textproduct: Chicago

KEY MESSAGES

- Patchy fog possible again late tonight into early Monday morning, some of which could be locally dense. - Seasonable temperatures to continue through mid-week.

- Confidence is increasing for a period of rain for the middle and later portions of the week.

DISCUSSION

Issued at 201 PM CDT Sun Sep 27 2026

High pressure continues to reside over the Great Lakes which is giving us another pleasant late September afternoon. These pleasant conditions are expected to persist through at least Tuesday afternoon as the high remains in place. Though, afternoon temperatures will be a degree or two each day as winds turn more southerly with highs in the lower to mid-70s and nighttime lows in the 50s. However, there will be another chance for some patchy fog again tonight into early Monday morning as winds ease and mostly clear skies allow for efficient radiational cooling. Similar to last night, forecast soundings do show moisture depths being rather shallow which means that fog tonight will likely be more scattered in nature and mainly confined to more low-lying areas. That said, anywhere fog does develop it could become locally dense with visibilities under a half mile. Given the low confidence on fog coverage have opted to hold off on any Dense Fog Advisories at this time, but something we will be monitoring closely.

The middle and later portions of the week continue to look active weather wise as the broad trough currently over the Pacific Northwest ejects into the Midwest. Guidance continues to show the trough splitting into two distinct features as the northern trough races east and the developing upper low off of northern CA breaks off and phases with what will be the remnants of Hurricane Polo over the southwest CONUS. As these features traverse into the Plains a frontal boundary is forecast to develop across the Plains and interact with advancing tropical moisture from the Gulf and the remnants of Polo. This should allow a broad area of rain to develop along this front and gradually get pushed east into the Midwest and Great Lakes ahead of the trough and upper low. However, since there will be timing differences between the northern trough and upper low there will mostly likely also be drastic timing differences on when the front and its associated rain will arrive from north to south. Right now the latest forecast trends continue to favor a later arrival of the front in northern IL and northwest IN with guidance currently favoring an arrival sometime in the Wednesday to Thursday timeframe. Though, with some stout warm advection expected to be ongoing ahead of the northern stream trough a few showers cannot be ruled out in portions of northern and northwest IL as early as Tuesday night.

Once the front and its associated rain does arrive, conditions do look favorable for a period of locally heavy rainfall given the stout forcing, modest instability, and ample moisture (PWATs nearing 2.0 inches). The big questions for us will be how long does the rain linger and where does the axis of heaviest rainfall set up. Unfortunately, these questions remain highly uncertain in the latest guidance as some are much more progressive with the front while others are slower and the axis of heaviest rain varies from WI to the Ohio River Valley. Bottom line, the middle to later part of the week will likely contain showers and a threat for heavier rainfall which could lead to localized flooding issues especially if the slower forecast solutions pan out. So make sure to stay updated on the forecast.

The northern stream trough and front are forecast to exit the region sometime on Friday or Friday night which should give us a break in the weather to start the weekend. However, another trough is forecast to be traversing across the northern Plains and should begin to phase with the slower moving upper low mentioned previously. Now where exactly this low goes as it phases will determine whether or not northern IL and northwest IN get into another period of rain this weekend or get missed to the south. With ensemble guidance varying greatly on where the low will track and how expansive the rain shield will be, confidence is obviously very low for this part of the forecast. So right now our official forecast will maintain some slight chance (15-20%) POPs through the weekend with hopes to offer better details as we get closer. Regardless, temperatures do look to turn towards slightly below average in the wake of the mid-week front with highs back in the 60s.

Yack

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1226 PM CDT Sun Sep 27 2026

Key Aviation Messages for the current TAF period:

- Lower vis and patchy fog possible once again Monday morning

For most of the TAF period, light northeast winds and VFR conditions are expected. Between sunset tonight and daybreak tomorrow, winds will slowly become north-northwesterly, if not variable altogether. With less higher cloud coverage, there is another risk for patchy fog developing tomorrow morning prior to daybreak. Confidence is low on the exact coverage of fog across the region. For now, kept mention of vis reductions down to MVFR levels, but LIFR/VLIFR conditions will be possible - especially for rural, low-lying terminals.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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