textproduct: Chicago

KEY MESSAGES

- Seasonable temperatures for late September are expected over the next several days.

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

DISCUSSION

Issued at 203 PM CDT Sat Sep 26 2026

A ridge of high pressure continues to reside over the Great Lakes in between the Nor'easter off the east coast and the broad troughing over the Pacific Northwest and Mountain West regions. This ridge is expected to remain in place through at least Monday which will keep seasonable temperatures (highs in the upper 60s to lower 70s and lows in the upper 40s and 50s) and primarily dry conditions around locally. However, there has been a band of some light showers and sprinkles in eastern IA associated with a shortwave disturbance rounding the base of the aforementioned western trough. While these showers/sprinkles will likely drift eastward tonight, the persistent east- northeast winds should keep drier air in place and prevent much of these showers/sprinkles from reaching the ground. That said, have opted to maintain a 10-20% chance for an isolated sprinkle near and west of I-39 late tonight into early Sunday morning just in case. Additionally, there is a chance for some patchy fog to develop around daybreak Sunday morning, mainly in northwest IN and adjacent areas of northeast IL. Given that cirrus clouds from the aforementioned showers in IA will spread east tonight, it is unclear how widespread and dense the fog will be as the clouds should help to ease some of the radiational cooling. For now have introduced some patchy fog to the forecast in northwest IN and portions of northeast IL where cloud cover is forecast to be at a minimum.

By Tuesday the broad trough over the Pacific Northwest and Mountain West is forecast to be ejecting into the upper Midwest. As it does so another trough/closed upper low is forecast to be traversing across southern California and the southwest CONUS. Latest ensemble and deterministic guidance favors that these two features will remain separate from one another which means that the Pacific Northwest trough should arrive first and bring with it some rain showers for the Tuesday night into Wednesday night timeframe. The southwestern trough/low is then expected to lag and arrive sometime towards the Thursday into Friday timeframe with additional showers and perhaps even some thunderstorms. However, it should be noted that guidance has been varying on the exact arrival time of these troughs and their associated rain thus making the specifics of this part of the forecast lower confidence. Despite the uncertainty, a period of wet weather looks likely mid to late week.

Behind these troughs, guidance is even more uncertain as to whether or not the upper level pattern will become more zonal (east-west oriented) or remain amplified heading into next weekend. If the zonal pattern becomes reality then we could be looking at another period of tranquil weather for next weekend, but if the amplified pattern materializes then things could remain active. With the large amount of uncertainty confidence beyond Thursday is obviously very low, but something we will be keeping an eye on. Regardless, seasonable temperatures are forecast to remain through at least next week.

Yack

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1230 PM CDT Sat Sep 26 2026

Key Aviation Messages for the current TAF period:

- Chances for patchy fog development around daybreak tomorrow near and southeast of KMDW

Light winds remain out of the southeast presently, but will slowly turn to the east-northeast as a lake breeze develops this afternoon. Precipitation currently over Iowa should remain west of the area through the forecast period. There is a non-zero chance for sprinkles/light rain to reach KRFD around and after midnight, but no impacts are expected so it was kept out of the TAF for now.

The main impact to be monitored is the risk for patchy fog development overnight. Winds will become light and variable, but higher level clouds from the showers in Iowa will continue to spread east overnight. There is a play they remain farther west which would increase chances for fog in the area, whereas the more east they spread reduces chances. For now, the best chances remain in northwest Indiana and near KGYY, though fog spreading as north as KMDW is possible, but confidence is lower. Any fog that does develop should mix out by mid morning for VFR conditions Sunday with light northeasterly winds.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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