776
FXUS63 KLOT 310600
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
100 AM CDT THU JUL 31 2025
KEY MESSAGES
- PERIODS OF RAIN ARE EXPECTED THIS EVENING AND OVERNIGHT AS A
SLOW-MOVING WEATHER SYSTEM MOVES OVERHEAD, SOME WITH LOCALLY
TORRENTIAL RAINFALL. FLOOD WATCH REMAINS IN EFFECT FOR AREAS
NEAR AND SOUTH OF I-80 THROUGH TONIGHT.
- HIGH WAVES AND RIP CURRENTS WILL LEAD TO A HIGH SWIM RISKS AT
ALL LAKE MICHIGAN BEACHES THROUGH AT LEAST FRIDAY AFTERNOON.
- COOLER TEMPERATURES AND MORE COMFORTABLE HUMIDITY LEVELS WILL
ARRIVE TOMORROW AND LAST THROUGH THE WEEKEND.
UPDATE
ISSUED AT 652 PM CDT WED JUL 30 2025
EARLIER WAVE OF HEAVY RAIN-PRODUCING SHOWERS AND THUNDERSTORMS
HAS MOVED EAST OF THE REGION. CURRENTLY, IN A BIT OF A LOCAL
ACTIVITY MINIMA, BUT WE'RE KEEPING A CLOSE EYE ON A FEW FEATURES.
AT THE SURFACE, A COMPOSITE OUTFLOW BOUNDARY/COLD FRONT HAS
SLIPPED SOUTH OF ABOUT THE I-80 CORRIDOR, WITH BREEZY NORTH TO
NORTHEASTERLY WINDS IN ITS WAKE. THIS FRONTAL ZONE SEEMS LIKE IT
CONNECTS UP TO PERHAPS A DIFFUSE AND LOCALIZED SURFACE LOW IN THE
VICINITY OF GALESBURG/PEORIA EMBEDDED WITHIN A BROADER INVERTED
TROUGH AXIS. WHILE THE AFOREMENTIONED FRONTS SOUTHWARD PROGRESSION
AND EARLIER CONVECTION HAVE BOTH WORKED TO DIMINISH OVERALL
INSTABILITY, IT'S CERTAINLY NOT GONE ENTIRELY. LATEST RAP/SPC
MESOANALYSIS DEPICTS A FEW HUNDRED J/KG OF MLCAPE NEAR AND SOUTH
OF THE FRONTAL ZONE, WITH NEARLY 1000 J/KG OF MUCAPE ANALYZED
ACROSS SOUTHERN LA SALLE-LIVINGSTON-FORD COUNTIES.
IN THIS VICINITY ACROSS THE SOUTHWEST QUARTER OF OUR FORECAST AREA,
VWPS, SURFACE OBSERVATIONS, AND LATEST RAP GUIDANCE INDICATE
STRONG VEERING OF THE LOW-MID LEVEL WIND FIELD, FROM EASTERLY
AT THE SURFACE TO WESTERLY AT 700-500 MB, INDICATIVE OF
PERSISTENT, BROAD WARM ADVECTION. ADDITIONALLY, A MODEST
SOUTHWESTERLY 700 MB JET ENHANCEMENT IS ALSO EVIDENT IN RECENT
MODEL ANALYSES, WHICH ESSENTIALLY IMPINGES UPON THE PREVIOUSLY-
MENTIONED SURFACE FRONTAL BOUNDARY. WITH PWATS STILL NORTH OF 2
INCHES IN THIS AREA, THERE'S SOME CONCERN THAT ADDITIONAL
SHOWERS WITH TORRENTIAL RAINFALL MAY CONTINUE TO DEVELOP THIS
EVENING AS DIVERGENCE WITHIN THE RIGHT ENTRANCE REGION OF A
QUASI-ZONAL JET STREAK ACROSS NORTHERN WI/MI SLOWLY INTENSIFIES.
