textproduct: Central Illinois

KEY MESSAGES

- Isolated Storm Threat Today: There is a very low chance (under 20%) for strong to severe thunderstorms to develop late this afternoon and evening near and north of the Interstate 74 corridor.

- Dangerous Heat Wave: A prolonged heat wave will bring dangerous heat index values of 100 to 105 degrees daily across all of central Illinois through Thursday.

- Holiday Weekend Storms: Daily thunderstorm chances return Thursday and last through the holiday weekend, bringing a risk of localized heavy rainfall and isolated strong storms.

DISCUSSION

Issued at 1230 PM CDT Sun Jun 28 2026

Near Term (This Afternoon and Evening): A diffuse surface warm front is currently analyzed across the area, stretching roughly from near the Quad Cities southeastward toward Terre Haute. Downstream of an energetic upper-level low pivoting over the western United States, a convectively enhanced shortwave is tracking across the Upper Mississippi Valley. These features will provide enough synoptic impetus to lift the surface front northward through this evening.

The primary forecast challenge revolves around a decaying Mesoscale Convective System (MCS) currently churning through eastern Iowa. High-resolution convective-allowing models (CAMs) suggest this system could maintain enough integrity to push an outflow boundary into our neck of the woods. If this outflow intersects the northward-lifting warm front near the I-74 corridor, it could serve as the focal point for new convective initiation later today. While the overall probability of occurrence remains very low (20% chance or less), the environment cannot be ignored; substantial thermodynamic instability paired with mid- level bulk shear exceeding 30 knots would easily support a strong to severe thunderstorm threat for any updrafts that manage to accelerate past a 750-mb warm layer.

Outside of this localized conditional threat along and north of the I-74 corridor, precipitation chances have fallen off a cliff for the rest of the CWA as the synoptic pattern amplifies.

Midweek Heat Wave (Monday through Thursday): The aforementioned Western trough is driving robust height rises downstream, cementing a potent, highly anomalous heat dome over the Ohio River Valley and the Midwest. This deep layer ridge will govern our weather through the middle of the week, resulting in a prolonged period of dangerous heat.

Daily high temperatures will consistently climb into the low 90s. While raw temperatures are noteworthy, the real story will be the oppressive humidity. Significant moisture pooling within the boundary layer will push dewpoints well into the 70s. NBM guidance heavily favors widespread heat indices climbing into the 100-105 F range daily from today straight through Thursday.

Given the lack of nocturnal relief and the multi-day duration of this event, an Excessive Heat Advisory remains in effect for all of central Illinois through Thursday.

End of Week and the Holiday Weekend: By Thursday into Friday, the core of the upper-level ridge is progged to migrate toward the Mid-Atlantic coast. This eastward shift will open the door to a broader, more active southwest flow aloft across the mid-Mississippi River Valley.

A series of subtle shortwaves ejecting out of the Southern Plains will traverse this flow, bringing a return of daily thunderstorm chances to the forecast. While deep-layer shear will be seasonally weak as the stronger jet dynamics stay well to our north, the combination of rich boundary-layer instability and minor synoptic ascent will be sufficient to warrant close monitoring. Isolated severe weather and heavy downpours cannot be ruled out late this week and heading into the holiday weekend.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 644 PM CDT Sun Jun 28 2026

A warm front is draped near the I-74 corridor this evening and continues to lift north. SE winds will veer to southerly with passage of the warm front. A few lingering MVFR ceilings near the warm front (mainly at CMI) will end as winds continue to veer over the next couple hours. Winds will set up out of the SSW by around midday Monday with gusts picking up into the 20-25 kt range.

ILX WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 7 PM CDT this evening for ILZ027>029-036- 037-040>042-047>054-061-062-066>068-071>073. Heat Advisory from 11 AM Monday to midnight CDT Thursday night for ILZ027>031-036>038-040>057-061>063-066>068-071>073.