textproduct: Central Illinois

KEY MESSAGES

- Several rounds of convection will impact central Illinois late tonight through Tuesday evening. While all severe weather hazards will be possible, damaging wind gusts in excess of 60mph will be the most likely.

DISCUSSION

Issued at 259 PM CDT Sun May 17 2026

*** Round One of Convection ***

19z/2pm regional analysis shows rapid thunderstorm development across eastern South Dakota ahead of a short-wave trough pivoting northeastward from Colorado. As this wave interacts with a frontal boundary draped across the Plains, widespread strong to severe convection will organize into one or more linear structures as it gradually shifts eastward later this evening into the overnight hours. CAMs exhibit a good deal of spread concerning both the coverage and timing of the storms as they cross the Mississippi River well after midnight. The NAM is the fastest model, showing cells potentially arriving across the far NW KILX as early as 09z/4am...while the HRRR continues to lag by a couple of hours. Regardless of which solution pans out, the storms will be entering an environment characterized by ample elevated instability, but weak deep-layer shear. NAM MUCAPEs range from 1500-2500J/kg ahead of the storms, while 0-6km bulk shear is just 25-30kt. Given the elevated nature of the convection, think the primary severe weather risk will be scattered damaging wind gusts from the Mississippi River eastward to the I-55 corridor through late morning.

*** Round Two of Convection ***

As the broken line of thunderstorms crosses I-55, it will begin to encounter an increasingly unstable airmass across east- central and southeast Illinois where HREF mean SBCAPEs reach 1500-2000J/kg. Once the storms become surface-based, all severe hazards will come into play east of I-55 Monday afternoon, including damaging wind gusts in excess of 60mph, hail larger than quarters, and isolated tornadoes. All model solutions show the storms exiting into the Ohio River Valley prior to sunset, followed by mainly quiet conditions across the board through the evening.

*** Round Three of Convection ***

A similar process to what will happen tonight will unfold Monday night as a vigorous short-wave noted on latest water vapor imagery over Nevada rounds the base of the Rockies trough and generates a renewed round of severe convection from Omaha and Des Moines southwestward to Kansas City late Monday afternoon and evening. These cells will also grow upscale into a line of storms, then progress eastward into west-central Illinois overnight. Damaging wind gusts and isolated tornadoes will be the primary hazards between about 06z/1am and 11z/6am from the Mississippi River eastward to the I-55 corridor.

*** Round Four of Convection ***

Outflow from the morning storms and/or the approaching synoptic cold front will be the primary focusing mechanisms for additional storms Tuesday afternoon and evening. While the exact generation zone is still in question, am continuing to focus attention on locations east of I-55 for the highest probability of damaging wind gusts, large hail, and a couple tornadoes. These storms will depart into Indiana by evening, followed by a period of cooler and much quieter weather conditions Tuesday night through Thursday before the next chance for showers arrives by the end of the week.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1243 PM CDT Sun May 17 2026

Southerly winds will gust 25-30kt at the central Illinois terminals this afternoon before gusts decrease slightly into the 20-25kt range after sunset. Models continue to show a strong nocturnal jet developing tonight, so have included 50kt of low-level wind shear at all sites accordingly. Aside from the wind, the other main aviation forecasting challenge will be the potential for convection late tonight into Monday morning. All models show storms firing across western Iowa into Kansas this evening...but the eastward evolution/timing of the convection remains somewhat in question. Based on overall consensus, it appears likely that a broken line of storms will push into the Illinois River Valley around dawn, then drop southeastward to the I-72 corridor toward midday. Given increasing confidence for convection, have included a TEMPO group for thunder at KPIA between 12z and 16z and at the I-72 terminals between 14z and 18z. With much richer boundary layer moisture in place, think ceilings will lower to MVFR within the convection.

ILX WATCHES/WARNINGS/ADVISORIES

None.