and 57 fatalities.85 Insurance industry analysts indicate that tornadoes, severe
thunderstorms, and related weather events (such as hailstorms, but not hurricanes or earthquakes)
have caused nearly 57%, on average, of all insured catastrophe losses in the United States in any
given year since 1953.86
Fatalities caused by tornadoes have declined significantly since the 1930s, generally because of
improved forecasting, warning systems, and increased public awareness of the tornado risk.
However, some researchers suggest that the decline is unlikely to continue and may have already
stopped.87 These findings attribute the stalled decline to increasing vulnerability due to
demographic factors, rather than shortcomings in tornado forecasts and warnings.88 The results
would suggest that there are limits to the number of potential lives saved by improvements in
forecasting ability and warning systems, and that social, behavioral, and demographic factors may
play an increasingly important role in tornado-related fatalities.89 Other stakeholders, however,
emphasize the need for increased investment in observations, computing power, research and
weather modeling to improve the nation’s resilience to severe weather.90
83 NOAA National Severe Storms Laboratory, at http://www.nssl.noaa.gov/primer/tstorm/tst_basics.html.
84 A. M. Best Company, Inc., Tornado Losses Approach Those of Hurricanes (Oldwick, NJ: A. M. Best Company,
2008), p. 1.
85 A. M. Best Company, Inc., p. 1 and Exhibit 4. According to the report, the costliest thunderstorm/wind/tornado event
occurred between May 2 and May 11, 2003, affecting 18 states and resulting in over $3.2 billion in insured damages. A
National Weather Service report released in March 2009 stated that preliminary damage estimates of the 2008 tornado
outbreak were $520 million; see U.S. Department of Commerce, National Oceanic and Atmospheric Administration,
National Weather Service, Super Tuesday Tornado Outbreak of February 5-6, 2008, March 2009, at
http://www.weather.gov/os/assessments/pdfs/super_tuesday.pdf.
86 A. M. Best Company, Inc., Tornado Losses Approach Those of Hurricanes, p. 7.
87 Walker S. Ashley, “Spatial and Temporal Analysis of Tornado Fatalities in the United States: 1880 - 2005,” Weather
and Forecasting, vol. 22 (December 2007), p. 1216.
88 Ibid.
89 See, for example, Dennis S. Mileti, Disasters by Design: A Reassessment of Natural Hazards in the United States
(Washington, DC: Joseph Henry Press, 1999), p. 226.
90 See, for example, “Advice to the New Administration and Congress: Actions to Make our Nation Resilient to Severe
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Most tornado fatalities occur within housing structures, unlike fatalities from floods, which occur
in vehicles. This indicates that people are more likely to seek shelter during tornadic events;
however, the risk of injury or death seems to depend on what type of housing stock they choose
for shelter. For example, one study indicated that manufactured houses, such as mobile homes,
are particularly vulnerable to tornadoes. Mobile home deaths accounted for an average of 44% of
all deaths caused by tornadoes between 1985 and 2005, and showed an increasing trend over the
20-year period.91 During the same time period, tornado-related deaths within permanent homes
fluctuated between 20% and 30%, and deaths in vehicles decreased to 9.9% of all tornado-related
deaths, on average.92 The study concluded that the high percentage of mobile homes in the
Southeast may be the key factor explaining why most tornado-related deaths occur in lower
Arkansas, Tennessee, and lower Mississippi River valleys. For example, 63% of the fatalities
during the 2008 Super Tuesday Tornado Outbreak occurred in manufactured houses.93
Since 1950, violent tornadoes were responsible for 67.5% of all tornado deaths in the United
States, yet comprise only 2.1% of all tornadoes.94 The number of fatalities caused by less violent
tornadoes is also significant, and some studies suggest that the percentage of fatalities caused by
less violent tornadoes has increased since the 1970s.95 (See below for an explanation of how
violent tornadoes and less destructive tornadoes are classified: the F-Scale and enhanced F-scale.)
Where and When Tornadoes Form
Tornadoes have been reported on all continents except Antarctica; however, they occur most
commonly in North America and particularly in the United States.96 They can occur in all 50
states but they form most commonly in three regions: (1) a swath of the Midwest extending from
the Texas Gulf Coastal Plain northward through eastern South Dakota (known as “Tornado
Alley”); (2) an area that extends across the Gulf Coastal Plain from south Texas eastward to
Florida (known as “Dixie Alley”); and (3) an area located in eastern Iowa, south-central Indiana,
western Pennsylvania, and central Arkansas (a smaller “tornado alley”). See Figure A-1.
Tornadoes occur mostly during spring and summer, and usually during the late afternoon and
early evening. However, they can occur on any day of the year, at any hour.97 The United States
averages approximately 1,000 tornadoes per year—the highest average annual number in the
world. (The actual number of recorded tornadoes per year varies, depending on the source of
information.) The first part of 2008 was relatively active, with 112 tornado-caused fatalities
between January and May, the eighth-deadliest January through May period since reliable record
keeping began in 1953.98 The total number of tornadoes reported in 2008 was 2,192, well above
(...continued)
Weather and Climate Change,” a document produced by the University Corporation for Atmospheric Research and
seven other stakeholder organizations; at http://www.ucar.edu/td/.
91 Ashley (2007), p. 1224.
92 Ibid.
93 U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Super
Tuesday Tornado Outbreak of February 5-6, 2008, March 2009.
