textproduct: Chicago

KEY MESSAGES

- Predominantly seasonable temperatures for this time of year are expected over the next several days.

- The next chances for widespread rainfall are forecasted to come mid-late this week.

DISCUSSION

Issued at 152 AM CDT Sun Sep 27 2026

A well-defined shortwave/vorticity maximum is evident over south-central Iowa on GOES water vapor imagery loops early this morning, slowly pinwheeling its way eastward towards the Mississippi River. Later today, this disturbance will eventually run into a wall of dry low-mid-level air and large- scale subsidence associated with an expansive ridge that is in place over the Great Lakes and portions of Ontario and Quebec, and most of its associated precipitation will disintegrate as a result. Essentially all 00Z and 03Z model guidance suggests that this will occur before this precipitation reaches our western CWA border, but still couldn't completely rule out a few sprinkles sneaking into our forecast area as far east as the I-39 corridor given that the driving wave looks a bit more robust on satellite and radar than this forecast guidance suggests it should be at the time of this writing.

Otherwise, temperatures today should be similar to yesterday's temperatures, with seasonable highs in the mid 60s to mid 70s once again. The coolest temperature readings will occur near Lake Michigan, courtesy of onshore flow. With little air mass change expected into tomorrow as a result of a persisting eastern CONUS Rex block, similar temperature readings should be observed again on Monday -- though tomorrow's high temperatures will likely be a smidge warmer than today's with another day of air mass warming and less upper-level cloud cover to contend with. Additionally, while this air mass remains quite dry, it may still nevertheless support patchy fog in portions of our area both this morning and again tonight into tomorrow morning wherever skies can remain relatively clear amidst light/calm winds within a strongly decoupled planetary boundary layer.

The Rex block will likely have broken down by Tuesday, though the same air mass will still remain in place across the region, and another day of incremental air mass warming could allow for some of our southern locales to make a real push towards 80F during the afternoon. The Rex block breaking down will also allow for a more progressive weather pattern to take shape across the CONUS during the middle and latter portions of the week. Of particular relevance for this time period will be a deep, positively-tilted longwave trough that is currently approaching the Pacific coast. This trough will evolve into a cut-off upper low over the Desert Southwest Monday into Tuesday while a separate shortwave component of this trough breaks off and continues to advance towards the Great Lakes.

This more progressive shortwave disturbance will help induce a blossoming axis of rain across an attendant frontal zone from the central Plains into the Upper Midwest Tuesday into Wednesday. There has been a notable slowing and westward trend with this frontal zone and the accompanying precipitation axis in both the GEFS and GDPS ensemble systems over the past 24 hours. If such a trend were to ultimately be realized, then precipitation likely wouldn't arrive in our forecast area until Wednesday morning at the earliest, and large portions of our area (particularly with southeastward extent) could potentially remain dry altogether on Wednesday. It remains to be seen whether the EPS will jump onto this trend as well, but for now, the NBM's PoPs have reasonably been slashed into the "chance" range across the southeastern half of our forecast area on Wednesday where the occurrence of precipitation on this day is in more doubt than it was previously.

Regardless of what happens on Wednesday, at least one period of rain still looks likely to occur here sometime during the latter half of the week as the aforementioned upper low and/or the remnants of Pacific Hurricane Polo work their way into the Midwest in some sort of fashion and induce an influx of combined Gulf and Pacific moisture into the region. Ensemble guidance still remains all over the place with the timing of this rainfall and overall rainfall totals for the region during this late week time frame, largely due to a considerable amount of uncertainty regarding an expected interaction/phasing attempt between the upper low and Polo's remnants and the subsequent ramifications that will have on the evolution of these atmospheric features into the latter half of the week. The latest NBM PoPs for our area currently peak in the "likely" category on Thursday with broad-brushed chance PoPs then continuing into the weekend. This looks reasonable for now, but will likely need refinement in the coming days as forecast trends become clearer.

Ogorek

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 650 AM CDT Sun Sep 27 2026

Key aviation forecast messages:

- Small pocket of LIFR to VLIFR stratus/fog at ORD/DPA this morning.

- Another round of patchy shallow fog possible early Monday morning (low confidence).

An area of fog that developed in southeast Wisconsin slowly expanded south into far northeast Illinois early this morning. The southern extent of this has since reached ORD and have accordingly introduced TEMPOs for 1SM VSBYs and BKN002 to account for this (and similar at DPA). Wouldn't be surprised if portions of the airfield end up < 1 SM and < 200 ft. How quickly this exits the area remains uncertain and have the TEMPO through 13Z for now. Will handle any additional updates tactically over the next couple of hours if it ends up holding together longer than expected. Can't fully rule out some lower CIGs approaching MDW, though confidence is not high enough to include for now.

The low clouds and fog is still expected to erode by mid morning with VFR conditions then forecast through the end of the period. We will have to keep an eye on the potential for another round of patchy fog development early Monday morning.

Winds will remain light through the morning around 5 kt out of the NNE. A weak lake breeze will move inland early this afternoon turning winds more northeasterly around 5-10 kt. Winds then ease after sunset, leaning NNW and at times variable overnight.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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