Historical records suggest, and meteorology usually dictates, that the chance for tornadoes in the United States increases as the Northern Hemisphere winter season comes to a close in March. Organized thunderstorms with the prospect of producing tornadoes peak in both frequency and intensity during the latter portion of the spring season, from late April through May. That is when increasingly unstable air masses can become entwined with faster and more dynamic westerly flow over large parts of the continental United States. As summer approaches, the stronger, deep-layer atmospheric westerly flow retreats northward, and the subtropical ridge axis expands across the southern United States and the Gulf of Mexico, effectively limiting the area where the most intense and organized thunderstorms can form.
The first months of 2008 were anything but typical of the long-term annual pattern for tornado activity just described. Severe weather began early in the year, and tornado events of substantial impact struck in January, February, and March. April had about as many tornadoes as more recent Aprils (2005-2007), before the spring tornado season came on with a vengeance in May and continued into June.
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