Marine Weather and Tide Forecast for Chicago, IL

UPDATE
Issued at 757 AM CDT Sat Jul 25 2026

Made some quick updates to our PoP and Wx grids this morning, mainly to boost and expand the spatial footprint of our mentionable PoPs and to explicitly mention drizzle in our forecast as ASOS/AWOS observations, webcam imagery, and radar observations indicate that the coverage of drizzle across our southern CWA is greater than previously expected. Low-level isentropic ascent will continue throughout the day, and forecast soundings from multiple models indicate that stratus depths should remain sufficiently deep for drizzle to persist across the southern half or so of our CWA through the remainder of this morning before the magnitude of isentropic ascent diminishes and the stratus is sufficiently eroded by the early-mid afternoon.

Ogorek

DISCUSSION
Issued at 209 AM CDT Sat Jul 25 2026

Today:

Stratus that is blanketing much of the Midwest is lurking just to our west early this morning. As low level moisture advection and weak isentropic ascent spread eastward into our area early this morning, so too should the stratus. Stratus seems likely to stick around through midday, with some gradual/partial clearing from the northwest this afternoon. Where cloud cover holds through much of the afternoon (most likely our southern and southeastern CWA), high temperatures should top out in the 70s.
Over western and northern CWA, highs could reach the lower 80s if overcast breaks for some partial sunshine early enough in the afternoon.

Tonight and Sunday:

As upper ridge amplifies over the central U.S. steamy air mass is progged to build northward and eastward across the Midwest, enveloping our area by early Sunday morning. Given the recent warm bias with the NBM, have nudged temps downward just a bit for Sunday, carrying highs primarily in the upper 80s/lower 90s. Despite deep mixing depicted in the HRRR and several other CAMs, the more likely scenario Sunday is a low level capping inversion being maintained as depicted by the GFS, NAM, and ECMWF. This stronger low level inversion also makes sense given the expectation of some mid-high level cloudiness.

The low level capping inversion should limit the ability of low level moisture to mix out while a large synoptic bank of mid 70 dewpoints overspreads the area. As we near peak evapotranspiration season, those mid 70 dewpoints will likely become corn-fed, resulting in locally upper 70 to perhaps even lower 80 degrees dewpoints. Heat indices over our western/southwestern CWA are expected to peak in the 100-105F range, just shy of our heat advisory criteria.

Sunday night through Monday night:

There will likely be subtle, low amplitude shortwaves cresting the upper ridge and potentially driving intense convection on the northeast and eastern periphery of the upper ridge. Any one of these subtle waves could ignite an MCS and lead to a larger convectively enhanced vort (MCV) developing. Unfortunately the predictability of these subtle features tends to be limited to only a day or so out, leading to a high degree of forecast uncertainty. There is a slight clustering of guidance that suggesting an MCS could drop south down the length of Lake Michigan Sunday night, which is certainly plausible, but so are about 1000 other scenarios. NBM pops looked reasonable with chance pops into about the eastern half of our CWA Sunday night.
Given the strong to extreme instability, severe weather would certainly be possible, though the stronger mid-upper level flow and resultant synoptically driven deep layer shear looks to be just to our north and east Sunday night. That of course doesn't account for any potential mesoscale enhancement to the shear by any potential MCV.

Assuming convection or convective debris don't retard heating, then Monday looks to be the hottest day with highs likely topping out in the low to mid 90s. Dewpoints in the mid 70s to lower 80s would likely push heat indices well over 100 degrees, potentially peaking near or just above 110 degrees given sufficient sunshine. Stronger southwest winds could lessen the mixing out of dewpoints over the urban heat island, leaving heat indices of 105F to possibly as high as 110F in play. No plans for any heat headlines for Monday at this point given the significant uncertainties regarding the potential for convection or convective debris, either of which could result in high temps and peak heat indices ending up lower than currently forecast (potentially significantly lower in spots).

Shortwave trough is expected to crest the ridge and move southeast across the Great Lakes region Monday night. This should drive surface cold front across our area Monday evening, which could serve as a focus for additional convection. There is substantial spread in guidance regarding the amplitude of this trough, with the ECMWF/EPS significantly stronger than GFS/GEM.
The strength of this trough will play a role in how much large scale forcing will accompany the front across our area Monday evening. It will also impact the magnitude of available shear.
Overall, forecast confidence in this time period is low as well, with possibilities ranging for intense supercells to no convection to anything in between the two extremes. Other than slightly increasing NBM pops over our eastern CWA Monday evening, no other changes were made.

Tuesday through Friday:

Northerly winds should usher in a less humid and cooler air mass Tuesday. The brisk northerly winds down the full fetch of the lake will likely result in building waves and we will likely be heading for Beach Hazard statements for this time frame.

While NBM does have a smattering of slight chance pops here and there during the middle and end of the upcoming work week, the pattern would favor dry conditions with little threat of rain.
Temps should be seasonable with high mostly in the low-mid 80s, but winds off the lake will likely keep those area only in the 70s.

- Izzi

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/
Issued at 615 AM CDT Sat Jul 25 2026

Main Concern:

- Lower CIGs , primarily during the day today

- 2-6SM BR early Sunday morning

MVFR CIGs in the ~1500-2500 ft range will develop this morning, likely lowest at RFD. There's still a chance for a period of IFR CIGs at RFD, but confidence was too low for mention in the TAF.
The exact timing of improvement back to VFR later today remains a lower confidence item. Early Sunday morning, conditions appear favorable for reduced VSBY in BR (possibly down to IFR in spots), particularly outside of Chicago. Added temporary MVFR VSBY to the RFD, DPA, and GYY TAFs for now. Expect light to moderate south to southwest winds through the period.

Castro

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...None.