126
FXUS63 KLOT 251918
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
218 PM CDT FRI JUL 25 2025
KEY MESSAGES
- SCATTERED SHOWERS/EMBEDDED STORMS WITH LOCALLY HEAVY RAINFALL
AND GUSTY WIND THREATS CONTINUE THIS AFTERNOON. ISOLATED
INSTANCES OF FLASH FLOODING REMAIN POSSIBLE.
- ADDITIONAL HEAVY RAIN PRODUCING THUNDERSTORMS POSSIBLE THIS
EVENING AND OVERNIGHT. INITIALLY WEST OF THE FOX VALLEY AND
THEN POSSIBLY EXPANDING AREA-WIDE OVERNIGHT. FLASH FLOODING
WILL BE POSSIBLE, AND IF TRENDS CONTINUE, ADDITIONAL FLASH
FLOOD WATCHES MAY BE NEEDED.
- MONITORING A STRONG TO SEVERE WEATHER THREAT ON SATURDAY
AFTERNOON, MAINLY NEAR AND SOUTH OF I-80.
- DANGEROUS HEAT WILL BUILD BACK ACROSS THE AREA ON MONDAY
(POSSIBLY AS EARLY AS SUNDAY), WITH PERIODS OF THUNDERSTORMS
OVER OR JUST NORTH OF THE AREA MONDAY THROUGH WEDNESDAY.
DISCUSSION
ISSUED AT 218 PM CDT FRI JUL 25 2025
THROUGH SATURDAY:
COMPACT AREA OF SURFACE CONVERGENCE--LIKELY WITH SOME MID-LEVEL
FLOW ENHANCEMENT--CONTINUES ACROSS PARTS OF THE CHICAGO AREA
EARLY THIS AFTERNOON, EXTENDING WEST ROUGHLY MIDWAY BETWEEN I-88
AND I-80. GOING THROUGH THE REST OF THE AFTERNOON, IT SEEMS
LIKE THIS MAY END UP BEING THE PRIMARY FOCUS FOR ADDITIONAL
DEVELOPMENT, WITH PERSISTENT DEBRIS CLOUD COVER FARTHER TO THE
SOUTH HAVING STUNTED INSOLATION AND INSTABILITY. ADDITIONAL
SHOWERS/EMBEDDED STORMS COULD FIRE AND FESTER ALONG ANOTHER
ZONE OF ENHANCED SURFACE CONVERGENCE TIED TO AN INLAND-OOZING
FRONT/PSEUDO LAKE BREEZE IN THE VICINITY OF I-90. INDIVIDUAL
STORM MOTIONS WILL REMAIN VERY SLOW, LIKELY AROUND 10 MPH OR
LESS, AND PROPAGATIVE EFFECTS WILL ADDITIONALLY LEAD TO NEAR
NET-0 STORM MOTIONS AT TIMES. SIMILAR TO WHAT WE SAW EARLIER
TODAY ACROSS PARTS OF CHICAGO, HIGHLY LOCALIZED RAINFALL AMOUNTS
OVER 2-3 INCHES WILL BE POSSIBLE.
ANOTHER COMPACT MCV, CURRENTLY INCHING NORTHEAST OUT OF THE
OTTUMWA AREA IN SOUTHEAST IOWA LOOKS TO BE THE MAIN FORCING
MECHANISM FOR ADDITIONAL STORMS THIS EVENING AND OVERNIGHT. NOT
SEEING A TON OF FLOW ENHANCEMENT ON REGIONAL VWPS, BUT SUSPECT
THIS IS BECAUSE THIS FEATURE IS CURRENTLY IN BETWEEN RADAR
SITES. HIRES GUIDANCE WHICH TYPICALLY HANDLES THESE TYPES OF
FEATURES FAIRLY WELL SHOWS A SIGNIFICANT INCREASE IN THE LOW-
LEVEL MASS RESPONSE (IN THE 850 TO 700 MB LAYER MOST NOTABLY)
THIS EVENING AND OVERNIGHT AS THIS FEATURE CONTINUES NORTHEAST.
CURRENT MOTION OF THIS FEATURE SEEMS TO HAVE IT TRACKING MOSTLY
WEST OF OUR FORECAST AREA, BUT THE GUIDANCE WHICH HAS RESOLVED
IT INSISTS ON AN EASTERLY COMPONENT DEVELOPING THROUGH THE
EVENING, AND PERHAPS WE'RE SEEING SOME OF THAT IN RECENT
SATELLITE LOOPS. SHOULD THIS OCCUR, CONCERN EXISTS FOR ANOTHER
BATCH OF SHOWERS AND STORMS WITH TORRENTIAL RAINFALL RATES WHICH
WOULD CARRY WITH THEM A LOCALIZED FLASH FLOOD THREAT. SEEMS
LIKE THIS FEATURE WOULD HAVE SOME FORWARD/NORTHEAST MOTION TO
IT, SO IT REMAINS UNCLEAR HOW WIDESPREAD AND PROLONGED ANY
CONVECTIVE TRAINING WOULD BE. FOR THE TIME BEING, HAVE DECIDED
TO HOLD OFF ON AN EXPANSION/ISSUANCE OF A NEW FLOOD WATCH FOR
OUR WEST GIVEN THE AFOREMENTIONED UNCERTAINTIES IN FAVOR OF
HANDLING THESE HEAVY RAIN EPISODES SIMILAR TO TYPICAL CONVECTIVE
SEVERE WATCHES. HOWEVER, IF TRENDS CONTINUE, FLOOD WATCHES MAY
BECOME NECESSARY LATER THIS AFTERNOON/EARLY EVENING, INITIALLY
WEST OF THE FOX VALLEY, AND THEN PERHAPS FARTHER SOUTH IF WARM
ADVECTION-DRIVEN ACTIVITY DEVELOPS OVERNIGHT.
