textproduct: Chicago

KEY MESSAGES

- Unseasonable warmth continues into Tuesday, warmest south of Interstate 80.

- Showers are likely Tuesday night into Wednesday morning and may be accompanied by isolated thunderstorms capable of gusty winds and small hail.

- Widespread rain is expected Thursday followed by a strong cold front and much cooler conditions Friday into the weekend.

DISCUSSION

Issued at 314 PM CDT Mon Oct 12 2026

After a brief delay in warming due to lingering high clouds, temperatures have since warmed well into the 80s to near 90! across the area, with both Chicago-O'Hare and Rockford having broken their daily high temperature records for October 12 with another hour or so of warming yet to go.

Heading into the evening hours, mid-level warm advection ahead of an approaching weak disturbance may lead to a stray shower or sprinkle developing over southeast Wisconsin and over Lake Michigan. Can't fully rule out this activity initiating over far northeast Illinois but have maintained a dry forecast with this update (< 15% chance) given very dry low-levels in place this far south. Continued southwesterly low-level flow will keep temperatures in the upper 60s to lower 70s overnight.

A weak surface low currently over southwest Minnesota will continue to drift east through central Wisconsin and northern lower Michigan by Tuesday morning. An associated weak surface trough/front will swing into the area and turn winds west to northwesterly. This will likely keep temperatures a bit cooler (but still unseasonably warm) for areas north of I-80 in the mid 70s to lower 80s. Farther south, temperatures may still manage to warm into the mid-upper 80s.

Showers and thunderstorms are expected to develop across west/northwest Illinois late Tuesday evening and expand across the area into the overnight hours. These storms will be on the cold side of the surface front (stalled out over central IL/IN) and will be elevated. While lapse rates are not overly impressive, modest MUCAPE (1000-1500 J/kg) may briefly overlap with 25-35 kt of shear within that elevated layer. Given overall weaker forcing, the potential for thunderstorms to grow tall enough to fully realize this environment is on the lower side, but can't fully rule out locally stronger storms capable of gusty winds and hail. If confidence increases in more robust storms the Level 1 of 5 threat for damaging hail may end up needing to be expanded farther east with later updates.

After a relative lull in precipitation later in the day on Wednesday another moisture laden system associated with energy ejecting off the Rockies will lift into the area late Wednesday night through Thursday bringing widespread showers and another chance for thunderstorms. The system will likely phase with an approaching vigorous mid-upper shortwave diving across the Upper Midwest that sends a strong cold front across the area. While some differences remain in the overall magnitude, ensembles remain consistent in depicting a period of much colder conditions occurring (relative to the near record warmth earlier in the week). Forecast overnight lows Friday and Saturday are in the 30s outside of Chicago which may bring our first potential for frost of the season if this trend holds. This cool spell may be relatively short lived as upper ridging builds back across the region allowing warmer temperatures to return next week.

Petr

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 626 PM CDT Mon Oct 12 2026

Key Aviation Messages for the current TAF period:

- A period of low-level wind shear (LLWS) is expected tonight

- A weak lake breeze is possible late tomorrow afternoon

Southwest winds remain breezy, presently, though wind gusts around 20 knots are becoming more sporadic. Wind gusts are expected to continue to diminish with prevailing southwest winds around 10 knots through the night. A westerly low level jet at 1500 feet is projected to strengthen this evening in excess of 40 knots. This will result in a period of LLWS at all TAF terminal sites. The jet is expected to weaken toward daybreak with surface winds beginning tomorrow out of the west and around 10 knots.

Conditions are expected to be fairly quiet and VFR through the day tomorrow. A lake breeze is expected to develop in the late afternoon/early evening, though be fairly light as it approaches Chicago terminals. Cloud cover is expected to increase after 00Z ahead of the next system. Cannot completely rule out some isolated to widely scattered showers prior to 06Z, but with greater chances after 06Z tomorrow night, decided to hold off adding it for now.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 4 AM CDT Tuesday for the IL nearshore waters.