120
FXUS63 KLOT 300613
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
113 AM CDT WED JUL 30 2025
KEY MESSAGES
- PERIODS OF THUNDERSTORMS THROUGH THURSDAY MORNING, SOME
POSSIBLY STRONG TO SEVERE WITH TORRENTIAL RAIN WHICH COULD
LEAD TO LOCALIZED FLASH FLOODING.
- POSSIBLE PERIOD OF PROLONGED HEAVY RAIN WEDNESDAY NIGHT INTO
EARLY THURSDAY MORNING.
- DANGEROUS SWIMMING CONDITIONS AT LAKE MICHIGAN BEACHES
WEDNESDAY AFTERNOON THROUGH FRIDAY.
DISCUSSION
ISSUED AT 252 PM CDT TUE JUL 29 2025
THROUGH TONIGHT:
IT HAS BEEN ANOTHER HOT AND HUMID DAY UNDER MOSTLY SUNNY SKIES,
THOUGH DEW POINTS HAVE TRENDED SLIGHTLY LOWER THEN YESTERDAY. HEAT
INDEX VALUES CURRENTLY IN THE UPPER 90S FOR MOST LOCATIONS WILL TOP
OUT AROUND 100, WITH SOME READINGS IN THE 100-105 RANGE ACROSS THE
WESTERN CWA AMID HIGHER LOW-LEVEL MOISTURE.
CUMULUS HAD STRUGGLED TO DEVELOP UNTIL THE PAST HOUR OR SO DUE TO
MODEST WARMING/CAPPING IN THE 6-8KFT LAYER. MIXING HEIGHTS HAVE
STARTED TO RISE INTO THAT LAYER, THOUGH SATELLITE IMAGERY AND VISUAL
APPEARANCE OF STUNTED CUMULUS SUGGEST CAPPING IS STILL HOLDING AS OF
3PM. A WEAK FRONT/BOUNDARY DRIFTING SOUTHWARD OVER SOUTHERN
WISCONSIN AS WELL AS A LAKE BREEZE OOZING INLAND FROM THE ILLINOIS
SHORE WILL BEGIN TO INTERACT WITH A MORE PRONOUNCED CUMULUS FIELD
OVER THE NORTHERN CHICAGO METRO DURING THE NEXT FEW HOURS. HAVE
MAINTAINED SLIGHT CHANCE POPS/THUNDER IN THIS AREA INTO EARLY EVENING
IN THE EVENT THE CAP DOES BREAK. IF THIS WERE TO OCCUR, MODEST DEEP-
LAYER SHEAR FOR LATE JULY WOULD BE SUFFICIENT FOR AN ORGANIZED
DISCRETE CELL OR TWO WITH LOCALLY STRONG WINDS AND TORRENTIAL
RAINFALL.
DRY CONDITIONS ARE EXPECTED THIS EVENING, AND POSSIBLY WELL INTO THE
NIGHT, AS WINDS SYNOPTICALLY VEER NORTH OR NORTHEASTERLY WHILE HIGH
PRESSURE BUILDS OVER THE FAR NORTHERN GREAT LAKES. PHASING OF A PAIR
OF WAVES THAT WILL BE CONVECTIVELY ENHANCED BY SUBSTANTIAL
CONVECTION OVER NEBRASKA AND IOWA TONIGHT WILL SHIFT EASTWARD ACROSS
IOWA THROUGH SUNRISE WEDNESDAY. A RESULTING LOW-LEVEL CONTRACTION OF
THE BAROCLINIC ZONE ALONG THE SOUTHERN EXTENT OF THE HIGH WILL
SETTLE SOMEWHERE OVER THE FORECAST AREA LATE TONIGHT. CANNOT RULE
OUT ISOLATED CONVECTION ALONG THIS BOUNDARY ACROSS THE WESTERN CWA
VERY LATE TONIGHT, THOUGH WEAK LOW-LEVEL FLOW NEARLY PARALLEL TO THE
FRONT WILL LIKELY PROVIDE INSUFFICIENT FORCING TO OVERCOME EXISTING
MUCIN REMAINING OVER NORTHERN ILLINOIS OVERNIGHT.
KLUBER
WEDNESDAY THROUGH TUESDAY:
PRIMARY FORECAST CONCERN REMAINS SEVERE THUNDERSTORM AND HEAVY
RAIN POTENTIAL THROUGH THURSDAY MORNING WITH SEVERAL ROUNDS OF
THUNDERSTORMS.
THUNDERSTORMS MAY BE ONGOING WEDNESDAY MORNING ACROSS THE
WESTERN CWA, POSSIBLY AS A SLOWLY WEAKENING LINE. ITS ALSO
POSSIBLE THAT WHILE THE LINE MAY WEAKEN DURING THE MID MORNING,
THAT IT MAINTAINS ENOUGH ORGANIZATION TO REINTENSIFY AS IT
MOVES ACROSS THE CHICAGO METRO AREA MIDDAY. SOME OF THESE STORMS
COULD POSSIBLY BECOME STRONG/SEVERE ALONG WITH HEAVY
RAIN/LOCALIZED FLOODING.
