599
FXUS63 KLOT 271131
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
631 AM 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 /12Z TAFS THROUGH 18Z MONDAY/
ISSUED AT 630 AM CDT SUN JUL 27 2025
KEY MESSAGES:
- SPOTTY SHOWERS AND POSSIBLY A FEW ISOLATED THUNDERSTORMS THIS
MORNING
- PERIOD OF MVFR CIGS POSSIBLE THIS MORNING
- EAST WIND SHIFT BEHIND A LAKE BREEZE LATE THIS MORNING
SHOWERS AND A FEW EMBEDDED THUNDERSTORMS ARE PERCOLATING ACROSS
THE WESTERN CHICAGO METRO AND POINTS WEST THIS MORNING. DO NOT
EXPECT THIS ACTIVITY TO BECOME TOO ORGANIZED, BUT IT MAY FESTER
INTO MID MORNING. A FEW CELLS HAVE BEEN ABLE TO PRODUCE
LIGHTNING BUT THIS HAS BEEN BRIEF BUT CANNOT RULE OUT SOME
ADDITIONAL ISOLATED STRIKES. POCKETS OF MVFR AND EVEN SOME IFR
ARE OCCURRING OUTSIDE THE TERMINALS AS WELL. DO NOT THINK THAT
THIS WILL BECOME MUCH OF AN ISSUE BUT SOME SCATTERED FRAGMENTS
AND POSSIBLY SOME INTERMITTENT BROKEN COVERAGE COULD OCCUR,
ESPECIALLY AS SHOWERS PERSIST. LIGHT AND VARIABLE WINDS WILL
BECOME STEADIER FROM THE EAST AND SOUTHEAST AS A LAKE BREEZE
PUSHES INLAND LATER THIS MORNING.
WINDS WILL DIMINISH AGAIN THIS EVENING AND MAY SEE SOME REDUCED
VISIBILITY, ESPECIALLY AWAY FROM THE TERMINALS. WILL NEED TO
FURTHER ASSESS LOWER CEILING AND VISIBILITY POTENTIAL BUT THIS
LOOKS TO AGAIN BE SCATTERED. THERE'S GROWING INDICATIONS THAT A
THUNDERSTORM COMPLEX WILL CROSS SOME PORTION OF THE AREA
TOMORROW MORNING, POSSIBLY IN A DECAYING STATE. THERE'S STILL
QUITE A BIT OF UNCERTAINTY SURROUNDING IT OCCURRING AND WHERE IT
WOULD TRACK SO OPTED TO LEAVE ANY MENTION OUT FOR NOW BUT SOME
MENTION WILL PROBABLY BE NEEDED IN LATER ISSUANCES.
MDB
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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