Weekend severe weather outbreak

The 2007 tornado season is off to a deadly start, as there have been nearly twice as many tornadoes as usual thus far (65 vs. 38) and many more deaths than usual (23 vs. 2). There is still the potential for a dangerous severe weather outbreak this weekend from the southern plains into the mid-south.

The synoptic setup has an enegetic shortwave (see 500 mb map) ejecting from the 4-corners Saturday morning. A strong low-level jet from the south (see the 850 mb map) will provide plenty of moisture and instability (note the directional shear between the low- and mid-levels). One of the keys for getting severe weather into the mid-south and points east this weekend will be the “50-50” low over Newfoundland. (50N, 50W). This trough position is key because as the shortwave ejects into the plains Saturday, the ridge of high pressure between the lows will increase in amplitude and create a block for the progression of the plains low. This will act to increase divergence at the upper levels downstream of the plains trough and cause it to become negatively tilted, which will cause the surface low to deepen and slow down by Saturday afternoon.

The region most likely to be affected by severe weather will be found under this area of increased divergence Saturday afternoon, which according to the GFS appears to be southern MO, AR, northern MS, and western TN. This region will also be beneath the left front quadrant of a jet streak, which will enhance the risk of severe weather. The storm will lose it upper air support as it runs into the block on Sunday, which may lessen the severe risk for the Southeast. The storm will occlude and weaken over the Great Lakes on Monday. The key for the severe weather to make it into the mid-south will be how much instability there will be when the upper level divergence passes overhead. Right now it looks as if the best instability will pass to our south, but we’ll have to wait and see.

The operational GFS continues to show a stormy pattern over the next couple of weeks with a return to colder weather during the first week of March.

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