textproduct: Chicago

KEY MESSAGES

- Much cooler and less humid today with dangerous swimming conditions at Lake Michigan beaches.

- Periods of warmth, humidity, and thunderstorms expected this weekend into early next week, with the potential for dangerous heat during the middle and end of next week.

DISCUSSION

Issued at 325 AM CDT Thu Jul 17 2025

Synoptic cold front is currently cutting across the area early this morning with a cooler and less humid air mass filtering into the area. Cold front is progged to continue pushing southward early this morning, likely pushing south and east of our CWA shortly after sunrise. Behind the front, there is an usually large area of post-frontal stratus for this time of year. Given the high sun angle, expect the stratus to lift and scatter out today, but be around long enough to hold temps in the 70s over most of the CWA. Onshore winds should keep temps mostly in the upper 60s this afternoon near the lake, quite a change from Wednesday's 90 degree heat and humidity!

Cold front is expected to lay east to west and stall out down state today as it becomes increasingly divorced from the its parent shortwave trough and surface low which will move off into Ontario. A southern stream disturbance responsible for the widespread convection over Kansas is expected to push eastward across Missouri and into southern Illinois early this evening. Widespread convection and developing cold pool with MCS over northeast Kansas will probably give the stalled boundary a bit more of a push farther south, making it unlikely that the southern stream wave will be able to push precip as far north as our southern CWA today.

Stalled boundary is progged to begin creeping back northward Friday into Friday night. The daytime hours Friday look to remain dry with comfortable humidity and warmer daytime temps. By Friday night into Saturday, the boundary is expected to try and push north of the CWA, but how far north the effective boundary gets will hinge on convection. Subtle, low amplitude waves rippling through the mostly zonal flow will probably become convectively enhanced and lead to an MCS pushing across the area in the later Friday night or Saturday time frame.

Moisture pooling near the boundary could lead to a reservoir of 2"+ precipitable water values in the area. While there is considerable uncertainty in the details, the general synoptic setup with such high moisture content air mass near a surface boundary that is largely aligned with the mid-upper level flow raises some red flags for heavy rainfall/flash flood potential. WPC continues to outlook our area for a heightened risk of flash flooding Saturday. Timing, placement, and eventual magnitude of any heavy rain/flash flood threat will hinge on mesoscale details that will become more apparent as the time period of concern nears.

It still looks like upper trough will dig into the Great Lakes and New England Saturday night into Sunday. Combination of the associated sfc high on the backside of this trough moving into the western Great Lakes and Saturday's convection could shove the effective boundary south of our CWA Sunday. If that scenario pans out, Sunday could end up dry, cooler, and less humid. There's still enough model spread to make it hard to justify removing NBM low end chances POPs for Sunday. If guidance continues to push boundary farther south, then later forecasts will likely be able to pull POPs from Sunday, particularly north of I-80.

That boundary is then expected to head back north as a warm front early next week as strong mid-upper level ridging develops over much of eastern 2/3 of the nation. Potential ring of fire pattern setup with MCS activity likely to impact the location of the front. Timing of any MCS and subsequent effects on the frontal placement this far out are beyond the current range of predictability. Our CWA looks to be on the northern flanks of the upper ridge, and any given day, we could end up south of the boundary and dealing with dangerous heat, or dealing with an MCS and associated flood/severe threat along or north of the boundary if prior MCS activity shoves it south.

Medium range guidance continues to advertise dewpoints in the lower 80s and temps in the 90s south of this boundary, so any days the boundary can push to our north next week could feature dangerous heat. Unlike the heat wave in June, this time around we look to be much closer to the ring of fire leading to much higher forecast uncertainty due to the possible (likely?) effects of convection.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 1258 AM CDT Thu Jul 17 2025

Key Messages:

- MVFR ceilings expected through tomorrow afternoon.

An expansive post-frontal MVFR stratus deck will remain in place over the region well into the daytime tomorrow. Some patches of IFR could trickle south of the Illinois-Wisconsin state line and affect RFD through the early-mid morning, but didn't have enough confidence in this yet to formally advertise IFR in the RFD TAF at the 06Z issuance time. The MVFR ceilings are currently favored to persist into the afternoon tomorrow before lifting to VFR and/or shifting south of the terminals.

Any gustiness at the beginning of the TAF period should subside prior to daybreak as winds settle into a northwesterly to north-northwesterly direction. A wind shift to a more easterly or northeasterly direction appears probable sometime between the late morning and early evening at each of our TAF sites.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from 9 AM CDT this morning through Friday morning for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement through Friday morning for INZ001- INZ002.

LM...Small Craft Advisory from 9 AM this morning to 4 AM CDT Friday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until 4 AM CDT Friday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.