801
FXUS63 KLOT 111710
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1210 PM CDT Fri Jul 11 2025

.KEY MESSAGES...

- Showers and thunderstorms likely late this afternoon into
  tonight, bringing potential for severe weather and heavy
  rainfall/flash flooding.

- Showers and thunderstorms are possible again Saturday
  afternoon and evening, especially southeast of I-55.

&&

.UPDATE...
Issued at 1210 PM CDT Fri Jul 11 2025

The main focus continues to revolve around the severe thunderstorm
and heavy rain threat later this afternoon and evening.

The remnant boundary from the early morning convection is now
beginning to lift back northward into northern IL and IN. South
of this boundary, thinning cloud cover is resulting in strong
heating and destabilization of a very moist low-level airmass (low
70s dewpoints). As this continues over the next few hours,
continued heating and destabilization will contribute to the
development of a very unstable airmass, with MLCAPE values of
2,000-3,000 j/kg.

This will certainly be priming the area for another round of
showers and thunderstorms later today into this evening. Our main
focus for the initial development of these storms will be to our
west across IA this afternoon. Over the past couple hours elevated
storms have been developing across northern parts of IA along the
nose of a remnant southerly low-level jet. This activity is
generally shifting to the east-northeast, and thus is likely not
impact our area in the near term. However, additional near surface
based storms are likely to develop farther south across
southwestern into south central parts of IA into early this
afternoon. This will occur as surface heating and destabilization
of the moist low-level airmass in continues in the vicinity of the
surface frontal boundary and just in advance of the main mid-
level trough axis and surface low.

The kinematic environment in which these storms develop will
feature clockwise turning hodographs amidst 700 to 500 mb
southwesterly flow of 30 to 40 kt. This will be supportive of
organized severe storms including supercells. As the afternoon
progresses, amalgamating outflows from individual storms is
expected to support a quick upscale growth into a forward
propagating QLCS across eastern IA into northwestern IL mid to
late this afternoon (likely after 4 PM). The main threat with this
QLCS across northern IL will be strong and damaging wind gusts
(potentially in excess of 75 mph). However, a tornado threat also
will exist into this evening, particularly across northwestern
portions of IL and eastern IA. With the increasing confidence in
this severe QLCS the SPC has increased the severe threat to a
level 3 of 5 across portions of northwestern IL. While a gradual
weakening trend in the QLCS is anticipated later this evening into
tonight with eastward extent across northern IL and northwestern
IN, the severe wind threat with these storms is likely to extend
through the Chicago metro area this evening (7 to 11 pm
timeframe).

Very heavy rainfall will also accompany these storms, and may
result in some renewed flash flooding across some of the same
areas that were hit with the very heavy rainfall in and around the
Rockford area last night. A flash flood watch thus continues for
northwestern IL through this evening.

KJB

&&

.DISCUSSION...
Issued at 328 AM CDT Fri Jul 11 2025

Water vapor imagery early this morning shows a trough over the
central Plains east into the Cornbelt with at least a couple of
mesoscale convective vorts (MCVs) embedded within the broader
synoptic trough. These MCVs will likely affect and complicate
our forecast for convective chances through tonight.

One MCV is pushing east of our CWA early this morning with a
much sparser coverage of showers across our CWA in the wake of
this feature. The convection associated with this MCV has laid
down an outflow boundary across our southern CWA west-northwest
toward the Quad Cities, though the "cold pool" associated meso
high in the wake of this MCS is quite weak.

Upstream, the first item of concern is an MCV and associated
small scale bowing line of severe storms approaching eastern IA.
The CAMs that have resolved this convection generally all show
it dissipating before reaching our CWA this morning. Given the
well defined MCV and only modestly less unstable air mass
downstream, there is concern it could persist longer than
guidance suggests with some showers and thunderstorms
potentially nearing our western CWA around sunrise. Seems
plausible that it should be on a weakening trend given the
weaker mid level lapse rates overturned by last evening`s MCS,
plan to carry some pops into the morning hours with highest pops
western CWA.

Another MCV was located over southeast Nebraska with this
feature more favorably timed to reach the Mississippi River
later this afternoon. Guidance generally forecasts 2-3K J/kg of
MLCAPE this afternoon across northern MO into IA and
northwestern IL. This magnitude of instability assumes that the
atmosphere is able to recover from morning convection and
convective debris.

Synoptically, southwest winds of 30-35kt at 6km is certainly
sufficient for organized and severe convection assuming the
above mentioned instability is realized. Also, there will likely
be some pockets of enhanced low-mid level flow, augmented by
MCVs which could create some areas of stronger deep layer shear
supportive of supercells.

