textproduct: Chicago

KEY MESSAGES

- Periods of heavy rain and scattered thunderstorms late tonight through Sunday night. Localized flash flooding will be possible where the heaviest rains occur over the same areas.

- Below normal temps early next week then dry weather expected mid/late next week.

DISCUSSION

Issued at 200 AM CDT Fri Sep 18 2026

The primary forecast focus and challenge remains timing and location of periods of heavy rain through Sunday.

Low clouds continue to slowly scatter out across southern Lake Michigan and into far northeast IL early this morning. This trend is expected to continue inland this morning though there will likely be mid level cloud cover later today. Net result should be at least some peaks of sunshine today. A weak wave may bring a few showers to western and northwest IL today, but much of the area is expected to remain dry. High temperatures today are little tricky, generally in the lower 70s north to upper 70s south but if enough sunshine materializes, these temps could be a few to possibly several degrees too cool. Northeast winds will allow waves to build into the 3 to 6 foot range at IL Lake Michigan beaches and the current beach hazard statement looks on track. East/southeast winds will increase again overnight into Saturday morning and another beach hazard statement may be needed.

Attention then shifts to what is expected to be developing convection across northern IA and southwest WI later this evening which is then expected to spread into northern IL early Saturday morning. Still some uncertainty for timing but the time period of concern seems to be focused on 09z to 15z Saturday. With precipitable water values during this time in the 2 inch range, very heavy rain is expected and if it trains over the same areas, flooding and localized flash flooding will be possible. While a flood watch may be need for parts of the area, this time period is still 24 hours away and there remains uncertainly for how far south into the cwa a flood watch may be needed. Therefore, no flood watch headline is planned this morning.

Trends for Saturday afternoon into Saturday night will likely be determined with how the convection Saturday morning materializes and there is some concern that additional thunderstorms will redevelop in the afternoon though there is quite a bit of uncertainty. SPC Day2 outlook has a level 1 severe risk for localized damaging winds for most of the area on Saturday. Another period of heavy rain is expected Saturday night into Sunday morning, but confidence for where this sets up is low. Though if it does take aim a the local area, then another flood watch could potentially be needed during this time period as well.

As winds turn northeast Monday into Tuesday, cooler air will spread across the region with high temperatures possibly only reaching the lower/mid 60s both days, well below normal for mid September. Persistent and at times, strong northeast winds will allow waves to build along the IL/IN Lake Michigan shore with high waves and dangerous currents expected during this time period. cms

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1247 AM CDT Fri Sep 18 2026

Key Aviation Forecast Messages:

- IFR-MVFR ceilings gradually lifting to VFR this morning, latest at RFD.

- SHRA/TSRA potential ramps up late tonight into Saturday morning, with MVFR/IFR conditions possible in heavy convective SHRA/TS.

Surface high pressure building east-southeast across Ontario and the northern Great Lakes was brining slightly drier north- northeast surface winds to the region early this morning, and helping to push the downstate frontal zone farther south. Area of IFR CIGs was gradually improving to low-MVFR from the northeast across the terminals at 0530Z, and surface/satellite obs and high-res model guidance support gradual continued improvement to high-end MVFR/VFR through this morning with RFD likely slowest to improve to VFR toward mid-morning.

Surface winds should turn more east-northeast during the day as the surface high continues to move off to our northeast, and eventually to more of an east-southeast direction tonight as an area of low pressure develops into IA. As this occurs, the frontal boundary to our south will gradually return northward as a warm front, with an expansion of SHRA/TSRA developing across the upper Mississippi Valley near and north of the surface low and warm front. While a few spotty showers can't be completely ruled out as early as this afternoon especially across northwest IL/southern WI, the greatest potential for more widespread and impactful heavy SHRA/TSRA across the terminals appears to be after 06Z for RFD and after 08/09Z for the Chicago terminals. A prolonged period of SHRA/TSRA could then persist into mid-morning Saturday, with MVFR/IFR VIS/CIG conditions.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from 5 AM CDT early this morning through this afternoon for ILZ006-ILZ103-ILZ104.

IN...None. LM...Small Craft Advisory from 7 AM this morning to 3 PM CDT this afternoon for Winthrop Harbor to Wilmette Harbor IL.