673
FXUS63 KLOT 101830
AFDLOT

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

.KEY MESSAGES...

- Showers and thunderstorms with embedded heavy rain and gusty
  winds through early afternoon, mainly west of the Fox Valley,
  with a chance for showers and thunderstorms this afternoon
  near and northwest of I-55.

- Periodic bouts of scattered, mainly afternoon/evening,
  thunderstorms through the first half of the upcoming work
  week, possibly followed by a few quieter days.

- Thunderstorms will be capable of torrential downpours the next
  several days, leading to the potential for a localized flash
  flood threat.

&&

.UPDATE...
Issued at 1154 AM CDT Sun Aug 10 2025

Warm advection driven convection that was quasi-stationary near
the MS River into southern Wisconsin early this morning finally
made headway eastward. There`s been a more decided weakening
trend in the parent convection over the past hour, but not
before dumping 1-2" of rain in spots from the Rockford metro to
northwest McHenry County. Outflow kicked out by the existing
convection is expected to be the focus mechanism to renewed
thunderstorm development farther east toward the I-55 corridor
this afternoon.

In the near-term, objective analysis indicated slightly better
wind shear profiles over far northern Illinois. This resulted
in a weak book-end vortex near Rockford early this hour that
should aid in a bit better storm organization over the far
northwest and northern suburbs over the next couple of hours,
with a threat for localized wind gusts to 40 mph in additional
to torrential rainfall rates.

Very warm to hot and humid conditions and better insolation
east of the debris cloud from the WAA convection has likely
eroded any remaining capping. With lingering outflow being the
primary trigger for lift, coverage may end up on the lower side
in closer to the heart of the Chicago metro (30-40% near/west of
the I-55 corridor). With that said, despite marginal mid-level
lapse rates (6C/km or less), fairly large MLCAPE could result in
strong updraft pulses and subsequent threat for localized strong
to marginally severe downburst wind gusts upon core collapses.
Off to the southeast of I-55, meanwhile, many areas will likely
remain dry, with a 10-20% or less chance of any showers/storms
making it into east central IL and northwest Indiana.

Farther west, the thoroughly worked over cold pool affected
environment (69F at RFD as of this writing) calls into question
the coverage of additional storms the rest of this afternoon. A
remnant MCV over south central Iowa lifting northeast could
serve as the impetus for renewed convection towards and after
sunset, though confidence is low. Will address our latest
thinking on convective trends tonight-Monday with the full
forecast issuance this afternoon.

Castro

&&

.DISCUSSION...
Issued at 254 AM CDT Sun Aug 10 2025

Through Monday:

Early this morning, an MCS is gradually lifting north across
southern WI and central Lake Michigan. This upwind-propagating
system brought hours of torrential rainfall to the Milwaukee
metro area prompting the issuance of a couple of Flash Flood
Warnings. South of this area of convection, recent obs suggest
and outflow boundary has pushed south of the state line into
far northern Cook County before curling back up into far
southern WI farther inland, and accordingly a few isolated
pockets of precip have been stirring along the state line
lately. RAP isentropic analysis depicts efficient moist upglide
south of and across the boundary through the low- and into the
mid-levels. Occasional showers and thunderstorms will remain
possible across our northern-most counties into the morning
around the outflow.

This upglide extends west beyond the Mississippi River along a
strengthening low level jet transporting rich deep moisture into
the river valley and wrapping into southern WI. The 00Z RAOB
out of DVN last evening recorded 1.9" of PWAT, and latest RAP
guidance suggests as many as 2.2" could build up around the
river by dawn. Along the nose of the LLJ, a low level confluent
axis resides across southern and eastern IA. Scattered
thunderstorms have recently begun to blossom in this general
area, including into northwest IL, and coverage is filling in
rather quickly. And even farther upstream, an MCV is spinning
eastward along the KS-NE state line beneath a low level
shortwave impulse, and coverage is quickly expanding into
southeast IA.

Expectations for the next few hours are for coverage to
continue expanding across IA and into northwest IL in the
warm/moist advective wing. It`s not clear how much coverage
we`ll see prior to dawn in our western CWA, but recent radar
trends suggest the I-88 corridor and points north, and west of
the Fox Valley will be the focus in our CWA for pockets of heavy
rain and thunder through daybreak. Lighter showers or sprinkles
will be possible farther south and east beneath a midlevel
stratus deck blanketing much of the area.

Around daybreak, the upstream MCV will begin pushing across IA.
Recent CAM guidance suggests the dynamic forcing into the very
thermodynamically favorable environment will trigger a
widespread plume of heavy rain ahead of the wave overspreading
IA and parts of northwest IL through the morning. Again it`s
unclear what coverage will look like into our CWA as the main
focus for heavy rain looks to align just outside of the area
closer to the river during the morning hours. The MCV will lift
northeast across the Mississippi and into central WI during the
afternoon. As it does though, the greater low level moisture and
forcing look to get pulled to our north, which could result in
a downward trend in coverage and intensity across our area
through the afternoon and into the evening. The highest chances
during the latter part of the day will remain west of the Fox
Valley. CAPE profiles are of the tall, skinny variety on
forecast soundings for this afternoon and shear remains poor, so
organized severe convection may be difficult to achieve. But
the moisture profile would be supportive of plentiful lightning
in storms and possibly wet downburst winds.

The stationary front that`s been hanging out to our northwest
for a couple of days now will inch closer into southern WI
tonight and additional showers/storms may fire along and south
of it through Monday. However, there`s quite the spread in model
solutions for Monday`s precip chances and I`m sure they`ll
influenced by what ends up happening today. So rode with the
NBM`s solution of widespread chance PoPs in Monday`s forecast
for the time being.

Temperatures this afternoon are expected to make it into middle
and upper 80s around most of the CWA. Heat indices may be as
warm as the middle 90s this afternoon. Conditions will likely be
kept cooler in our northwest with the greater cloud cover and
precip chances expected. There`s uncertainty in just how much
cooler; the forecast calls for lower 80s but there`s support to
be stuck in the 70s out west today. Similar conditions are
anticipated on Monday with highs in the 80s and heat indices up
in the 90s.

Doom


Monday Night through Saturday:

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 coverage during this period 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 mostly bring an end to
the daily shower and storm chances, although the front might end
up getting hung up in our far south on Wednesday yielding some
additional chances for a few afternoon showers and storms.

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

&&

.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.

&&

$$

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