Wednesday, July 21, 2010

Thursday Potential N. IL and S. WI

Been trying to hold off on getting excited about this one, as these July days love to just vanish or fall into the mushy HP and MCS category, however this one has kept my interest, and actually increased in potential since I first noticed it. I would go as far as to say there is a fairly good chance at a couple tornado producing supercells in southern Wisconsin and northern Illinois tomorrow.

My current threat area for a few tornadoes is near Interstate 39, between Highways 20 in Illinois and 18 in Wisconsin, or an area from Rockford, IL to Madison, WI and about 50 miles west of that.

Should be a round or two of convection along the synoptic warm front across northern Illinois and eastern Iowa overnight and tomorrow morning, lifting north into Wisconsin eventually. South of this convection, at least partial clearing should allow for fairly rapid destabilization along and south of the warm front in northern Illinois, and eventually southern Wisconsin. A strong low level jet will increase out of the SSW and aid in eventual development of scattered severe thunderstorms ahead of the surface low and weak surface boundary in far northwest Illinois and southwest Wisconsin. Storms should move east across the area, in a highly sheared environment with 0-1 km helicity values approaching 250-300 m2/s2, especially near the warm front in southern Wisconsin. Given the strong directional shear, and should ample instability be realized in the warm sector tornadic supercells should be a decent threat. Hard to nail down a specific target at this point as the position of the surface boundaries could be greatly effected by overnight and early period convection, but an area just west of Janesville, WI looks to be a solid starting point.

I'll likely update again in the morning as things become more clear, and will hang in DeKalb for the better part of the morning as I'm not convinced northern IL won't see a significant threat and I would hate to get suckered too far north and miss something close to home. More than likely, where the warm front ends up is where you will find me.

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