155
FXUS63 KLOT 260608
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
108 AM CDT SAT JUL 26 2025
KEY MESSAGES
- TORRENTIAL RAIN PRODUCING THUNDERSTORMS (WITH RATES GREATER
THAN 2 INCHES PER HOUR) ARE LIKELY LATE THIS EVENING AND
OVERNIGHT AND MAY LEAD TO FLASH FLOODING.
- THE FLASH FLOOD WATCH HAS BEEN EXPANDED TO INCLUDE ALL AREAS
ALONG AND NORTH OF I-80 OVERNIGHT THROUGH SATURDAY AFTERNOON
TO ACCOUNT FOR THIS POTENTIAL.
- STRONG TO SEVERE THUNDERSTORMS MAY DEVELOP SATURDAY AFTERNOON,
MAINLY NEAR AND SOUTH OF I-80 (SEVERE THREAT LEVEL 1 OF 5).
- DANGEROUS HEAT AND HUMIDITY RETURNS MONDAY (POSSIBLY AS EARLY
AS SUNDAY) FOLLOWED BY PERIODS OF THUNDERSTORMS TUESDAY INTO
WEDNESDAY.
MESOSCALE
ISSUED AT 612 PM CDT FRI JUL 25 2025
RECENT RADAR AND SATELLITE IMAGERY DEPICT A COMPACT MCV AND
ASSOCIATED BATCH OF SHOWERS AND STORMS CROSSING THE MISSISSIPPI
RIVER NEAR THE QUAD CITIES AT PRESS TIME. MEANWHILE, ANOTHER,
LARGER, MCV IS PRESENT IN NORTHEASTERN KANSAS. BOTH MCVS ARE
MOVING ALONG/WITHIN A BROAD FRONTAL ZONE DRAPED ACROSS THE
MIDDLE MISSISSIPPI RIVER VALLEY.
OVER THE NEXT 3 TO 6 HOURS, THE FLOW FROM 850 TO 700 MB IS
EXPECTED TO INCREASE MARKEDLY ACROSS CENTRAL AND NORTHERN
ILLINOIS IN RESPONSE TO THE COMBINED EFFECTS OF BOTH
APPROACHING MCVS. SUCH INCREASING FLOW ATOP THE STALLED FRONTAL
BOUNDARY (E.G., ISENTROPIC ASCENT) SHOULD LEAD TO A
CORRESPONDING INCREASE IN COVERAGE OF SHOWERS AND STORMS THIS
EVENING AND ESPECIALLY OVERNIGHT, PRIMARILY NEAR AND PERHAPS
NORTH OF I-80 (NOSE OF THE LOW-LEVEL JET). WITH PWATS OF 2.25 TO
LOCALLY 2.4" IN PLACE ALONG THE FRONTAL BOUNDARY, HOURLY RAIN
RATES OF 2 TO LOCALLY 4 INCHES PER HOUR ARE EXPECTED WHERE
THUNDERSTORMS ARE MOST PERSISTENT TONIGHT SUPPORTING A THREAT
FOR FLASH FLOODING.
OF PARTICULAR CONCERN IS A SCENARIO WHERE STORMS MATERIALIZE IN
A TRAINING FASHION OVER THE I-55 CORRIDOR WHERE 3 TO LOCALLY 6
INCHES OF RAIN HAS ALREADY FALLEN IN THE PAST 24 HOURS. IN SUCH
A SCENARIO, SIGNIFICANT FLASH FLOODING WOULD BE A DISTINCT
THREAT, INCLUDING SUBSTANTIAL FLOODING OF BASEMENTS, ROADWAYS,
AND FIELDS, AS WELL AS RAPID INCREASES IN RIVER LEVELS
(PARTICULARLY THE DUPAGE AND DES PLAINES RIVERS AND ASSOCIATED
TRIBUTARIES IN DUPAGE, COOK, AND WILL COUNTIES). THIS IS A VERY
CONDITIONAL SCENARIO, BUT ONE NOT TO IGNORE.
ALL THINGS CONSIDERED, OPTED TO EXPAND THE FLOOD WATCH FOR ALL
AREAS ALONG AND NORTH OF I-80. WITH ADDITIONAL ROUNDS OF STORMS
EXPECTED AFTER DAYBREAK TOMORROW, FELT THE COURSE OF LEAST
REGRET WAS TO ALSO EXTEND THE EXPIRATION TIME OF THE FLOOD WATCH
THROUGH TOMORROW EVENING. FOR ANYONE LIVING IN A FLOOD PRONE
AREA, ENSURE YOU HAVE MULTIPLE WAYS TO GET WARNING INFORMATION
INCLUDING AT LEAST ONE WAY TO WAKE YOU UP.
