792
FXUS63 KLOT 220510
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1210 AM CDT Tue Jul 22 2025

.KEY MESSAGES...

- Dangerous heat and humidity expected Wednesday into Thursday.

- Weather pattern may turn stormy at times beyond Thursday.

&&

.DISCUSSION...
Issued at 344 PM CDT Mon Jul 21 2025

Through Tuesday Night:

Quiescent weather has largely been observed across the area
today due to northern Illinois and northwest Indiana being
positioned on the southwestern periphery of a surface high over
the Great Lakes. While there is a dearth of obvious large-scale
forcing mechanisms in our immediate vicinity, there is a broad,
slanted low-level confluence zone evident on GOES visible
satellite imagery across central Illinois and Indiana where 70+
degree surface dew points have helped facilitate a large,
congested cumulus field. Within this confluence zone, spotty to
isolated showers may be observed prior to the loss of diurnal
heating roughly as far north as a Dixon, IL to Fowler, IN line,
so have slight chance PoPs going through the remainder of the
afternoon across the southwestern third of our CWA to account
for this.

Tonight, we`ll need to keep an eye out for possible convection
blossoming to our west and northwest within a broad zone of warm
air advection/isentropic ascent along the northeastern rim of
an EML plume. If saturation is achieved in the right level(s) of
the troposphere to activate the elevated instability, then at
least scattered showers and storms would be likely to develop
and could end up propagating in the general direction of our
forecast area. Amongst the 12Z CAM runs, only the FV3 and WRF-
NSSL depict such a scenario. The former of those CAMs is
notorious for being overly aggressive with convection in these
kinds of setups, while the latter`s overnight convective output
appears to, in part, be driven by a modeled MCV originating from
a modeled MCS in southern Minnesota/northern Iowa that is non-
existent in reality this afternoon, so it`s hard to put much
stock into either of these CAM runs. Would also tend to think
that, in the absence of a notable low-level jet near our CWA,
any convection that develops would struggle to sustain itself
through the night and into tomorrow morning unless it is able
to generate a decent cold pool, which is far from a guarantee.
Thus, in all likelihood, it appears that our forecast area will
remain dry tonight through the daytime tomorrow, but there is a
low (roughly 10-15%) probability scenario where showers and/or
storms occur in our CWA tomorrow, so model and observational
trends will need to be monitored closely.

If increased cloud cover from this lower probability scenario
doesn`t materialize, then tomorrow should be warmer than today
with highs generally in the mid-upper 80s. A few locations could
even reach the low 90s, too. It will be more humid as well in
most locations as return flow on the backside of the departing
Great Lakes surface high allows for the richer low-level
moisture farther downstate to slosh northeastward. We`ll need
to keep an eye on upstream convection to our west tomorrow
afternoon and evening for any signs of MCV development as that
would provide a better chance for showers and storms to persist
after sunset tomorrow and at least get close to our forecast
area tomorrow night/early Wednesday morning. Bumped Tuesday
night`s PoPs up a bit from the NBM initialization, which seemed
too low, but have withheld on introducing mentionable
precipitation chances into the forecast grids for the time being.

Ogorek


Wednesday through Monday:

Not much change in thinking for the upcoming heat Wednesday and
Thursday. Over the past 24 hours there may have been a slight
downward tick in guidance max temps Wednesday and Thursday, but
really no meaningful change to expected impacts of the upcoming
heat and humidity. Generally looking for 925mb temps to top out
in the 24-27C range both days, which would support highs mostly
in the mid 90s. Evapotranspiration (ET) should help push
dewpoints into the mid 70s to lower 80s, with enough of a
southwest wind to likely push those high dewpoints into city and
the heat right up to the lakefront.

Planning to continue the extreme heat watch another forecast
cycle or two. Cook County and the city of Chicago look very
likely to reach their 2 day extreme heat warning criteria of
105-110 degree afternoon heat indices. Elsewhere, reaching the
110F+ heat index criteria for an extreme heat warning is going
to be dependent on ET pushing dewpoints close to or into the
lower 80s. Currently looks like a slam dunk for widespread heat
advisory criteria outside of Cook County with possible
marginal/low end warning criteria in spots, unless ET
overperforms.

As upper ridge begins to break down late Thursday into Friday, a
weak, probably outflow augmented, cold front is expected to drop
southward into northern Illinois Thursday night. Strongest
synoptic forcing for convection should largely remain to our
north Thursday night. Absent any un-forecastable at this range
enhancement to deep layer shear by an MCV, deep layer shear
looks quite weak Thursday night which raises doubts about how
quickly convection will weaken after sunset. In the absence of
an MCV to help augment forcing/shear, it`s possible that the
coverage of showers/storms in our CWA could grow pretty sparse
Thursday night. Not confident enough in this scenario to make
any changes to NBM POPs, but certainly plausible that NBM POPs
are too high and many areas may escape meaningful rain Thursday
night, especially the farther south you go.

Where the composite outflow boundary/cold front ends up on
Friday will dictate temps/heat indices as well as better chances
for afternoon showers/storms. Near/ahead of the front, moisture
pooling and ET could lead to very high dewpoints and another day
of potentially dangerous heat. At this point, it looks like this
would primarily be in our southern CWA. At least scattered
afternoon convection will likely develop near the front, though
deep layer shear looks weak, so organized severe threat appears
low at this point.

Upper ridge is likely to re-strengthen over the central Plains
over the weekend into early next week with our CWA on the
northeast fringes of the upper ridge. Most likely scenario is
the effective sfc boundary and edge of the extreme heat/humidity
will be just to our south and southwest, but it could be close.
Low amplitude impulses rotating around the flanks of the upper
ridge could also provide for at least occasional shower/storm
chances this weekend into early next week.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1210 AM CDT Tue Jul 22 2025

VFR conditions will prevail through the period. Lower-end VFR
cumulus should develop today as moisture begins to filter over
the area. Light ENE winds will gradually veer SSE through
daybreak as a surface high over Lower Michigan expands
southward. SSE winds around 10 knots are expected today and
tonight, though a boundary from lake- modified air may settle
near or possibly over ORD/MDW and shift winds ESE this
afternoon.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Extreme Heat Watch from Wednesday morning through Thursday
     evening for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...None.
LM...None.

&&

$$

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