619
FXUS63 KLOT 110828
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
328 AM 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.

&&

.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 /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1205 AM CDT Fri Jul 11 2025

SHRA/TSRA should move east of our eastern terminals within the
first couple hours of this TAF cycle with storms setting up well
south of the terminals the rest of tonight. Another disturbance
over western IA is expected to continue eastward through the
morning hours and we will need to monitor trend for possible
inclusion of SHRA and possibly a TSRA threat late
morning/midday. Confidence is too low for including in the TAFs
yet, but will continue to monitor trends.

Better threat of SHRA/TSRA is expected to develop during the
mid-late afternoon hours closer to the MS River. This activity
could organize into a line and push eastward affecting the
terminals late Friday afternoon or Friday evening. Didn`t make
any changes to the inherited timing of the TEMPO, but as trends
become clearer, some adjustments to timing and possible upgrade
to a TEMPO could be needed.

In the wake of he current storms winds should become light and
tend to favor east or southeast this morning. Most guidance
wants to surge warm front northward and result in wind shift to
south-southwesterly early this morning. Given the coverage of
storms to the south, have delayed that wind shift timing in the
TAFs a few hours as these storms should tend to keep the front
from surging north overnight.

Finally, there is a small break in the mid-high level cloudiness
between storm clusters that could result in a period of clearing
prior to sunrise. If skies do clear out, can`t rule out some IFR
or low end MVFR CIGS developing for a short time this morning.
Confidence is too low to include anything at this time and will
also be monitoring these trends this morning.

- Izzi

&&

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

&&

$$

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