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March 4, 2010 / Dave Gorham

Southern States Cooler and Wetter – and Free of Tornadoes?

[Just as this post was about to be uploaded to YourWeatherBlog an amended storm report surfaced that included a lone, weak tornado in California on February 27. Not wanting the facts to stand in the way of a good severe weather article, we’re posting the article in its original state while at the same time bringing you the latest and most correct information.]

No tornadoes? It’s true. February, 2010 goes into the record books as unique because not a single tornado was recorded anywhere in the United States for the first time since records have been kept by the National Weather Service in 1950.

Tornadoes, of course, are products of severe thunderstorms and thunderstorms are formed by rapidly rising columns of warm, moist air. In the summer months it’s not uncommon for these columns to be independent of frontal systems. In the winter however, it frequently takes the clash of cold dry air from Canada or the Arctic with the relatively warm air in place over the southern states to lead to thunderstorms, some of which are capable of producing tornadoes. Ask a resident of the South about the thunderstorms last month and you’ll not likely find much to talk about. Once again, we can thank El Niño.

The signature characteristic of El Niño for the United States is cooler, wetter and cloudier conditions for the southern states — the exact location where wintertime thunderstorms, and thus tornadoes, would typically be found. But as El Niño-driven Pacific storms have moved one after the next across the South, there has been little opportunity for the warm, rising columns of air needed to initiate and fuel thunderstorms, let alone tornadoes.

Photo: NOAA archive. (Look familiar?)

Does this mean anything about anything? Short answer: No. Longer answer: There is no evidence to suggest a February free of tornadoes points to a severe season with significantly fewer than a normal number of tornadoes, though most years that start with a fewer number typically finish with an average or just below average number of tornadoes.

However, perhaps not. As we head into an El Niño-influenced severe weather season, our ImpactWeather StormWatch team reports that all early signs point to increased severe activity. Just as with the increased frontal systems passing across the southern states over the past couple of months, this trend is expected to continue into the summer. And as the atmosphere over the southern states begins to percolate with more columns of rising warm, moist air more thunderstorms should be the result. On top of that, literally, the southern jet stream will drive these frontal systems into the warm, unstable air masses from the Plains to the Mid-Atlantic. And the jet stream is key to severe weather! Ask any student of severe thunderstorms and they will tell you that when you add a strong jet stream on top of a thunderstorm the likelihood of a severe thunderstorm increases.


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