510
FXUS63 KLOT 271740
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1240 PM CDT SUN JUL 27 2025
KEY MESSAGES
- PATCHY FOG, SOME LOCALLY DENSE, TO DEVELOP EARLY THIS MORNING.
- DANGEROUS HEAT AND HUMIDITY MAY RETURN ON MONDAY.
- PERIODS OF THUNDERSTORMS RETURN MONDAY THROUGH WEDNESDAY, SOME
POSSIBLY STRONG TO SEVERE WITH TORRENTIAL RAIN WHICH COULD
LEAD TO FLASH FLOODING.
- DANGEROUS SWIMMING CONDITIONS AT LAKE MICHIGAN BEACHES
WEDNESDAY INTO THURSDAY.
DISCUSSION
ISSUED AT 417 AM CDT SUN JUL 27 2025
THROUGH MONDAY:
A SUBTLE MID-LEVEL IMPULSE IS PASSING THROUGH EASTERN IL AND
NORTHWEST IN THIS MORNING WHICH HAS CONTINUED TO DEVELOP SOME
ISOLATED TO WIDELY SCATTERED SHOWERS AND THUNDERSTORMS NEAR THE
IL- IN LINE. GIVEN THE WEAK WIND SHEAR THESE SHOWERS/STORMS WILL
REMAIN BELOW SEVERE LIMITS, BUT THE HUMID AIR MASS (PWATS
AROUND 2.0 INCHES) WILL MAKE ANY OF THIS ACTIVITY CAPABLE OF
HEAVY RAINFALL. WHILE THESE SHOWERS/STORMS WILL TAPER OVER THE
NEXT 2-3 HOURS AS THE AFOREMENTIONED IMPULSE EXITS, A WEAK
CONVERGENCE ZONE WILL REMAIN PARKED ACROSS OUR FAR SOUTHERN CWA
(AREAS ALONG AND SOUTH OF A PONTIAC TO RENSSELAER LINE) TODAY
WHICH MAY PROVIDE JUST ENOUGH SUPPORT TO KICK OFF AN ISOLATED
SHOWER AND/OR STORM THIS AFTERNOON.
OUTSIDE OF THE RAIN POTENTIAL, THERE IS ALSO SOME PATCHY FOG
DEVELOPING ACROSS NORTHWEST IL THIS MORNING. SO FAR THE FOG HAS
REMAINED FAIRLY SHALLOW, BUT WITH TEMPERATURE-DEW POINT
DEPRESSIONS CONTINUING TO DECREASE SUSPECT VISIBILITIES WILL
LOWER WITH SOME POCKETS OF DENSE FOG (VISIBILITIES <1/4 MILE)
POSSIBLE. RIGHT NOW DENSE FOG IS SPOTTY ENOUGH TO HOLD OFF ON A
DENSE FOG ADVISORY, BUT IF TRENDS WORSEN THEN ONE MAY BE NEEDED.
REGARDLESS, ANYONE WITH EARLY MORNING TRAVEL PLANS SHOULD BE
PREPARED FOR REDUCED VISIBILITY AND USE CAUTION UNTIL THE FOG
ERODES TOWARDS MID-MORNING.
OTHERWISE, THE MAIN FORECAST CONCERN THROUGH MONDAY WILL BE THE
BUILDING HEAT AND HUMIDITY AS A MID-LEVEL RIDGE MOVES INTO THE
GREAT LAKES. TODAY HIGH TEMPERATURES ARE STILL FORECAST TO BE IN
THE UPPER 80S TO AROUND 90 WHICH SHOULD CAP HEAT INDICES IN THE
MID TO UPPER 90S THIS AFTERNOON. AS WE HEAD INTO MONDAY THE
INCREASING HUMIDITY AND SLIGHTLY WARMER TEMPERATURES (HIGHS IN
THE LOW TO MID-90S) WILL PUSH HEAT INDICES IN THE 100-110 DEGREE
RANGE.
