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Posted at 3:15 PM ET, 03/ 8/2011

Mardi Gras storm risk & the new tornado alley

By Jason Samenow
tornado-risk-new-orleans.jpg
Forecast probabilities of tornadoes today. Source: NOAA.

UPDATE (4:30 p.m., Wednesday):: Unfortunately, tornadic storms materialized in the vicinity of New Orleans with three twister reports . At New Orleans International Airport, Accuweather reports a wind gust was clocked at 77 mph and incredibly, in just half an hour, three inches of rain fell. Fortunately, the line of fierce storms was polite enough to tear through the Big Easy around 6:30 a.m. this morning, in the wake of most of the Mardi Gras festivities.

For information on research suggesting the expansion of "tornado alley" into parts of the South, keep reading after the jump...

UPDATE (11 p.m. Tuesday) A new larger tornado watch was issued (replacing the one issued earlier) which now includes New Orleans, and remains in effect through 3 a.m.

Originally posted 3:15 p.m. (Tuesday, 3/9): While Fat Tuesday revelers get their buzz tonight on Bourbon Street, forecasters are buzzing about the severe thunderstorm risk encompassing Louisiana and surrounding states in the Deep South. NOAA's Storm Prediction Center says these areas, including New Orleans, stand a slight to moderate risk of dangerous storms capable of producing damaging winds, large hail, and tornadoes.

Says NOAA's Storm Prediction Center:

THE THREAT OF SUPERCELLS DEVELOPING INCLUDING TORNADOES ARE LIKELY BEGINNING BY LATE THIS AFTERNOON AND CONTINUING TONIGHT ACROSS LA INTO SOUTHERN MS. THERE IS THE RISK OF STRONG TORNADOES.

A tornado watch has been issued for large parts of central and southern Louisiana and southern Mississippi through 8 p.m. central but just north of New Orleans. Despite no watch for New Orleans, the NWS cautions overnight storms could be severe there with damaging winds and torrential rain (a flood watch is in effect), so unprepared late night partiers may get doused and blown around.

It's not surprising to see the tornado watch situated just north of the Big Easy (the southern border of the watch is on the opposite side of Lake Pontchartrain) where tornadoes are generally rare.

tornado-alley.jpg
Density calculations of average annual tornadoes within 25 mi of any point. Source: P. Dixon et al., Bulletin of the AMS (2010)

Despite the lack of historic twister activity around New Orleans, tornado climatology indicates they become much more common due north into south central Mississippi and expanding northeast and northwest from northern Alabama across northern Louisiana, southwest Tennessee and into eastern Arkansas.

In fact, a recent study, "Tornado Risk Analysis: Is Dixie Alley an extension of Tornado Alley?", argues this part of the U.S. should really be considered part of tornado alley traditionally defined to encompass primarily the Great Plains. The study, led by Grady Dixon at Mississippi State University (published in the Bulletin of the American Meteorological Society), concludes:

...there are large portions of the Southeast that experience more tornadoes than the rest of the country. It appears that Tornado Alley and "Dixie Alley" are part of a single large region of high tornado risk...

and...

Placement of the maximum tornado density in Mississippi, along with other regions of relative maxima across the Southeast, may warrant modification of the traditional tornado risk map that focuses only on the Great Plains

The study arrives at similar conclusions to the research conducted by Michael Frates at the University of Akron discussed by Steve Tracton in a post back last April.

Frates concluded:

...Dixie Alley has the highest frequency of long-track F3 to F5 tornadoes, making it the most active region in the United States. ... Based on this analysis, colloquial tornado alley fails to represent the areas of highest activity in the United States, indicating that a more comprehensive analysis of additional tornado alleys in the United States by the NWS may be needed in the future.

Perhaps the work of Frates and this latest contribution by Dixon et al. will help raise awareness of the tornado risk in the South and dispel the myth that the Great Plains is the U.S.'s epicenter of tornado activity.

By Jason Samenow  | March 8, 2011; 3:15 PM ET
Categories:  Latest, Thunderstorms, U.S. Weather  
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Next: PM Update: Rain arrives Wednesday afternoon

Comments

I would hate for it to storm on all those partiers.
Most of them are so drunk, if they look up at the sky under heavy pouring rain they may drown like domesticated turkeys.

Posted by: FIREDRAGON47 | March 8, 2011 4:25 PM | Report abuse

I saw on the map that there is a area of higher frequency of tornadoes right around the DC area. I don't remember any tornados occurring in this area, so when did the last ones occur? What seasons do tornadoes occur in this area? Is there a possibility that they might occur this year?

Posted by: Byzantium1453 | March 8, 2011 8:49 PM | Report abuse

@Byzantium1453

Tornadoes are not at all unusual around here- we probably average 1 or 2 in the area each year. There was a confirmed tornado in Baltimore just this past November, and tornadoes touched down in the region in June 2008, in 2004 when the remnants of Hurricane Ivan came through, and of course there were the destructive tornadoes in College Park (September 2001) and La Plata (April 2002).

Posted by: Jason-CapitalWeatherGang | March 8, 2011 10:56 PM | Report abuse

Almost any part of the U.S. east of the Rockies can be considered a potential tornado belt, depending on what time of the year it is. Florida, for example, experiences a lot of tornadoes and severe weather in the winter, when Gulf low-pressure-systems and their attandent cold-fronts push into warm, humid air entrenched in the state. Parts of the Deep South also experience severe weather in the winter and early spring for the same reason. Then, of course, as the sun moves north with the season, so does the average zone of where warm, humid air clashes with cold- Canadian air in low-pressure systems and cold-fronts.....and the tornado belt gradually moves northward. By June and July, it has moved, with the jet stream, pretty much up to the north-central and northeastern states, near the Canadian border. In the Plains states, sometimes it actually moves into Canada. Regina, Saskachewan and Edmonton, Alberta, have both seen destructive tornadoes...and Edmonton is at some 54 degrees north, far from the Gulf's source of warm, humid air.

Posted by: MMCarhelp | March 9, 2011 9:19 PM | Report abuse

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