textproduct: Central Illinois

KEY MESSAGES

- An intense, multi-hazard severe thunderstorm event appears likely this evening into tonight. Areas along and west of a Taylorville to Champaign line are in an Enhanced Risk (Level 3 of 5) of severe storms, while areas to the east are in a Slight Risk (Level 2 of 5). Storms are likely to impact areas west of I-55 between 5-9 PM, then spread east of I-55 late this evening.

- The storms could produce locally heavy rainfall of 2-4". Scattered instances of flooding are possible, especially in urban, low- lying, or other poor drainage areas. A Flood Watch has been issued for areas along and north of Shelby to Edgar County.

- Cooler air returns this weekend, and lows in the 30s may result in frost development both Saturday night and Sunday night.

DISCUSSION

Issued at 134 PM CDT Fri Apr 17 2026

*** Severe Storms Likely This Evening-Tonight ***

The 'bottom line up front': the forecast largely remains on track, with an intense, multi-hazard severe thunderstom event this evening into tonight.

Sfc low is positioned just north of Duluth, MN as of 18z/1pm, with the attendant cold front extending south near Minneapolis, MN, to Des Moines, IA, to Wichita KS. In the last hour, storms have erupted across eastern IA along a pre-frontal trough, which extends roughly from the IA/WI border to near Kirksville, MO. This is an evolution that CAMs highlighted in recent days. Locally, temps have pushed to near 80F while dewpoints are still in the mid 60s. Continued advection of the EML and deeper moisture content have led to instability rising over 2000 J/kg, and PWATs over 1.2". Vis satellite shows a Cu field quickly expanding across the ILX CWA, and any capping is weak or non- existent. Still, the best source of forcing in the short-term will be that pre-frontal trough noted to our north/west. For our area, isolated supercells cannot be ruled out this afternoon, and any mature storms that develop will pose a risk of all severe hazards.

The more widespread severe threat is expected to come as the cold front approaches and upscale growth of thunderstorms occurs. As these storms move into IL, a highly unstable airmass will still be in place, with increasing low-level shear as the LLJ ramps up after 00z/7pm. This line of storms could produce scattered damaging winds, with gusts over 75 mph possible. Line- embedded tornadoes remain a concern, as 0-1km SRH values surge above 150 J/kg in response to the LLJ.

CAM timing has not changed much from previous iterations, although there is some thought that cold pool dominant storm structures may surge east a smidge faster than CAMs currently suggest. Our latest thinking is that the line will impact areas west of I-55 between 5-8 PM, then continue east and reach portions of the I-57 corridor by 9- 10 PM. The main line of storms should reach the IL/IN border by about 05z/midnight CT. Scattered showers may linger for a few hours behind the main line of severe storms.

*** Flooding Potential ***

In light of recent rains and unseasonable PWATs (95th+ percentile) present with this system, concern is growing that at least scattered flash flooding will occur with tonight's storms. Despite the relatively progressive forward motion of the line, the last few cycles of HREF LPMM guidance have consistently highlighted an area roughly from the IL River to the I-72/Danville corridor where localized rainfall amounts of 2-4" are possible. 3-hr Flash Flood Guidance is less than 2" in most locations, and a recent mesoscale precipitation discussion from WPC notes storms will be highly efficient rainfall producers with rates of 1-2" per hour. A Flood Watch was issued to capture the counties where scattered instances of flooding seem most likely to arise.

*** Rest of the Weekend ***

Following tonight's FROPA, breezy northwest winds will persist into Sat. Highs are likely to be cooler over the weekend, in the 50s or low 60s. No major changes to the frost/freeze outlook for Saturday night and Sunday night. Frost chances could extend as far south as I- 70 both nights, but the chances for frost generally increase with northward extent. The gridded forecast reflects this. While some spots, particular those in low-lying areas, could drop below 32F, a widespread freeze appears unlikely.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1210 PM CDT Fri Apr 17 2026

Only minor changes were made to the TAFs with this update, with a slight adjustment to the TSRA timing this evening (bringing thunder to each terminal an hour faster than previously forecast). Overall, this TAF period is still expected to be characterized by breezy south winds this afternoon, then a line of strong storms progressing west to east this evening into tonight along a cold front. After the initial line moves through, showers could linger for a few hours before coming to an end. MVFR ceilings also linger behind the front, but ceilings should improve to VFR towards Sat morning. Winds turn northwesterly following the frontal passage and remain breezy (20-25 kt gusts).

ILX WATCHES/WARNINGS/ADVISORIES

Flood Watch from 6 PM CDT this evening through late tonight for ILZ027>031-036>038-040>057-061.