BORCHARDT
DISCUSSION
ISSUED AT 238 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:
TWO PRIMARY CONCERNS ARE HIGH TEMPS/HEAT INDICES SUNDAY AND
MONDAY AND THEN THUNDERSTORM CHANCES THROUGH MIDWEEK.
THUNDERSTORMS MAY BE ONGOING SATURDAY EVENING WITH A LINE MOVING
SOUTHEAST THROUGH THE CWA. SOME OF THESE WOULD HAVE THE
POTENTIAL TO BE STRONG TO LOCALLY SEVERE ALONG WITH TORRENTIAL
RAINFALL. AS TRENDS LOOK NOW, THIS ACTIVITY SHOULD BE MOVING
INTO THE SOUTHERN CWA BY THE OVERNIGHT HOURS AND POSSIBLY ENDING
OVERNIGHT/BY DAYBREAK SUNDAY. WHILE THERE REMAINS A CHANCE OF
THUNDERSTORMS SUNDAY, ESPECIALLY ACROSS CENTRAL IL AND CENTRAL
IN, SUNDAY MAY END UP MAINLY DRY FOR MOST OF THE AREA AND IF
THAT MATERIALIZES, HIGH TEMPERATURES WILL LIKELY REACH THE UPPER
80S TO AROUND 90 FOR MOST AREAS. COMBINED WITH DEWPOINTS IN THE
MID/UPPER 70S, PEAK HEAT INDICES IN THE 100-105 RANGE WOULD BE
POSSIBLE. IT APPEARS THERE WILL BE A LAKE BREEZE KEEPING THE
AREAS CLOSE TO LAKE MICHIGAN COOLER SUNDAY AFTERNOON.
CONFIDENCE FOR THUNDERSTORM TIMING/LOCATION COVERAGE DECREASES
SUNDAY NIGHT THROUGH MONDAY NIGHT. THERE WILL LIKELY BE
THUNDERSTORM COMPLEXES ROAMING ACROSS THE UPPER MIDWEST OR
WESTERN GREAT LAKES, WITH SOME POTENTIAL FOR THESE TO REACH AT
LEAST THE NORTHERN CWA. HOWEVER, THERE CURRENTLY IS NOT MUCH
GUIDANCE/ENSEMBLE SUPPORT FOR PRECIP INTO THE LOCAL AREA AND THE
BLENDED POPS ARE MAINLY DRY. IF THIS DRY FORECAST PANS OUT, THEN
HIGH TEMPS ON MONDAY WILL LIKELY BE IN THE LOWER/MID 90S WITH
DEWPOINTS BACK INTO THE UPPER 70S MAYBE AROUND 80. THIS WOULD
PUSH PEAK HEAT INDICES INTO THE 105-110 RANGE. IF THESE
CONDITIONS APPEAR MORE LIKELY AS THIS TIME PERIOD APPROACHES,
SOME HEAT HEADLINE MAY BE NEEDED.
THUNDERSTORM CHANCES WILL INCREASE TUESDAY INTO WEDNESDAY AS A
COLD FRONT MOVES SOUTH ACROSS THE GREAT LAKES REGION. GIVEN THE
EXPECTED STRENGTHEN OF THIS FRONT, A FASTER ARRIVAL INTO
NORTHERN IL TUESDAY AFTERNOON/EVENING SEEMS PLAUSIBLE. WHETHER
THE FRONT SLOWS/STALLS ACROSS THE AREA TUESDAY NIGHT/WEDNESDAY
IS UNCLEAR. THUNDERSTORMS WILL BE POSSIBLE DURING THIS TIME,
POSSIBLY A FEW ROUNDS WITH PERHAPS THE MAIN ROUND ON WEDNESDAY.
STRONG/SEVERE STORMS AND TORRENTIAL RAIN/LOCALIZED FLOODING
WOULD BE POSSIBLE.
NORTHERLY WINDS THEN LOOK TO INCREASE INTO THE 15-25 MPH RANGE
OVER THE LAKE WEDNESDAY INTO WEDNESDAY NIGHT, WHICH WOULD LIKELY
LEAD TO HIGH WAVES AND DANGEROUS SWIMMING CONDITIONS FOR LAKE
MICHIGAN BEACHES.
