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Tornado Alley is renowned for the frequency and ferocity of its twisters. But the Great Plains are not usually in nature’s crosshairs when it comes to the most widespread U.S. tornado outbreaks,a new analysis shows.
Drawing on data provided by the Storm Prediction Center (SPC),www.weathernow24.com found that the most far-flung outbreaks of the last 70 years — defined as the ones affecting the most states — tended to focus east of the Mississippi River,rather than in the classic Tornado Alley region.
While the Great Plains sees plenty of tornadoes,these are often in regional clusters,such as those connected to the dry line that often sloshes across Texas,Oklahoma and Kansas in May and June.
The large,powerful upper-level storms needed for the most far-flung outbreaks are often fast movers,which means that the low-level moisture required for tornadic supercells may not get entrained until the system is east of the Great Plains. This puts the region between the Mississippi River and the Appalachians at particular risk of the most widespread tornado swarms.
The massive tornado outbreak of April 12-13,2020,is one of the largest in U.S. history. A total of 140 tornadoes were confirmed by the National Weather Service. From Texas to Maryland,ten states experienced at least one tornado during this outbreak,which caused 32 deaths and damaged or destroyed many hundreds of homes.
Three tornadoes from the outbreak have been confirmed at EF4 strength,two in Mississippi and one in South Carolina. Thirteen others reached EF3 strength.
Texas saw six tornadoes,all of them EF1 strength. It's the only state affected by Sunday's outbreak that lies within the Great Plains corridor considered the heart of Tornado Alley.
Where the Biggest Outbreaks Tend to Strike
Although the April 12-13 outbreak involved 10 states over two days,the most widespread outbreaks since 1950 (when reliable data begins) were even more impressive in their reach. The top 30 "outbreak days",as measured in 24-hour periods starting and ending at 7 a.m. EST (8 a.m. EDT),all involved at least 10 states.
The states hit most often in the biggest outbreaks are almost all east of the Mississippi River. The most frequently hit states include Illinois and Kentucky (each hit in 24 of the top 30 widespread outbreak days),Tennessee (23 of 30),Indiana and Missouri (each 22 of 30),Mississippi (19 of 30),Alabama and Arkansas (each 18 of 30),and Ohio (16 of 30).
Only two of these states,Arkansas and Missouri,lie west of the Mississippi.
In contrast,Kansas — which got more tornadoes per square mile than any other U.S. state from 2000 to 2015 — reported a tornado in only 11 of the top 30 outbreak days since 1950. Texas had at least one tornado in just 12 of the top 30 outbreak days,and Oklahoma in 13.
The states above are identified based on where each tornado began,according to Patrick Marsh,chief of the science support branch for SPC. Typically,says Marsh,when a tornado moves from one state into another,that second state will get a subsequent tornado,thus adding it to the outbreak’s total tally of states.
While there is no formal definition,a tornado outbreak is often defined as a sequence of at least six tornadoes separated by less than six hours. At www.weathernow24.com,we consider an event with at least 20 tornadoes to be an outbreak.
The two biggest outbreaks in U.S. history — the Super Outbreaks of 1974 and 2011 — each ravaged a vast area from the Midwest to the Deep South. The outbreak day centered on April 3,1974,brought tornadoes to 13 states. The one centered on April 27,2011,spun off tornadoes in 17 states,the most of any outbreak day on record. (The preceding day – April 26,2011 – saw tornadoes in 12 states.)
“It doesn't surprise me,” Marsh said,“but it just goes to show how ‘big’ those days were.”
The smaller size of states east of the Mississippi is another factor in their outsized representation,as is their typically slender east-west orientation (except for Kentucky and Tennessee). “The [Southeastern states] are narrow,longitudinally speaking,so an outbreak that transverses just the state of Oklahoma would hit three to five states in the southeast,” Marsh said.
“In the big events,the [upper-level storm] is lifting northeast,so the storm motions take the storms across the Southeast and then up the spine of the Appalachians,” said Marsh. “Look at the orientation of the storm reports from [April 27,2011,and April 12,2020]. They show a general west-east across Mississippi and Alabama and then a pivot northeast across Alabama to Georgia.”
In some of these large-scale outbreaks,especially those toward the central part of the U.S.,cold air associated with the upper low can generate weak tornadoes well behind the surface front. This is one reason that states such as California and Idaho can show up on the map above.
The Five Most Widespread Outbreak Days on Record
(by number of states involved)
April 27,2011 (17)
AL,AR,FL,GA,IN,KY,MD,MS,MO,NC,NY,OH,PA,SC,TN,VA,WV
May 6,2003 (15)
AL,AR,AZ,GA,IL,KS,KY,LA,MS,MO,ND,OK,SC,TN,TX
May 25,2011 (13)
AL,AR,CA,IA,IL,IN,KS,KY,LA,MO,OH,TN,TX
April 11,1979 (13)
AL,AR,ID,IN,KS,KY,LA,MO,MS,NE,OK,TN,TX
April 3,1974 (13)
AL,GA,IL,IN,KY,NC,OH,MI,MS,NY,TN,VA,WV
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