632
FXUS63 KLOT 272322
AFDLOT

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

.KEY MESSAGES...

- Chance of thunderstorms this evening southeast of I-55.

- 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.

&&

.UPDATE...
Issued at 600 PM CDT Fri Jun 27 2025

A recent hand surface analysis revealed a cold front (really a
west to northwesterly wind shift) extending from Chicago to
Macomb, IL. Meanwhile, radar imagery from KILX depicts
northward-surging outflow boundary (from prior thunderstorms in
central Illinois) extending from from Mount Sterling to
Bloomington, IL. Between both features is a narrowing low-level
confluence zone, in which 1-minute GOES-19 satellite imagery
depicts gradually clumping cumulus clouds. The airmass in the
confluence zone is unstable with temperatures in the mid to
upper 80s and dew points in the lower 70s contributing to 2000
to 2500 J/kg of MLCAPE.

Over the next few hours, the expectation is for at least
isolated showers and storms to develop within the narrowing
confluence zone, focusing on an axis from Fairbury, IL to
Remington, IN. The primary time window looks to be from 7 to 11
PM or so, though am noting the first signs of development
already just west of Bloomington, IL. With little to no mid- to
upper-level flow, any shower or storm would be prone to quickly
collapsing, supporting a threat for torrential downpours and
gusty outflow winds. There is a 5% chance of a damaging
microburst provided a storm grows unusually tall, especially
given DCAPE near 1000 J/kg within the confluence zone.

Based on the forecast outlined above, did opt to boost PoPs to
the 40 to 60% range across our southern tier of counties, again
focused in the 7 to 10 PM time window. Will send out updated
forecast products shortly.

Borchardt

&&

.DISCUSSION...
Issued at 201 PM CDT Fri Jun 27 2025

Through Saturday Night:

This morning`s thunderstorms are moving eastward out of the
forecast area into Indiana. Meanwhile, a cold front is currently
descending southeastward across the forecast area, which shows
up fairly clearly on visible satellite imagery. As the area
destabilizes this afternoon, there is a chance that an
additional round of showers and storms can form along the front,
likely southeast of Interstate 55. Given the moist air mass
with dew points over 70 across the area, localized downpours are
possible over areas that already saw some rain earlier today.
Shear profiles do not look particularly exciting, although
recent SPC mesoanalysis has around 500-1000 J/kg of DCAPE.
Strongest storms could potentially have localized stronger wind
gusts, mainly southeast of Interstate 57. However, the front is
expected to continue its movement east through the afternoon so
any shower and storm threat should diminish quickly as it exits
this evening.

Surface high pressure will build into the area overnight and
into Saturday. Dewpoints will drop back into the 60s (finally!)
with high temperatures tomorrow the mid to upper 80s. A lake
breeze is expected to develop in the morning which will help
keep the lake shore in the mid 70s. Subsidence and drier air
mass should keep things dry through Saturday.

Attention will turn to northern Iowa and southern Minnesota
late Saturday afternoon and evening. High res guidance is
suggesting a potential MCS could develop and move southeastward.
However, there is fairly decent consensus that it weakens as it
crosses the Mississippi River during the night. Perhaps it
could produce a stronger outflow that makes its way down to
northern Illinois in the early hours of Sunday morning, but
confidence is low. For now, a dry forecast remains through
Sunday morning with overnight low temperatures in the upper 60s
and lower 70s.

DK

Sunday through Friday:

As surface high moves off to the east Sunday, southerly winds
will send the hot and humid air mass back northward into our
area. Given the progged sfc gradient from the due south, there
could be a weak afternoon lake breeze along the Illinois north
shore Sunday afternoon, otherwise look for highs mostly near or
in the lower 90s. Dewpoints rising back into the low-mid 70s are
expected to push heat indices back up to around 100 degrees.

Forecast soundings Sunday afternoon show the atmosphere
moderately to strongly unstable with little if any cap left
during the afternoon. Strongest synoptic forcing should remain
well north/west of our CWA during the day Sunday, however
guidance suggests the TUTT cell (tropical upper tropospheric
trough) over the southeastern United States will drift
northwestward potentially getting close enough to our CWA to
bring the smattering of scattered afternoon/evening convection
north into portions of our CWA Sunday afternoon and evening.

At least periodic chances of showers and thunderstorms will
continue Sunday night into Monday as an upper trough digs into
the upper Mississippi Valley and Great Lakes. Still a little
early for specifics, but highest chances look to be Monday
afternoon/early evening as cold front moves across the area.

Upper trough sets up over the Great Lakes Tuesday and Wednesday
with associated Canadian high pressure ushering in a much less
humid air mass and more seasonable temperatures (80s). Medium
range guidance suggests that the upper trough will be replaced
by upper ridge that gradually builds eastward into the region
later next week. An eventual return to warmer and more humid
conditions is expected late in the week along with shower and
thunderstorms chances.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 622 PM CDT Fri Jun 27 2025

Only forecast concern this period is the winds.

Westerly winds will gust into the 20kt range through sunset then
quickly diminish through the evening and likely become light and
variable for most locations. Where wind directions persist and
probably for ORD/MDW, winds will turn light northerly overnight
and possibly light northeast by daybreak. A lake breeze is
expected Saturday, turning winds easterly for ORD/MDW/GYY.
Further inland, winds will favor a light southeast or southerly
direction. cms

&&

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

&&

$$

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