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 157 AM CDT Sat Sep 26 2026

A Rex block comprised of expansive ridging/high pressure over the Great Lakes, eastern Ontario, and southwestern Quebec and a deep coastal low/nor'easter just off the Mid-Atlantic coast will lead to tranquil conditions here locally over the weekend and into early next week. Seasonable temperatures for late September with highs in the mid 60s to mid 70s and lows in the mid 40s to mid 50s can be expected while the blocking pattern is in place and keeps the same air mass locked in over the region.

One prominent shortwave presently over the central Plains will attempt to lift northeastward into the western Great Lakes tonight but will be stymied as it encounters the aforementioned Great Lakes ridge. A steady feed of dry low-mid-level air along the southern flank of the ridge should erode the associated shower activity as it attempts to enter our forecast area while its main forcing source is quickly weakened. A few of the latest high-resolution model runs suggest that these showers/sprinkles could get as far east as the I-39 corridor or so late tonight into Sunday morning before fading for good, though the majority of the latest forecast guidance generally favors this activity staying fenced off just to the west of our westernmost counties.

By midweek, the eastern CONUS Rex block will have broken down, allowing for a more progressive weather pattern to take shape. A deep, positively-tilted longwave Pacific 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 progress towards the Great Lakes. As this shortwave disturbance arrives in the region in tandem with a frontal zone and an influx of column moisture, a period of rain appears likely to occur in our area sometime late Tuesday into Wednesday. There is a fair amount of ensemble support for a modest amount of instability building into our area during this time period, suggesting that there could be some potential for thunderstorms. However, this support is far from unanimous at this point.

How things will play out for late Wednesday and beyond will hinge heavily upon a complex interaction between the aforementioned upper low and the remnants of Pacific Hurricane Polo. Ensemble guidance broadly favors another period of rain (or perhaps just one, drawn-out period of rainfall beginning with the late Tuesday/Wednesday precipitation) occurring here during the latter half of the week as these atmospheric features lift northeastward towards the Midwest and induce a poleward surge of combined Gulf and Pacific moisture. Unsurprisingly though, there is still quite a bit of spread across the latest global ensemble suites in exactly when any additional rainfall may occur here and how much rain we'll get. The broad-brushed chance and lower-end likely PoPs offered up by the NBM for the first few days of October thus seem to look appropriate for now.

Ogorek

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1245 AM CDT Sat Sep 26 2026

There remains a very low potential for patchy shallow BR (MIFG) outside of Chicago early this morning, however, the potential remains too low to include with this update. Otherwise VFR conditions are forecast through the period with increasing high cloud coverage expected through midday.

Light ENE to at times variable winds early this morning may briefly turn ESE after daybreak before returning to an east direction late morning and back to ENE in the wake of a weak lake breeze in the afternoon around 5-10 kt. Winds then ease again after sunset, becoming light and variable under 5 kt through early Sunday morning.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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