Rising humidity follows early Wednesday morning thunderstorms

 

A HEAT ADVISORY will go into effect for all of northern Illinois and southern Wisconsin beginning Wednesday morning as afternoon heat index values surpass 100 degrees, rising as high as 105-108 degrees during the afternoon. But before that, isolated showers and thunderstorms will be possible early in the morning.

A remnant low pressure system, known as an MCV - mesoscale convective vortex - developed late Monday night following the weakening of a storm complex over the Plains. Moving through Iowa during the afternoon Tuesday, this low has produced isolated showers and thunderstorms across eastern Iowa. While most of this activity is moving north, development to the northeast is possible across southwest and southern Wisconsin overnight. The majority of precipitation should remain north of the state line early Wednesday, but a few isolated showers and thunderstorms can't be ruled out south of the state line during that time as well.

The best time frame for this isolated activity would be centered around 5am-8am. As a warm front lifts north late morning any precipitation that developed along the front should also push north. This will pave the way for a climb in temperatures and rise in humidity.

A complex of thunderstorms will develop well to our northwest Wednesday night which may help push an outflow boundary/cold front closer to the Stateline Thursday. Depending on how quickly this boundary moves in will determine how hot - and humid - the afternoon gets. Right now, the forecast high for Thursday is in the low 90s with a heat index approaching 105 degrees. 

Scattered showers and thunderstorms are then likely late Thursday as the cold front passes through, with a few storms lingering into Friday.