In addition, an axis of deeper moisture (precipitable waters
>1.8") is pooled from Iowa east across northern Illinois. The
high PWATs and deep warm cloud depths will again set the stage
for convection today into tonight to be very efficient heavy
rain producers, likely in excess of 2"/hour with the stronger
storms.

Any training of convection in this environment could easily
result in localized rainfall totals over 4". The strongest
heavy rainfall signal in HREF guidance is across eastern IA into
the western half of northern IL, including the Rockford metro
area, which was hit hard Thursday. While there is still
uncertainty to the precise areas of where the heaviest rain will
fall today into tonight, yesterday`s heavy rainfall has knocked
flash flood guidance across Winnebago and Boone Counties down
to around an inch or less in an hour. While the mesoscale
details are still a bit uncertain, the very low tolerance for
any additional heavy rainfall supports the issuance of a flood
watch (for flash flooding) for Winnebago and Boone Counties.
While Ogle and Lee Counties generally tend to be less flash
flood prone, they also saw heavy rainfall Thursday and are at
risk for more heavy rainfall today into this evening, so
planning to hoist a flood watch for these areas into this
evening.

Given sufficient destabilization, it is also possible that
there could be a corridor of greater severe threat. Low LCLs,
MCV augmented stronger shear, and the potential for any
lingering outflow boundaries could lead to a localized supercell
potential with an attendant tornado threat. Convection could
easily grow upscale into a QLCS with a threat for a more
concentrated corridor of damaging winds and perhaps some QLCS
tornadoes.

Further delineating the more precise areas of any potential
greater severe weather and/or heavy rainfall/flash flood
potential will be easier later this morning as it becomes
clearer how morning convection/debris will augment instability.
In addition, we should be able to better time out the southeast
Nebraska (or any other MCV). It is possible that the flood watch
may need to be expanded.

A more pronounced northern stream shortwave trough is progged to
sweep across the upper Mississippi Valley and upper Great Lakes
Saturday. There`s quite a bit of spread on where the effective
surface front will be Saturday, with some guidance suggesting
it`ll be east of our CWA, while other models still have in
draped across our southeast CWA Saturday afternoon. Despite poor
lapse rates, moderate to possibly strong instability ahead of
the front should lead to another round of convection Saturday
afternoon and evening. The stronger mid-upper level flow looks
to be confined to areas well behind the sfc front, so absent of
any MCVs, shear profiles look to be quite weak Saturday
afternoon. So if there is convection in our CWA Saturday, it
would likely be southeastern CWA and with mostly just an
isolated pulse type severe threat, if any.

Didn`t have a lot of time to look at the longer term period with
so much active weather in the short term, but it does look like
we should get a break in the convective chances and humidity
Sunday and Monday in the wake of Saturday`s shortwave trough and
associated cold front.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1210 PM CDT Fri Jul 11 2025

Key Aviation Messages for the current TAF period:

- Winds will remain light and southerly, though there is slight
chance for winds to flip briefly southeasterly along a lake breeze
late this afternoon at ORD/MDW - but confidence remains low

- Scattered to numerous showers and thunderstorms late afternoon
through the overnight

VFR with light southwesterly winds early this afternoon. Recent
models are suggesting that a weak lake breeze could move inland late
in the afternoon. The expectation is that this could turn the winds
more southerly at Chicago terminals. However, there is a chance for
a brief window for southeasterly winds at ORD and MDW. Confidence
remains low and winds should remain at or below 8 knots so it was
left out of the TAF presently.

With the amount of moisture present, depending on how far that it
moves inland, the lake breeze could have enough forcing to trigger
an isolated shower or thunderstorm late this afternoon. But the main
area of concern will be a line of showers and storms coming out of
Iowa and moving eastward through the afternoon and evening. There is
still a lot of discrepancy in models on the exact timing of the
storms, which obviously lowers confidence. There is a (30 percent)
chance for isolated storms to form out ahead before the main line
moves through after around 22Z for RFD and 00Z for Chicago terminals.

Lingering showers are possible through the overnight as winds become
light, though it is possible that things could completely dry out.
Cigs are expected to lower after daybreak for MVFR conditions
Saturday morning. There is a (30 percent) chance for showers to
redevelop along and ahead of a front southeast of RFD tomorrow
afternoon. For now, thunder was left out and it was kept as a PROB30
mention for the time being.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 10 PM CDT this evening for ILZ003-ILZ004-
     ILZ008-ILZ010.

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

&&

$$

Visit us at weather.gov/chicago