textproduct: Chicago

KEY MESSAGES

- Periods of showers and/or drizzle at times this weekend, with highest rain chances this morning.

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

DISCUSSION

Issued at 246 AM CDT Sat Oct 31 2026

Early morning surface analysis indicates a warm front stretching southeast across the middle and lower Missouri River Valleys, which then becomes nearly stationary across far western Kentucky. Farther to the east, high pressure was centered across the central and eastern Great Lakes region. GOES water vapor imagery continues to depict a negative-tilt short wave lifting slowly northeast across Kansas and Oklahoma. A broad region of low-level southerly return flow was noted ahead of this feature into the upper Midwest, where theta-E advection and isentropic ascent atop the elevated warm frontal zone was resulting in a WNW-ESE band of showers from southwest MN to portions of our area.

Across northern IL/northwest IN, these showers were currently focused along/south of the I-80 corridor, with decreasing intensity with eastward extent away from the better southerly low-level jet and atop somewhat drier easterly low-level flow. The expectation is that these showers will continue to lift slowly north across the area this morning, though with some gradual weakening due to dampening of the approaching short wave trough and an eastward translation of an upper jet streak over the Lakes to our northeast. This should result in decreasing shower coverage and intensity by this afternoon, although the presence of the elevated baroclinic zone and weak forcing within a fairly moist thermodynamic profile will likely maintain some threat for at least isolated light showers at times into this evening. Overall however, afternoon and early evening conditions should be fairly decent with cloudy skies, temperatures in the upper 50s/low 60s and light east-southeast winds.

On Sunday, a secondary short wave (currently over the TX/OK Panhandle region) is forecast to track into the region, increasing precipitation chances again mainly from the lower Ohio Valley into central IL/IN. There is quite a bit of spread in model guidance with how far north precipitation will spread, with NBM pops pretty minimal across the LOT forecast area. Have included some low-chance pops across our southern tier of CWA counties during the day for this however, in collaboration with WFOs ILX/IND. Can't completely rule out a shower or sprinkle farther north, though have not introduced pops north of roughly the U.S. 24 corridor at this time.

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 brining 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 appear too high especially over our eastern CWA Monday into Monday night, and have trimmed them a bit in our east but may see these decrease further with later runs.

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.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 628 AM CDT Sat Oct 31 2026

Main Item of Note:

- Periodic -SHRA/sprinkles

There are again no significant flying concerns through the period at the TAF sites, with primarily VFR conditions and east to southeast winds. Shower activity across the area as of this writing will result in occasional light rain through mid to late morning. Additional isolated to widely scattered -SHRA/sprinkles can't be ruled out at any point this afternoon through early Sunday. However, with low confidence on specific timing details, and little/no impacts expected, withheld additional precip mention from the TAFs.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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