ON SATURDAY, SHOWERS WITH EMBEDDED THUNDERSTORMS WILL LIKELY BE
ONGOING DURING THE MORNING WITH SOME GRADUAL DOWNTICK IN
COVERAGE THROUGH THE MORNING. CONTINUED WAVES OF EMBEDDED
STORMS COULD LEAD TO ISOLATED STREAKS OF HEAVIER RAINFALL
AMOUNTS.
FORECAST EVOLUTION INTO THE AFTERNOON REMAINS SOMEWHAT UNCLEAR.
IT SEEMS LIKELY THAT PERSISTENT CLOUD COVER WILL HOLD
TEMPERATURES DOWN NORTH OF I-80, BUT THERE IS A SIGNAL FOR
CLEARING TO TAKE HOLD FARTHER TO THE SOUTH. WITH DEWPOINTS IN
THE LOW TO MID 70S, ANY ISOLATION WILL LEAD TO A QUICK INCREASE
IN INSTABILITY. GUIDANCE INDICATES MODERATE TO PERHAPS EVEN
STRONG INSTABILITY MATERIALIZING DURING THE AFTERNOON SOUTH OF
I-80 (AND INCREASING WITH SOUTHWARD EXTENT). WHILE SHEAR DOESN'T
LOOK PARTICULARLY HIGH, A VERY MODEST MID-LEVEL FLOW
ENHANCEMENT IS FORECAST TO YIELD PERHAPS 30-35 KTS OF DEEP LAYER
SHEAR, WITH SOME DEGREE OF HODOGRAPH CURVATURE IN THE LOW-
LEVELS. GIVEN THE MAGNITUDE OF INSTABILITY PROGGED TO BE IN
PLACE, SEMI-ORGANIZED MULTICELL/TRANSIENT SUPERCELLS WOULD BE
THE PRIMARY STORM MODE. CURRENT SPC LEVEL 1/5 SEVERE RISK
APPEARS REPRESENTATIVE OF THE CURRENT UNCERTAINTY/COVERAGE
COMBO, BUT WILL NEED TO KEEP AN EYE ON THIS GOING FORWARD.
CARLAW
SATURDAY NIGHT THROUGH FRIDAY:
WILL BE ISSUED SHORTLY.
AVIATION /18Z TAFS THROUGH 00Z SUNDAY/
ISSUED AT 105 PM CDT FRI JUL 25 2025
THE PRIMARY FORECAST CONCERN IS THUNDERSTORMS WITH BEST TIME
WINDOWS BEING THIS AFTERNOON, EARLY SATURDAY MORNING AND AGAIN
SATURDAY AFTERNOON/EVENING.
SCATTERED THUNDERSTORMS FORMED NORTH OF JOT TO MDW A FEW HOURS
AGO AND ARE SLOWLY SAGGING SOUTH AND THIS APPEARS TO BE THE
FAVORED AREAS FOR THUNDERSTORMS THROUGH MID AFTERNOON. BUT WITH
OUTFLOW BOUNDARIES AND A LAKE BREEZE BOUNDARY, ADDITIONAL
THUNDERSTORMS ARE POSSIBLE NORTH OF THIS CURRENT ACTIVITY, WHICH
WOULD INCLUDE ORD/DPA. OPTED TO INCLUDE TEMPO THUNDER MENTION
FOR ALL OF THE CHICAGO TERMINALS. ALL OF THIS ACTIVITY IS
EXPECTED TO DISSIPATE LATE THIS AFTERNOON.
ATTENTION THEN TURNS TO POSSIBLE CONVECTION OVERNIGHT INTO THE
DAYBREAK HOURS SATURDAY MORNING. A WAVE MOVING OUT OF MO IS
EXPECTED TO ALLOW ADDITIONAL THUNDERSTORMS TO DEVELOP THIS
EVENING SOUTHWEST OF THE AREA, WHICH WILL MOVE NORTHEAST
ACROSS NORTHERN IL OVERNIGHT. STILL SOME TIMING AND LOCATION
UNCERTAINTY, BUT INCREASING CONFIDENCE WARRANTS TEMPO MENTION
FOR THIS TIME PERIOD. ANOTHER PERIOD OF THUNDERSTORMS IS
EXPECTED SATURDAY AFTERNOON AND ADDED PROB MENTION FOR THE
ORD/MDW 30 HOUR TAFS. ANY OF THESE THUNDERSTORMS WILL BE CAPABLE
OF PRODUCING TORRENTIAL RAIN WHICH MAY REDUCE VIS/CIGS TO
IFR/LIFR.
MAINLY EASTERLY WINDS ARE EXPECTED AT THE CHICAGO TERMINALS
THIS AFTERNOON BECOMING LIGHT SOUTH/SOUTHEAST TONIGHT,
SOUTHWEST SATURDAY MORNING THEN BACK TO SOUTHEAST SATURDAY
AFTERNOON. HOWEVER, WIND DIRECTIONS WILL BE IMPACTED BY
THUNDERSTORMS/OUTFLOWS. WINDS OVERNIGHT MAY BECOME LIGHT AND
VARIABLE. CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...FLOOD WATCH UNTIL 6 PM CDT THIS EVENING FOR ILZ012-ILZ013-
ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...FLOOD WATCH UNTIL 6 PM CDT THIS EVENING FOR INZ001.
LM...NONE.
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