textproduct: Chicago

KEY MESSAGES

- An active weather pattern will get underway on Sunday with periodic thunderstorms forecasted through at least the middle of the upcoming week. Some storms could be severe and may also result in flash flooding, especially on Sunday and Monday.

DISCUSSION

Issued at 330 AM CDT Sat Aug 8 2026

Through Tonight:

A regional surface analysis taken at 0800Z depicted a cold front extending from just east of Wisconsin's Door Peninsula southwestward towards the Quad Cities. This front will continue to drift southeastward through the remainder of this morning before stalling out across central Illinois and Indiana this afternoon. Sufficient low/mid-level moisture has resulted in the presence of an elevated modestly unstable layer rooted near 800-850 mb along and ahead of the front, and isentropic upglide has taken advantage of that to support the presence of isolated showers and a few storms across the frontal zone. This already sparse convective coverage should diminish further over the next few hours as the magnitude of the ongoing isentropic ascent weakens with time, so will carry slight chance PoPs through a little past sunrise to account for any festering isolated shower/storm activity near the front.

Also monitoring the potential for fog this morning -- primarily across our southern counties, where a few AWOS sites have already reported some visibility reductions. Think that the mid- and high-level cloud cover ahead of the front should be able curtail things enough to keep the fog from getting too out of hand and prevent a repeat of yesterday morning's dense fog, but will continue to monitor observational trends closely.

Most, if not all, of our forecast area should find itself north of the aforementioned front come mid-afternoon and along the northern periphery of a surface ridge. As a result, dry conditions with more comfortable humidity levels should be experienced today -- particularly with northward extent -- amidst primarily low-mid 80s high temperatures. Mid-upper 70s highs are forecasted in the immediate vicinity of Lake Michigan, where a lake breeze is expected to sharpen by early this afternoon. Skies today should be mostly clear outside of any diurnal cumulus, though they could have a milky look to them later on in the day as smoke originating from wildfires in the Pacific Northwest spreads over the area once again.

Sunday through Friday:

Tomorrow, the stalled frontal boundary will lift back northward as a warm front, ushering evapotranspirationally-enhanced 70s dew points back into the area. The return of this muggy air mass will set the stage for a prolonged, convectively active period of weather across the region that will begin on Sunday and continue well into next week. However, confidence in forecast specifics regarding the thunderstorm potential during this period inherently remains low given the involvement of a muggy summertime air mass and subtle forcing mechanisms, as well as the influence of one convective cycle on the next and the subsequent domino effects that follow.

The initial wave of convective activity that we'll need to keep an eye on will develop well to our west and northwest tonight as a shortwave ripple embedded within stout west-northwesterly flow aloft acts upon the northeastern periphery of the inbound EML plume. It's not yet clear at this time whether this activity will evolve into an MCS or whether it will remain disorganized as it propagates towards the east-southeast. At any rate, this activity could reach northern Illinois as early as Sunday morning, so that is when mentionable PoPs and thunderstorm chances are first introduced into our going forecast. The influence of this initial round of convection and the associated cloud debris on destabilization efforts puts into question where and when additional convection may develop later on in the day on Sunday.

Assuming that the air mass that will be in place doesn't get convectively overturned to a significant degree by this initial activity, a strengthening low-level jet and another subtle shortwave diving into the Midwest suggest that storms are likely to redevelop somewhere in the general region late Sunday afternoon or evening. With northern Illinois and northwest Indiana appearing to be favorably positioned relative to these atmospheric features, would think that it is more likely than not that we'll see some degree of thunderstorm activity across our forecast area in this late day through Sunday night time frame, so have largely kept the higher PoPs brought in by the NBM for this time period. While the specific details regarding Sunday's thunderstorm threat remain uncertain, increasing instability and strengthening deep-layer wind shear will support the potential for severe thunderstorms with damaging winds and hail. The moist air mass will also support torrential rainfall rates with any storms, so some threat for flash flooding could also materialize somewhere in the region.

With the muggy air mass and primary frontal zone not likely to budge much by Monday, Monday will similarly be a day that will be favorable for thunderstorms with threats for severe weather and flash flooding in the region. Forecast specifics for Monday are even less clear than the muddled picture for Sunday, owing to its dependence on how Sunday's convective cycle plays out and where any surface boundaries get laid out. It's then rinse and repeat for Tuesday, and possibly Wednesday as well, as additional shortwave disturbances are expected to zip into the region and generate additional rounds of convection within the relatively stagnant moist and unstable air mass.

With time, this air mass and the associated active quasi- stationary frontal zone should eventually get suppressed southward and relegate the thunderstorm activity to our south along with it. Ensemble guidance suggests that this could happen as early as Wednesday, but it's more likely to take until Thursday or Friday for this happen.

Ogorek

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 630 AM CDT Sat Aug 8 2026

Key aviation forecast messages for the 12Z TAFs:

- Patchy MVFR possible early this morning, though coverage appears lower than previous forecasts.

- Lake breeze turns winds NE at GYY midday and likely for MDW mid-afternoon. Low confidence in a wind shift at ORD.

- SHRA/TSRA chances return later Sunday morning/midday. Low confidence in timing and coverage prior to end of current ORD/MDW 30 hour forecasts.

Surface cold front was roughly from MKE to MLI at issuance time, and will push through the Chicago metro terminals in the next couple of hours. Winds will turn northwest with speeds around 10 kts behind the front. While patchy MVFR clouds can't be ruled out through mid-morning, GOES IR satellite imagery depicts lower coverage of clouds than earlier expected and thus have pulled MVFR ceiling mentions from TAFs.

High-res model runs continue to indicate lake breeze winds will turn northeast at GYY around midday, and likely at MDW by mid- afternoon. Guidance remains in decent agreement that the boundary will remain east of ORD however, with winds remaining northwest there until winds turn light south-southeast at all sites after sunset this evening as weak surface high pressure slides across the area.

Moisture/instability returns Sunday as winds turn south- southwest and become somewhat breezy during the morning. It's likely there will be thunderstorm development late tonight across parts of IA/MN/WI on a southwesterly low-level jet, and some of this may approach the Chicago terminals toward the very end of the current ORD/MDW 30-hour forecasts. Confidence is low enough with the coverage and timing however to leave out of the forecast at this time, though would expect some mention of SHRA/TSRA by midday with later forecasts.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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