textproduct: Chicago

KEY MESSAGES

- A passing storm system may provide periods of rain across the area during the middle of the week, favoring areas south of Interstate 80.

- Expect seasonable temperatures through next weekend.

DISCUSSION

Issued at 129 PM CDT Sun Oct 26 2025

A Rex block extending from southern Hudson Bay to the southern Great Plains will slowly drift eastward through Monday. With the main upper-level low remaining well south of the area, mostly clear skies will prevail amid seasonable temperatures.

As the upper low (ridge) drifts eastward to the southeastern states (western Quebec) by Tuesday, a mid-level low dislodging from a broader trough over the northwest U.S. will unravel while attempting to phase with the eastern low. Consensus guidance is gradually coming into better agreement that the trough/low will pivot from the Missouri River Valley on Tuesday to over central Kentucky and Tennessee by Wednesday night. Remaining mid-level moisture in the wake of the current low to the south will begin to advect back northwestward ahead of the approaching wave through the day Tuesday and especially on Wednesday. Upper-level support from the entrance region of a northwestward directed upper-level jet streak over Minnesota will support precip generation west of the area by Tuesday morning.

Tuesday afternoon onward, the shield of rain should expand northeastward and the northward toward our area. However, just how far into our forecast area the rain expands remains unclear, as extensive low-level drying from a psuedo-Hudson Bay high to the northeast is a common trap scenario for under-performing rain in our area. Blended NBM PoPs are bullish with likely (>55%) values as far north as I-80 and mid-range chance (>40%) values toward the Wisconsin state line by Tuesday night, with a continuation of PoPs all the way into Thursday. While parts of the area may ultimately see rain, there remain very realistic scenarios where much of our area remains completely dry as the system passes to our south. We'll let future forecast shifts tighten up the gradient in PoPs as we get closer.

Interactions with the current southern low, the incoming western trough/low, and Hurricane Melissa should trend toward a longwave trough across the Northeast U.S. late in the week. Ensemble guidance then hints at a potential omega block developing late next weekend into the subsequent week. Our area would be situated on the western edge of the main longwave trough to the east, yielding sporadic bouts of scattered rain showers and perhaps some lake effect rain showers late week through next weekend. Temps will remain seasonable during this period as arctic air masses remains well north of the area.

Kluber/Borchardt

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1211 PM CDT Sun Oct 26 2025

There are no aviation weather concerns for the 18Z TAF period.

E winds around 10 kt will occasionally gust as high as around 20 kt before subsiding to mostly below 10 kt for the evening and night. E gusts to around 20 kt will pick up mid-late morning. Expect VFR throughout the period.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 10 PM this evening to 7 PM CDT Tuesday for the IL nearshore waters.