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INSIGHTi
Tornadoes: Background and Forecasting
December 13, 2021
Background
Severe thunderstorms and tornadoes affect communities across the United States every year, causing
fatalities, destroying property and crops, and disrupting businesses. Tornadoes are narrow, violently
rotating columns of air that extend from the base of a thunderstorm to the ground, sometimes producing
winds that exceed 300 miles per hour. Tornadoes have been reported on all continents except Antarctica;
however, they occur most commonly in North America and particularly in the United States, which
reports approximately 1,200 tornadoes per year. Tornadoes occur across the United States but form most
commonly in three regions, shifting seasonally: (1) southern Plains (e.g., Texas, Oklahoma, Kansas), (2)
Gulf Coast, and (3) northern Plains and upper Midwest (e.g., North and South Dakota, Nebraska, Iowa,
Minnesota). Depending on geographic location, tornadoes occur mostly during spring and summer
(Figure 1) and usually during the late afternoon or early evening. However, tornadoes can occur at any
time. For example, a deadly storm system with several reported tornadoes, including a “significant long
track tornado,” touched ground overnight in Arkansas, Illinois, Kentucky, Missouri, Mississippi, and
Tennessee between December 10 and 11, 2021. Preliminary estimates report that peak winds of the long-
track tornado reached between 158 and 206 miles per hour. Aspects of the event were rare, such as its
timing, potential track length, and geographic location.
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Figure 1. U.S. Tornadoes Daily Count and Running Annual Trend
Source: National Oceanic and Atmospheric Administration (NOAA), Storm Prediction Center, “Daily Counts and Annual
Running Trend.”
Classification
Experts estimate the strength or wind speed of a tornado by examining the damage it caused rather than
by measuring actual wind speeds during an event. The Fujita, or F-scale, estimation method, developed in
1971, was used for over three decades, but its limitations prompted the development and adoption of a
new scale in 2007, called the enhanced F-scale, or EF-scale (Table 1). The EF-scale is intended to be a
more robust and precise method of assessing a tornado’s strength, and it uses 28 different types of damage
indicators, such as building type, structures, and trees.
Table 1. Enhanced F-Scale
EF Number
3-Second Gust (mph)
Damage Level
0
65-85
Weak
1
86-110
Moderate
2
111-135
Significant
3
136-165
Severe
4
166-200
Extreme
5
Over 200
Catastrophic
Source: NOAA, Storm Prediction Center, “Enhanced F Scale for Tornado Damage.”
Notes: Three-second gusts are estimated at the point of damage based on a judgment of 8 levels of damage to the EF-
scale’s 28 indicators. These estimates vary with height and exposure. The 3-second gust is not equivalent to wind speeds
measured in standard surface observations.
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Forecasting, Detection, and Communication
Exactly how and why tornadoes form is not completely understood. Tornado formation is believed to be
dictated mainly by storm-scale conditions in and around rotating thunderstorms with well-defined
circulation. The National Oceanic and Atmospheric Administration’s (NOAA’s) National Weather
Service (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). NWS provides weather, water, and climate
forecasts and warnings for the United States, its territories, adjacent waters, and ocean areas. In addition,
several NWS programs, including the National Severe Storm Laboratory, focus on 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 observations, numerical weather
prediction models, and ensemble forecasting (running several models at one time) to determine if
atmospheric conditions, temperature, and wind flow patterns may lead to the formation of severe weather.
SPC issues three-day forecasts (convective outlooks) on a daily basis and mesoscale discussions of severe
thunderstorm potential for the next six hours, with an emphasis on the next one to three hours, as
warranted.
If conditions favorable for either multiple tornadoes or a single intense tornado continue to develop, SPC
issues a tornado watch that typically lasts six to eight hours. Such watches alert the public, emergency
managers, storm spotters, broadcast media, and local WFOs that conditions have become favorable for
the development of tornadoes. SPC aims to issue watches at least two hours before the first tornado event.
Forecasters and storm spotters recognize certain storm features from visual cues, such as the rear-flank
downdraft, and particular patterns in Doppler radar images, such as the tornadic vortex signature (a
region of intense concentrated rotation). WFOs issue tornado warnings when a tornado has been sighted
or indicated by weather radar. The warning contains specific language about areas at risk, time frames,
specific hazards, recommended safety precautions for those at risk, and the WFO issuing the warning.
Communication
Several methods exist to communicate warnings to the public, including outdoor warning sirens, local
television and radio stations, cable television systems, cell phone applications, and NOAA Weather Radio
All Hazards (NWR). NWS maintains and operates NWR, a nationwide network of radio stations
broadcasting continuous weather information directly from the nearest WFO 24 hours a day, 7 days a
week. NWR works with the Emergency Alert System, an automated system that allows NWS warnings to
be disseminated over broadcasters, satellite digital audio services, direct broadcast satellite providers,
cable television systems, and wireless cable systems.
Considerations
It is not clear whether the average number of tornadoes each year has changed over time, due to reporting
issues, and if climate change has, or may, impact tornado frequency or intensity overall or in certain
circumstances (see here for more about the differences between weather and climate). Congress may
consider whether and how federal agencies should continue research into potential connections between
climate change and tornado activity and whether there are ways to mitigate any climate change impacts.
Additional considerations for Congress may include how social, behavioral, and demographic factors play
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a role in tornado-related fatalities and federal responsibilities in preventing and responding to damages
from windstorms, such as tornadoes.
Author Information
Eva Lipiec
Analyst in Natural Resources Policy
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff
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