textproduct: Central Illinois

KEY MESSAGES

- Thunderstorm chances (30-50%) return as early as Wednesday afternoon, with damaging winds and hail as the main threats. Flash flooding is not a risk.

- Pleasant and dry weather is anticipated from Thursday into Saturday, followed by an approaching heat dome early next week that could drive heat index values up to 100 degrees.

UPDATE

Issued at 944 PM CDT Tue Aug 25 2026

Some of the new CAMs are showing more light showers overnight across more of central Illinois and into east central IL, and also into the morning hours. So made some adjustments to add some 20 pops in the forecast for parts of central and east central IL. Rest of forecast looks, but did increase chance pops for tomorrow late afternoon. Update should be coming soon.

DISCUSSION

Issued at 1248 PM CDT Tue Aug 25 2026

Quiet and low-impact weather will persist through late afternoon under the influence of a mid-level ridge axis and lingering surface high pressure anchored over the eastern Great Lakes region. Temperatures remain just below typical late-August normals, with afternoon highs peaking near 80F beneath scattered high cirrus.

The pattern transitions quickly tonight as low-level warm air advection (WAA) and broad-scale isentropic ascent ramp up across central Illinois. An amplifying shortwave trough over the northern Plains will pivot eastward into the Upper Midwest overnight, driving an increasing 30-35 kt 850 mb low-level jet (LLJ). Moisture transport focusing along the nose of this LLJ will drive a corridor of theta-e advection and synoptic forcing across the CWA. Recent convective- allowing models (CAMs) continue to trend more aggressive with nocturnal shower potential overnight into early Wednesday morning. Forcing for ascent will be further bolstered by a pre- frontal trough propagating into western Illinois after Midnight before reaching the I-57 corridor toward daybreak. Expect scattered showers to focus primarily north of the I-70 corridor through sunrise Wednesday. The combination of expanding mid-level cloud cover, persistent WAA, and sporadic precipitation will limit nocturnal radiational cooling, holding overnight lows in the mid-to-upper 60s.

A more noticeable pattern shift arrives Wednesday afternoon as the shortwave trough and associated surface cold front track into the Great Lakes. Moderate boundary layer destabilization is expected ahead of the advancing front as a narrow plume of low-70s dewpoints pools over the region, helping yield MLCAPE values around 2500 J/kg by mid-afternoon. Overlapping this buoyancy with ~30 knots of 0-6 km bulk shear could favor organized multi-cell and semi-discrete supercellular structures along and ahead of the boundary. Deep- layer shear vectors oriented largely parallel to the initiating frontal zone alongside forecast mesoscale soundings featuring straight or subtle veer-back-veer hodographs point to a large hail or damaging wind threat over an isolated tornado risk.

CAM coverage signals remain somewhat mixed Wednesday afternoon. The primary upper-level vort max will be displaced well to our north, leaving synoptic-scale forcing over central Illinois somewhat poor and potentially limiting total storm coverage. Furthermore, limited moisture return ahead of the front keeps total QPF low. With forecast PWATs around 1.5 inches and progressive storm motion, the flash flooding risk from convective rainfall is quite low.

Following frontal passage Wednesday evening, cool and dry conditions take hold for the end of the workweek into the weekend. An elongated Central U.S. upper ridge will build east, driving an expansive surface high pressure system and a continental polar (cP) airmass across the Midwest and Great Lakes. Expect comfortable conditions with afternoon highs hovering right around 80F Thursday and Friday. Lowering dewpoints into the 50s will support efficient radiational cooling overnight, dropping low temperatures into the mid-to-upper 50s.

By early next week, medium-range global ensembles indicate a notable warming trend taking shape. A strong upper-level trough making landfall in the Pacific Northwest will amplify and nudge the central CONUS ridge axis into the Midwest. The combined presence of a 588594 dam upper height dome and persistent low-level return flow on its western flank will drive highs back into the upper 80s to low 90s. With dewpoints rebounding back into the upper 60s to around 70F, maximum heat index values could approach triple digits by Monday and Tuesday.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

Issued at 649 PM CDT Tue Aug 25 2026

VFR conditions will prevail at all sites through the period...outside of any thunderstorms. High clouds are expected tonight but lower mid clouds will move over the sites overnight with possible high based showers. Even though possible, will not add showers for overnight given if they occur, it will not reduce cig or vis. After a lull during the morning hours, a cold front will approach. More CU will develop ahead of the front in the afternoon. Storms are possible, but based on timing of some of the CAMS, think PROB30 only needed at SPI, DEC, CMI. However will have VCTS at PIA and BMI. Chance of storms will be only in afternoon.

Winds will be light and variable through the night then become more southwest tomorrow morning. Winds become more southwest around 10kts when the front approaches and then become west- northwest behind the front. Wind speeds behind the front will over 10kts.

ILX WATCHES/WARNINGS/ADVISORIES

None.