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June 26, 2024
Tornadoes: Background and Forecasting
Tornadoes are narrow, violently rotating columns of air,
Classification
extending from the base of a thunderstorm to the ground,
Experts estimate the strength or wind speed of a tornado by
that affect communities across the United States every year.
examining the damage it caused rather than by measuring
Tornadoes can cause fatalities and injuries, destroy property
actual wind speeds during an event. The Fujita, or F-scale,
and crops, and disrupt businesses. For example, a weather
estimation method, developed in 1971, was used for over
system on April 26-28, 2024, produced over 200 tornadoes,
three decades, but its limitations prompted the development
high winds, and large hail and caused multiple deaths and
and adoption of a new scale in 2007, called the enhanced F-
injuries across the Midwest and South, according to
scale, or EF-scale (Table 1). The EF-scale uses 28 different
preliminary estimates.
types of damage indicators, such as building type,
structures, and trees.
Tornadoes have been reported on all continents except
Table 1. Enhanced F-Scale
Antarctica. They occur most commonly in North America,
particularly in the United States, which reports
EF Number
3-Second Gust (mph)
approximately 1,200 tornadoes per year. Tornadoes occur
0
65-85
across the United States but form frequently in three
1
86-110
regions: (1) southern plains (e.g., Texas, Oklahoma,
2
111-135
Kansas), (2) Gulf Coast (e.g., Alabama, Florida, Louisiana,
3
136-165
Mississippi), and (3) northern plains and upper Midwest
4
166-200
(e.g., North and South Dakota, Nebraska, Iowa, Minnesota).
Tornadoes occur mostly during spring and summer (Figure
5
Over 200
1) and usually during the late afternoon or early evening.
Source: NOAA, Storm Prediction Center, “Enhanced F Scale for
However, tornadoes can occur at any time.
Tornado Damage.”
Notes: EF = Enhanced F-scale. A 3-second gust is estimated at the
point of damage based on the EF-scale’s 28 damage indicators. The 3-
second gust is not equivalent to wind speed measured in standard
surface observations.
Figure 1. U.S. Tornadoes Daily Count and Running Annual Trend
Source: NOAA, Storm Prediction Center, “Daily Counts and Annual Running Trend,” June 23, 2024.
Notes: The 2024 U.S. daily occurrence and trend for tornadoes is shown in red, and the average daily and annual trend (2005-2015) is shown
in gray.
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Tornadoes: Background and Forecasting
Forecasting, Detection, and
be disseminated by broadcasters, satellite digital audio
Communication
services, direct broadcast satellite providers, cable
Exactly how and why tornadoes form is not completely
television systems, and wireless cable systems.
understood. Tornado formation is believed to be dictated
mainly by conditions in and around rotating thunderstorms
Figure 2. Selected Components of Certain
with well-defined circulation. The Secretary of Commerce,
Thunderstorms
acting through the National Oceanic and Atmospheric
Administration’s (NOAA’s) Administrator, has authority
for weather forecasting and for issuing storm warnings (15
U.S.C. §313), including tornado forecasting and warnings.
The National Weather Service (NWS) provides weather,
water, and climate forecasts and warnings for the United
States and its territories, adjacent waters, and ocean areas.
Several other NOAA programs, including the National
Severe Storm Laboratory, also focus on tornado research to
improve observations, modeling, and instrument
development, among other activities.
Forecasting and Detection
Severe thunderstorm and tornado forecasts are made by the
NWS Storm Prediction Center (SPC) and by local weather
forecast offices (WFOs). SPC forecasters use weather
Source: NOAA, National Weather Service, “The Supercell Pt.2.”
observations, numerical weather prediction models, and
Notes: A forward flank downdraft is the leading part of a supercell,
ensemble forecasting (running several models at one time)
with most of the heavy precipitation. A rear flank downdraft is a region
of dry air subsiding on the back side of, and wrapping around, a
to determine if atmospheric conditions, temperature, and
cyclone (NOAA NWS, “Field Guide Glossary”).
wind flow patterns may lead to the formation of severe
weather. SPC issues three-day forecasts (convective
Congressional Considerations
outlooks) on a daily basis and mesoscale discussions of
Congress continues to express interest in improving
severe thunderstorm potential for the next six hours, with an
forecasting, detection, and communication related to
emphasis on the next one to three hours, as warranted.
tornadoes. For instance, in the 118th Congress, Members
have introduced bills (e.g., S. 1284 and H.R. 6093) to
If conditions favorable for either multiple tornadoes or a
amend a NOAA tornado program to “rapidly” improve
single intense tornado continue to develop, SPC issues a
tornado forecasts, predictions, and warnings and to evaluate
tornado watch, which typically lasts six to eight hours. Such
and potentially update the current tornado rating system.
watches alert the public, emergency managers, storm
The bills also would require a pilot program for tornado
spotters, broadcast media, and local WFOs that conditions
hazard communication, including the improvement of
have become favorable for the development of tornadoes.
social, behavioral, economic, risk, and communication
SPC aims to issue watches at least two hours before the first
sciences, among other actions.
tornado event.
Due to reporting issues, experts are unable to determine if
Forecasters and storm spotters recognize certain storm
the average number of tornadoes each year has changed
features from visual cues, such as the forward or rear flank
over time. For example, an increase in the number of
downdraft (Figure 2), and particular patterns in Doppler
reported EF0-EF1 tornadoes since the 1990s is likely due to
radar images, such as the tornadic vortex signature (a
better detection, greater media coverage and verification
region of intense concentrated rotation). WFOs issue
efforts, more storm spotting and chasing, a growing
tornado warnings when a tornado has been sighted or
population, and the advent of cell phone cameras. In
indicated by weather radar. The warning contains specific
addition, the science is unclear on if climate change has
language about areas at risk, time frames, specific hazards,
impacted, or may impact, tornado frequency or intensity
and recommended safety precautions for those at risk.
overall or in certain circumstances (see here for more about
Communication
the differences between weather and climate). Congress
may consider whether and how federal agencies should
Several methods exist to communicate warnings to the
continue research into potential connections between
public, including outdoor warning sirens, local television
climate change and tornado activity and whether ways exist
and radio stations, cable television systems, cell phone
to mitigate any climate change-related impacts.
applications, and NOAA Weather Radio All Hazards
(NWR). NWS maintains and operates NWR, a nationwide
network of radio stations broadcasting continuous weather
Eva Lipiec, Specialist in Natural Resource Policy
information directly from the nearest WFO 24 hours a day,
IF12695
7 days a week. NWR works with the Emergency Alert
System, an automated system that allows NWS warnings to
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Tornadoes: Background and Forecasting
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