textproduct: Central Illinois

KEY MESSAGES

- Dense fog will reduce visibility to a quarter mile or less across portions of central Illinois through mid-morning. Drive with caution and allow extra stopping distance.

- Isolated showers and possibly a brief funnel cloud or two are possible this afternoon and evening south of I-72, but the overall severe storm threat is very low.

- Summerlike heat returns this weekend with highs approaching 90 degrees in parts of central Illinois, followed by periodic chances for thunderstorms next week along a stalled front.

DISCUSSION

Issued at 201 AM CDT Fri Sep 11 2026

Dense Fog This Morning... Early this morning, surface high pressure centered over lower Michigan extends southwestward into central Illinois. Light and variable winds, combined with clearing skies from the west, have set the stage for strong radiational cooling. Given the influx of low-level moisture from recent rainfall, dense fog has developed in several areas. As the ridge axis shifts eastward over the next several hours, a developing southeasterly pressure gradient may act to displace or dissipate the fog from east to west. In the interim, a Dense Fog Advisory remains in effect for portions of central Illinois, specifically from the lower Illinois River Valley eastward through Springfield. Will monitor observational trends closely for any needed expansions through mid-morning.

Upper Low This Afternoon and Evening... Water vapor imagery early this morning reveals a potent mid- level vort max digging southeastward across Oklahoma and Arkansas. Model guidance suggests this wave will close off at 500mb as it lifts northeastward across southern Missouri and into southern Illinois by late this evening and overnight. Strong differential cyclonic vorticity advection (dCVA) ahead of this feature will peak over central and southern Illinois mid- to-late this afternoon. This forcing for ascent will align with the right-entrance region of a modest 70-kt upper-level jet streak, interacting with a weakly unstable environment. The GFS is advertising MLCAPE values near 500 J/kg across central Illinois by Friday afternoon. However, forecast soundings depict elongated, 'skinny' CAPE profiles, which should limit updraft strength. Combined with negligible deep-layer shear, the overall severe threat remains very low. In fact, weak updrafts may struggle to grow deep enough to support lightning. Despite the low severe risk, the proximity of the closed upper low does present a conditional threat for funnel clouds. The RAP highlights a few pockets of non-supercell tornado parameters around 1-3 grazing our southern CWA border, though the primary focus for this activity remains closer to the core of the low within the LSX forecast area.

Saturday and Sunday... By Saturday, the closed upper low is projected to open into a trough as it slowly moves eastward along the lower Ohio Valley. A few lingering light showers are possible east of I-57, but the vast majority of central Illinois will remain dry. To the northwest, a cold front will advance across the Upper Midwest on Saturday. An associated thermal ridge, characterized by 850mb temperatures of 18-20C, will surge across central Illinois ahead of the front. This will support a quick rebound in temperatures, with widespread highs in the upper 80s and a few locations hitting 90. As the front stalls over the region on Sunday, a sharp thermal gradient will develop across the local area. Highs north of the Illinois River are forecast to sit around 80, while southern areas south of I-70 soar into the lower 90s. While a few light showers or sprinkles cannot be ruled out near the front, a prominent 700mb warm nose will keep deep convection capped.

Monday through Wednesday... An active and unsettled weather pattern is anticipated for the first half of next week. A high-amplitude upper-level wave is progged to lift from the Pacific Northwest across the Canadian Rockies Sunday night into Monday. In response, a deep surface low will develop and trek from the Canadian Prairies toward Hudson Bay between Monday and Tuesday. An associated warm front will lift across the Great Plains Sunday night, acting as a focus for nocturnal convection. Some decaying activity could linger into portions of central Illinois as the warm front lifts northward through the area on Monday. A better opportunity for widespread showers and thunderstorms arrives Tuesday into Wednesday as the trailing cold front pushes into and stalls across the Midwest. A sharp north-to-south temperature gradient will establish itself across the CWA, with highs ranging from the upper 70s north to the lower 90s south. This period will require close monitoring for potential severe weather, as the upper-level jet dips into the Great Lakes, boosting deep-layer shear across the stalled boundary.

Thursday and Friday... Unsettled weather looks to persist late next week as the baroclinic zone is progged to linger across the Midwest, serving as a focus for daily precipitation chances. Model solutions begin to diverge regarding the exact placement of this frontal boundary by Thursday and Friday; the GFS lifts the front northward into the Great Lakes, whereas the ECMWF and GEM keep the boundary stalled directly over the Midwest. While details regarding timing and location remain low-confidence, the primary takeaway is that daily chances for showers and thunderstorms will continue near the front. Temperatures will remain highly dependent on the front's position, but current guidance suggests a range from the 70s north to the upper 80s or low 90s south.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 456 AM CDT Fri Sep 11 2026

High pressure over the Great Lakes is resulting in light and variable winds early this morning which has allowed patchy fog and low stratus to develop. This may linger an hour or two beyond 12Z, then expect VFR conditions to return the remainder of the period. ESE/SE winds are expected to prevail today, then southerly tonight. Winds should remain below 10 kt through the period.

ILX WATCHES/WARNINGS/ADVISORIES

Dense Fog Advisory until 9 AM CDT this morning for ILZ036-040- 041-047>051.