419
FXUS63 KLOT 281017
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
517 AM CDT SAT JUN 28 2025
KEY MESSAGES
- AFTER A BIT OF A REPRIEVE FROM HEAT AND HUMIDITY TODAY
(ESPECIALLY NEAR THE LAKE), ANOTHER HOT AND HUMID DAY IS
EXPECTED SUNDAY.
- THREATS FOR MAINLY AFTERNOON/EVENING STORMS RETURNS SUNDAY
INTO MONDAY.
UPDATE
ISSUED AT 517 AM CDT SAT JUN 28 2025
UPSTREAM RADAR MOSAICS DEPICT A WELL-DEFINED MCV ALONG THE
MINNESOTA/IOWA BORDER ASSOCIATED WITH REMNANT CONVECTION THAT
ORIGINATED WEST OF SIOUX FALLS OVERNIGHT, WITH ANOTHER POSSIBLE
MCV FARTHER NORTH OF THE TWIN CITIES. GUIDANCE TO VARYING
DEGREES HAS PICKED UP ON THESE FEATURES WITH A NOTABLE 700 MB
WIND MAX ADVERTISED ACROSS NORTHERN IOWA. THE AIRMASS AHEAD OF
THESE FEATURES ACROSS CENTRAL/NORTHEAST IOWA IS FORECAST TO
DESTABILIZE NOTABLY THIS MORNING AS 70S DEWPOINTS RETURN
NORTHWARD, ALTHOUGH IT'S UNCLEAR IF CONVECTIVE INHIBITION WILL
DIMINISH PRIOR TO THE PASSAGE OF THE MAIN LOBE OF ASCENT. AS
SUCH, CAM GUIDANCE REMAINS MIXED AS TO WHETHER ROBUST CONVECTION
WILL MANAGE TO RE-DEVELOP LATER THIS MORNING AND AFTERNOON
UPSTREAM. AS NOTED IN THE OVERNIGHT AFD, A SUBSTANTIAL GRADIENT
IN MUCAPE WILL BE IN PLACE IN OUR LOCAL VICINITY, WITH
INSTABILITY DROPPING OFF QUICKLY WITH SOUTH AND EAST EXTENT INTO
ILLINOIS ALONG WITH INCREASED INHIBITION AND GENERALLY
DIMINISHED DEEP LAYER SHEAR.
IF A RENEWED COMPLEX GETS GOING TO OUR NORTHWEST, AND IT'S
AGAIN UNCLEAR IF THIS WILL OCCUR, THE INCREASINGLY HOSTILE
ENVIRONMENT FOR MCS MAINTENANCE LOCALLY SUGGESTS CONVECTION
SHOULD WEAKEN AND/OR HAVE A PROPENSITY TO TURN DUE SOUTH TOWARDS
THE MISSISSIPPI RIVER. STILL, THE CLOSE PROXIMITY OF THESE
FEATURES SUGGESTS A GUSTY OUTFLOW THREAT CAN'T ENTIRELY BE RULED
OUT ACROSS PERHAPS THE WESTERN THIRD OF THE CWA THIS AFTERNOON
GIVEN THE MAGNITUDE OF THE DCAPE RESERVOIR, CONDITIONAL ON MCS
DEVELOPMENT. IT'S ENTIRELY POSSIBLE NOTHING TRANSPIRES, BUT
SOMETHING FOR THE DAYSHIFT TO KEEP AN EYE ON FOR LATER THIS
MORNING. NO CHANGES TO THE FORECAST AT THIS TIME.
CARLAW
DISCUSSION
ISSUED AT 241 AM CDT SAT JUN 28 2025
A MODEST COLD FRONT HAS PUSHED THROUGH ALMOST ALL OF THE REGION
AND HAS SENT DEWPOINTS "TUMBLING" INTO THE LOW TO MID 60S NORTH
OF THE KANKAKEE RIVER. EVEN WITH THE DRIER AIRMASS IN PLACE,
LIGHT WINDS, CLEAR SKIES, AND A NEAR-SURFACE HYDROLAPSE MAY
PROMOTE SOME PATCHY, SHALLOW FOG WEST OF THE FOX VALLEY THROUGH
MID MORNING. HIGHS TODAY WILL ACTUALLY LIKELY BE NEAR OR EVEN A
FEW DEGREES WARMER THAN YESTERDAY, EXCEPT NEAR THE LAKE WHERE A
ROBUST LATE MORNING/AFTERNOON LAKE BREEZE WILL HOLD TEMPERATURES
IN THE MID 70S.
AN EARLIER SEVERE, BUT COMPACT, MCS ACROSS SE SOUTH DAKOTA HAS
EXHAUSTED OUTFLOW AND WEAKENED CONSIDERABLY. LATEST HIGH-
RESOLUTION GUIDANCE, TO VARYING DEGREES, SUGGESTS AN ASSOCIATED
MCV COULD ENCOURAGE RENEWED CONVECTIVE DEVELOPMENT LATER THIS
MORNING AS THE ASSOCIATED VORTICITY MAXIMA ENCOUNTERS A
DESTABILIZING ENVIRONMENT ACROSS NORTHERN IOWA. SHOULD AN
ADDITIONAL MCS MATERIALIZE, A NOTABLE INSTABILITY GRADIENT AND
SHARPLY-DIMINISHING DEEP LAYER SHEAR WITH SOUTHWARD EXTENT
SUGGESTS CONVECTION SHOULD WEAKEN PRIOR TO ARRIVAL INTO OUR
AREA, BUT SOMETHING WE'LL KEEP AN EYE ON FOR LATER THIS
AFTERNOON.