HOWEVER, THERE IS A GROWING CONCERN THAT AN ORGANIZED CLUSTER
OF THUNDERSTORMS MAY MOVE THROUGH PORTIONS OF THE MISSISSIPPI
VALLEY LATE TONIGHT INTO MONDAY MORNING. GUIDANCE CONTINUES TO
BE IN GOOD AGREEMENT ON THE CLUSTER DEVELOPING IN NORTHERN MN
THIS EVENING AND THEN TRACKING SOUTH-SOUTHEAST ALONG THE
MISSISSIPPI RIVER VALLEY AS THE BUILDING RIDGE TURNS THE
STEERING FLOW. THE UNCERTAINTY, HOWEVER, IS WITH THE CLUSTER'S
INTENSITY AS IT NEARS NORTHWEST IL AS VIRTUALLY ALL GUIDANCE
WANTS TO WEAKEN THE STORMS BEFORE THEY REACH THE IL-WI LINE. THE
ISSUE IS THAT THE WARM AND HUMID AIR MASS OVERHEAD TONIGHT WILL
ALLOW SOME FAIRLY DECENT INSTABILITY TO REMAIN IN PLACE,
ESPECIALLY ACROSS EASTERN IA AND WESTERN IL, WHICH SHOULD BE
MORE THAN SUFFICIENT TO SUPPORT A MATURE STORM CLUSTER AND BRING
IT RIGHT THROUGH THE WESTERN HALF OF OUR CWA EARLY MONDAY
MORNING. HENCE HAVE DECIDED TO INCREASE THUNDERSTORM CHANCES
INTO THE 25-30% RANGE FOR AREAS WEST OF THE FOX VALLEY. IF THE
STORMS DO EVOLVE AS EXPECTED, THEN THE CLOUD DEBRIS AND STORM
OUTFLOW WILL LIKELY KEEP HIGH TEMPERATURES AND THE RESULTANT
HEAT INDICES A FEW DEGREES COOLER THAN CURRENTLY FORECAST. IT IS
FOR THIS REASON THAT WE HAVE DECIDED TO HOLD OFF ON ISSUING ANY
HEAT HEADLINES FOR MONDAY AS THERE IS A CHANCE WE COULD REMAIN
BELOW THE FORMAL 105 DEGREE HEAT INDEX CRITERIA. SO WHILE WE
CONTINUE TO MONITOR TRENDS TODAY, IT WOULD NOT BE A BAD IDEA TO
PREPARE FOR DANGEROUS HEAT ON MONDAY AND ENSURE YOU TAKE PROPER
PRECAUTIONS.
MONDAY NIGHT THROUGH SATURDAY:
THE AFOREMENTIONED MID-LEVEL RIDGE IS EXPECTED TO REMAIN IN
PLACE THROUGH TUESDAY WHICH WILL KEEP THE POTENTIAL FOR
DANGEROUS HEAT AND HUMIDITY AROUND. HIGHS ON TUESDAY ARE ONCE
AGAIN FORECAST TO GET INTO THE LOWER 90S WITH PEAK HEAT INDICES
IN THE 100-105 RANGE. HOWEVER, SIMILAR TO MONDAY THERE IS THE
POTENTIAL FOR ANOTHER THUNDERSTORM COMPLEX TO TRAVERSE ACROSS
THE REGION WHICH COULD RESULT IN SLIGHTLY COOLER CONDITIONS THAN
FORECAST. GIVEN THAT THE OVERALL ENVIRONMENT SUPPORTS THE
GREATER STORM COVERAGE TO REMAIN NORTH OF THE IL-WI LINE ON
TUESDAY CLOSER TO THE BETTER FORCING, POPS WERE TRIMMED BACK TO
AROUND 30% FOR AREAS NEAR THE IL-WI LINE AND THEN TAPERED TO
AROUND 20% FOR THOSE SOUTH OF I-80 BUT STILL THINK SOME
SCATTERED STORMS ARE POSSIBLE IN NORTHERN IL.
