Severe Weather POSSIBLE For Southern Illinois

As I'm sure many of you have heard, there is potential for severe weather Wednesday afternoon and evening for parts of Southern Illinois. At the time of this writing (Tuesday afternoon), there are still questions about who has the greatest potential for seeing severe storms.

We mentioned in the last blog entry that a pattern shift would occur early in the week, returning us to a more "zonal flow" which in essence, will bring a somewhat active weather pattern to ILMOKY. In the 500mb charts, we've been watching a shortwave trough cross the peak of a ridge centered over the Southern Plains. As the shortwave and accompanying 500mb to 700mb jet streak moves east/southeast across the Northern Plains and into the Midwest, a line of storms (MCS or Mesoscale Convective System) will develop and produce severe weather Tuesday from Nebraska to western Indiana. It's possible a few storms from the MCS make it to I-64 in Southern Illinois early Wednesday, but the severe weather threat should have waned by that point.

HiRes NAM 500mb chart at 09Z Wednesday morning.

As the shortwave trough continues east, a trailing surface cold front will drop south throughout the day Wednesday into Southern Illinois. That's where the chance for severe weather will come in. Ahead of the cold front, instability (CAPE) will pool with values 2500J/kg +, high amounts of moisture can be found with models indicating that PWats may push 2", in addition to a somewhat strongly sheared environment. With all that in play, there are two limiting factors that still may inhibit severe weather potential in Southern Illinois. 1) Early morning shower and storm activity could lead to some stabilization and lingering cloud cover 2) Some models are indicating a strong "cap" at 700mb.

Skew-T from Cape Girardeau, Missouri at 4PM Wednesday

Most models agree the "cap" will erode and convection will occur in at least parts of Southern Illinois. Comparing soundings (like the one above) in Cape Girardeau, to that of Paducah, and Evansville is how we've come to the conclusion that the best chances of severe weather in Southern Illinois is likely to be along and east of I-57/I-24. Outside of this area where the cap is stronger, storms are still possible, and if they can punch through, they would likely go severe.

Timing for Southern Illinois has moved up from previous model runs with storms moving in as early as 3PM along I-64 and will drop south with the line possibly growing to the west late afternoon and into the evening. With the ingredients we have, the threat will be large hail, damaging winds, and possibly an isolated tornado, though the threat for tornadic activity looks small at this point.

- Nick