302
FXUS63 KLOT 270224
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
924 PM CDT Thu Jun 26 2025

.KEY MESSAGES...

- Chances of showers and t-storms will continue at times tonight
  into Friday.

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

&&

.UPDATE...
Issued at 924 PM CDT Thu Jun 26 2025

After a busy evening with gusty to locally damaging
thunderstorms over northeastern IL and northwestern IN,
attention turns to the line of showers and storms now
approaching the Mississippi River. Water vapor imagery depicts
the parent upper-level short initially responsible for
initiating thunderstorms across Iowa and Minnesota earlier this
afternoon now lifting east-northeastward toward Lake Superior.
With the forcing so far removed from the line of storms, any
renewed development into Illinois will be dependent on slabwise
lifting atop the cold pool. Recent radar trends from KDVN
depicting the leading edge of the cold pool racing ahead of the
convective cells, implying the slope of the cold pool and
associated strength of forcing along the leading edge is
decreasing. Such an evolution is not at all surprising given a
lack of strong low-level shear ahead of the line of storms (the 00Z
DVN RAOB sampled around 20kt of 0-3km shear, which is hardly
enough to balance a summertime cold pool). Taken altogether, the
expectation is for the gust front to continue moving ahead of
the line of storms, leading to a gradual decay of the overall
system as it moves eastward into northern Illinois.

Given there is still some 800 to 1000 J/kg of DCAPE and 2000
J/kg of MUCAPE ahead of the cold pool, it will take some time
for the entire system to fully decay. As a result, still expect
at least a wind shift with gusty west winds followed by
scattered showers and storms to make it to I-39, and perhaps as
far east as the Fox Valley. Chances that thunderstorms hold
together as far east as Chicago seem pretty low (20% or less).
Taken together, no additional severe weather is expected
tonight.

Updated products will be sent soon.

Borchardt

&&

.DISCUSSION...
Issued at 249 PM CDT Thu Jun 26 2025

Another day of hot and humid conditions across the area this
afternoon. Some isolated showers and thunderstorms have brought
some temporary relief to a few places this afternoon, but the
majority of the area is experiencing another day with
temperatures in the 90s and heat index readings near or just
above 100 degrees. Atmosphere is hot, humid, and uncapped, which
has allowed for some "air mass" convection to pop this
afternoon. The threat of these isolated to widely scattered
storms could linger through sunset, but unless we get some
amalgamation of cold pools to force additional storms, coverage
should remain pretty isolated through sunset.

Water vapor imagery shows a well defined shortwave over the
northern Plains moving gradually east to the upper Mississippi
Valley. More organized convection has been developing in
association with this shortwave from southern MN south into
central IA. This convection is expected to continue to shift
gradually eastward toward the MS River the rest of the
afternoon. Stronger shear associated with this shortwave is
north of our latitude, so despite the instability, anticipate
this convection should fall apart after sunset as it crosses the
MS River into northwest IL. Kept the highest pops later this
evening into the early overnight over our northwestern CWA, with
much lower chances for this activity surviving long enough to
bring any organized rain chances over our eastern CWA.

Composite outflow/weak synoptic front is expected to move across
our CWA tomorrow. Strongest forcing associated with the
shortwave trough is expected to pass well north of the area, but
with air mass expected to become moderately to strongly unstable
and uncapped in advance of this boundary tomorrow. Certainly
plausible that scattered thunderstorms could develop ahead of
this front tomorrow afternoon, particularly across NW IN and our
east central IL counties. Slightly drier and more stable
conditions in the wake of the front should result dry weather
over our northwest CWA.

Transient surface high will move across the western Great Lakes
Saturday with lower dewpoints making the still seasonably warm
temperatures more tolerable. The position of the high should
also allow for a prominent lake breeze to move well inland
during the afternoon, providing for nature provided AC for areas
closer to the lake.

That high will scoot off the east of the region Sunday with
southerly winds likely to drag the heat and humidity back north
into the area. Highs on Sunday should top out in the low-mid 90s
with heat indices again near to just over 100 degrees. Southern
flanks of another shortwave trough passing north of the area
Sunday should lead to increasing chances of showers and storms
later Sunday afternoon into Sunday evening. Stronger shear and
better threat for organized convection should remain to our
north, but the air should be very moist and moderately to
strongly unstable again Sunday afternoon, so if convection makes
it into the area, wet microbursts and torrential downpours
would be a threat.

Longwave trough is progged to dig into the Great Lakes Region
early next week with much more comfortable humidity, especially
by Tuesday and Wednesday. Temps will remain seasonably warm
away from the lake, but not as hot as many of the days we`ve
experienced recently.

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 629 PM CDT Thu Jun 26 2025

Forecast concerns include...

Chance of thunderstorms early this evening.
Chance of thunderstorms northwest IL late this evening.
Possible mvfr cigs Friday morning.

Isolated thunderstorms continue to develop and lift northeast
across the area early this evening and this activity is expected
to continue through sunset. Brief heavy rain and locally gusty
winds will be possible with the strongest storms.

A line of thunderstorms across IA will move into northwest IL
late this evening and are expected to steadily weaken as they
move toward and then past RFD. Have maintain prob thunder for
RFD for this activity but most guidance suggests thunder chances
are decreasing for the Chicago terminals and have removed
thunder. Timing for any lingering showers and a possible
northwest wind shift looks to be later, in the early morning
hours and adjusted timing accordingly.

There is a chance of thunderstorms late Friday morning through
mid afternoon, mainly southeast of the terminals, possibly as
far north at MDW/GYY. Confidence is low for both coverage and
location and maintained a dry forecast for sites on Friday.

Most guidance brings an mvfr deck into at least northwest IL in
the predawn hours with it persisting through mid morning and
have included mvfr cigs at RFD. Further east, confidence is low
with the expectation that the precipitation will weaken and
possibly dissipate. Some mvfr level cloud cover is possible at
the Chicago terminals Friday morning and trends will need to be
monitored.

A lake breeze is slowly moving northeast, back toward the lake
currently. This wind shift is not expected to reach ORD/MDW with
south/southwest winds continuing through sunset. However,
prevailing winds may become southerly or briefly south/southeast
for a few hours later this evening, before turning back to the
southwest overnight. Winds will turn west/southwest Friday
morning with gusts into the lower 20kt range possible. Winds
will diminish with sunset Friday evening and turn to the
northwest. Winds may shift to the north or northeast late Friday
evening for the Chicago terminals. cms

&&

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

&&

$$

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