textproduct: Chicago

KEY MESSAGES

- There will be periods of showers and thunderstorms Wednesday into Wednesday night, locally heavy rainfall is possible and could lead to some flooding concerns.

- Seasonably warm and mostly dry conditions are expected late in the week into the upcoming weekend.

- A return to a potentially more active northwest flow pattern is expected next week.

DISCUSSION

Issued at 130 PM CDT Mon Aug 3 2026

High pressure currently centered over Illinois is progged to slowly move east to New England by Tuesday afternoon. The ridge axis may be close enough to the area still tonight to allow for some patchy ground fog to develop, especially across rural areas of northwest Indiana in closer proximity to the surface ridge. Fog and low cloudiness isn't expected to be nearly as widespread as it was this morning, but opted to include some patchy fog in the grids over far southern and southeastern portions of the CWA. Tuesday's weather will be similar to today's; partly cloudy and warm with rather comfortable humidity levels. While there may be a lake breeze again Tuesday afternoon, a bit more of a southwesterly gradient should limit the inland extent and likely confine it to the IL shore.

Attention will turn to the trough currently over the northern Rockies. This trough is expected to translate eastward across the northern Plains and into the upper Mississippi Valley by mid week. At the surface, a cold front currently across the central into the northern Plains will gradually move eastward in concert with the upper trough and into the Midwest. While high pressure to our south across the Gulf Coast states should limit access to Gulf moisture, there is a reservoir of moisture pooled ahead of this front with widespread dewpoints in the 70s and PWATs 1.75" to nearly 2". As the front settles southeastward toward our region, so too should this axis of richer atmospheric moisture.

Unfortunately, there will likely be several smaller scale vorticity maxima embedded within the broader trough, which could complicate prediction of the timing and placement of better convective and heavy rainfall chances, especially as these small embedded shortwaves potentially become convectively enhanced. There is a signal in guidance that one such wave will support convection developing over the Cornbelt region Tuesday afternoon, with most guidance suggesting this activity could persist through the night and arrive in our northwestern CWA late Tuesday night/early Wednesday morning in a weakening state. Extensive cloud cover and potentially rain with this feature is expected to overspread especially the northern half of our area Wednesday morning with cloud cover and potentially showers lingering into the afternoon, limiting potential heating/destabilization.

Global models are in pretty good agreement in depicting another embedded shortwave trough supporting more widespread convective development Wednesday afternoon across the southern Cornbelt/lower Missouri River valley. This, potentially convectively enhanced vort, is then expected to track eastward into Illinois Wednesday evening leading to an increase in coverage and intensity of rainfall. Given the high PWATs pooled ahead of this boundary; heavy rainfall would certainly be a threat, but there are a lot of moving pieces to this atmospheric puzzle, so forecast confidence is still on the lower end of the spectrum regarding where and ultimately if a heavy rainfall/flooding threat will materialize in our area. If a sizable MCS does roll across the area Wednesday night, that would likely push the effective surface boundary south, potentially out of our CWA. This could result in better rain chances Thursday ending up south of I-80 or potentially farther south into central Illinois. For now, given the lower confidence, I didn't stray from the aggressively high precip chances in the NBM for Thursday, but if confidence increases in a formidable MCS moving across the area Wednesday night, then the focus for redevelopment Thursday could end up much farther south.

Mainly dry and seasonably warm weather is expected late in the week through much of the upcoming weekend. There are indications in the medium range guidance that we could get into a potentially more active northwest flow regime heading into next week with thunderstorm chances returning.

- Izzi

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 636 PM CDT Mon Aug 3 2026

No significant aviation weather concerns through the 00Z TAF period.

Surface high pressure ridge will continue to drift off to the southeast tonight, though the lingering weak pressure gradient will support largely light variable/southeast winds overnight. The exception will be associated with the lake breeze for the Chicago area terminals, where winds will initially be easterly near 10 kt early this evening.

Light winds may become more south-southwest toward daybreak, then mainly southwest near 10 kt during the day. While not overly strong, this should keep the afternoon lake breeze boundary east of ORD/MDW. VFR conditions are expected through the period, though initial FEW/SCT cu development Tuesday morning may initially develop around 2 kft but should quickly rise from there. Mid-high level clouds will then approach in association with a weak surface cold front and mid-level disturbance Tuesday evening. Any SHRA/TSRA potential is expected to be beyond the end of this forecast period, late Tuesday night or Wednesday morning.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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