Monday, August 2, 2010

Marginal Potential in NW Illinois 8/3

Another marginal late-season tornado threat, this time in northwest Illinois. I'm going to be brief on this one, at least for tonight until things become more clear as to how they'll unfold.

Currently, an E-W oriented MCS is lined up near Interstate 80 from northern Illinois west into central Iowa. This slow moving sub-severe MCS, though at times capable of producing a heavy rain and an associated flooding threat should continue into the morning as it slowly chugs into north central Illinois. In it's wake, an outflow reinforced warm front will be located from southeast Iowa, into north central Illinois.

Instability along and south of the warm front should be more than adequate for severe convection redeveloping during peak heating, with MLcape values around 3000-3500 j/kg possible by mid-afternoon. Assuming minimal back building occurs thru the overnight hours and into the morning, in the wake of the MCS additional convection should begin to develop during the early afternoon. As this convection develops near the Quad Cities area and moves southeast it should attain severe levels. Given sufficient hodographs in the 0-3 km range, rotating storms are possible, at this point most likely in an area from Peoria to Bloomington, IL. The main limiting factor may be the 0-1 km range. Directional shear is conducive to storm rotation, but speed shear should be fairly limited so baring outflow boundary interaction (which is a valid threat) the storms may lack the *umph* to produce tornadoes.

All that said, given sufficient instability and the presence of the outflow reinforced warm front the afternoon hours warrant attention, likely along Interstate 74 from the Quad Cities towards Bloomington, as mentioned above.

If I were most concerned about one issue, it's the nagging feeling that tomorrow could end up a subsidence induced bust. If subsidence lingers in the wake of the MCS too long, this coupled with marginal forcing along the warm front due to weak surface flow, convection could be suppressed in the high shear zone until later in the period, if at all. Close attention will need to be paid to this issue as all the hodograph turning in the world will do us no good if we can't first develop a thunderstorm. Still, I'll pay attention.

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