textproduct: Central Illinois

KEY MESSAGES

- Patchy frost is forecast near and east of I-57 tonight. While the chance is low (20-40%), those with tender vegetation should consider covering their plants to prevent damage.

- Scattered thunderstorms are forecast Monday into Wednesday. There is a low (5-14%) chance of a severe thunderstorm both Sunday and Monday into Monday night.

DISCUSSION

Issued at 141 AM CDT Sat May 2 2026

***** COOL WITH A FEW SHOWERS IN SOUTHEAST IL TODAY *****

At 130am, the axis of a shortwave trough extended from Ontario southwest into the Central Plains. Ahead of this feature, a few showers linger in southeast Illinois, while scattered clouds are present elsewhere. This will limit radiational losses through the morning, though patchy frost is possible (15-30% chance) in areas that see the most breaks in this cloud cover, with the highest chances near and north of roughly I-74.

With 850mb temperatures plunging to around -2 degC this afternoon and numerous diurnal cumulus limiting surface heating, temperatures are forecast to climb only into the mid to upper 50s - around 15 degrees below normal for early May. With cloud cover diminishing in the evening, temperatures may fall enough overnight to result in patchy frost in east-central Illinois, though as surface high pressure shifts southeastward during the night winds will return from the southwest. The National Blend of Models (NBM) advertises a 20-50% chance overnight low temperatures fall below 36 degF south of I-72 in east-central Illinois, so a mention of patchy frost was added to the forecast there, though we'd want to see confidence come up before pulling the trigger on a Frost Advisory.

***** WARMER, FEW STORMS, EARLY NEXT WEEK *****

Tomorrow into Monday, the upper level trough responsible for our cool, showery weather over the past several days will finally lift northward into the Hudson Bay, while another shortwave dives southeast across the Upper Midwest and Great Lakes. This combination will promote southwesterly flow over the local area ahead of a weak cold front on Sunday, allowing temperatures to climb into the 70s. The jury's out on whether or not this front will provide sufficient lift to trigger showers and storms Sunday afternoon, but the chance appears low with the NAMNest the only CAM depicting storms. NBM/LREF (Low Resolution Ensemble Forecast) ensemble mean depict several hundred J/kg of SBCAPE (highest southwest) along with 45-55 kt of 0-6km shear, which would allow any storm that does form to become strong. Even so, SPC's day 2 outlook would suggest the chance is low (5-14%).

The front is forecast to stall over our area Sunday night, and linger through at least Tuesday night. With moisture convergence and a series of weak disturbances riding along the boundary, periods of showers and storms are likely (50-70% chance). With any breaks in cloud cover, a few thousand J/kg SBCAPE could materialize near and south of the front on Monday, potentially fueling a couple stronger storms that could generate locally severe hail and/or wind. However, confidence in this is low due to weak forcing. The best opportunity for storms might actually come Monday night as the LLJ ramps up over the area. SPC has our entire area outlooked in a level 1 of 5 (marginal) risk for severe weather.

***** BELOW NORMAL TEMPS RETURN MID WEEK *****

A stronger shortwave trough is forecast to arrive on Wednesday, giving the area a stronger push of cool advection. The LREF gives a 30-50% chance that 850mb temperatures drop below -2 degC, which would favor high (low) temperatures in the mid-upper 50s (upper 30s to low 40s) Wednesday and Thursday. The parent upper level trough is forecast to park itself over the Great Lakes and Northeast late next week, which would foster perpetual cool, northwest flow across the Midwest. WPC's cluster analysis suggests the greatest source of uncertainty is in the size/amplitude of this upper level trough heading into next weekend. About 30% of models have above or near normal temperatures, while the remaining 70% depict some flavor or below normal; for reference, typical highs in early May are in the low-mid 70s. Given lack of deep-layer moisture in this northwest flow regime, heavy rainfall would not be favored, though a few light showers will be possible (10-25% chance) with any shortwaves passing through the area.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/

Issued at 1235 AM CDT Sat May 2 2026

VFR conditions will prevail at all sites next 24hrs. PIA and BMI will see broken cigs around 7kft through the night while SPI, DEC, and CMI will be scattered...per satellite loops. Scattered clouds around 6kft are expected at all sites today before clearing out in the evening. Winds will be light now but then become northerly during the day, with speeds of 7-9kts. Then light and variable winds expected in the evening.

ILX WATCHES/WARNINGS/ADVISORIES

None.