625
FXUS63 KLOT 112347
AFDLOT

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

.KEY MESSAGES...

- Additional thunderstorm chances late this evening through
  Tuesday. Any storms that form will carry a threat for locally
  heavy rainfall.

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

- Chances for showers and storms early next week.

&&

.DISCUSSION...
Issued at 237 PM CDT Mon Aug 11 2025

Through Saturday...

As an axis of moisture has drifted northward, this morning`s
showers and storms have gradually eroded. However, new cells
have started to develop down around Danville, IL. Isolated
showers and storms may still fester through this afternoon, but
will mainly be confined south and east of Interstate 55.

A broad upper level trough is draped over the Plains this
afternoon with a surface low located near Topeka, Kansas. While
showers and storms are ongoing around that area of cyclonic
flow, attention is being drawn to the remnants of a MCV from
last night`s storms that is presently in northeast Missouri. As
an associated vort maxima moves northeastward, renewed
convection is expected to develop into northern Illinois late
this evening and into tomorrow morning. Models are still
struggling to capture its current status in Missouri which
lowers overall confidence in timing and coverage of storms.
However, as southwesterly winds increase closer to midnight,
shower and storm coverage is expected to blossom mainly north
and west of Interstate 57 through early Tuesday morning.

The previously mentioned upper level trough is expected to
start to move eastward through the overnight and Tuesday and
kick a very subtle and diffuse surface front eastward tomorrow.
Increased coverage of showers and storms can be expected as it
progresses through the forecast area on Tuesday. With better
forcing and steeper low level lapse rates along the front, there
is a chance for localized strong and potentially damaging
winds. The Storm Prediction Center went ahead and added a level
1 out of 5 ("marginal") risk for severe weather tomorrow mainly
for portions of northeast Illinois closer to the lake as well as
northwest Indiana. But with precipitable water amounts near 2
inches, the main hazard from these storms will remain the risk
for torrential downpours and localized flooding for localized
areas where training of storms occurs.

Models are suggesting that the frontal passage may linger into
Tuesday night before exiting to the east, but there is lower
confidence in the exact timing of showers and storms
dissipating. High res guidance is suggesting a weak midlevel
wave may pass over the forecast area on Wednesday as the
synoptic trough moves away from the area, which led to the blend
putting slight chance PoPs south and east of Interstate 57 in
the afternoon. Given the weak forcing, confidence is low, but
felt no reason to remove it currently. Regardless, cooler more
seasonable temperatures should develop behind the front through
the end of the week creating momentary less humid conditions.

As high pressure ridge develops on Friday and Saturday, 850 mb
temperatures are expected to climb once again into the 20C range
as southerly winds help draw precipitable water amounts near
two inches back into the region. This will increase humidity
once again and have temperatures start climbing back into the
90s, with the potential for heat indices back into the 90s and
potentially low 100s (especially on Saturday).

Models are still suggesting a weak wave may ride over top of
the upper level ridge on Saturday which may translate into
showers and storms cropping up in the northern portion of the
forecast area. Depending on the strength of the high, its
possible that later model runs keep the wave farther north and
remove PoPs. While confidence leans towards much of the area
remaining dry, but no changes were made to the NBM at present.

DK

Sunday and beyond...

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.

DK/Doom

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 647 PM CDT Mon Aug 11 2025

Main Concern:

- Potential for a few rounds of SHRA/TSRA from late this evening
  through Tuesday afternoon and their associated timing.

A weak disturbance pushing in from western Illinois has resulted
in the development of widely scattered showers and thunderstorms
from north of GBG to near C75 as of this writing. Additional
scattered showers and isolated to scattered TS are favored near
the terminals late this evening and early overnight, supporting
VCTS mention and TEMPO SHRA. The possible exception with this
initial timeframe will be RFD, which may remain north of the
lead activity. Then in the pre-dawn hours of Tuesday through
the mid to late morning, a stronger disturbance (currently over
south central IA) is expected to track towards northwest
Illinois, bringing additional scattered showers and storms. Felt
confident enough for VCTS at RFD for this, but only confident
enough in PROB30s for the Chicago metro terminals.

Finally, an approaching cold front should touch off scattered
gusty storms over northeast IL and northwest IN Tuesday
afternoon. There appears to be a decent chance these storms miss
RFD to the south, with a better chance of direct impacts at the
Chicago metro terminals supporting another PROB30 mention there.

Light winds tonight, generally from the south-southwest, will
become west-southwest on Tuesday. A period of MVFR CIGs is
likely at RFD from mid morning through midday, while VFR
conditions are expected at the other TAF sites outside of TS
effects on VSBY and CIGs.

Castro

&&

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

&&

$$

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