595
FXUS63 KLOT 251115
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
615 AM CDT Sat Jul 25 2026

.KEY MESSAGES...

- Heat and humidity will return to the region Sunday and
  continue into Monday, potentially requiring headlines for at
  least some areas.

- Some potential for thunderstorms late Sunday through Monday
  night, but predictability in timing, location, and coverage of
  storms is still very low.

&&

.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.

&&

$$

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