textproduct: Chicago

KEY MESSAGES

- Frost is expected tonight across northwest Indiana and especially east of I-55 in Illinois.

- An extended period of dry and unseasonably mild weather is expected the upcoming work week and likely into next weekend as well.

DISCUSSION

Issued at 202 AM CDT Sat Oct 17 2026

Lake effect showers are expected to end early this morning as inversion lower in the wake of the departing mid-upper level trough. Lake convergence band will gradually shift west this morning with lake effect cloudiness spreading into portions of northeast IL toward sunrise. Lake cloudiness is expected to decrease later this morning with sunny skies expected elsewhere. The sunshine and MUCH lighter winds will make today's highs in the 50s feel milder than Friday's 50s.

Early this evening, high pressure will be centered over Indiana and eastern IL. The clear skies, dry boundary layer, and light/calm winds should set the stage for strong radiational cooling. The high pressure system is progged to shift slowly east tonight, likely allowing a light south to southwest wind to develop over northern CWA, with areas east of I-55 into northwest Indiana probably remaining more decoupled with light or nearly calm winds until sunrise Sunday. Have sided with the slightly chillier MAV guidance for low temps tonight, with lows in the 30s away from the urban heat island. The coldest lows should be east of I-55 and in the typically colder valley and cold air drainage areas of northern IL where lows in the low to mid 30s are expected.

The most favorable area for frost tonight will be east of I-55 into northwest Indiana and after coordination with neighboring WFOs, we have opted to hoist a frost advisory there. Farther north and west, there are questions about how widespread frost will get given the increasing winds. Plus, some of these areas may see an end to the growing season with freezing temps this morning. The day shift can take a closer look at this and could potentially need to expand the frost advisory a bit farther north/west.

Otherwise, the weather looks incredibly tranquil Sunday through the upcoming work week. Many of the days one may be hard pressed to even spot a cloud in the sky! Temperatures Sunday will moderate back closer to normal into the low-mid 60s. Monday will warm a few more degrees, then by Tuesday through Saturday afternoon highs are largely expected to be in the 70s. This is particularly impressive at a time of year when the daily average highs are falling quite quickly. By the middle and end of the week the average highs are in the upper 50s, so the 70s we're forecasting will be a good 15-20F above average!

- Izzi

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1217 PM CDT Sat Oct 17 2026

There are no key concerns to highlight for the 18Z TAF period.

SW winds below 10 kt through the night will gust to around 20 kt for the late morning and afternoon on Sunday before subsiding for the evening. Expect VFR with largely clear skies through the period.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

IL...Frost Advisory from 3 AM to 8 AM CDT Sunday for ILZ023-ILZ032- ILZ033-ILZ039-ILZ108.

IN...Frost Advisory from 3 AM CDT /4 AM EDT/ to 8 AM CDT /9 AM EDT/ Sunday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 3 PM CDT this afternoon for the IL and IN nearshore waters.