THE COLD FRONT WILL BE MOVING SOUTH ACROSS THE AREA WEDNESDAY
AND THE MORNING/MIDDAY STORMS MAY AID THE FRONT PUSHING INTO
THE SOUTHERN CWA INTO THE AFTERNOON. ADDITIONAL THUNDERSTORMS
ARE EXPECTED TO DEVELOP WEDNESDAY AFTERNOON. DEPENDING ON WHERE
THE FRONT ENDS UP WHEN STORMS DEVELOP, THERE IS THE POTENTIAL
FOR SUPERCELL THUNDERSTORMS ACROSS THE SOUTHERN THIRD OR SO OF
THE CWA WEDNESDAY AFTERNOON, WHICH WOULD THEN ALLOW FOR ALL
MODES OF SEVERE WEATHER. DESPITE THE NORTHEAST WINDS FURTHER
NORTH, IF ANY ORGANIZED THUNDERSTORMS CAN DEVELOP/PERSIST, THERE
WOULD REMAIN AT LEAST AN ISOLATED DAMAGING WIND POTENTIAL
FURTHER NORTH.
THERE IS NOW A CONSENSUS FOR ADDITIONAL RAIN, MOST LIKELY HEAVY
RAIN WEDNESDAY NIGHT INTO THURSDAY MORNING ACROSS MUCH OF THE
AREA, PARTICULARLY ALONG/NORTH OF I-80. A SLOW MOVING SECONDARY
LOW IS EXPECTED TO MOVE ACROSS THE AREA ALLOWING FOR A PROLONGED
PERIOD OF RAIN. PRECIPITABLE WATER VALUES DURING THIS TIME ARE
IN THE 2 TO 2.25 INCH RANGE WITH SEVERAL MODELS SHOWING QPF
AMOUNTS ANYWHERE FROM 2 TO 4 INCHES, MUCH OF THIS FALLING IN 6
TO 12 HOURS. CONSIDERATION WAS GIVEN TO A FLOOD WATCH FOR THIS
TIME PERIOD BUT IT IS STILL 30-36 HOURS OUT AND THERE REMAINS
QUITE A BIT OF UNCERTAINTY. HAVE INCREASE QPF DURING THIS PERIOD
BUT IT MAY NOT BE ENOUGH IF THE CURRENT TRENDS MATERIALIZE.
WHILE SCATTERED THUNDERSTORMS WOULD REMAIN POSSIBLE WEDNESDAY
NIGHT, THE HEAVY RAIN AND POSSIBLE FLOODING REMAINS THE MAIN
THREAT DURING THIS TIME PERIOD.
NORTHEASTERLY WINDS STEADILY INCREASE WEDNESDAY AFTERNOON AND
MAY BE GUSTING INTO THE 30-35 MPH RANGE WEDNESDAY NIGHT. DID
INCREASE SPEEDS/GUSTS, BUT THEY MAY NEED TO BE INCREASED FURTHER
WITH LATER FORECASTS. THESE NORTHEAST WINDS WILL ALLOW FOR HIGH
WAVES AND DANGEROUS SWIMMING CONDITIONS AT LAKE MICHIGAN BEACHES
THROUGH FRIDAY.
MUCH LESS HUMID CONDITIONS ARE EXPECTED THROUGH SUNDAY AND THEN
HUMIDITY LEVELS BEGIN TO INCREASE AGAIN EARLY NEXT WEEK. HIGHS
IN THE 70S ARE EXPECTED THURSDAY/FRIDAY AND THEN HIGHS WILL
LIKELY REACH 80 SATURDAY AND THEN BACK INTO THE 80S SUNDAY INTO
EARLY NEXT WEEK. CMS
AVIATION /06Z TAFS THROUGH 12Z THURSDAY/
ISSUED AT 113 AM CDT WED JUL 30 2025
FORECAST CONCERNS FOR THE TERMINALS...
- SEVERAL PERIODS OF SHOWERS AND THUNDERSTORMS WEDNESDAY. SOME
OF WHICH COULD BE STRONG WITH GUSTY WINDS AND TORRENTIAL
RAINFALL.
- BREEZY EASTERLY WINDS BEHIND COLD FRONT WEDNESDAY AFTERNOON
WITH GUSTS OF 20-25 KTS.
- LOWERING CEILINGS THROUGH THE PERIOD WITH MVFR CONDITIONS
EXPECTED THROUGH WEDNESDAY NIGHT. IFR CEILINGS POSSIBLE.
