textproduct: Central Illinois

KEY MESSAGES

- A 60-80% chance of scattered thunderstorms exists throughout the day. This evening, especially after 4 PM and primarily north of Interstate 70, there is a Slight Risk (Level 2 out of 5) for severe weather. - Late Friday evening, areas generally west of Interstate 57 face a Slight Risk (Level 2 of 5) for another round of severe weather.

- Significantly cooler weather returns this weekend. There will be a low (20-30%) chance for frost Saturday night north of Interstate 70, and a higher chance area-wide Sunday night.

DISCUSSION

Issued at 313 AM CDT Wed Apr 15 2026

Two primary areas of convective activity are visible on the radar this morning. One area is situated across southeast Iowa and northern Illinois, near the main synoptic front. The second area is lifting across the Missouri Ozarks. Both are fueled by a low-level jet and shortwave energy ejecting ahead of the main upper trough, which is currently moving across the lee of the Rockies.

Convective tops are notably warming over the Ozarks as this system encounters drier mid-level air, weaker instability, and reduced surface moisture convergence. As the shortwave/MCV continues its northeast track out of the Ozarks and into the Mid-Mississippi Valley later this morning, it is expected to interact with outflow boundaries from decaying northern storms that will have drifted south of the main synoptic front.

The net effect will be an area of re-blossoming convection over central Illinois this morning. While a tenuous low-level jet may sustain this activity into the early afternoon, the severe threat remains low due to relatively poor lapse rates and shallow CAPE profiles.

Thunderstorm activity is expected to briefly lull this afternoon as the shortwave/MCV departs and subsidence occurs, leaving a somewhat worked-over warm sector. However, this lull may be short-lived due to quickly increasing synoptic ascent within a difluent zone ahead of the main upper-level shortwave moving into the lower Missouri valley.

Conditions are expected to become more favorable for severe weather, particularly west of Interstate 55, where CAPE profiles will become chubbier as mid-level lapse rates steepen ahead of the main upper-level shortwave and near a residual Elevated Mixed Layer (EML).

With better CAPE and deep-layer shear of 30 knots or greater, supercell and multicell clusters are possible across west- central Illinois. Modeled hodographs from the HREF Mean suggest straight or N-shaped profiles, which are more indicative of splitting supercells capable of producing large hail and damaging winds.

While it is uncertain if these storms will be surface-rooted or elevated, an intensifying low-level jet this evening will increase Storm Relative Helicity (SRH) and result in a more curved hodograph. This introduces a tornado threat if storms manage to become surface-rooted.

Regardless of storm initiation/evolution for the late afternoon through early evening period, CAM output becomes more consistent by tonight that additional storms upstream will congeal and grow upscale into a linear MCS overtime as the effective surface boundary sags southward into central Illinois.

In addition to the severe weather threat, there is a low potential for localized flash flooding, particularly if storms repeatedly track over the same area. The latest HREF QPF LPMM highlights a narrow area in east-central Illinois that could see 3-5 inches of rain through Thursday morning. This is notable, as current 6-hour flash flood guidance suggests that general flooding issues could arise after 2.5-3 inches of rain.

A break from storm activity is expected across the entire region on Thursday, driven by synoptic-scale subsidence following the departure of the upper trough.

However, the threat of thunderstorms returns over the weekend, specifically beginning Friday night and lasting into Saturday. This renewed activity is tied to a compact shortwave trough lifting across the Mid-Mississippi Valley, preceding a more robust, pivoting trough and its associated cold front. Despite the frontal passage occurring during a diurnally unfavorable time, the parameter space for CAPE and shear appears sufficiently volatile to support an organized risk of severe weather which lasts through the overnight hours as the convection grows upscale along the front.

A significant cooling trend is expected from Saturday night through Monday following the passage of the cold front, as the previous week's warm, moist air mass is displaced. Latest NBM guidance indicates a concern for widespread frost, with low temperatures Saturday and Sunday night anticipated to fall into the 30s.

By the middle of next week, temperatures are forecast to quickly rebound into the lower 70s. This warming trend is due to an upper-level pattern exhibiting some blockiness, allowing an amplified ridge axis to build into the region.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 101 PM CDT Wed Apr 15 2026

Multiple rounds of showers and thunderstorms will push across the central Illinois terminals through the TAF period. Confidence in timing and coverage of storms remains low, resulting in several periods of either TEMPO or PROB30 for TSRA through tonight. Away from storms, VFR conditions are expected. Southwest winds will continue through the period, gusting to 25 kt at times, shifting to the west Thursday morning.

ILX WATCHES/WARNINGS/ADVISORIES

None.