CLOUD-BEARING FLOW IS EXCEPTIONALLY SLOW ACROSS PARTS OF OUR
AREA, WHERE SURFACE EASTERLIES BECOME WESTERLY IN THE MID
LEVELS, YIELDING NEARLY STATIONARY CELL MOTIONS. NEARLY MOIST
ADIABATIC PROFILES WILL PROBABLY YIELD LITTLE IN THE WAY OF
ADDITIONAL LIGHTNING ACTIVITY, BUT DEEP WARM CLOUD DEPTHS
(NEARLY 15 KFT) WILL FACILITATE CLASSIC LOW ECHO CENTROID
SHOWERS WITH DECEPTIVELY HIGH RAIN RATES. HAVE ALREADY SEEN THIS
WITH ONGOING SHOWERS ACROSS PARTS OF BUREAU COUNTY WHERE
30-MINUTE RAINFALL RATES HAVE LOCALLY EXCEEDED 1.5 INCHES.
RECENT HRRR RUNS APPEAR TO HAVE CAUGHT ONTO THINGS FAIRLY WELL,
AND CONSISTENTLY DEPICT AN AXIS OF HEAVY RAINFALL DEVELOPING
ACROSS PARTS OF LA SALLE, LIVINGSTON, AND FORD COUNTIES THIS
EVENING. GIVEN EVERYTHING MENTIONED ABOVE, DID NOT FEEL
COMFORTABLE LETTING THE INHERITED FLOOD WATCH GO AT 7 PM, SO
DID EXTEND THIS THROUGH LATE TONIGHT AND EXPANDED IT WESTWARD.
GREATEST CONCERN THROUGH MID-EVENING IS ACROSS THAT LA SALLE TO
FORD COUNTY AXIS, BUT ADDITIONAL EMBEDDED HEAVY SHOWERS MAY
BLOSSOM EASTWARD LATE THIS EVENING AND OVERNIGHT. HAVE ALSO KEPT
THE I-80 CORRIDOR COUNTIES IN THIS EXTENDED WATCH FOR THIS
REASON. CAN'T ENTIRELY RULE OUT HAVING TO MAKE ANOTHER NORTHWARD
EXPANSION TO THE WATCH, BUT AT THIS TIME, IT SEEMS LIKE THE
EFFECTIVE BOUNDARIES MAY SAG FAR ENOUGH SOUTH TO KEEP THE MAIN
THREAT FOR ADDITIONAL HEAVY RAIN SOUTH OF CHICAGO.
CARLAW
DISCUSSION
ISSUED AT 226 PM CDT WED JUL 30 2025
THROUGH THURSDAY:
RECENT RADAR IMAGERY DEPICTS SHOWERS AND STORMS MOVING EASTWARD
ACROSS THE AREA AT PRESS TIME AHEAD OF AN MCV/SHORTWAVE MOVING
EASTWARD ACROSS THE MISSISSIPPI RIVER. MEANWHILE, A SECOND AREA
OF SHOWERS AND EMBEDDED THUNDERSTORMS STRETCHES FROM FAR
NORTHERN IOWA EASTWARD THROUGH MUCH OF SOUTHERN WISCONSIN ALONG
THE NORTHERN FLANK OF THE MCV AND ALONG A BROAD FRONTAL ZONE
DRIFTING SOUTHWARD THROUGH THE GREAT LAKES. BETWEEN BOTH
FEATURES, RELATIVELY TRANQUIL CONDITIONS RESIDE WITH BROKEN
CLOUD COVER AND TEMPERATURES IN THE MID 70S.
