textproduct: Chicago

KEY MESSAGES

- Scattered showers and thunderstorms are expected late this evening and overnight, especially south of Interstate 88. The strongest storms may be capable of small hail. - Additional showers and a few thunderstorms will move through the region on Thursday.

- Friday will feature blustery northwest winds and highs in the 50s. Areas of frost and locally near-freezing conditions are favored away from Lake Michigan Friday night.

- After the brief period of chilly weather, there will be a warming trend early next week.

DISCUSSION

Issued at 315 PM CDT Tue Oct 13 2026

Today was yet another unseasonably warm mid-October day across Chicagoland. The weak cold front that moved into the area earlier this morning has since stalled out south of I-80 which has resulted in a notable temperature gradient from northwest (mid 70s) to southeast (upper 80s). A couple of areas of altocumulus (one overhead and another crossing the Mississippi River) associated with narrow pockets of mid-level moisture have managed to produce isolated showers/sprinkles. Think this activity will remain fairly isolated through the early evening hours but can't rule out a few showers or a rumble of thunder.

Focus continues to be on the potential for scattered showers and thunderstorms later this evening through the overnight hours. Model guidance has remained varied on the timing and placement of the greatest coverage of showers, likely owing to differences in the handling of the timing and placement of subtle vort lobes and areas of drier air between plumes of increasing moisture. This is especially evident in regional satellite and radar imagery showing large gaps between ongoing convection and areas of ACCAS. Nevertheless, the presence of sufficient synoptic forcing within the right entrance of the upper jet and moist air lifting atop a stalled surface boundary support an eventual increase in shower and storm coverage overnight, and potentially developing as early as 8-9 PM if some of the earlier hi-res guidance verifies. Overall weaker mid-level lapse rates should limit storm strength overall even in the presence of favorable shear for hail. Thus, will continue to message the potential for small hail with the strongest storms with the severe potential remaining mainly southwest of the area.

The threat of lightning quickly wanes toward daybreak Wednesday with shower coverage beginning to focus south of the area. However, residual low to mid-level moisture in the presence of weak ascent may support lingering light rain or drizzle through part of the morning. Expanding high pressure across the western Great Lakes will eventually shift this south of the area toward midday, with dry conditions forecast through the remainder of the afternoon and evening hours. Persistent north to northeasterly winds will bring temperatures back to more seasonable levels with highs in the 60s for most of the area and lower 70s well south of I-80.

This brief break in precipitation will be fairly short lived as energy ejecting off the Rockies will phase with a wave diving southeast toward the Upper Midwest late Wednesday night into Thursday. An associated developing low pressure system will move across the area bringing widespread showers and a chance of thunderstorms on Thursday.

Petr

Friday Onward:

As the surface low pulls away (and deepens toward 990 mb in southern Ontario), the arrival of a strong (nearly 1040 mb) surface high pressure system into the central Plains will set the stage for an impressive pressure gradient across the Great Lakes. Accordingly, do expect a windy day on Friday with early BUFKIT mixing profiles supporting frequent gusts of 30 to 40 mph. Such strong winds will facilitate aggressive low-level cold air advection in the wake of the cyclone with mean 850mb temperatures plummeting 10 to 15 degrees C in 24 hours (to near or below 0C). Taken together, Friday will feel every bit like October with highs in the 50s, blustery northwest winds, and showers of leaves (not water) flying through the air. With that said, a notable temperature difference between Lake Michigan water and the fall airmass will support lake effect rain showers mainly east of Gary.

There remains ensemble agreement that the surface high will slide directly over the Great Lakes Friday night into Saturday morning setting the stage for ideal radiational cooling and the coldest night of the autumn season so far. Assuming no major changes in forecast guidance, would expect areas of frost and even locally freezing temperatures across the region right in line with climatology (the first freezing temperatures away from Lake Michigan typically occur in the October 11 to 20 timeframe).

Thereafter, ensemble guidance advertises the surface high stalling over the Atlantic Seaboard and the northward-shift of the dominant upper-level jet into Canada supporting gradually rising temperatures into next week.

Borchardt

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 626 PM CDT Tue Oct 13 2026

Key Aviation Messages for the current TAF period:

- Showers and scattered thunderstorms expected this evening and through the overnight.

- Prevailing MVFR cigs are expected tomorrow.

Westerly winds continue to diminish under VFR clouds presently. Isolated to scattered showers and storms are ongoing in eastern Iowa. This activity is expected to continue to move eastward this evening and through the overnight. There is lower confidence in coverage of storms as it moves in prior to midnight. However, no changes were made to the timing of the PROB30/TEMPOs as coverage is expected to expand around and after midnight as better forcing moving in. There is moderate confidence in showers and storms continuing through around daybreak. There is a slight chance for lingering light showers/drizzle to fester through 12Z, but generally most showers should be sinking south by then with any thunder activity diminishing. An MVFR stratus deck is expected to develop in the wake of the precipitation with northeast winds between 10 to 15 knots. Its possible that cigs below 2000 ft lift for a brief window to 2500-3000 ft in the afternoon. But with lower confidence in the timing of higher cigs as well as guidance keeping MVFR cigs through 00Z, opted to prevail MVFR cigs around 2000 feet through the end of the TAF.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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