926
FXUS63 KLOT 290616
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
116 AM CDT TUE JUL 29 2025
KEY MESSAGES
- DANGEROUS HEAT AND HUMIDITY WITH A HEAT ADVISORY IN EFFECT
THROUGH EARLY THIS EVENING.
- PERIODS OF THUNDERSTORMS TUESDAY THROUGH THURSDAY MORNING,
SOME POSSIBLY STRONG/SEVERE WITH TORRENTIAL RAIN WHICH COULD
LEAD TO LOCALIZED FLASH FLOODING.
- DANGEROUS SWIMMING CONDITIONS AT LAKE MICHIGAN BEACHES
WEDNESDAY INTO THURSDAY, POSSIBLY THROUGH FRIDAY NIGHT.
DISCUSSION
ISSUED AT 230 PM CDT MON JUL 28 2025
THROUGH THIS EVENING:
WE REMAIN IN A HOT AND HUMID AIRMASS TODAY, WITH THESE CONDITIONS
FUELING ISOLATED CONVECTION ACROSS NORTHEAST ILLINOIS AND NORTHWEST
INDIANA CLOSER TO THE LAKE. FORCING FOR THIS CONVECTION HAS BEEN
TIED TO A WEAK LOCALIZED COLD FRONT/OUTFLOW FROM AN MCV THAT CROSSED
SOUTHERN WISCONSIN EARLY THIS MORNING. THIS BOUNDARY HAS BEEN
DRIFTING SOUTHWARD TODAY AND IS CURRENTLY LOCATED FROM NEAR ROCKFORD
TO NORTHWEST COOK COUNTY. MEANWHILE, AN OUTFLOW-ENHANCED LAKE BREEZE
HAS SURGED WESTWARD ACROSS THE CHICAGO METRO, WITH THE ORIGINAL LAKE
BREEZE DRIFTING SOUTHWARD OVER NORTHWEST INDIANA. EARLY AFTERNOON
AIRCRAFT OBS AND SATELLITE TRENDS INDICATE THAT MID-LEVEL DRYING AS
WELL AS SUBTLE MID-LEVEL WARMING ARE SUPPRESSING MOST OF THE
THUNDERSTORM ACTIVITY. THIS TREND SHOULD CONTINUE THROUGH THE
REMAINDER OF THE AFTERNOON, BUT WITH A GENERALLY UNCAPPED
ENVIRONMENT STILL IN PLACE SOUTH AND WEST OF THE BOUNDARIES,
ISOLATED CONVECTION REMAINS A POSSIBILITY INTO EARLY EVENING OVER
THE SOUTHWEST HALF OF THE CHICAGO METRO EASTWARD TO AROUND THE
KANKAKEE RIVER VALLEY IN NORTHWEST INDIANA. BEYOND THE DIURNALLY
DRIVEN ISOLATED STORMS THROUGH EARLY EVENING, DRY CONDITIONS ARE
EXPECTED INTO THE EARLY OVERNIGHT HOURS.
HAVE MAINTAINED THE HEAT ADVISORY FOR THE ENTIRE AREA THIS AFTERNOON
EVEN AS THE LAKE BREEZE AND ISOLATED STORMS HAVE PROVIDED SOME
RELIEF FROM THE HEAT CLOSER TO THE LAKE (PRIMARILY COOK AND LAKE
COUNTY IL). HEAT INDEX VALUES BEHIND THE BOUNDARIES AND STORMS ARE
STILL UP TO 100F AS OF 2PM. WITH REMAINING DAYTIME HEATING AND DEW
POINTS STILL IN THE MID 70S, HEAT INDEX VALUES COULD STILL REACH
105F AGAIN. ELSEWHERE OVER THE ADVISORY AREA, CURRENT HEAT INDEX
VALUES OF 100-105F ARE ON TRACK TO MEET ADVISORY CRITERIA OF 105F OR
HIGHER FOR MOST LOCATIONS.
KLUBER
EARLY TUESDAY THROUGH MONDAY:
PRIMARY FORECAST CONCERN IS THE POTENTIAL FOR AT LEAST A FEW
ROUNDS THUNDERSTORMS THROUGH THURSDAY MORNING.
THUNDERSTORMS ARE EXPECTED TO DEVELOP ACROSS SOUTHERN MN/
NORTHERN IA THIS EVENING AND QUICKLY MOVE SOUTHEAST REACHING
NORTHWEST IL IN THE PREDAWN HOURS IN A DECAYING PHASE. HOW FAST
THESE DISSIPATE IS UNCERTAIN AND CURRENT GUIDANCE IS GENERALLY
IN AGREEMENT THAT THIS ACTIVITY WILL DISSIPATE ACROSS THE
NORTHERN CWA. GIVEN THE COVERAGE OF ACTIVITY THIS MORNING, WHICH
WAS HIGHER THAN ALL GUIDANCE WAS SHOWING 24 HOURS AGO, HAVE
INCREASED POPS TO HIGH CHANCE ACROSS THE NORTHERN CWA AROUND
DAYBREAK WITH POPS THEN STEADILY DIMINISHING THROUGH MID/LATE
MORNING. ADJUSTMENTS WILL BE NEEDED WITH LATER FORECASTS AS
TRENDS EMERGE. WITH THE EXPECTED WEAKENING TREND, AN ISOLATED
STRONG, POSSIBLY SEVERE STORM WOULD BE POSSIBLE ACROSS THE
NORTHWEST CWA.
