textproduct: Central Illinois

KEY MESSAGES

- There is an Enhanced Risk (Level 3 of 5) of severe thunderstorms near and west of the Illinois River Friday afternoon-evening. A Slight Risk (Level 2 of 5) extends from the IL River to a Shelbyville-Danville line. Damaging winds, large hail, and tornadoes are all possible.

- Low temperatures in the 30s could lead to frost developing both Saturday night and Sunday night. This could harm any sensitive early season vegetation.

DISCUSSION

Issued at 214 PM CDT Thu Apr 16 2026

*** Rest of Today ***

Mostly sunny across IL this afternoon, save for some corridors of scattered diurnal cumulus. Temps remain above normal once again, with sfc temps in the low/mid 70s as of 18z/1pm and the expectation being these will climb into the upper 70s this afternoon. IL sits between a few sfc features including a 1006-mb sfc low over the Great Lakes and a 1022-mb sfc high over the SE CONUS.

A subtle shortwave lifting across the Ozarks is interacting with modest instability (1000-2000 J/kg of SBCAPE), which has led to scattered thunderstorms developing. Effective shear values are under 30 kts. This activity is progged to stay just south of the ILX CWA, but felt it worth a mention since it will be a close call along the US-50 corridor. A Marginal Risk (level 1 of 5) of severe storms was added for areas south of I-64, mainly for hail potential with these storms. These storms should push out of IL by 02-03z/9-10pm CT. Expect mostly clear skies and mild temps tonight, with lows in the mid 50s (nearly 10-15 degrees above normal).

*** Friday Severe Storm Potential ***

The biggest change to highlight for Friday's severe storms is a trend towards faster/earlier timing. An upper trough, currently evident in IR satellite imagery over Montana, will result in lee cyclogenesis tonight. Into Fri, guidance depicts a sfc low over northern MN with a cold front extending south-southwest into KS. The ILX CWA will be firmly within the expansive warm sector of this system, where breezy south winds will advect higher moisture content, with sfc dewpoints in the low/mid 60s. The resultant instability values will be high, ranging from 2000-3000+ J/kg (increasing with NW extent).

Forecast soundings show only minimal capping remaining by early afternoon, and the latest CAMs seem to be latching onto a pre- frontal trough NW of the IL River as an initiating boundary. While sfc convergence looks uninspiring, only having weak capping in place does lend credence to the CAM solutions depicting storm development earlier than previously forecast (potentially as early as 20z/3pm). Forecast hodographs appear quite supportive of large hail, conditional upon a supercellular storm mode, with muted low-level shear (0-1 kt shear less than 15 kts) and a straight, elongated hodograph through the mid- levels. However, with linear forcing, weak capping, and right- moving supercell storm motions nearly parallel to the initiating boundary, the expectation is that quick upscale growth will occur which would limit the large hail potential to some degree and instead favor a damaging wind/line-embedded tornado threat. The window for any supercells would be as storms initially form during the afternoon NW of the IL River. While any supercell will be capable of producing tornadoes, the supercell tornado threat is highest NNW of the ILX CWA, closer to the sfc low where backed sfc winds and higher low-level helicity will be present.

By mid-evening, CAMs almost uniformly depict a linear storm mode, nearing the IL River by 00z/7pm. By that time, the LLJ will be ramping up, leading to a considerable increase in 0-1 km shear (30+ kts) and storm-relative helicity (300+ J/kg). These conditions could support strong line-embedded tornadoes, but will likely require localized surges/bows within the line to obtain a favorable shear vector orientation for mesovort development. The severe threat will gradually wane into the night as storms progress east, but based on the slightly faster storm timing the Slight Risk (level 2 of 5) was expanded as far east as a Danville-Shelbyville line, with a Marginal Risk further east from there.

In summary, the thinking is storm development by mid-afternoon NW of the IL River, with supercells posing a risk of all hazards. Storms are expected to quickly grow upscale, with a squall line then bringing a risk of scattered damaging winds and embedded tornadoes. Latest model guidance has this line of storms near the IL River at 00z/7pm. Storms are unlikely to reach areas east of I-55 until after 01-02z/8-9pm. Monitor forecast updates, as the specific timing could still change by a few hours.

*** Frost/Freeze Potential This Weekend ***

Following FROPA on Sat and the departure of precip, the main concern will be a colder airmass offering frost/freeze potential. An expansive sfc high will be present over the Plains Sat night, then shift east towards the northern Ohio Valley Sun night. Mostly clear skies and light winds should allow lows to fall into the 30s. The probability of frost or freeze increase with northward extent, but anyone with sensitive vegetation will need to be prepared to take action this weekend. For Saturday night, the probability of lows below 36F is 60-80% north of a Macomb-Bloomington line, decreasing to 20-40% along and south of I-72. For Sunday night, the probability of temps below 36F is 70-80% along the I-74 corridor, decreasing to 30-40% along the I-70 corridor. The probability of a freeze is lower, below 50% area-wide both nights. While not impactful, highs will also be cooler over the weekend, in the 50s or low 60s.

*** Trending Warmer Next Week ***

Into next week, temps rebound with highs back into the 70s. The upper pattern will be characterized by NW flow locally, with upper disturbances passing by to our north. Elsewhere, height rises start to build over the western Plains ahead of a cut-off low off the west coast. With some weaker disturbances possible within the NW flow regime, light precip chances can't be ruled out next week, but there isn't an overwhelming signal to latch onto at this range and the blended guidance kept PoPs below 15% Mon-Wed. The extended range outlook (Days 8-14) continues to favor above normal precip across IL (33-50% chance).

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1214 PM CDT Thu Apr 16 2026

Scattered diurnal cumulus lingers early this afternoon, but has generally lifted above 3000 feet. VFR conditions are therefore favored to prevail through the rest of the period, with breezy southwest winds this afternoon becoming light/variable overnight, then developing out of the southeast Friday morning. The terminals should be dry this period, but thunderstorm chances loom Friday night.

ILX WATCHES/WARNINGS/ADVISORIES

None.