Illinois Convection Accelerates Toward Evening MCS Wind Threat — SevereWX

Illinois Convection Accelerates Toward Evening MCS Wind Threat

The Storm Prediction Center has issued Mesoscale Discussion 1742, flagging a growing severe weather risk across Illinois, far southwest Indiana, and western Kentucky. Isolated thunderstorms are bubbling up in an extraordinarily unstable atmosphere, with MLCAPE soaring to 4500-5000 J/kg per recent soundings.

Right now, a lone cell over south-central Illinois looks unlikely to turn severe, as it's offset from key low-level convergence along a surface boundary stretching from western Kentucky through central Illinois. But don't count it out yet—forecasters are eyeing fresh development along that boundary, boosted by a subtle mid-level disturbance sliding in from Iowa.

Recent HRRR/RRFS model runs are nailing the trends, showing this initial activity fading before new storms fire up between 3-5 PM CDT (20-22 UTC). GOES satellite loops back this up, revealing cumulus towers strengthening along the boundary in southern Illinois and nearby areas.

Once ignited, these storms should tap into prime conditions: supercell composite parameters (SCP) of 8-12, supporting organized clusters. Expect rapid upscale growth into a mesoscale convective system (MCS) by early evening, packing damaging straight-line winds. SPC pegs peak gusts at 55-70 mph, with potential to 90 mph, hail up to 1.25 inches, and low-end tornado risk.

A tornado watch carries 60% odds this afternoon into evening, as timing hinges on widespread initiation. The affected polygon spans from roughly St. Louis northward into central Illinois and east into the lower Ohio Valley.

Stay prepared: Review your severe weather safety plan, keep an eye on radar via local NWS offices or apps, have a shelter ready, and heed any watches or warnings. This could evolve quickly—don't wait for the watch to break out power lines.