textproduct: Central Illinois

KEY MESSAGES

- Light snow accumulations are likely tonight as a clipper system moves through, with a dusting up to 2 inches of new snowfall forecast for areas north of I-70.

- A significant warming trend begins tomorrow. Temperatures are forecast to climb into the 60s Thursday then reaching the 70s by the weekend.

UPDATE

Issued at 731 PM CDT Tue Mar 17 2026

Regional radar mosaics look impressive in coverage this hour, though surface observations suggest the leading edge of the snow is around Cedar Rapids IA at this point. Our in-progress weather balloon flight confirms there is still quite a bit of dry air that will need to be overcome, which recent HRRR forecast soundings suggest should take place by late evening over areas northwest of the Illinois River. However, those soundings only show about a 2-3 hour window with decent low level moisture. Latest NBM guidance shows about a 50-60% chance of more than an inch of snow generally north of a Macomb-Bloomington line, though the experimental NBM5.0 brings it down to more like 15-25%. Whatever snow does fall will melt quickly Wednesday morning, with temperatures well above freezing by 10 am.

Recent updates mainly focused on updated PoP's for timing the start and end of the snow.

Geelhart

DISCUSSION

Issued at 107 PM CDT Tue Mar 17 2026

The primary concern in the short term is the arrival of a fast- moving clipper system this evening. Initial lift will go into saturating a very dry airmass above 800 mb. High-res guidance shows the onset of light snow beginning across west- central Illinois between 6 PM and 9 PM, overspreading the remainder of the CWA through midnight. The 12Z HRRR and RAP suggest a narrow but potent band of frontogenetical forcing, supporting snowfall accumulations of 1-2 inches in a narrow band, likely near or north of I-74. Outside of the band of stronger forcing, lighter snowfall rates will reduce accumulations to a dusting to half inch for many areas. Pressure gradient tightens tonight, resulting in southeast winds gusting 20-25 mph at times. These winds will be strong enough to produce drifting in open areas.

Another clipper targets the region Wed night to Thu morning, however this currently is shown to track farther north than tonight's wave. Low to mid level warm advection beginning Wed will favor rain as the primary precip type across central IL. Latest NBM probabilities indicate less than a 20% chance of measurable rain due to limited moisture return and better forcing off to our northeast Thu morning.

High confidence exists in an unseasonably warm period late week through the upcoming weekend. Upper-level ridging over the western United States amplifies and shifts toward the Midwest, allowing mid- level heights to rise substantially. NBM deterministic values show high confidence in a sharp warming trend, and ECMWF EFI values of 0.6 to 0.8 for Fri-Sun indicate an unusually warm airmass for March. Gridded guidance shows highs reaching the 60s on Thursday, and 70s Fri-Sun. Regarding trends for the end of the period, medium range guidance indicates a frontal passage early next week, returning temperatures to more seasonable levels.

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AVIATION

(For the 00z TAFs through 00z Wednesday Evening) Issued at 610 PM CDT Tue Mar 17 2026

Ceilings expected to lower to about 5kft this evening, as a fast- moving storm system brings about a 3-4 hour period of light snow. Will continue with the PROB30 for lower visibilities/ceilings, but probabilities for a short IFR level visibility are higher from KPIA-KCMI (about 30-40%). Once the snow moves out, a period of gusty south winds is expected, diminishing around midday and gradually turning more westerly.

Geelhart

ILX WATCHES/WARNINGS/ADVISORIES

None.