One of the biggest headlines in weather last year was the record number of tornadoes that occurred in April. Over 700 tornadoes formed putting April well above it's monthly average. This year, however, has been a little more quiet. Looking back at the past three months this year and comparing them to last year, March 2012 started off strong with the first outbreak of weather occurring actually on the 29th of February and then continuing the first couple days in March.
Both April and May were fairly quiet months when looking back to last year as well as the three year average. The biggest explanation for this would be the more 'summer-like' jetstream pattern this year. Typically during the spring months the jetstream runs through the middle of the country. This year, it's been further north. This has kept the warm air in place which doesn't allow thunderstorms to build. In order to get air to rise it must be warmer than the air that surrounds it. If the surrounding air is as warm or warmer, then air parcels at the surface can't rise therefore limiting thunderstorm development. It looks like the warm air will remain in place through the weekend with the threat for stroner storms in the Plains and upper Midwest.
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