AS HAPPENED TODAY, THE WIND FIELD WILL LIKELY BE DISTURBED WITH
ANY WEAKENING SHOWERS/STORMS, WHICH WILL LEAD TO A LAKE BREEZE
TUESDAY AFTERNOON AND SIMILAR TO TODAY AND ON SUNDAY, ISOLATED
THUNDERSTORMS WILL AGAIN BE POSSIBLE TUESDAY AFTERNOON NEAR THE
LAKE BREEZE. THERE IS ALSO THE POTENTIAL FOR A FEW THUNDERSTORMS
AWAY FROM THE LAKE BREEZE, ESPECIALLY ACROSS NORTHWEST IL. THERE
DOESN'T SEEM TO BE MUCH OF A FOCUS, BESIDES THE LAKE BREEZE BUT
WILL CONTINUE LOW CHANCE POPS FOR THE ENTIRE AREA.
GUIDANCE HAS LOWERED HIGH TEMPS BY SEVERAL DEGREES FOR TUESDAY
WITH HIGHS NOW GENERALLY IN THE UPPER 80S/AROUND 90. DEWPOINTS
WILL LIKELY BE SIMILAR TO TODAY, GENERALLY MID 70S. THIS WOULD
ALLOW FOR PEAK HEAT INDICES IN THE 95-100 RANGE. POSSIBLE FOR
SOME AREAS TO GET INTO THE 100-105 RANGE, ESPECIALLY ACROSS THE
SOUTHWEST CWA AND CURRENTLY NOT PLANNING ANY HEAT HEADLINES FOR
TUESDAY.
A COLD FRONT WILL MOVE SOUTH ACROSS THE AREA TUESDAY NIGHT INTO
WEDNESDAY. HOW MUCH THUNDERSTORM COVERAGE TUESDAY NIGHT THROUGH
WEDNESDAY NIGHT IS QUITE UNCERTAIN AS THIS FRONT MAY PUSH THE
BULK OF THE HEAVIER THUNDERSTORMS INTO AT LEAST THE SOUTHERN
CWA, POSSIBLY SOUTH OF THE CWA. CONFIDENCE IS VERY LOW AND
CURRENT BLENDED POPS ARE HIGH CHANCE TO LIKELY AND THIS SEEMS
REASONABLE FOR NOW. STRONG/SEVERE THUNDERSTORMS ARE POSSIBLE,
ESPECIALLY TUESDAY NIGHT INTO WEDNESDAY AS WELL AS HEAVY RAIN
AND LOCALIZED FLASH FLOODING.
THERE APPEARS TO BE A FEW PUSHES OF NORTHERN WINDS, ONE ON
WEDNESDAY AND ANOTHER ON THURSDAY THAT WILL EVENTUALLY PUSH ALL
OF THE PRECIP CHANCES SOUTH OF THE CWA BY THURSDAY AFTERNOON.
DEWPOINTS WILL DROP INTO THE 60S THURSDAY AND INTO THE 50S ON
FRIDAY BRINGING MUCH LESS HUMID CONDITIONS.
THE STRONGER NORTHERLY WINDS WILL ALLOW FOR HIGH WAVES AND
DANGEROUS SWIMMING CONDITIONS AT LAKE MICHIGAN BEACHES FROM
WEDNESDAY AFTERNOON THROUGH AT LEAST THURSDAY NIGHT AND POSSIBLY
INTO FRIDAY. CMS
AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 116 AM CDT TUE JUL 29 2025
FORECAST CONCERNS FOR THE TERMINALS...
- PATCHY FOG IN NORTHWEST IN AND EASTERN IL TONIGHT.
- SCATTERED SHOWERS AND THUNDERSTORMS POSSIBLE LATE TONIGHT
INTO EARLY TUESDAY MORNING, ESPECIALLY IN NORTHWEST IL.
- BRIEF PERIOD OF BREEZY EASTERLY WINDS BEHIND SHOWERS/STORMS
TUESDAY MORNING.
- ANOTHER PERIOD OF SHOWERS AND THUNDERSTORMS POSSIBLE TUESDAY
NIGHT INTO WEDNESDAY.
