NWS Chicago Decision Support Briefing - Forecast

332
FXUS63 KLOT 252319
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
619 PM CDT Fri Apr 25 2025

.KEY MESSAGES...

- Brisk north winds tonight followed by seasonably cool and dry
conditions for Saturday.

- A strong warming trend into Monday with highs in the lower 80s
for Monday.

- A chance for strong to potentially severe thunderstorms as a
cold front moves across the region Monday night into Tuesday
next week.

&&

.DISCUSSION...
Issued at 307 PM CDT Fri Apr 25 2025

Through Sunday:

Showers and isolated storms ending late this afternoon over
northwest IN and east central IL as the shortwave trough and
wave of low pressure move east of the area. Increasing nly
winds are then expected as low pressure deepens across the
lower Great Lakes while a 1030 mb high builds into ern MN and nw
WI. Nly wind gusts of 30-35 mph will be possible near the lake.
Temps will fall into the 40s by Saturday morning.

High pressure will then move across nrn IL and nrn IN Saturday
afternoon and night. Sly winds and warm advection will then
begin on Sunday. High temps Saturday will range from near 60F
toward central IL to around 50F at the lake followed by mid to
upper 60s away from the lake for Sunday.

MG

Sunday Night through Friday:

High pressure will hang on for one more day as temperatures
begin to moderate Sunday. This moderation will be courtesy of
southerly winds and rising heights aloft as a ridge starts to
build into the upper Midwest. Humidity and warmth then increase
Monday as a low pressure system approaches the western end of
Lake Superior. Thunderstorms are expected over the northwestern
half of the CWA Monday morning as a waning LLJ leans over east
into the region. MUCAPE and effective shear appears weak on the
GFS, so any storms look to be sub-severe at this time. The
morning storm potential will be followed by a late morning and
afternoon lull with some potential scattering of the clouds. A
line of storms then looks to form over Iowa and translate east
with time Monday night. GFS soundings support some severe threat
with 1500 to 2000 J/kg of MLCAPE, effective shear of 40 knots
and modest low to mid lapse rates ranging from 6 to 7 C/km.
0-6km shear boundary parallel shear seems to suggest a linear
storm threat with some QLCS potential given cyclonic hodograph
curvature and 0-3km shear around 40 knots. Trends will be
monitored through the weekend to pin down more accurate timing
and threats for Monday. A second threat window also looks
possible Tuesday pending how quickly a boundary clears us
Tuesday morning and afternoon.

Beyond, high pressure moves in mid week, keeping conditions
quiet, before a warm front approaches bringing rain chances
toward the end of the extended period.

CM

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 620 PM CDT Fri Apr 25 2025

Aviation forecast concerns for the 00Z TAFs:

- Blustery N to NE winds into Saturday, with gusts 25 kts or
slightly higher tonight gradually easing tomorrow.

- Period of MVFR ceilings likely from mid-evening into late
morning/midday Saturday.

Surface low pressure was over south-central lower Michigan early
this evening, with a cold front trailing southwest across
IN/southern IL. Blustery north winds gusting 25-30 kt were
occurring on the back side of this departing low, and will
persist tonight before gradually easing through the day
Saturday. Wind direction is currently NNE for ORD/MDW, but may
back to 350-ish for a time later tonight before shifting more
NNE and NE Saturday. Northeast winds should diminish quickly
Saturday evening as surface high pressure approaches.

Cloud wise, an extensive area was upstream across WI, and should
spread back into the terminals through early-mid evening. Bases
may initially be below 1500 ft, but will likely gradually rise
late tonight into Saturday morning before eventually scattering
to VFR by midday/early afternoon. Can`t rule out some brief IFR
bases along/over Lake Michigan which could affect KGYY later
this evening. Also may be a few isolated sprinkles along/north
of about the I-90 corridor from KRFD to KORD this evening,
though nothing which should be of any significance.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 PM CDT Saturday for the IL and IN
nearshore waters.

&&

$$

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