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 127 PM CDT Sat Oct 17 2026

Following a chilly start to the day, sunny skies are boosting temperatures back through the 50s early this afternoon. Meanwhile, light winds (~5 mph or less) to nearly calm conditions persist as the center of surface high pressure slides eastward into the Mid-Mississippi Valley. This light wind regime will continue tonight under clear skies as the high shifts into the Ohio Valley. Consequently, another chilly night is expected across the area, particularly away from the Chicago urban heat island. Overnight lows are projected to bottom out in the low to mid-30s (40s in the city), with frost formation possible once again.

No changes were made to the ongoing Frost Advisory in effect for areas generally east of I-55, as this region remains the most favored for continued nocturnal decoupling with little to no surface wind through daybreak Sunday morning. Conversely, areas north of the advisory area may see light southerly winds develop prior to daybreak as the center of the surface high shifts further east. Accordingly, while temperatures will be comparable, subtle boundary layer mixing may be sufficient to limit widespread frost formation outside the current advisory area.

Following another cool morning, pleasant weather is expected across the region Sunday, though southwesterly winds will turn slightly breezy by the afternoon. Afternoon highs should rebound into the low-to-mid 60s under mostly sunny skies. Thereafter, dry and unseasonably mild weather will persist through the upcoming week as high pressure dominates the Great Lakes. Temperatures will trend upward into the mid-to-upper 60s on Monday, peaking in the low-to-mid 70s (warmest inland) from Tuesday onward. While remaining below record thresholds (record highs in the lower 80s at Chicago and Rockford), these temperatures are well above the seasonal average high of ~60 degrees.

KJB

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 540 PM CDT Sat Oct 17 2026

VFR with no significant concerns through the period. Light to moderate south-southwest winds tonight will increase by mid to late Sunday morning, gusting to around 20 kt until close to sunset, when they will diminish to 10 kt or less.

Castro

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...None.