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.

UPDATE

Issued at 900 PM CDT Fri Apr 17 2026

The tornado watch in effect until 11 pm has been expanded eastward to the Indiana border and as far south as Cumberland and Clark counties. The flood watch continues until 4 am for central IL north of I-70 for 1-2 inches of rain per hour with locally 2-4 inches possible. The QLCS squall line currently was approaching I-55 and racing ENE at 50-60 mph and forecast to remain severe as it moves into east central IL during next 1-2 hours. Several tornado and severe thunderstorm warnings along this line of storms. In addition to possible tornadoes along this squall line, 1-2 inch hail and damaging winds over 70 mph a risk, along with the heavy rainfall. The latest CAMs have line of storms gradually weakening during overnight after 1-2 am as it moves into southeast IL.

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 thunderstorm 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 /00Z TAFS THROUGH 00Z SUNDAY/

Issued at 620 PM CDT Fri Apr 17 2026

Line of TSRA present along the IL/IA border as of 2315z, moving west in time, with scattered TSRA ahead of the line. Main concern for this TAF period will be timing out the storms, which could bring severe wind gusts and large hail. Based on radar trends, slowed the timing by an hour or two at each terminal. Storms should be past CMI by 06z. These storms are associated with a front which will swing winds to northwesterly and a brief period of MVFR ceilings along/behind the front. Winds will remain gusty into Saturday, while ceilings improve and skies clear into the day.

ILX WATCHES/WARNINGS/ADVISORIES

Flood Watch until 4 AM CDT Saturday for ILZ027>031-036>038- 040>057-061.