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A Comparison of Tornado Statistics from Tornado
Alley and Dixie Alley
Alan Gerard and John Gagan NWS WFO Jackson, MS
And
John Gordon NWS WFO Louisville, KY
Tornado Alley?
www.cdli.ca/CITE/tornadoes_alley.htm
Tornado Alley Definitions:
• Region in the middle of the United States where more tornadoes occur than anywhere else in the world.
• Most tornadoes in the United States form in an area called "Tornado Alley". This area includes parts of Texas, Oklahoma, Kansas, and Nebraska. www.windows.ucar.edu/tour/link=/earth/Atmosphere/tornado/alley.html
• Where is tornado alley? geography.about.com/library/faq/blqztornadoalley.htm
• “The United States “Tornado Alley”, stretches from northwest TX across OK and through northeast KS.”
Fawbush and Miller 1948
The weather situation is discussed by Capt. Robert C. Miller (left) and Lieut. Col. E. J. Fawbush
Perception
• “Historically, the term “tornado alley” has been used loosely, and the locations indicated are often based on personal perception rather than scientific data. (“The Tornado Nature's Ultimate Windstorm” pg 278 Grazulis)
• Wizard of Oz
Origins of Tornado Alleys. • Personally spoke to Doswell, Grazulis, Dan
McCarthy, Joe Schaeffer, Howie Bluestein, Fred Ostby, and Allen Pearson about the origins of Tornado Alley.
• Searched many books (including Flora and internet sources)
• No one knew the origins of Tornado Alley.• Found papers from Kelly 78 et al., Concannon
2000 et al., Grazulis books, and no definitive answer.
Fawbush and Miller
• On Feb 15, 1952, Fawbush and Miller set up a new project called, "Tornado Alley," in which a concentrated study of severe weather activity was made over an area extending from Lubbock, Texas to Eastern Colorado and Nebraska.
• This was done in stages, the 1st being from LUB-END, & 2nd from END north to the Nebraska line.
Many Different Alleys
• “Evidence of smaller tornado alleys across the United States based on a long track F3 to F5 tornado climatology study from 1880 to 2003" (Broyles and Crosbie 2004)
• “There is more than one tornado alley. Any area that seems to experience above average tornado frequency is eventually labeled a “tornado alley.” There are dozens of such regions. (Grazulis The Tornado Nature's Ultimate Windstorm pg 278).
Dixie Alley Origins
• AR/TN OUTBREAK: March 21-22, 1952 28 tornadoes * 204 deaths
First tornado watch ever issued by NWS
• MS Delta Outbreak Feb 21, 1971: 10 tornadoes 121 deaths, 1524 injuries
Dixie Alley
• However…Pearson…
Named Dixie Alley after 71 Outbreak in MS Delta (no formal documentation that we can locate).
Jeopardy Question?
Which alley is under the greatest threat of tornadoes for the entire year?
A.Plains Tornado Alley
B.Dixie Alley
Depiction of Plains and Dixie Tornado Alleys
• Plains Alley
– Size: 407,734 sq mi
– Population: 17,824,023
• Dixie Alley
– Size: 276,890 sq mi
– Population: 24,948,345
Plains Alley and Dixie Alley Breakdown
Population Data from the 2000 Census
Methodology
• Our own depictions of Tornado Alley and Dixie Alley are “personal”
• Data is from 1950 to 2003 and is derived from the SPC “ONETOR” database– “A close approximation to actual tornadoes.”
