654
FXUS63 KLOT 102001
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
301 PM CDT Sun Aug 10 2025

.KEY MESSAGES...

- Periodic bouts of scattered showers and isolated to scattered
  thunderstorms can be expected through the first half of the
  upcoming work week, probably followed by a few quieter days.

- Any stronger thunderstorms the next few days will be capable
  of producing torrential downpours and localized flooding.

&&

.DISCUSSION...
Issued at 301 PM CDT Sun Aug 10 2025

Through Monday:

In the wake of the earlier showers and storms, convective coverage
has greatly diminished as of this writing, owing to likely
subsidence amidst marginal mid-level lapse rates. Isolated to
widely scattered showers and perhaps a few isolated thunderstorms
should be the theme for the rest of the afternoon hours. Can`t
completely rule out an isolated thunderstorm as far southeast as
the Kankakee River Valley of northwest Indiana over the next
few hours.

Looking off to the west of the MS River, a slow moving MCV over
eastern Iowa has recently resulted in the initiation of scattered
convection. Given the worked over and more stable conditions over
much of northern Illinois, along with this wave approaching
after dark, how much bonafide deep convection is able to
develop in our CWA is a big question mark. The next shift will
be able to better refine things per observational trends this
evening. Given the uncertainty, weak lapse rates, unfavorable
timing, and only modest forcing, kept thunder chances in the
slight chance range through tonight and into Monday morning. Our
highest forecast PoPs (30-50%) for shower activity regardless
of thunder chances/coverage are once again focused
near/northwest of I-55.

The slow moving cold front up across the upper MS Valley will
gradually progress southeast through Monday. Specifics regarding
shower and thunderstorm trends in the afternoon are unclear.
However, outflow could push the effective boundary farther
south resulting in better chances for scattered showers and
thunderstorms farther south than previous forecast thinking.
Exactly how things play out overnight into the morning hours
will help determine how the rest of the day goes.

All in all, weak forcing, possible cloud cover effects, and
increased uncertainty necessitated lowering PoPs down into the
30-40% range through the day on Monday. If and where thunderstorms
develop, coverage now appears more likely to be widely scattered
in nature. Marginal mid-level lapse rates and sub marginal deep
layer shear should further limit the threat for any damaging
downburst winds on Monday. Slow storm motions and high PWATs will
keep the threat for localized torrential downpours and flooding
going, however. Aside from the uncertain convective details on
Monday, it will be another warm and humid day, especially near and
southeast of I-55 (upper 80s to around 90F).

Castro


Monday Night through Sunday:

The potential for continued off-and-on rounds of showers and
thunderstorms will persist through Tuesday night/Wednesday as an
axis of high theta-e air with PWATs near 2 inches remains
nearly quasistationary overhead. Several perturbations will
slingshot their way through fairly active southwest flow during
this time frame, as a robust shortwave drops out of southern
Canada and sweep through the northern Plains.

Timing each of these individual perturbations remains
challenging, as, to some degree, convective augmentation will
take place each day with successive rounds of thunderstorm
development. That said, there is a decent signal in latest model
guidance that a synoptic wave (positively tilted in most
guidance) may impinge on the region Monday evening and
overnight, yielding an uptick in broad warm and moist advection
and an associated increase in shower and some thunderstorm
activity. The ensemble signal remains fairly strong that the
greatest potential coverage during this period overnight Monday
night into Tuesday morning will remain near and northwest of
I-55, within the core of the modeled deepest tropospheric
moisture.

While 925-850 mb southwesterly flow will be seasonably
strong (near 35 kts at times), not currently seeing signs of
particularly robust convergence axes which would spell more of a
concerning heavy rain and flood scenario. Some training of
individual convective clusters will be possible with cloud-bearing
flow generally southwest to northeast, aligned parallel with the
main moist axis, but nothing currently stands out as overly
concerning from a widespread flash flood perspective into Tuesday.
Guidance remains in decent agreement that a cold front will press
south through the region during the Tuesday night/Wednesday
timeframe. This should shift the shower and thunderstorm chances
southward Tuesday afternoon and eventually bring an end to the
daily convective potential. If the front gets hung up in our far
south on Wednesday, isolated afternoon showers and thunderstorms
will be possible.

Moisture and instability looks like it`ll end up sloshing back
across the region towards next weekend. Current extended
guidance suggests that our region may remain on the northern
periphery of lingering subsidence with a belt of stronger
cyclonic mid and upper level flow displaced across northern
Wisconsin and the UP of Michigan. This may end up keeping the
more appreciable storm chances to our north, although there are
some signals that the mid-level ridge could break down towards
the Saturday night - Monday time frame ahead of another front
which could open us up to the next round of MCS chances.

Carlaw/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 130 PM CDT Sun Aug 10 2025

Key Messages:

- Continued potential for periodic showers and storms through
  the TAF period.


A slow-moving frontal zone draped over the area will continue to
provide the opportunity for periodic showers and storms at and
near the terminals through much of the TAF period, though
confidence in specific details remains low on the whole. The
going PROB30 groups reflect the most likely time frames for
showers or thunderstorms to affect each respective TAF site,
though in reality, there is at least a 15-20% for showers at
pretty much any time during the current TAF period. It appears
that there will probably be a relative lull in the potential
for thunderstorms late tonight into tomorrow morning, though
isolated to perhaps widely scattered thunderstorms should remain
possible into this evening and again during the daytime
tomorrow. Either way, the low predictability of forecasting
showers and storms at point locations in this weather pattern
will likely necessitate tactical amendments to the TAFs as
trends become apparent.

There is also a potential for sub-VFR conditions at times
during the TAF period. This would mainly be during any
thunderstorms or more robust showers, but MVFR ceilings could be
observed at RFD during dry periods as well. Didn`t have enough
confidence in that occurring during a specific period time to
carry a prevailing MVFR mention in the RFD TAF at this time,
however.

Ogorek

&&

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

&&

$$

Visit us at weather.gov/chicago