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Posted at 06:28 PM ET, 03/07/2011

Active U.S. spring severe weather season likely


(Image of tornado. Source: NOAA.)
The predictions of several independent long-range forecast outlets support a very busy severe weather season across the middle portion of the U.S. from east of the Rockies to near the East Coast. This means an enhanced risk of thunderstorms capable of producing damaging winds, large hail, and tornadoes.

In its spring outlook issued this morning, State College, Pa.-based AccuWeather warned severe weather would be “more active than normal.”

“The zone of greatest concern for above-normal severe thunderstorm activity this spring ... lies from Arkansas and Missouri into Tennessee and Kentucky, areas that have been targeted already by several severe outbreaks the last week of February,” wrote AccuWeather.

While NOAA’s Climate Prediction Center [CPC] does not issue a spring thunderstorm outlook, its 90-day temperature outlook would suggest the kind of elevated north to south temperature contrast necessary to fuel big storms in the transition zone.

As Paul Yeager, weather blogger for America Online, described:

[CPC’s] forecast for March through May calls for temperatures that are likely to be warmer than average in the southern part of the country and cooler than average in the northern part of the country. Thus, the contrast will be more intense than in a typical year.

Yeager also noted the 2011 severe weather season is off to a fast start:

Through Monday, there have been about 800 preliminary reports of damage from severe weather in 2011, more than 2.5 times greater than the number through the first two months of 2010. January and February 2010 generated 307 reports, including just one February tornado.

Meteorologist Dave Tolleris, who forecasts for commercial purposes, released his spring outlook yesterday - headlining his Powerpoint presentation: “Much above normal severe weather season.” Similar to NOAA’s CPC, Tolleris is generally forecasting colder than normal temperatures across the northern tier of the U.S. and persistently warm temperatures relative to average across the Deep South. AccuWeather’s spring temperature forecast shows the same general pattern.


(AccuWeather’s spring temperature forecast.)
So there is strong agreement in the temperature pattern predicted by both commercial and government outlets. This is a reflection of the La Nina pattern expected to persist through the Spring, even if its intensity wanes as some computer models suggest.

During La Nina, a strong northern jet stream brings reinforcing shots of cold air to the northwest and north central portions of the U.S. while a ridge of subtropical high pressure to the southeast pumps warm air over the South. And it’s the places caught in the middle that have a heightened risk of severe thunderstorms.

Will the heightened risk of severe storms extend to the mid-Atlantic and D.C. metro region, which, from the look of things, seem to be a bit south and east of the biggest temperature contrast and storm dynamics?

Ken Reeves, Director of Forecasting Operations at AccuWeather, told me: “...the dynamics [supporting the enhanced risk of severe weather in the middle Mississippi Valley and into portions of the Tennessee and Ohio Valleys] will change and the reasons for the higher risks farther west will not necessarily translate eastward, especially the farther east you go. I currently do not see any stand out factors that point to a higher or lower likelihood for severe weather around Washington D.C.”

While some of the best dynamics for storms characteristically pass to the mid-Atlantic’s west and northwest during La Nina, the warm, unstable air ahead of such systems can still fuel big storms here. For example, several severe weather outbreaks occurred in the spring of 2008, the last La Nina event.

See these posts recapping storms from 2008:

Inside the Suffolk, Va. Tornado

Recap: June 4 Severe Weather Outbreak

Severe Storms Strike Again

We’ll take a closer look at the La Nina/severe weather link in the mid-Atlantic in a future data-driven post...

By  |  06:28 PM ET, 03/07/2011

Categories:  Latest, Thunderstorms, U.S. Weather

 
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