textproduct: Central Illinois

KEY MESSAGES

- Frost potential tonight...there is a 30 to 50% chance of conditions favoring frost formation north of I-70 late tonight with the highest probabilities near the I-74 corridor east of I-39.

- Thursday showers and gusty winds...Scattered (30-40%) showers and isolated (20%) storms are expected near the I-74 corridor Thursday afternoon and evening. Any storms are expected to remain sub-severe, but a few gusts of 30 to 40 mph are possible.

DISCUSSION

Issued at 105 PM CDT Wed May 6 2026

Tonight: Frost Potential The overarching pattern through Friday remains dominated by upper-level troughing centered over the Great Lakes, driving a period of below-normal temperatures and a couple of light precipitation chances. At the surface, a 1020mb high pressure system, initially over Kansas and Nebraska, will track east across the mid Mississippi and lower Ohio Valleys overnight.

The main concern tonight is the potential for frost. A brief window of favorable radiational cooling--clear skies and light or calm winds--is expected as the ridge axis moves overhead. This will allow temperatures to rapidly fall through the 40s, with some spots dipping into the upper 30s. Mid-level warm air advection will spread back across the region behind the ridge axis, potentially leading to increasing cloud cover from the west late tonight and helping temps level off.

The joint probability for temperatures below 38degF and cloud cover less than 30% (the threshold favorable for frost formation) ranges from 30-50% north of I-70, with the highest probability along and north of the I-74 corridor east of I-39. While confidence in widespread frost formation is currently too low for a headline, we will continue to monitor the situation. If confidence increases later this evening, a Frost Advisory may be required.

Thursday and Friday: Scattered Showers/Storms On Thursday, a series of low-amplitude shortwave troughs will dig across northern Illinois, bringing a chance of scattered showers and isolated thunderstorms to the region. Forecast soundings show steep low and mid-level lapse rates (an inverted 'v' profile), which will efficiently mix down winds in any convective precipitation. However, low equilibrium heights are expected to limit the overall strength and depth of convection. The latest 12Z HRRR model suggests the potential for gusts up to around 30 knots within any showers or storms. PoPs are highest (30-40%) along the I-74 corridor, with dry conditions becoming favored further south.

A slightly deeper shortwave trough will dig across the region on Friday, with a weak surface low reflection moving across central Illinois Friday afternoon and evening. Instability is expected to be quite limited due to poor mid-level lapse rates, with pockets of MLCAPE peaking around 300-400 J/kg ahead of the surface low. Meanwhile, deep layer shear will be moderately strong, around 50 knots. Given the modest synoptic-scale forcing and generally weak instability, convection is currently expected to remain sub-severe on Friday.

Weekend Forecast: Brief Warmup Followed by a Strong Front The upper trough will briefly retreat on Saturday as shortwave ridging moves overhead, setting the stage for a dry start to the weekend. Temperatures will rebound nicely into the mid to upper 70s area-wide. This warmth will be short-lived, however, as a cold front is forecast to drive across the region Saturday night into Sunday. Gulf moisture ahead of the front will be shunted well to our south, and coupled with unfavorable diurnal timing, the front is expected to push through most of central Illinois with weak instability. However, depending on the frontal speed, there may be a brief window Sunday afternoon across southeastern Illinois where a better thermodynamic environment (750-1000 J/kg of MLCAPE and moderate shear) could support a few strong to severe storms before the system exits into Indiana. Behind the front, expect a dry start to the week with temperatures returning to below-normal values by Monday.

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/

Issued at 547 PM CDT Wed May 6 2026

High pressure will move in from the west this evening, causing winds to quickly drop off and go light/variable through sunrise. Southwest winds will increase Thursday morning and gust near 25 kt into afternoon. Scattered -SHRA could occur near the I-74 corridor late morning through afternoon, and covered this with a PROB30 at KPIA-KBMI-KCMI. An isolated -TSRA can't be ruled out but probabilities are too low to mention at this point.

ILX WATCHES/WARNINGS/ADVISORIES

None.