March 2014 Tornado Drought May Set Record

For the second consecutive year, U.S. March tornado counts are among the lowest on record.

Through March 24, only four tornadoes have been confirmed across the nation. According to The Weather Channel's severe weather expert Dr. Greg Forbes (Facebook | Twitter), only March 1951 featured fewer tornadoes through the first 24 days of March, dating to 1950.

The lowest March U.S. tornado count on record dating to 1950 was six tornadoes in March 1951, according to Forbes. If the rest of the month features at most one additional tornado, we would set a record low for the month.

If this sounds familiar, March 2013 was the least tornadic March in 35 years, with only 18 tornadoes. 

March continues a slumbering tornado trend for the year. A preliminary 49 tornadoes have been tallied in 2014, which is roughly one-third of the 10-year average-to-date of 156 tornadoes.

"Twenty other years since 1950 have had fewer tornadoes (than 2014) thus far (through March 20)," says Forbes. "There had been just eight tornadoes thus far in 1969. The 41 tornadoes in February 2014 have kept the year from being closer to a record-low pace."

Why the Tornado Drought?

Essentially the same pattern responsible for a persistently cold and snowy winter in parts of the U.S. has also, for the most part, squashed the threat of severe weather so far.

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 19-20, 2013: EF5 in Moore, Okla.
• May 31, 2013: EF3 in El Reno, Okla.

How long can 2014 stay quiet?

The Spring Ramp-Up Looms

Through the middle of the upcoming week, the pattern does not look favorable for widespread severe thunderstorms capable of producing tornadoes anywhere in the U.S., as another southward jet stream dip plunges into the eastern U.S.

Beginning Wednesday, a flow of warmer, more humid air should flow northward into at least the southern Plains and Lower Mississippi Valley.

At the same time, a pair of jet-stream-level disturbances should push into the Plains, Ohio Valley and Great Lakes.

These ingredients should trigger at least an uptick of severe thunderstorms in parts of the Plains and Lower Mississippi Valley Thursday and Friday. For more details on the severe threat, check out our Severe Tracker page.

As you can see in the bar graph at right, on average, the U.S. witnesses a marked ramp-up of tornadoes in April as warmer, more humid air flows farther north to intercept under the still-energetic polar jet stream.

A corridor from the southern Plains to the Tennessee Valley is typically in the highest risk for severe thunderstorms in early April, however, those are just climatological averages. Early April severe weather can stretch into the Upper Midwest, Ohio Valley and even parts of the East. 

Some of the nation's worst tornado outbreaks have occurred in April, including the infamous Superoutbreaks of 1974 and 2011. 

http://www.weather.com/news/tornado-central/march-tornadoes-may-set-record-low-20140321