A COMPLEX PERIOD OF WEATHER IS EXPECTED AT THE TERMINALS OVER
THE NEXT 30 HOURS. TO START, PATCHY FOG (SOME DENSE WITH
VISIBILITY AROUND 1/2-1/4 MILE) HAS CONTINUED TO DEVELOP ACROSS
NORTHWEST IN AND ADJACENT PORTIONS OF EAST AND NORTHEAST IL
WHERE SURFACE DEW POINTS REMAIN IN THE LOWER 70S. THE FOG SHOULD
BE ABLE TO PERSIST THROUGH MUCH OF THE NIGHT BEFORE GRADUALLY
IMPROVING AFTER 10Z.
OUR ATTENTION THEN TURNS TO THE LINE OF SEVERE THUNDERSTORMS
ONGOING IN SOUTHERN MN AND EASTERN IA THAT IS EXPECTED TO MOVE
INTO NORTHWEST IL AROUND 08-09Z TONIGHT. THE STORMS ARE EXPECTED
TO ARRIVE IN A DECAYING STATE AS THEY OUTRUN THE BETTER
INSTABILITY AND ENCOUNTER A POCKET OF DRIER AIR ALOFT. HOWEVER,
THERE ARE A COUPLE OF CONVECTIVELY DRIVEN LOW PRESSURE CENTERS
WITHIN THE STORM COMPLEX THAT SHOULD ALLOW AT LEAST SOME
THUNDERSTORMS TO REACH RFD BEFORE THEY COMPLETELY DISSIPATE AND
BECOME SCATTERED SHOWERS. THEREFORE, HAVE OPTED TO CONVERT THE
PROB30 TO A TEMPO GROUP AT RFD TO ACCOUNT FOR THIS EXPECTATION.
WHILE OVERALL STORM SEVERITY SHOULD DIMINISH BEFORE THE STORMS
REACH RFD, STILL CANNOT RULE OUT SOME LOCALLY GUSTY WINDS IN
EXCESS OF 30 KTS. AS FOR THE CHICAGO AREA TERMINALS, THE STORMS
SHOULD WEAKEN ENOUGH THAT THEY WILL BE IN THE FORM OF SCATTERED
SHOWERS IF THEY MAKE IT AT ALL. SINCE CONFIDENCE ON THE COVERAGE
OF SHOWERS AT THE CHICAGO TERMINALS IS STILL UNCERTAIN, HAVE
OPTED TO MAINTAIN THE PROB30 GROUPS AT DPA, MDW, AND ORD FOR NOW
BUT WILL CONTINUE TO ASSESS TRENDS AND UPDATE AS NEEDED. THAT
SAID, STILL CANNOT COMPLETELY RULE OUT THAT AN ISOLATED
LIGHTNING STRIKE COULD BE SEEN IN THE VICINITY OF THE CHICAGO
TERMINALS BETWEEN 11Z AND 13Z TUESDAY MORNING.
REGARDLESS OF WHAT HAPPENS WITH THE SHOWERS/STORMS, A MESO-HIGH
ON THE BACK SIDE OF THE STORM COMPLEX IS EXPECTED TO PIVOT
THROUGH NORTHEAST IL AND NORTHWEST IN EARLY TUESDAY MORNING.
THIS SHOULD RESULT IN A 2-3 HOUR PERIOD OF BREEZY (10-12 KT)
EASTERLY WINDS BEFORE WINDS EASE BACK INTO THE 5-10 KT RANGE AND
DIRECTIONS TURN SOUTHWEST TOWARDS MIDDAY. OTHERWISE, IT LOOKS
AS A MOSTLY DRY DAY WILL FOLLOW THE MORNING RAIN CHANCES WITH
VFR CONDITIONS EXPECTED THROUGH THE PERIOD. THAT SAID, GUIDANCE
HAS TRENDED A FEW DEGREES COOLER WITH TEMPERATURES TODAY DUE TO
THE BROADER CLOUD COVER FROM THE MORNING SHOWERS/STORMS WHICH IN
TURN LOOKS TO LOWER THE LIKELIHOOD THAT A LAKE BREEZE WILL
DEVELOP AND MAKE IT TO ORD AND MDW. WHILE IT IS STILL NOT
COMPLETELY OUT OF THE QUESTION THAT A LAKE BREEZE DEVELOPS, THE
NEW CONSENSUS IS THAT WINDS SHOULD BE REMAIN WEST-SOUTHWEST
THROUGH 06Z TUESDAY NIGHT BEFORE TURNING NORTHEAST BEHIND A COLD
FRONT AND THE TAFS HAVE BEEN ADJUSTED TO REFLECT THIS THINKING.
FINALLY, THERE IS YET ANOTHER POTENTIAL FOR SHOWERS AND
THUNDERSTORMS TUESDAY NIGHT INTO WEDNESDAY AS A COLD FRONT MOVES
THROUGH THE AREA. GIVEN THE DRIER AIR FORECAST TO DEVELOP
OVERHEAD TODAY, SUSPECT ANY SHOWERS STORMS WILL HOLD OFF UNTIL
AFTER THIS TAF PERIOD SO NO FORMAL MENTIONS HAVE BEEN ADDED BUT
WILL REASSESS FOR FUTURE ISSUANCES.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page