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When and Where Do Tornadoes Occur? |
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Tornadoes that hit Washington, D.C. suburbs in September 2001
illustrate a fact often stated by weather forecasters: "Tornadoes
can and do happen any time of
Figure 1. Tornado Alley
includes parts of Texas, Oklahoma, Nebraska, eastern Colorado and
western Iowa, and is characterized by a high frequency of strong
and violent tornadoes and a relatively consistent season from year
to year.the year in just about any location." No
one knows this better than National
Severe Storms Laboratory Research Meteorologist Dr. Harold
Brooks, who has studied the climatology of severe weather in the
United States to better understand when and where tornadoes are
most likely to occur. By understanding the threat posed by tornadoes
in the United States, particularly the threat of strong and violent
tornadoes, forecasters can more accurately predict them and communities
can better prepare for them.
Through his study of tornado climatology, Brooks has revealed
several important things. First, tornadoes occur most often in
the central United States, commonly known as Tornado Alley. Second,
the central plains have a repeatable annual tornado cycle, with
the highest probability of tornado occurrence in the springtime.
Finally, areas outside of tornado alley do not have a typical tornado
season and experience fewer tornadoes. These findings have several
implications.
In his study, Brooks used a data set of tornado reports from 1921
to 1995. He particularly focused on significant tornadoes, those
rated F2 or greater on the Fujita
Damage Scale. Even though only about 10 percent of tornadoes
are significant, these tornadoes are responsible for the majority
of deaths caused by tornadoes in the country, with violent tornadoes - those
rated F4 or higher - claiming 67 percent of the total. In addition,
these tornadoes typically cause millions of dollars in damage costs
when they occur.
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Figure 2. The Fujita scale, or F scale, categorizes tornado severity
based on observed damage to man-made structures and not on recorded
wind speeds. |
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The primary area of the United States in which significant tornadoes
occur most often is in an L-shaped region from Iowa to Colorado
to Texas (see Figure 1), with the highest threat in Oklahoma. In
addition, this area has a consistent season each year - from April
through mid-June, with the most tornadoes normally occurring in
May. These two facts - the conjunction of high frequency of strong
and violent tornadoes and the relative consistency of the season
from year to year from north Texas up into western Iowa - provide
a natural, objective way to define Tornado Alley.
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Figure 3. Lubbock, Texas has an annual cycle of tornado occurrence
with a tight peak in the spring that is typical of the Plains. |
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The concept of Tornado Alley may be very important for the emergency
management community, Brooks said.
"It is relatively easy to keep awareness up in a region where
events happen frequently and where the threat is confined to a
relatively short period of time," he said. "In addition,
it is typically easier to recruit volunteer storm spotters in such
an area and to maintain their enthusiasm."
Tornado near Oklahoma City, OK, May 3,
1999.
Photo by Daphne Zaras, NOAA Public awareness was high, for instance, during the May 3, 1999
Oklahoma City tornado. Despite damaging almost 8,000 structures,
fewer than 40 direct fatalities occurred. In addition to timely
and accurate warnings from the National Weather Service and live
coverage from local television and radio stations, people in the
path of the tornadoes knew how to respond and did it.
In
contrast, heightening awareness in an area where tornadoes rarely
occur or occur over a broader season of the year is much more difficult,
Brooks suggests. In fact, he believes a lack of public awareness
in areas of the country where the threat of a tornado on any particular
day is low is one reason for many of the high death toll events
that have occurred in the past 20 years. During that period of
time, only two of the 22 tornadoes in the United States have caused
at least eight fatalities (representing the highest 10 percent
of death tolls) in Tornado Alley. Those are the Andover, Kansas
tornado on April 26, 1991, which had its fatalities in a trailer
park, and the May 3, 1999, Oklahoma City tornado, which was the
(inflation adjusted) biggest property damage tornado in U.S. history.
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Figure 4. Hattiesburg, Miss. is typical of the southeastern United
States with no discernible annual cycle for tornadoes, even though
the total number of tornadoes for the year is only 20 percent
less than Lubbock. |
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While almost one-fourth of all significant tornadoes occur in
Tornado Alley, only nine percent of the major killers have, Brooks
said. Thus, he found, the vast majority of high fatality tornadoes
in recent years have occurred in areas such as the southeastern
United States where tornadoes are an especially rare event on any
given day.
"In these areas, people have not thought about what they
are supposed to do to seek shelter from a tornado, and they typically
don't make good decisions under pressure," Brooks said. "When
their basic state of awareness is very low, they are less likely
to respond to a warning."
Emergency managers who must prepare their communities for the
rare significant tornado face a significant challenge.
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Adapted from Clues
from Climatology: When and Where Do Tornadoes Occur? by
Keli Tarp, NOAA, 10/8/01
Additional information about the Severe Thunderstorm Climatology
project can be found at http://www.nssl.noaa.gov/hazard
The National
Severe Storms Laboratory,
in Norman, Oklahoma conducts research to improve accurate and
timely forecasts and warnings of hazardous weather events such
as blizzards, ice storms, flash floods, tornadoes, and lightning.
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