94 Ashley (2007), p. 1217.
95 Ibid., p. 1218.
96 National Severe Storms Laboratory, at http://www.nssl.noaa.gov/primer/tornado/tor_faq.shtml.
97 National Severe Storms Laboratory, at http://www.nssl.noaa.gov/primer/tornado/tor_climatology.html#.
98 NOAA, National Climatic Data Center (Asheville, NC), “State of the Climate: Tornadoes, Spring 2008,” at
(continued...)
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the 10-year average according to NOAA 99 In contrast, 2009 was one of the five quietest years for
the decade, although tornadoes in April, June, and July exceeded the past three-year average.100
Figure A-1. Map Showing the Number of Recorded Tornadoes Greater than F3 in the
United States Between 1950 and 1998
Source: Federal Emergency Management Agency (FEMA), at http://www.fema.gov/plan/prevent/saferoom/
tsfs02_torn_activity.sht. According to FEMA, the map is based on NOAA, Storm Prediction Center statistics.
How and Why Tornadoes Form
A tornado is a narrow, violently rotating column of air that extends from the base of a
thunderstorm to the ground. Tornadoes develop from severe thunderstorms in warm, moist,
unstable air along and ahead of cold fronts. There are two types of tornadoes, those that come
from a supercell thunderstorm and those that do not.101
Tornadoes that form from supercell thunderstorms are the most common, usually the largest, and
the most dangerous. In supercell thunderstorms, a rotating updraft is essential to development of a
tornado. Rotation of the updraft can be caused by wind shear, which occurs when winds at two
different levels above the ground blow at different speeds or in different directions. An invisible
(...continued)
http://www.ncdc.noaa.gov/sotc/?report=tornadoes&year=2008&month=15&submitted=Get+Report
99 NOAA, National Climatic Data Center (Asheville, NC), “State of the Climate: Tornadoes, Annual 2008,” at
http://www.ncdc.noaa.gov/sotc/?report=tornadoes&year=2008&month=13&submitted=Get+Report.
100 NOAA, National Climatic Data Center (Asheville, NC), “State of the Climate: Tornadoes, Annual 2009,” at
http://www.ncdc.noaa.gov/sotc/?report=tornadoes&year=2009&month=13&submitted=Get+Report. Reported
tornadoes as of the end of November, 2009.
101 NOAA National Severe Storms Laboratory, “A Severe Weather Primer: Questions and Answers about Tornadoes,”
at http://www.nssl.noaa.gov/primer/tornado/tor_basics.html.
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tube of air begins to rotate horizontally, and rising air within the thunderstorm tilts the rotating air
from horizontal to vertical, resulting in rotation that extends through much of the storm. Once the
updraft is rotating and being fed by warm, moist air flowing in from the ground level, a tornado
can form. The mechanisms that cause tornadoes to form from supercell storms are not known
precisely, and it is not currently possible to predict which supercell thunderstorms will produce
tornadoes and which will not. Based on observations, approximately 20% of supercell
thunderstorms produce tornadoes.102
A non-supercell tornado forms from a vertically spinning parcel of air near the ground, about 1-10
kilometers in diameter, that is caused by wind shear from a warm, cold, or sea breeze front, or
from a dryline—the interface between warm, moist air and hot, dry air. When an updraft moves
over the spinning parcel of air and stretches it, a tornado can form. This type of tornado formation
commonly occurs in eastern Colorado, where cool air descending from the Rocky Mountains
toward the west collides with hot dry air from the Great Plains.103 Land-falling tropical storms
and hurricanes can also generate non-supercell tornadoes.
Classifying Tornadoes: The F-Scale
The Fujita or F-scale was developed to provide a method for estimating the intensity of tornadoes,
and was intended to relate the degree of damage to the intensity of wind.104 The original F-scale
was used for over three decades, but its limitations prompted the development of a new scale,
called the enhanced F-scale, or EF-scale. The EF-scale is intended to be a more robust and precise
method of assessing tornado damage than the original F-scale. The EF-scale calibrates tornado
damage using 28 different types of damage indicators, such as the type of construction (e.g.,
anchored versus unanchored houses, mobile homes, schools, garages, barns, skyscrapers,
transmission towers, and others).105 Even with the improvements over the original F-scale, the
EF-scale represents only estimates of wind speed, based on damage, and not measurements of
actual wind speeds in tornadoes.106 Actual tornado wind speeds are still largely unknown. Table
A-1 compares the original F-scale with the EF-scale currently used by meteorologists and wind
engineers.
Table A-1. F-Scale and Enhanced F-Scale for Tornado Damage
Original F-scale
Wind Speed (mph)
Enhanced F-scale
Wind Speed (mph)
F-0 45-78 EF-0 65-85
F-1 79-117 EF-1 86-110
F-2 118-161 EF-2 111-135
F-3 162-209 EF-3 136-165
F-4 210-261 EF-4 166-200
F-5 262-317 EF-5 Over
200
Source: NOAA Storm Prediction Center, at http://www.spc.noaa.gov/faq/tornado/ef-scale.html.
102 Ibid.
103 NOAA National Severe Storms Laboratory, “A Severe Weather Primer: Questions and Answers about Tornadoes.”
104 Dr. Ted Fujita developed the scale in 1971.
105 NOAA Storm Prediction Center, at http://www.spc.noaa.gov/faq/tornado/index.html#f-scale3.
106 NOAA Storm Prediction Center, at http://www.spc.noaa.gov/faq/tornado/ef-scale.html.
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Author Contact Information
Peter Folger
Specialist in Energy and Natural Resources Policy
pfolger@crs.loc.gov, 7-1517
Acknowledgments
Aisha Reed, a former intern with CRS, contributed to this report.
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