• Focused on strong/violent tornadoes
• Data not normalized for population or spatial extent
Tornadoes by F-Scale
Tornadoes By F-Scale
0
1000
2000
3000
4000
5000
6000
0 1 2 3 4 5
F-Scale
To
rnad
oes
Plains
Dixie
Strong & Violent Tornadoes by Month
Strong/Violent Tornadoes
0
100
200
300
400
500
600
700
800
900
1000
January February March April May June July August September October November December
Month
Torn
adoe
s Plains
Dixie
Killer Strong & Violent Tornadoes by Month
Killer Strong/Violent Tornadoes
0
10
20
30
40
50
60
70
80
90
100
Janu
ary
Febr
uary
Mar
ch
April
May
June July
Augu
st
Sept
embe
r
Oct
ober
Nove
mbe
r
Dece
mbe
r
Month
Torn
adoe
s Plains
Dixie
Strong & Violent Tornadoes by Time of Day
Strong/Violent Tornadoes By Time (CST)
0
50
100
150
200
250
300
350
400
450
12:0
0 AM
1:00
AM
2:00
AM
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AM
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AM
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6:00
AM
7:00
AM
8:00
AM
9:00
AM
10:0
0 AM
11:0
0 AM
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0 PM
1:00
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2:00
PM
3:00
PM
4:00
PM
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PM
6:00
PM
7:00
PM
8:00
PM
9:00
PM
10:0
0 PM
11:0
0 PM
Time (CST)
Torn
adoe
s Plains
Dixie
Killer Strong & Violent Tornadoes by Time of Day
Strong/Violent Killer Tornadoes By Time (CST)
0
5
10
15
20
25
30
35
40
12:0
0 A
M
1:00
AM
2:00
AM
3:00
AM
4:00
AM
5:00
AM
6:00
AM
7:00
AM
8:00
AM
9:00
AM
10:0
0 A
M
11:0
0 A
M
12:0
0 PM
1:00
PM
2:00
PM
3:00
PM
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PM
6:00
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7:00
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PM
9:00
PM
10:0
0 PM
11:0
0 PM
Time (CST)
Torn
adoe
s Plains
Dixie
Strong & Violent Tornado Path Length
Strong and Violent Tornado Path Length Comparison
0
100
200
300
400
500
600
700
<= 0.2 0.3 to 1.1 1.2 to 4.7 4.8 to 12.8 12.9 to 26.6 >= 26.7
Path Length (sm)
To
rnad
oes
Plains
Dixie
Tornado Deaths
Tornado Deaths
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Plains Dixie
All Tornadoes
Strong/Violent Tornadoes
Strong & Violent Tornado Fatalities
Fatalties
0
100
200
300
400
500
600
700
Month
Fata
litie
s Plains Fatalities
Dixie Fatalities
Post NWS Modernization
• In addition to the longer term statistics, felt it would be revealing to compare to post-NWS modernization.– Increased number of tornadoes since
deployment of 88D.• Increased ability to see potentially tornadic storms.• More emphasis on verification/storm data.
• Look at statistics from 1998-2004, focusing particularly on strong tornadoes.
Methodology
• Used same definitions of “Tornado Alley” and “Dixie Alley.”
• Looked at total tornadoes, strong tornadoes, strong tornado days, and numbers of tornadoes, by month.
• Data taken from the NWS storm data/verification homepage.– Subject to potential biases
• County based versus “track” based• Population density and county size potential biases
same as longer term dataset.
Summary of Stats
• Dixie Alley had about 1.5 times as many strong tornadoes as Tornado Alley (338 vs 206) while Tornado Alley had more tornadoes overall (2278 vs. 1703)
• Killer tornadoes much more prevalent in Dixie Alley area than Tornado Alley– 66 vs 24
• Dixie Alley had twice the number of “outbreak days”– 9 days with 10+ strong tornadoes versus 5 in
Tornado Alley
Strong Tornadoes 98-04
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
1 2 3 4 5 6 7 8 9 10 11 12
Month
Avg
Mo
nth
ly #
Tornado Alley
Dixie Alley
Seasonal Trends
• Tornado Alley has a pronounced peak in Spring– Much lesser threat
rest of year
• Dixie Alley has a more consistent threat January-May, with a large peak in November
• 98-04 period does not well reflect tropical cyclone tornado threat, particularly in Dixie Alley area (e.g., Katrina, Rita)
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
30.0%
35.0%
40.0%
Jan
Fe
b
Ma
r
Ap
r
Ma
y
Jun
e
July
Au
g
Se
p
Oct
No
v
De
c
Dixie Alley
Tornado Alley
Percentage of Annual Strong Tornado Days
Annual Trend• 76% of strong tornado days in Tornado Alley
occur in a three month period (April, May, June)– Adding March in takes the value to nearly 90%
• The highest percentage over a three month period in Dixie Alley is 46% (March, April, May)– The next two highest three month periods are
February, March, April (41%) and November, December, January (37%)
• Eight out of 12 months (Oct-May) have at least 5% of the annual strong tornado days for Dixie Alley– Only 5 out of 12 for Tornado Alley (Mar-June, Oct)
Conclusions• Confirms validity of Plains Tornado Alley
– Not meant to diminish magnitude of risk in this area• Highlights magnitude of tornado risk in Dixie
Alley area– Strong/violent tornadoes as prevalent– Killer tornadoes much more prevalent
• Temporal aspect of threat much different in Dixie Alley versus Plains Tornado Alley– Threat most of year – killer strong/violent
tornadoes not uncommon in late fall/winter months!!
– While a peak in strong/killer tornadoes occurs in late afternoon and evening, a much higher risk exists in the 9 pm to 5 am timeframe than in the Plains Tornado Alley.
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