Wednesday, June 06, 2007

Dangerous Thursday

It appears that all the ingredients are coming together to possibly create a widespread, significant severe weather outbreak across the region on Thursday. An unseasonably strong storm system is developing across the northern plains today, and this is expected to drive a strong cold front through the Upper Midwest tomorrow. Wind shear values are terrifyingly high, which leads to a high probability of severe thunderstorms. The entire region is currently under a moderate risk for severe storms. The most likely scenario would see thunderstorms rapidly developing across eastern Minnesota and Iowa early tomorrow afternoon. Tornadoes, some of which could be strong, will be possible with early supercells that develop. These discrete storms are then expected to form into a rapidly-moving squall line which is then forecast to move across Wisconsin late Thursday afternoon and evening. Widespread damaging winds are possible with this line, an event known as a 'derecho'.

There are two factors that could limit thunderstorm development. The first is the convection that is expected to affect the Dakotas and Minnesota tonight. A widespread severe weather event is anticipated there, but it is forecast to remain well to our north. However, if any of this precipitation makes it to southern Wisconsin, or debris clouds linger across the region tomorrow morning, the severe threat will be reduced. At this time, this is not expected to happen. The second inhibiting factor is the wind shear. While strong wind shear favors severe thunderstorms, in this case it is possible that the shear will actually be too strong, and the storms will be ripped apart before they can fully develop. Basically, this will probably be one of those "all or nothing" scenarios. If storms get going, they will likely be some really bad ones. If one of these two factors mentioned above comes into play, we may not get much of anything. Despite this, severe weather looks likely tomorrow afternoon, and if everything comes together right, it could be a bad day.



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