textproduct: Chicago

KEY MESSAGES

- Less humid today with temperatures in the mid 70s to lower 80s.

- Showers and thunderstorm chances return Friday and Friday night with the greatest coverage in the late afternoon and evening. Primary hazards include strong winds and hail in addition to localized downpours.

DISCUSSION

Issued at 332 AM CDT Thu Aug 20 2026

Showers and thunderstorms have pushed southeast of the area early this morning with surface high pressure beginning to expand across the region in their wake. Accordingly, expect decreasing cloud coverage through the morning. Can't fully rule out patchy shallow fog development toward daybreak, mainly across interior northern Illinois. Temperatures are then forecast to warm into the upper 70s to lower 80s today away from the cooler lakeshore areas. With dewpoints likely mixing down into the 50s and lower 60s, heat indices should remain in check, making for a pleasant weather day and a much needed break in what has been a relentless and humid weather pattern.

Attention then turns to Friday when a series of waves will move within the broader west northwest upper level flow across the region. This will bring the potential for showers and thunderstorms back to the area Friday morning through Friday night. The first of which may arrive in the form of a decaying cluster of showers and thunderstorms in the morning out of Wisconsin associated with a wave currently over southern Saskatchewan/Manitoba. Several CAMs suggests the majority of this activity may erode entirely before it reaches the I-88/80 corridors given the very dry antecedent airmass.

While it may erode with southward extent, this first "round" may help to locally increase the low to mid level moisture which could play a role in additional development later into the afternoon and evening when a trailing wave/vort lobe drifts across the area. This wave may prove sufficient for initiating a new round of convection across northwest IL ahead of an approaching surface cold front in the afternoon that then moves across the area into the evening. Increasing mid-level flow, especially across areas southwest of the Chicago metro, suggests that at least strong to locally severe storms may be able to form along the front. It is possible that a Level 1 of 5 threat for severe weather ends up needing to be expanded east to include portions of northern Illinois with later updates if this signal persists.

A stronger mid-upper wave is then progged to dive across the Great Lakes into Saturday. While low-level moisture will be be decreasing in the wake of Friday's storms and trailing cold front, a low-level convergence axis paired with mid-level forcing with the wave may be sufficient to result in at least isolated shower and storm development inland of Lake Michigan and have maintained 15-20% chances with this update to account for this potential.

Dry conditions are then forecast early next week as upper ridging builds back into the region. Seasonably cool temperatures are expected on Sunday with highs in the low to mid 70s due to continued onshore flow. Temperatures then very gradually increase through midweek back into the lower 80s. A disturbance moving atop the upper ridge across Ontario may swing an associated surface cold front across the region in the Tuesday-Wednesday timeframe bringing our next chance for showers and thunderstorms to the region.

Petr

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 618 AM CDT Thu Aug 20 2026

High pressure building over the area will bring VFR conditions with periods of upper-level cloud cover throughout the forecast window. Light N to sometimes NNW winds around 5 knots at TAF issuance will settle NNE/NE within an hour or two this morning. A lake breeze will increase speeds to around 10 knots late morning through the afternoon. Light and VRB winds around or less than 5 knots tonight will favor the SE quadrant in the evening and SW quadrant overnight into Friday morning.

A decaying TS complex over Iowa tonight may bring an increase in mid-level cloud cover and possibly residual SHRA across northern Illinois late Friday morning into early afternoon. Chances for ORD/MDW are too low to include in the TAF at this time.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.