Why the Tornado Drought?
Essentially the same pattern responsible for a persistently cold and snowy winter in parts of the U.S. also, for the most part, squashed the threat of severe weather through the first three weeks of March.A pronounced southward dip in the polar jet stream has frequently driven cold air into the Gulf of Mexico, as a powerful northward diversion of the jet stream has persisted in the eastern Pacific Ocean and western U.S. This is the polar opposite of a pattern which would favor severe weather in the southern states during winter.
As a result, deeper, richer Gulf moisture can't flow northward into the southern U.S. ahead of a strong jet-stream level disturbance. Shallow, meager moisture, with weak instability lends itself to damaging straight-line winds in any severe thunderstorms that have developed, rather than tornadic supercells.
However, history shows that a slow start to the year doesn't signal a quiet period is ahead. Both 2012 and 2013 featured at least 400 less U.S. tornadoes than the 10-year average. Despite that apparent tornado drought, we had the following destructive events:
- Mar. 2-3, 2012: EF4 in Henryville, Ind.; EF3 in West Liberty, Ky.
- May 15, 2013: EF4 in Granbury, Texas
- May 19-20, 2013: EF5 in Moore, Okla.
- May 31, 2013: EF3 in El Reno, Okla.
- Nov, 17, 2013: EF4 in Washington, Ill.
http://www.weather.com/tv/tvshows/americas-morning-headquarters/march-tornadoes-may-set-record-low-20140321
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