textproduct: Chicago

KEY MESSAGES

- Periods of showers and/or drizzle at times through the weekend.

- Additional periods of wet weather likely next week, with warmer than average weather early/mid week.

DISCUSSION

Issued at 115 PM CDT Sat Oct 31 2026

Through the rest of this afternoon and into the evening, a diffuse zone of cyclonic shear vorticity (currently across central Illinois) will shift northward across the area. Some high-based convective cloud structures are already evident with this feature within zone of locally steeper mid-level lapse rates and resultant non-zero MUCAPE for parcels rooted above about 800 mb. Hard to say if forcing will be sufficient to result in a localized blossoming additional shower activity given limited hires support for this, but given the modestly increased instability, can't rule out some isolated brief downpours if sufficient forcing is realized. The lightning threat remains relatively low given progged ELs warmer than about -10 C.

Through the rest of the evening and overnight, while general isentropic upglide is generally forecast to weaken, a moist airmass with several mid-level perturbations drifting around upstairs will result in at least a low chance for sprinkles/showers persisting through the night. Through Sunday morning, another shortwave--this one currently pushing across eastern Oklahoma--will arrive in our south. This additional wave of more coherent forcing should result in a modest expansion of light precipitation once again, particularly for areas south of about I-80 into the afternoon. Soundings farther to the north still look supportive of some very light precip, so have added a mention of sprinkles across northern Illinois during the afternoon as a result.

Much farther to the west, a long-wave trough is progged to eventually consolidate from several disturbances off the west coast, including the current closed low off of the Pacific Northwest. Guidance is in reasonable agreement in bringing the long-wave trough across the Rockies Sunday night with a deepening surface low near 1000 mb developing across the Plains of KS/NE. This is expected to induce increasing, broad southerly low-level flow across the eastern Plains and Midwest Monday. Best moisture return and precipitation chances look to initially be largely along/west of the Mississippi River, though a period of warm advection induced showers appears possible into parts of northern IL/WI during the day. Current NBM pops still appear too high across the area Monday into Monday night, although the areal extent has trended downwards a bit.

Confidence then decreases moving into mid-week with the evolution of the upper trough, and the track/trends of the primary surface low. Global ensembles continue to show significant spread in the details of the evolution of the primary circulation within the upper trough (the former Pacific NW closed low) and the degree of phasing with a northern-stream disturbance dropping southeast across the Canadian Prairies. This has significant impacts on the track, intensity, location and potential occlusion of the primary surface low from Monday night onward. At this time, it does appear that the early/mid- week period will be somewhat unsettled with chances of more widespread rain. Potentially windy and continued warmer than average conditions are also likely especially Monday into Tuesday. Of lowest confidence is perhaps how quickly the system departs mid-late week, and the magnitude of any intrusion of cooler air as it does.

Carlaw/Ratzer

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 637 PM CDT Sat Oct 31 2026

The primary forecast concern this period remains the chance and timing of showers.

Showers currently northeast of the terminals will continue to moving east/northeast and away from the terminals this evening. Additional scattered showers across western and central IL are expected to slowly move north and across the terminals overnight into Sunday morning, though there remains uncertainty for coverage and duration. Current prob mention has this potential handled well for now. There may be additional low chances for showers again Sunday afternoon and Sunday night into early Monday morning, but confidence is too low to include any mention with this forecast.

Cigs are expected to remain vfr through the period. However, guidance is showing mvfr cigs late Sunday night into Monday morning and this will need to be monitored with later forecasts.

East/southeast winds under 10kts will turn more southeasterly tonight and then southerly Sunday morning. Wind directions may favor south/southwest Sunday afternoon, before turning back to the south/southeast with sunset Sunday evening. cms

LOT WATCHES/WARNINGS/ADVISORIES

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