THE TAF PERIOD WILL START ON A QUIET NOTE WITH LIGHT EASTERLY
WINDS EXPECTED TONIGHT AND OTHERWISE JUST INCREASING CLOUD
COVER. THAT SAID, THE FORECAST WILL BECOME MUCH MESSIER HEADING
INTO WEDNESDAY MORNING AS THE COMPLEX OF STORMS ONGOING IN IA
BEGINS TO APPROACH. CURRENT THINKING REMAINS GENERALLY THE SAME
AS BEFORE WITH SHOWERS AND THUNDERSTORMS ARRIVING AT RFD AROUND
13-14Z AND THEN THE CHICAGO AREA TERMINALS AROUND 15-16Z.
HOWEVER, IS THE CHANCE (<20%) THAT SOME ISOLATED TO SCATTERED
SHOWERS AND/OR STORMS COULD DEVELOP AHEAD OF THE MAIN COMPLEX AS
THE PLUME OF DEEPER MOISTURE ADVECTS INTO NORTHERN IL AND
NORTHWEST IN AFTER 09Z TONIGHT. GIVEN THAT THE BETTER
INSTABILITY NOT EXPECTED TO ARRIVE UNTIL JUST BEFORE THE
AFOREMENTIONED COMPLEX THE THINKING IS THAT ANY LEADING ACTIVITY
SHOULD STRUGGLE AND FALL APART RATHER QUICKLY. IT WAS THIS
UNCERTAINTY THAT HAS LEAD TO THE MAINTAINING OF THE PROB30S FOR
THE MORNING ROUND OF SHOWERS/STORMS, BUT SUSPECT IF TRENDS
CONTINUE THEN AN UPGRADE TO A TEMPO MAY BE WARRANTED.
IN THE WAKE OF THE MORNING SHOWERS/STORMS, A BRIEF LULL IN
ACTIVITY MAY DEVELOP AS THE ATMOSPHERE TRIES TO DESTABILIZE FOR
THE AFTERNOON. THOUGH, THE HUMID AIR MASS AND PRESENCE OF AN
UPPER DISTURBANCE OVERHEAD WILL LIKELY KEEP SCATTERED SHOWERS
AROUND THROUGH VIRTUALLY THE ENTIRE AFTERNOON. IT IS FOR THIS
REASON THAT SHRA WAS KEPT IN THE TAFS. THAT SAID, THE NEXT ROUND
OF SHOWERS AND THUNDERSTORMS LOOKS TO GET UNDERWAY AFTER 19Z
WHEN ANOTHER DISTURBANCE PIVOTS IN AND INTERACTS WITH THE
DESTABILIZING ATMOSPHERE. GIVEN THAT THE THUNDER COVERAGE WITH
THE SECOND ROUND IS A BIT MORE UNCERTAIN IN ADDITION TO EXACT
TIMING, HAVE OPTED TO EXTEND THE PROB30 BY AN HOUR AT ALL SITES.
REGARDLESS, ANY SHOWERS AND STORMS THAT DEVELOP TODAY WILL HAVE
THE POTENTIAL TO HAVE LOCALIZED GUSTY WINDS AND TORRENTIAL
RAINFALL WHICH COULD LEAD TO INSTANCES OF FLOODING ESPECIALLY
IF STORMS REDEVELOP OVER THE SAME AREA REPEATEDLY. WHILE THE
THUNDER THREAT IS EXPECTED TO MOVE SOUTH OF THE TAF SITES AFTER
00Z, SHOWERS ARE EXPECTED TO PERSIST THROUGH WEDNESDAY NIGHT
BEFORE RAIN TAPERS THURSDAY MORNING.
OUTSIDE OF THE RAIN, WINDS WILL INCREASE BEHIND A COLD FRONT
WEDNESDAY AFTERNOON RESULTING IN 20-25 KT GUSTS THROUGH THE
REMAINDER OF THE PERIOD. THOUGH DIRECTIONS WILL REMAIN
NORTHEASTERLY THROUGHOUT. CEILINGS ON THE OTHER HAND WILL
GRADUALLY LOWER THROUGH THE PERIOD WITH MVFR CONDITIONS EXPECTED
TO DEVELOP BY WEDNESDAY EVENING. ONCE MVFR CEILINGS ARRIVE THEY
ARE EXPECTED TO PERSIST INTO THURSDAY MORNING WITH THE POTENTIAL
FOR IFR CEILINGS TO DEVELOP WEDNESDAY NIGHT.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...BEACH HAZARDS STATEMENT FROM THIS AFTERNOON THROUGH FRIDAY
AFTERNOON FOR ILZ006-ILZ103-ILZ104.
IN...BEACH HAZARDS STATEMENT FROM THIS AFTERNOON THROUGH FRIDAY
AFTERNOON FOR INZ001-INZ002.
LM...SMALL CRAFT ADVISORY FROM 1 PM THIS AFTERNOON TO 9 AM CDT
FRIDAY FOR THE IL AND IN NEARSHORE WATERS.
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