OVER THE COMING HOURS, THE EXPECTATION IS FOR THE MAIN AXIS OF
SHOWERS AND STORMS TO SHIFT EASTWARD OUT OF OUR AREA ONLY TO BE
REPLACED BY THE SECONDARY AREA OF SHOWERS AND EMBEDDED STORMS
DROPPING SOUTHWARD OUT OF WISCONSIN. A BROAD AREA OF UPPER-
LEVEL DIFFLUENCE IN THE RIGHT ENTRANCE REGION OF A PASSING JET
STREAK IN TANDEM WITH PWATS NEAR 2" SHOULD AFFORD SURPRISINGLY
EFFICIENT RAIN RATES THIS EVENING PARTICULARLY WHERE POCKETS OF
INSTABILITY STILL RESIDE. WITH THAT SAID, THE SIGNAL FOR
WIDESPREAD, TORRENTIAL RAINFALL TONIGHT IS MUCH LOWER THAN IT
HAD LOOKED 12 HOURS AGO (LIKELY AS A RESULT TO A DIFFERENT
EVOLUTION OF CONVECTION TODAY THAN PERHAPS EXPECTED). AS A
RESULT, WILL GO AHEAD AND TRIM THE FLOOD WATCH WHERE RAIN HAS
ALREADY PASSED AND LET THE EVENING SHIFT REMOVE THE REST LATER
TODAY. NOTE THERE MAY STILL BE A VERY LOW-END THREAT ON A VERY
LOCALIZED BASIS (CALL IT A 5% CHANCE FOR FLOODING AT ANY GIVEN
LOCATION). MEANWHILE, INCREASING NORTH TO NORTHEASTERLY FLOW OFF
THE LAKE WILL LEAD TO BUILDING WAVES ON LAKE MICHIGAN,
RESULTING IN A HIGH SWIM RISK THIS EVENING ONWARD AT ALL LAKE
MICHIGAN BEACHES.
TOMORROW, THE BACK EDGE OF THE SHORTWAVE WILL SHIFT EASTWARD
CAUSING SHOWERS TO TAPER FROM NORTHEAST TO SOUTHEAST. SHOWERS OR
SPRINKLES MAY LAST THE LONGEST AT LAKESHORE LOCATIONS WHERE
ONSHORE FLOW AND TUMBLING 850MB TEMPERATURES MAY AFFORD MODEST
LAKE INSTABILITY BEFORE DRY AIR RUSHES INTO THE REGION. OUTSIDE
TAPERING RAIN CHANCES, TOMORROW WILL FEEL QUITE DIFFERENT
COMPARED TO THE PAST WEEK WITH HIGHS ONLY IN THE LOW TO MID 70S
AND MUCH LOWER HUMIDITY LEVELS. CONTINUED NORTH TO NORTHEASTERLY
ONSHORE WINDS WILL MAINTAIN A HIGH SWIM RISK AT ALL LAKE
MICHIGAN BEACHES. FINALLY, HAVE INTRODUCED NEAR-SURFACE SMOKE TO
THE FORECAST OWING TO UPSTREAM OBSERVATIONS OF 2-5 MILE
VISIBILITIES ACROSS THE UPPER GREAT LAKES (THAT AIRMASS WILL
COME OUR WAY BEHIND THE FRONT TOMORROW).
