textproduct: Chicago

KEY MESSAGES

- Areas of shallow fog may become locally dense early this morning outside of Chicago.

- A corridor of training thunderstorms may set up near and south of a Pontiac to Fowler line this afternoon into the evening. Primary hazards include gusty winds and torrential rainfall which may lead to flash flooding.

DISCUSSION

Issued at 340 AM CDT Wed Aug 19 2026

Residual low-level moisture early this morning has allowed shallow fog to develop across portions of the area, including interior northern Illinois and into portions of northwest Indiana. The lowest visibilities are focused mainly where the coverage of high clouds extending north into the area from convection in southwest Illinois is lowest. Generally expect at least patchy fog to persist through daybreak, some of which could be locally dense near low-lying areas and open fields.

The surface cold front that initiated convection last evening has since washed out over the region. Instead of a true frontal passage, the surface flow may simply flop northerly after daybreak this morning for most of the area. Surface flow may remain southerly across central IL/IN, however, which will likely set up a corridor of surface convergence near and south of a Pontiac to Rensselaer line. This may lead to at least isolated shower and thunderstorm development this afternoon depending on the strength of the low-level convergence amidst a largely uncapped environment. Additionally, a reinforcing lake breeze boundary is expected to drift inland today which could serve as another lifting mechanism for at least a few showers into portions of the Chicago metro. Also can't fully rule out lightning with any of the deeper showers north of the Kankakee/Illinois Rivers.

Meanwhile a mid-level wave and associated vort currently over southern South Dakota early this morning is forecast to drift east southeast over the area today. This feature may bring the necessary forcing to increase coverage of showers and thunderstorms along the aforementioned surface convergence axis. Increasing PWATs ahead of this wave will support rainfall rates in excess of 1-2 inches per hour with the heaviest showers/storms. Hi-resolution guidance has remained fairly consistent in developing this corridor of heavy showers and thunderstorms over central Illinois and Indiana, including portions of the local area, but have wobbled north or south by a county or two. Due to the increased susceptibility to flooding over much of our area, have elected to hoist a Flood Watch for Livingston, Ford, Iroquois, and Benton Counties this afternoon and evening where confidence is higher in heavy rainfall occurring.

Surface high pressure builds into the area on Thursday bringing a much needed break from the shower and thunderstorm activity. Lower dewpoints paired with temperatures in the mid 70s to around 80 will make for a pleasant day compared the the more humid airmasses we have been experiencing the past couple of weeks.

High temperatures through the end of the week will remain near to slightly below normal, in the mid 70s to lower 80s. Additional waves moving within the upper flow Friday into the upcoming weekend may bring additional showers and storms to the area at times (30% chance). Dry conditions are then favored into early next week as upper ridging builds into the region.

Petr

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1252 AM CDT Wed Aug 19 2026

A weak and diffuse cold front currently over central Wisconsin will drift southward across northern Illinois this afternoon. Meanwhile, high pressure will drift eastward toward the western Great Lakes. The combination of these two features will result in a gradual veering of the winds (at around 5 knots or less) from W early this morning, NNW by daybreak and NNE by mid- morning. A lake-enhanced wind shift will then allow winds to settle NE early this afternoon and through the remainder of the period, with speeds 10 to 15 knots at times this afternoon.

Light/calm winds, passing mid/upper-level clouds, and plenty of surface-layer moisture will allow fog and/or LIFR stratus to develop across interior northern Illinois early this morning through sunrise. Have included TEMPO LIFR visibility at RFD and IFR visibility at DPA from 10-14Z, though both sites could see VLIFR visibility. Have also include VFR BR at ORD/MDW during the same window, with a low chance (under 30%) that fog well northwest of the core of the Chicago metro drifts toward ORD/MDW as IFR stratus around daybreak.

With the abundant low-level moisture in place ahead of the cold front, diurnal mixing should yield a period of MVFR ceilings at all sites mid to late morning. Stratocumulus will then lift into VFR levels and scatter through the afternoon, with drier and nearly SKC conditions tonight.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch from 1 PM CDT this afternoon through late tonight for ILZ032-ILZ033-ILZ039.

IN...Flood Watch from 2 PM EDT this afternoon through late tonight for INZ019.

LM...None.