Analysis And Reconstruction Of The 1974 Tornado Super Outbreak - RMS Special Report.pdf

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Analysis and Reconstruction of
the 1974 Tornado Super Outbreak
RMS Special Report
TM
Risk Management Solutions
I
NTRODUCTION
The Super Outbreak of tornadoes that occurred on April 3-4, 1974 was the most intense and
widespread tornado outbreak in recorded history. In total, 148 tornadoes spanned 13 states
producing about 900 square miles (2331 square km) of tornado damage in less than 18 hours. This
report reviews the event’s meteorological and damage characteristics, describes the impact
subsequent research had on tornado risk models, and examines the property losses possible if the
outbreak were to occur today.
1
1974 Super Tornado Outbreak
April 2, 2004
Copyright
2004
Risk Management Solutions, Inc. All Rights Reserved.
M
ETEOROLOGICAL
C
HARACTERISTICS
Looking back, the atmospheric characteristics preceding the super outbreak provided clear evidence
of the high potential for widespread tornado development. On the evening of April 2, 1974 a deep
area of low pressure (983 millibars) over the Colorado-Kansas border created strong winds blowing
from the south over the lower Mississippi River Valley. This forced high-humidity air northward
from the Gulf of Mexico to the lower Midwest and Ohio Valley states. This moist layer of air was
“capped” by warm, dry air, which limited thunderstorm development and created highly unstable
atmospheric conditions. Recognizing this, the National Oceanic and Atmospheric Administration
(NOAA) issued preliminary severe weather alerts on the morning of April 2 for much of the central
and southern plains states.
During the morning of April 3, the center of low pressure moved northeastward to the Iowa-
Illinois border. As the air warmed up during the day and winds converged near the ground, an
explosive and sudden outbreak of thunderstorms developed around 2:00 pm Central Daylight Time
(CDT). Three major squall lines of thunderstorms oriented in a southeast-northwest direction
developed (see Figure 1). The most northerly line covered central Illinois, the central line extended
from northwestern Kentucky to central Indiana, and the southern line ran along the Tennessee-
North Carolina border. Most of the tornadoes were produced by individual thunderstorm cells
within these lines. The individual tornadoes moved northeastward at 40-60 mph (65-95 km/hr),
while the larger scale squall-line systems advanced toward the southeast.
Figure 1: Satellite image of the Eastern U.S. at 21:00 GMT on April 3,
1974
Up to 15 destructive tornadoes occurred simultaneously during the Outbreak. Among the suite
of tornadoes produced, the longest life of any single tornado was two hours and five minutes.
Ninety percent of the tornadoes, however, lasted less than 40 minutes. Many of these tornadoes
were part of ‘families’ or a sequence of tornadoes spawned in succession by a single thunderstorm
cell. Dr. Ted Fujita identified 30 such tornado families that comprised 74% of the Outbreak’s
2
1974 Super Tornado Outbreak
April 2, 2004
Copyright
2004
Risk Management Solutions, Inc. All Rights Reserved.
tornadoes and resulted in 98% of the 315 deaths. The longest-lasting tornado family existed for
nearly five hours, while the average life was approximately two hours.
The total path length of all tornadoes combined was approximately 2,600 miles (4,185 km),
covering a total area of approximately 900 square miles (2331 square km). Dating back to the late
1800s, no other single-day event has surpassed the super outbreak in terms of the number of
tornadoes or the area they affected.
3
1974 Super Tornado Outbreak
April 2, 2004
Copyright
2004
Risk Management Solutions, Inc. All Rights Reserved.
E
VENT
D
AMAGE
C
HARACTERISTICS
Following the super outbreak, Dr. Ted Fujita and his colleagues initiated the most extensive aerial
tornado survey ever conducted. The aerial survey was complemented by the collection of vast
amounts of engineering data on the performance of individual structures subject to wind and debris
loads. It took Fujita’s team nearly 10 months to confirm the characteristics of each of the tornadoes
in the event depicted in Figure 2. The F-scale classification statistics of the Outbreak are shown in
Table 2.
Figure 2: Map of the 1974 Tornado Super Outbreak
4
1974 Super Tornado Outbreak
April 2, 2004
Copyright
2004
Risk Management Solutions, Inc. All Rights Reserved.

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