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 /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 630 AM CDT Fri Sep 18 2026

Key Aviation Forecast Messages:

- A few VFR light SHRA possible midday/early afternoon at RFD.

- SHRA/TSRA expected to develop across the region predawn Saturday. 06Z model guidance is slower with a somewhat smaller footprint of convection than earlier runs, leading to somewhat lower confidence especially in TS coverage/timing early Sat.

Surface high pressure continues to build east-southeast across Ontario and the northern Great Lakes region this morning, with northeast low-level flow advecting drier air into the region. This has allowed continued lifting and scattering of earlier MVFR deck across the terminals, with even RFD now just above 3 kft. Can't completely rule out some brief patchy 2500-3000 foot bases this morning after sunrise, but overall VFR conditions are expected through this evening. Northeast surface winds are expected to increase into the 10-15 kt range this morning and eventually turn more easterly. Some spotty decaying SHRA across eastern IA may reach RFD by midday if they don't dissipate first. Have added a VFR PROB30 for light showers for this.

Surface low pressure is progged to develop from the High Plains northeastward toward the upper-Mississippi Valley through tonight, turning local winds southeast and pulling the frontal boundary currently over downstate IL/IN back north into the forecast area as a warm front into Saturday morning. As this occurs, a southwesterly low level jet develops into the IA/MN/WI border region, providing ascent and returning moisture which is expected to result in widespread SHRA/TSRA development over parts of the region. 00Z model runs were in decent agreement in developing a decent sized footprint of SHRA/TS over northern IL predawn Saturday, though have noted a significant slowing and decrease in that footprint in many of the 06Z model runs. Am a bit hesitant to back off too much on SHRA/TSRA forecast early Saturday given decent forcing and moisture at play, though did push timing back a bit later from the 06Z TAFs. This may need additional adjustment with later updates if 12Z guidance suite continues to indicate a similar trend as the 06Z set. Would expect at least some MVFR CIG/VIS conditions in association with SHRA/TS that occur in this scenario.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement until 3 PM CDT this afternoon for ILZ006-ILZ103-ILZ104.

IN...None. LM...Small Craft Advisory until 3 PM CDT this afternoon for Winthrop Harbor to Wilmette Harbor IL.