MUCH COOLER AND LESS HUMID CONDITIONS ARE THEN EXPECTED FOR LATE
WEEK. CMS
AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
ISSUED AT 108 AM CDT SAT JUL 26 2025
FORECAST CONCERNS FOR THE TERMINALS...
- WAVES OF SHOWERS AND THUNDERSTORMS TONIGHT THROUGH SATURDAY
EVENING.
- PERIODS OF MVFR CEILINGS WITH SHOWERS AND STORMS, OTHERWISE
VFR CONDITIONS.
THE FORECAST THROUGH SATURDAY EVENING WILL BE QUITE MESSY AS
SEVERAL DISTURBANCES MOVE THROUGH RESULTING IN OFF AND ON
PERIODS OF SHOWERS AND THUNDERSTORMS. THE FIRST DISTURBANCE WILL
CONTINUE TO MOVE OVERHEAD THROUGH 07-08Z WHICH WILL MAINTAIN
SCATTERED SHOWERS OVER THE TAF SITES WITH MORE ORGANIZED SHOWERS
AND STORMS ALONG AND SOUTH OF A PNT TO VPZ LINE. AS THIS FIRST
WAVE EXITS A BRIEF LULL SHOULD MATERIALIZE AS SOME SINKING AIR
BEHIND THE SYSTEM LIMITS THE FORCING FOR REDEVELOPMENT, THOUGH
SOME ISOLATED SHOWERS MAY FESTER THROUGH THE NIGHT DUE TO THE
HUMID AIR MASS.
THE SECOND DISTURBANCE IS EXPECTED TO PIVOT INTO THE AREA
TOWARDS DAYBREAK BRINING WITH IT ANOTHER ROUND OF SHOWERS AND
THUNDERSTORMS. THIS DISTURBANCE IS CURRENTLY (AS OF 557Z)
TRACKING ACROSS NORTHERN MO WITH JUST SOME ISOLATED SHOWERS
ONGOING, BUT AS WE NEAR DAYBREAK THE INCREASING INSTABILITY
SHOULD ALLOW FOR THE COVERAGE OF SHOWERS AND STORMS TO INCREASE
ESPECIALLY AT THE CHICAGO AREA TERMINALS AND POINTS SOUTH. ONCE
THE SHOWERS/STORMS ARRIVE THEY SHOULD PERSIST THROUGH MID-
MORNING BEFORE EXITING AND LEAVING US WITH ANOTHER PERIOD OF OFF
AND ON PASSING SHOWERS THROUGH THE MORNING.
THE THIRD AND FINAL DISTURBANCE (CURRENTLY OVER EASTERN KS) IS
EXPECTED TO PIVOT INTO THE AREA THIS AFTERNOON WHICH WILL ONCE
AGAIN LEAD TO THE DEVELOPMENT OF SHOWERS AND THUNDERSTORMS
MAINLY ALONG AND SOUTH OF A VYS TO VPZ LINE. HOWEVER, SOME
ISOLATED TO WIDELY SCATTERED SHOWERS AND STORMS MAY STILL PASS
OVER THE CHICAGO AREA TERMINALS SATURDAY AFTERNOON (MAINLY AFTER
19Z). STORMS THAT DEVELOP WITH THIS THIRD WAVE WILL HAVE THE
POTENTIAL TO BECOME STRONG TO SEVERE WITH A THREAT FOR DAMAGING
WINDS (IN EXCESS OF 40KTS) AND HEAVY RAINFALL WHICH MAY RESULT
IN LOCALIZED FLOODING. SHOWERS AND STORMS WILL FINALLY BEGIN TO
EXIT THE AREA AFTER 00Z SATURDAY EVENING WITH DRY WEATHER
EXPECTED TO CLOSE OUT THE TAF PERIOD.
OUTSIDE OF THE RAIN, VFR CONDITIONS CAN GENERALLY BE EXPECTED
THOUGH PERIODS OF MVFR CEILINGS WILL BE POSSIBLE WITH ANY
SHOWERS AND/OR STORMS. LIGHT SOUTHEAST WINDS TONIGHT WILL BECOME
SOUTHWESTERLY ON SATURDAY AND INCREASE INTO THE 8-10 KT RANGE
THROUGH THE AFTERNOON. WHILE SPEEDS WILL EASE SATURDAY EVENING,
DIRECTIONS WILL REMAIN SOUTH-SOUTHWEST THROUGH THE END OF THE
PERIOD.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...FLOOD WATCH THROUGH THIS EVENING FOR ILZ003-ILZ004-ILZ005-
ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...FLOOD WATCH THROUGH THIS EVENING FOR INZ001.
LM...NONE.
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