956
FXUS63 KLOT 271124
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
624 AM CDT Fri Jun 27 2025

.KEY MESSAGES...

- Isolated to widely scattered thunderstorms possible this
  afternoon, mainly east of I-55/57. Gusty downburst winds a
  threat with the strongest storms.

- After a bit of a reprieve from heat and humidity Saturday
  (especially near the lake), another hot and humid day is
  expected Sunday.

- Additional threats for mainly afternoon/evening storms Sunday
  and Monday.

&&

.DISCUSSION...
Issued at 319 AM CDT Fri Jun 27 2025

Through Saturday:

Outflow from earlier convection across Iowa is pressing east of
the I-57 early this morning. While almost all of the earlier
shower and thunderstorm activity has diminished locally, a
tongue of enhanced mid-level moisture is currently drifting east
of the Mississippi River. Along the eastern flanks of this
corridor, scattered elevated convection will be possible as
several low-amplitude impulses ripple northeastward. Will hold
onto some chances for showers/storms through the morning hours
as this mid-level moist axis and associated vorticity maxima
scoot eastward.

With the passage of the aforementioned outflow this morning,
convergence along an incoming cold front this afternoon doesn`t
look particularly significant, with the surface wind field
forecast to homogenize somewhat with mixing. Additionally,
soundings show considerable drying in the 800-500 mb layer
occurring through the day west of about I-55, along with
generally nebulous/weak forcing mechanisms in the region. As a
result, while areas roughly along and east of a La Salle to
Waukegan line will become moderately unstable and uncapped this
afternoon, shower/storm coverage later today remains a bit
uncertain. The hesitancy for high resolution guidance to convect
suggests that dry air entrainment into incipient updrafts may
play a role in curtailing coverage, particularly the farther
west across the CWA you go. With better mid-level moisture in
place south and east of about I-57, and global guidance
generally producing precipitation, continue to hold onto
"chance" PoPs in this region during the afternoon. Limited deep
layer shear suggests pulse/"airmass" convection and a potential
for isolated strong downbursts with 30+ C surface-700 mb theta-e
deficits in place.

Any lingering convection will diminish this evening, with the
cold front pressing through the rest of the CWA. Surface high
pressure will drift across the region on Saturday, which will
result in a brief reprieve from the mid/upper 70s dewpoints.
Weak gradient flow will allow a lake breeze to push inland
through the late morning and afternoon, holding temperatures
across immediate lakeside locales in the mid 70s.

Carlaw


Saturday Night through Thursday:

High pressure will scoot east Saturday night, with southerly
return flow gradually re-establishing across the region. The
eastern terminus of a robust elevated mixed layer is forecast to
build across western Iowa and northern Wisconsin, with robust
convective development possible in the vicinity of a warm front
Saturday afternoon/evening across central Minnesota. Extended
high-res guidance indicates a propensity for convection to grow
upscale into an MCS into Saturday night. Convective inhibition
will likely be quite high locally Saturday night, with a
significant MUCAPE gradient forecast off to our north and west.
As a result, any MCS should weaken notably with time through the
night. However, with 850-300 mb thicknesses and forward-
propagating Corfidi vectors oriented south to southeast, if a
well developed, severe MCS manages to materialize upstream, it
wouldn`t be totally out of the question for an outflow and
weakening showers/storms to make a run into the northern half of
the CWA into early Sunday morning given the somewhat close
proximity of initial MCS development. This remains a lower
likelihood outcome at this point, however.

Heat/humidity looks to increase on Sunday as southwesterly
breezes kick in with highs pushing back into the low to perhaps
mid 90s. Dewpoint trends during the afternoon are a bit
uncertain, as forecast soundings depict significant drying in
the 850-700 mb layer which could facilitate downward mixing in
the afternoon, particularly north of I-80. Additionally, there
is a potential for southeasterly winds to develop near the lake
in the afternoon which could offer additional cooling to
immediate lakeshore locales, particularly into Lake county,
IL. Current forecast shows peak heat indices near and locally
above 100 degrees, but the above-mentioned items result in some
uncertainty. Isolated to widely scattered thunderstorms are
possible on Sunday afternoon with the strongest storms once
again posing a downburst wind threat.

A notable trough will send another cold front slicing through
the region on Monday with additional shower and thunderstorm
chances. The front looks to scour the worst of the heat and
humidity out for a bit, with more seasonable temperatures
Tuesday and Wednesday along with lower humidity. Southwest
return flow will being to pump higher dewpoint air back into the
region towards the end of next week.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 624 AM CDT Fri Jun 27 2025

- Small TSRA chance (~15-30%) near the Chicago area terminals
  in the afternoon, with higher chances off to the southeast.

MVFR CIGs are likely to spread into north central IL this
morning through midday, including at RFD, though these CIGs
will probably not make it into the Chicago area before
scattering and/or lifting of cloud bases.

Breezy west-southwest winds will develop this morning ahead of a
cold front. The 230-260 deg wind direction and dry air moving in
aloft this afternoon should generally serve to limit the
coverage of convective development ahead of the front near the
TAF sites early this afternoon. Recent forecast soundings
suggest that MDW and particularly GYY have a low, but
relatively higher chance, than ORD and DPA of isolated TS
developing nearby. Opted to err on the side of caution with a
PROB30 for MDW and GYY. Can`t completely rule out TS as far NW
as DPA-ORD, but chances there are around 20%-not high enough for
mention in those TAFs.

Winds will likely shift to northwest behind the front early
this evening, with a low chance for a shift to northeast. Expect
light/VRB winds outside of Chicago tonight into early Saturday.
Winds are expected to shift to light north-northeast by mid-
late Saturday morning, though speeds should remain solidly below
10 kt.

Castro

&&

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

&&

$$

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