BORCHARDT
THURSDAY NIGHT THROUGH WEDNESDAY:
SURFACE HIGH PRESSURE WILL REMAIN STATIONED OVER THE GREAT
LAKES FRIDAY THROUGH THE WEEKEND. ABUNDANT SUNSHINE CAN THUS BE
EXPECTED ACROSS THE REGION, THOUGH IT'S POSSIBLE THAT LINGERING
WILDFIRE SMOKE ALOFT COULD CONTINUE TO GIVE THE SKY A MILKY LOOK
AT TIMES. ANTICYCLONIC FLOW AROUND THE SURFACE HIGH WILL
REINFORCE THE COOLER AND DRIER AIR MASS OVER THE REGION, LEADING
TO CONTINUED BELOW TO NEAR-NORMAL TEMPERATURES (HIGHS IN THE
70S TO LOW-MID 80S AND LOWS IN 50S TO MID 60S) AND COMFORTABLE
HUMIDITY LEVELS. SYNOPTIC CONDITIONS FAVOR LITTLE TO NO
PRECIPITATION IN OUR AREA THROUGH THE WEEKEND. HOWEVER, THE
UNSEASONABLY COOL LOW-LEVEL AIR (850 MB TEMPERATURES IN THE
BOTTOM 10% OF REGIONAL SOUNDING CLIMATOLOGY) SPREADING OVER A
MILD LAKE MICHIGAN WITH WATER TEMPERATURES STILL IN THE 70S
COULD SUPPORT A CONDITIONAL POTENTIAL FOR ISOLATED LAKE-INDUCED
SHOWERS OR DRIZZLE THURSDAY NIGHT INTO FRIDAY, PENDING THE
PRESENCE OF SUFFICIENT LOW-LEVEL MOISTURE AND THE DEVELOPMENT OF
ANY LOW-LEVEL CONVERGENCE ZONES. BOOSTED POPS A BIT FOR THIS
TIME FRAME TO ACCOUNT FOR THIS POSSIBILITY, BUT HAVE MAINTAINED
A DRY FORECAST FOR NOW GIVEN THE LINGERING UNCERTAINTIES AND
ONLY MARGINAL LAKE EFFECT PARAMETERS IN PLAY.
MEDIUM RANGE GUIDANCE CONTINUES TO INDICATE THAT THE GREAT
LAKES SURFACE HIGH WILL DEPART GOING INTO EARLY NEXT WEEK,
OPENING THE DOOR FOR WARMER AND MORE HUMID CONDITIONS TO RETURN,
ALONG WITH OUR NEXT CHANCES FOR PRECIPITATION AS UPPER-LEVEL
SHORTWAVE DISTURBANCES START TO GET FUNNELED BACK INTO THE
REGION. HOWEVER, THERE IS STILL A FAIR AMOUNT OF SPREAD AMONGST
THE LATEST ENSEMBLE OUTPUT REGARDING HOW QUICKLY THIS WILL
OCCUR AND JUST HOW WARM IT MIGHT GET NEXT WEEK, ALONG WITH THE
TIMING OF ANY POTENTIAL SHOWERS OR STORMS NEXT WEEK. THIS
SPREAD APPEARS TO BE TIED TO THE STRENGTH AND TRAJECTORY OF A
REINFORCING UPPER-LEVEL TROUGH DIVING INTO THE GREAT LAKES
AND/OR NORTHEASTERN CONUS FROM CANADA, WHICH WILL INFLUENCE THE
EXTENT TO WHICH AN UPPER-LEVEL RIDGE OVER THE SOUTHWESTERN
CONUS IS IMPEDED AS IT ATTEMPTS TO SLIDE EASTWARD. REGARDLESS,
THE OVERALL TREND SHOULD STILL BE TOWARDS WARMER AND MORE HUMID
CONDITIONS NEXT WEEK, AND THIS TREND IS REFLECTED IN THE LATEST
NBM OUTPUT. THE NBM'S POPS, HOWEVER, ARE LIKELY TOO BROAD-
BRUSHED AS A RESULT OF THE ENSEMBLE SPREAD, AND WILL LIKELY NEED
REFINEMENT AS TRENDS EMERGE IN ENSEMBLE GUIDANCE.
OGOREK
AVIATION /06Z TAFS THROUGH 12Z FRIDAY/
ISSUED AT 100 AM CDT THU JUL 31 2025
FORECAST CONCERNS FOR THE TERMINALS...
- LINGERING SHOWERS OVERNIGHT MAINLY SOUTH OF A VYS TO VPZ LINE.
- MVFR CEILINGS TO PERSIST THROUGH DAYBREAK, MAINLY AT THE
CHICAGO AREA TERMINALS, WITH POCKETS OF IFR POSSIBLE AT TIMES
ESPECIALLY WITH ANY RAIN.
- BREEZY NORTHEAST WINDS TO PERSIST THROUGH THURSDAY AFTERNOON.