THE HEAT WILL BEGIN TO BREAK TUESDAY NIGHT INTO WEDNESDAY AS A
COLD FRONT BEGINS TO MOVE INTO THE AREA WHICH WILL BRING THE
BEST CHANCES FOR WIDESPREAD SHOWERS AND THUNDERSTORMS TO THE
AREA. GIVEN THAT THERE WILL BE NO SHORTAGE OF MOISTURE AND
INSTABILITY PRESENT, THERE IS THE POTENTIAL FOR A FEW STORMS
DURING THIS TIME TO BE STRONG TO SEVERE DESPITE WIND SHEAR
LOOKING TO BE MORE MODEST. IF SEVERE STORMS DO MATERIALIZE THE
MAIN THREAT LOOKS TO BE DAMAGING WINDS BUT TORRENTIAL RAINFALL
WILL ALSO BE A CONCERN WHICH COULD LEAD TO INSTANCES OF
FLOODING.
SHOWER AND STORM CHANCES WILL GRADUALLY DIMINISH HEADING INTO
THURSDAY AS THE FRONT EXITS. THOUGH, WINDS BEHIND THE FRONT ARE
EXPECTED TO BECOME QUITE BREEZY ESPECIALLY OVER THE LAKE WHICH
WILL RESULT IN BUILDING WAVES AND DANGEROUS SWIMMING CONDITIONS
WEDNESDAY THROUGH AT LEAST THURSDAY. ON THE BRIGHT SIDE, THE AIR
MASS BEHIND THE FRONT WILL BE MUCH LESS HUMID WITH DEW POINTS
FORECAST TO BE IN THE 50S. TEMPERATURES DURING THIS TIME WILL
ALSO BE MUCH COOLER WITH HIGHS CURRENTLY FORECAST TO BE IN THE
70S THURSDAY INTO NEXT WEEKEND.
YACK
AVIATION /18Z TAFS THROUGH 00Z TUESDAY/
ISSUED AT 1240 PM CDT SUN JUL 27 2025
THE PRIMARY CONCERNS FOR THE 18Z TAF PERIOD INCLUDE:
* PERIODIC MVFR CIGS POSSIBLE THIS AFTERNOON
* POTENTIAL FOR RAIN AND STORMS MONDAY MORNING, GREATEST AT RFD
A DENSELY SCATTERED CLOUD DECK BETWEEN 2 AND 3 KFT AGL HAS BEEN
FILLING IN PERIODICALLY TO BRING FLIGHT CATEGORIES DOWN TO
MVFR. WHILE PREDOMINANTLY VFR IS EXPECTED THIS AFTERNOON,
PERIODS OF MVFR WILL REMAIN POSSIBLE.
A DECAYING SYSTEM OF SHOWERS AND THUNDERSTORMS WILL APPROACH
NORTHERN IL FROM THE NW AROUND DAWN ON MONDAY. THERE ARE
UNCERTAINTIES IN COVERAGE AS THE WAVE MOVES ACROSS DURING THE
MORNING. THE GREATEST POTENTIAL FOR MORNING SHOWER/THUNDER
COVERAGE WILL BE AT RFD WHERE A PROB30 WAS INTRODUCED DURING
THIS TAF CYCLE. THE CHICAGOLAND TAFS WERE KEPT DRY FOR NOW AS
PROBS APPEAR TOO LOW TO INCLUDE, BUT TRENDS WILL BE CLOSELY
MONITORED THROUGH THE NIGHT. A PERIOD OF MVFR WOULD ALSO BE
POSSIBLE EARLY MONDAY IF THE RAIN HOLDS TOGETHER.
MEANWHILE, EASTERLY WINDS BELOW 10 KT WILL VEER TO SOUTHERLY
EARLY THIS AFTERNOON AND REMAIN AS SUCH THROUGH THE NIGHT.
MONDAY WILL FEATURE SW WINDS BELOW 10 KT INTO THE LATE AFTERNOON
BEFORE A LAKE BREEZE MAY APPROACH THE TERMINALS TURNING WINDS
TO NW THEN POSSIBLY TO NE TOWARD THE END OF THE 30-HR TAF
PERIOD. ADDITIONALLY, WINDS MAY GO NE FOR A TIME MONDAY MORNING
ASSOCIATED WITH THE RAIN POTENTIAL. EVEN IF THE RAIN DOESN'T
QUITE MAKE IT INTO CHICAGO, OUTFLOW FROM NEARBY CONVECTION MAY
ALSO RESULT IN A SHIFT TO NE.
DOOM
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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