654
FXUS63 KLOT 111758
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1258 PM CDT Mon Aug 11 2025

.KEY MESSAGES...

- Chances for showers and isolated storms today, mainly south of
  I-80. Additional chances late this evening and overnight and
  again on Tuesday. Any storms will carry a threat for locally
  heavy rainfall.

- Uncomfortable heat and humidity will return for this weekend
  with heat indices in the 90s to near 100 degrees, especially
  on Saturday.

- Chances for additional showers and storms build during the
  early part of next week.

&&

.DISCUSSION...
Issued at 308 AM CDT Mon Aug 11 2025

Through Tuesday:

Regional VWPs and recent objective analysis reveals a fairly
pronounced 850-700 mb shortwave trough axis overhead, with west
to northwest winds at DVN, and strong southwesterlies locally
and northward into southeast Wisconsin. The associated vort max
is continuing to shift northeastward, and generally more
subsident flow in the wake of this feature looks to settle
across the northern portions of the forecast area this morning.
The primary low to mid-level moist axis is forecast to set up
predominantly along and south of about I-80 where the "highest"
PoPs today have been focused.

An initial region of warm/moist upglide is forecast to push
across the southern half of the CWA through mid to late
morning. Ascent isn`t forecast to be particularly robust, but
sufficient to lift and moisten a (currently) warm/dry 900-800 mb
layer. The spread in guidance regarding the handling of
cooling/moistening of this layer is substantial (recent HRRR
runs have been totally dry, for example). However, given the
ongoing development of stratus in the 5- 8 kft layer across
west-central Illinois and eastern Missouri, elected to focus
some chance PoPs to account for potential shower development
later this morning south of I-80. Moist adiabatic lapse rates
through the entire column suggest lightning potential should
remain pretty low, although will continue to carry some "slight"
chance mentions just in case.

Additional shower/isolated storm development will be possible
this afternoon, again predominantly south of I-80 within the
main moist and unstable axis. Farther to the north, subtle/weak
confluence axes could still serve as foci for some shower
development, but coverage is expected to remain more isolated in
nature.

For this evening and overnight, focus turns to the ongoing mass
of convection across south-central Kansas, where a
convectively-augmented vort max/MCV is forecast to develop. As
is typical with these features, guidance is having a difficult
time either (a) resolving this feature in the first place and/or
(b) determining its ultimate trajectory as it interacts with a
broader, postively-tilted trough across Iowa. Latest rough guess
is that the associated mid-level mb wind enhancement could try
to focus across northwest Illinois late this evening/overnight
and serve as a renewed focus for showers and storms. Ultimately,
really didn`t make any changes to the inherited PoPs from the
previous shift during this period, with high-end chance/likely
values north and west of a La Salle to Waukegan line, and
diminishing with east extent from there. If the 700 mb wind
enhancement is as significant as some guidance suggests
(incoming 06z HRRR), we`d need to keep an eye on a locally
strong/gusty wind threat given limited near-surface static
stability owing to low to mid 70s surface dewpoints.

Assuming things don`t remain muddied up through Tuesday morning,
somewhat better coverage of showers and storms is possible late
Tuesday morning into the afternoon as a synoptic-scale trough
axis swings through. Torrential downpours and some training will
be possible as cloud-bearing mean flow will remain WSW-ENE,
aligned with the main trough and moist axis.

Carlaw


Tuesday Night through Sunday:

Following the long-awaited passage of a cold front Tuesday and
Tuesday night, mostly quiet conditions should follow for
Wednesday. The front will get hung up across south-central IL
during the day. Guidance keeps our southern CWA largely uncapped
just north of the front on Wednesday and some models resolve a
few daytime showers well south of I-80. But instability looks
modest at best and forcing and moisture will be heavily lacking,
especially above the boundary layer, so it may be difficult to
get anything more than some bubbly cumulus out of this airmass.
In the forecast, stuck with the NBM`s slight chances near and
south of US Rt 24.

Expect seasonably warm conditions for the middle of the week.
Highs both Wednesday and Thursday are forecast to reach the
lower and middle 80s. High pressure inching into the Great Lakes
from the north should provide more sun than clouds, and the
drier near-surface conditions should omit much of a heat index
factor both days. Late in the week, high pressure aloft
building over the SE US will bring upper height rises to the
region warming things up a few degrees toward the weekend. Highs
on Friday are forecast in the upper 80s with lower 90s possible
again by Saturday. We should be back in a southerly flow regime
for the weekend which may drive afternoon heat indices toward
the 100 degree mark, especially on Saturday.

Medium-range guidance resolves a "ridge rider" low pressure
system moving across the upper Midwest this weekend around the
edge of the upper high. A highly unstable airmass is expected to
build up south of the storm track with the GFS and Euro
resolving 4,000-5,000 J/kg of MUCAPE around the region this
weekend. However, the local area should remain shielded by the
upper high keeping the main moisture feed and forcing mechanisms
displaced to our west and north. Model soundings exhibit modest
boundary layer moisture, hence the high instability, but deep
layer moisture should be heavily lacking and there are no
apparent features nearby to trigger that instability. Ensemble
QPF output is pretty quiet this weekend through Saturday, so the
forecast was left largely dry through then.

Confidence in precip builds at the beginning of next week when
the upper high will retreat to the south allowing for the
lingering frontal boundary in the upper Midwest to drop south
across our CWA. There`s some uncertainty on when exactly the
front actually gets into our area, but it should be by late
Sunday. It then looks like the front will hang out somewhere in
northern/central IL on Monday before departing to the south for
Tuesday. Accordingly, chances for rain and embedded thunder look
pretty good during the Sunday-Monday timeframe, especially late
Sunday-Monday. Shear during this time looks generally very poor
but, given the high instability, will have to keep an eye on
the potential for severe convection early next week.

Doom

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1258 PM CDT Mon Aug 11 2025

The main concerns for the 18Z TAF period include:

* Several windows for possible showers and thunderstorms through
  Tuesday

* MVFR ceiling potential, mainly at RFD Tuesday

Morning MVFR and patchy IFR ceilings have dissipated/scattered
for the most part, though some broken patches may linger toward
RFD for a short time early this afternoon. Southwest winds 6-10
kt with a few sporadic gusts in the 15-20 kt range will continue
this afternoon.

The thunderstorm forecast remains a challenge through the
period. Current shra/tsra remains well south of the terminals
though a few bouts of light rain may approach GYY by mid
afternoon. Otherwise, the focus for additional shra/tsra will
be a circulation over northern Missouri that will reach the
local area this evening. Have continued with a PROB30 for this
activity but narrowed the time window as it appears forcing will
come in waves. A second wave of shra/tsra is expected toward or
just after daybreak Tuesday with another wave more closely tied
to the passage of a cold front later Tuesday afternoon. Timing
refinement will likely be needed, but have decent confidence in
the overall precipitation trend in the forecast. Winds will
decrease tonight and remain southwest or even south-southwest
before increasing modestly Tuesday morning. Shra/tsra activity
may disrupt the prevailing wind field depending on how organized
it can become. Expect prevailing VFR ceilings outside of
precipitation, but MVFR may develop at RFD on Tuesday behind the
passing front.

MDB

&&

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

&&

$$

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