A MID-LEVEL CIRCULATION AND ITS ASSOCIATED FRONTAL ZONE
CONTINUE TO DEVELOP SHOWERS ACROSS PORTIONS OF NORTHEAST IL AND
NORTHWEST IN. WHILE THESE SHOWERS ARE EXPECTED TO GRADUALLY
PIVOT SOUTHWARD OVERNIGHT, SOME REDEVELOPMENT AS FAR NORTH AS
I-80 (ROUGHLY A VYS TO VPZ LINE) IS POSSIBLE WHICH COULD PROLONG
SHOWERS AT GYY. GIVEN THAT THIS REDEVELOPMENT IS LOWER
CONFIDENCE HAVE OPTED TO MAINTAIN A DRY FORECAST AT GYY (AND
EVERYWHERE ELSE) AFTER 08Z WHEN THE CURRENT SHOWERS MOVE OUT.
THAT SAID, THE LINGERING HUMIDITY WILL MAKE ANY SHOWERS VERY
EFFICIENT RAIN PRODUCERS WHICH COULD RESULT IN INSTANCES OF
FLOODING IN ADDITION TO REDUCED VISIBILITIES. REGARDLESS,
SHOWERS ARE EXPECTED TO CONCLUDE ACROSS THE AREA BY MID-MORNING
AREAWIDE.
AS SHOWERS TAPER OVERNIGHT, THE MAIN CONCERN WILL TURN TO THE
MVFR CEILINGS WHICH ARE EXPECTED TO PERSIST THROUGH DAYBREAK
THURSDAY MORNING BEFORE SCATTERING OUT. THOUGH, RECENT SATELLITE
TRENDS DO SHOW THAT THE BULK OF THE MVFR MAY REMAIN MOSTLY OVER
THE CHICAGO AREA TERMINALS AND POINTS SOUTH. FURTHERMORE, SOME
IFR CEILINGS ARE STILL BE SEEN IN PORTIONS OF THE AREA BUT THESE
LOOK TO BE MAINLY TIED TO THE AFOREMENTIONED SHOWERS. SO EXPECT
ANY IFR TO REMAIN MORE SPOTTY AND TO ERODE AS SHOWERS TAPER.
OTHERWISE, EXPECT BREEZY NORTHEAST WINDS TO PREVAIL THROUGH
THURSDAY AFTERNOON WITH GUSTS IN THE 20-25 KT RANGE BEFORE
SUBSIDING TO AROUND 10 KTS THURSDAY EVENING AND NIGHT.
FINALLY, THERE IS A LOW CHANCE (<20%) THAT SOME ISOLATED SHOWERS
REDEVELOP THURSDAY NIGHT ALONG A LAND BREEZE BOUNDARY. WHILE THE
IS A FAIR AMOUNT OF LAKE INDUCED INSTABILITY, UNCERTAINTY
REMAINS IF SUFFICIENT SATURATION WILL OCCUR TO ACTUALLY GENERATE
SHOWERS. THAT SAID, IF SHOWERS DEVELOP THEY WOULD MAINLY IMPACT
ORD, MDW, AND GYY AND SHOULD TAPER BY 12Z FRIDAY. GIVEN THE
UNCERTAINTY HAVE DECIDED TO MAINTAIN DRY TAFS FOR NOW BUT DID
INTRODUCE A SCT050 MENTION TO TEASE THIS POTENTIAL.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...BEACH HAZARDS STATEMENT THROUGH FRIDAY AFTERNOON FOR ILZ006-
ILZ103-ILZ104.
FLOOD WATCH UNTIL 4 AM CDT EARLY THIS MORNING FOR ILZ019-
ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107-
ILZ108.
IN...BEACH HAZARDS STATEMENT THROUGH FRIDAY AFTERNOON FOR INZ001-
INZ002.
FLOOD WATCH UNTIL 4 AM CDT /5 AM EDT/ EARLY THIS MORNING FOR
INZ001-INZ002-INZ010-INZ011-INZ019.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CDT FRIDAY FOR THE IL AND IN
NEARSHORE WATERS.
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