HIGH PRESSURE WILL BEGIN TO SCOOT EAST OF THE AREA ON SUNDAY,
WHICH WILL OPEN US BACK UP TO WARM/MOIST SOUTHERLY LOW-LEVEL
TRAJECTORIES. THERE'S A FAIR AMOUNT OF SPREAD IN GUIDANCE
REGARDING THE HANDLING OF DEWPOINT TRENDS DURING THE AFTERNOON,
WITH THE TYPICAL DEEPER MIXERS (HRRR, RAP, HIRESARW) TRADING OFF
LOWER PM DEWPOINTS FOR WARMER TEMPERATURES. WITH 925 MB
TEMPERATURE CLIMATOLOGY SUGGESTING SOME MID 90S COULD BE IN
PLAY, ELECTED TO BOOST HIGHS A BIT ON SUNDAY AND KNOCK A FEW
DEGREES OFF THE DEWPOINTS. THIS STILL RESULTS IN PEAK HEAT
INDICES NEAR/LOCALLY ABOVE 100 DEGREES, EXCEPT PERHAPS RIGHT AT
THE LAKE ON THE ILLINOIS SIDE AS WINDS MAY END UP BACKING JUST
ENOUGH TO DELIVER SLIGHTLY COOLER TEMPERATURES TO IMMEDIATE
LAKESIDE LOCALES.
THE ENVIRONMENT ON SUNDAY IS FORECAST TO BECOME MODERATELY
UNSTABLE, WITH ANY CAPPING ERODING BY LATE-MORNING/MIDDAY. LARGE
SCALE FORCING REMAINS NEBULOUS, BUT A NEARBY LOW-AMPLITUDE
TROUGH AXIS, ANY NUMBER OF LINGERING VORT MAXIMA, AND SURFACE
CONVERGENCE NEAR THE LAKE COULD SERVE AS INITIATING MECHANISMS
FOR ISOLATED TO PERHAPS WIDELY SCATTERED CONVECTION. GIVEN THIS,
DID NOT FEEL COMFORTABLE WITH A PRECIP-FREE FORECAST, EVEN GIVEN
NO QPF FROM OVERNIGHT CAMS AND MOST GLOBAL GUIDANCE. DID,
HOWEVER, TRIM POPS BACK A BIT THOUGH, WITH MOST LOCATIONS LIKELY
TO REMAIN DRY THROUGH THE DAY. IF STORMS ARE ABLE TO DEVELOP AND
SUSTAIN, A MICROBURST THREAT WOULD BE IN PLAY GIVEN THE HOT
CONDITIONS AND DCAPE VALUES IN EXCESS OF 1000 J/KG.
ADDITIONAL CONVECTION MAY DEVELOP UPSTREAM SUNDAY EVENING AS A
MORE ROBUST TROUGH AXIS SWINGS ACROSS MINNESOTA, WITH SOME
POTENTIAL FOR THIS TO SCRAPE PORTIONS OF THE FORECAST AREA
OVERNIGHT. DEPENDING ON HOW THINGS PLAY OUT THROUGH MONDAY
MORNING, A SLIGHTLY BETTER CHANCE FOR MORE ORGANIZED STRONG-
SEVERE CONVECTION MAY MATERIALIZE ON MONDAY AS THE
AFOREMENTIONED TROUGH PRESSES INTO WISCONSIN AND EASTERN IOWA
AND AN ASSOCIATED COLD FRONT SLIPS THROUGH THE REGION.
A BRIEFLY COOLER AND LESS HUMID AIRMASS WILL RETURN ON TUESDAY
AND WEDNESDAY, BUT MOISTURE WILL START TO PILE BACK INTO THE
AREA THROUGH THE END OF THE WEEK AND UPCOMING JULY 4TH HOLIDAY
WEEKEND. MEDIUM RANGE GUIDANCE AND ENSEMBLES SUGGEST THAT AT
LEAST PERIODIC SHOWER AND THUNDERSTORM CHANCES MAY RETURN TO
THE REGION, PARTICULARLY OVER THE WEEKEND.
CARLAW
AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
ISSUED AT 1136 PM CDT FRI JUN 27 2025
ONLY ITEM OF NOTE:
- TIMING OF STRONGER (NEAR 10 KT) LAKE INFLUENCED WINDS AT ORD,
MDW, AND GYY ON SATURDAY AFTERNOON.
AN OVERALL WEAK PRESSURE GRADIENT WITH HIGH PRESSURE BUILDING
OVER LAKE MICHIGAN EARLY SATURDAY AFTERNOON WILL RESULT IN
LIGHT WINDS THROUGH THE PERIOD, EXCEPT FOR LAKE BREEZE ENHANCED
EAST AND NORTHEAST WINDS AT THE NEAR LAKE TERMINALS. CONFIDENCE
IS HIGH IN SPEEDS WELL UNDER 10 KT PRIOR TO THE LAKE BREEZE
PASSAGE, DESPITE GUIDANCE VARIANCE IN WIND DIRECTION SPECIFICS.
EXPECTING THE LAKE BREEZE TO PUSH THROUGH GYY IN THE LATE
MORNING AND ORD AND MDW IN THE EARLY AFTERNOON, THOUGH SOME
GUIDANCE HAS A BIT LATER TIMING THAN INDICATED IN THE TAFS.
WINDS WILL BECOME LIGHT SOUTHEAST AFTER SUNSET SATURDAY EVENING.
VFR CONDITIONS WILL PREVAIL THROUGH THE CURRENT TAF CYCLE.
CASTRO
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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