783
FXUS63 KLOT 050135
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
835 PM CDT FRI SEP 4 2026
KEY MESSAGES
- CHANCE OF SCATTERED SHOWERS AND THUNDERSTORMS TONIGHT.
- GRADUALLY TURNING COOLER AND LESS HUMID THIS WEEKEND, THOUGH
REMAINING SEASONABLY WARM.
- MODERATE TO HIGH SWIM RISK EXPECTED LATE SATURDAY INTO EARLY
SUNDAY DURING THE FINAL WEEKEND OF BEACH SEASON.
UPDATE
ISSUED AT 835 PM CDT FRI SEP 4 2026
LOW-CONFIDENCE CONVECTIVE FORECAST TONIGHT, THOUGH THE
ATMOSPHERE MAY BE STARTING TO SHOW IT'S HAND WITH NEW CELLS
DEVELOPING NEAR THE IL/WI BORDER NORTHWEST OF CHICAGO AT MID-
EVENING. HAVE CANCELLED THE HEAT ADVISORY AS TEMPS/HEAT INDEX
VALUES LOWER AFTER SUNRISE, AND ALSO ALLOWED THE BEACH HAZARD
STATEMENT TO EXPIRE WITH WINDS/WAVES DECREASING SIGNIFICANTLY
FROM EARLIER TODAY.
GOES VAPOR IMAGERY SHOWS A MINOR MID-LEVEL SHORT WAVE TOPPING
THE RIDGE AXIS OVER NORTHEAST IA/SOUTHWEST WI EARLY THIS
EVENING. A CLUSTER OF THUNDERSTORMS OVER NORTHEAST IA (LIKELY
AIDED BY ASCENT FROM THE SHORT WAVE) HAD WEAKENED SIGNIFICANTLY
WITH SUNSET, THOUGH RECENT RADAR IMAGERY INDICATES NEW
CONVECTIVE CELLS DEVELOPING ALONG THE IL/WI NORTHWEST OF
ROCKFORD. DVN 00Z RAOB INDICATES ~4800 J/KG OF MUCAPE (~5000
J/KG IN SPC RAP MESOANALYSIS DOWNSTREAM INTO NORTHERN
IL/NORTHWEST IN). DESPITE THIS, WARM PROFILES/WEAK CAPPING IN
LOW LEVELS AND OTHERWISE WEAK LOW-LEVEL FORCING HAD NOT BEEN
SUPPORTING EVEN MUCH IN THE WAY OF LOW-LEVEL CUMULUS ACROSS THE
FORECAST AREA LATE THIS AFTERNOON. HOWEVER, THE MID-LEVEL WAVE
APPEARS TO BE ENHANCING ASCENT, INCREASING EFFECTIVE SHEAR AND
MOISTENING THE PROFILE SUFFICIENTLY FOR SOME THREAT OF
CONVECTIVE DEVELOPMENT INTO THE AREA THROUGH THE REMAINDER OF
THE EVENING AND EARLY OVERNIGHT HOURS. THOUGH CAM GUIDANCE HAS
STRUGGLED WITH THE DETAILS OF DEVELOPMENT/PLACEMENT OF STORMS,
THERE HAS BEEN A SOMEWHAT CONSISTENT SIGNAL FOR AN UPTICK IN
THUNDERSTORM COVERAGE FROM FAR NORTH/NORTHEAST IL INTO NORTHWEST
IN TONIGHT. DESPITE SOME SHALLOW STABILIZATION OF THE BOUNDARY
LAYER AFTER SUNSET, THE DEGREE OF AVAILABLE INSTABILITY, ~30 KTS
OF EFFECTIVE SHEAR AND RAP FORECASTS DCAPE OF ~1000 J/KG WOULD
SUGGEST THESE STORMS COULD BECOME LOCALLY STRONG/SEVERE WITH A
DAMAGING WIND THREAT ESPECIALLY IF ANY ORGANIZED COLD POOL CAN
DEVELOP. CONFIDENCE IS LOW IN JUST HOW CURRENT CELLS NORTHWEST
OF ROCKFORD WILL EVOLVE, BUT THEY'LL BEAR WATCHING. THUNDERSTORM
DEVELOPMENT HAS BEEN A LITTLE SLOWER THAN SOME OF THE GUIDANCE,
THOUGH IT STILL LOOKS AS IF STORMS SHOULD EXIT THE CWA TO THE
SOUTHEAST PRIOR TO DAYBREAK.
OTHERWISE, A WEAK LAKE-ASSISTED COLD FRONT WAS PUSHING SLOWLY
SOUTHWEST ACROSS THE NORTHERN HALF OF THE CHICAGO METRO AT THIS
HOUR, AND THIS MAY FURTHER SLIGHTLY DESTABILIZE THE BOUNDARY
LAYER AS SLIGHTLY COOLER (THOUGH STILL HUMID) AIR OOZES INLAND.
EVEN SOUTH OF THE FRONT, HEAT INDICES HAVE FALLEN INTO THE 90S
WITH SUNSET AND THUS ALLOWS CANCELLATION OF THE HEAT ADVISORY A
LITTLE EARLY.
RATZER
DISCUSSION
ISSUED AT 314 PM CDT FRI SEP 4 2026
BREEZY EAST-NORTHEASTERLY WINDS THIS MORNING IN THE WAKE OF THE
OUTFLOW BOUNDARY HAVE SIGNIFICANTLY IMPACTED THE ABILITY TO
HEAT UP THROUGH EARLY AFTERNOON. THIS HAS PARTICULARLY BEEN TRUE
ACROSS FAR NORTHERN/NORTHEASTERN IL, WHERE TEMPERATURES AS OF 1
PM RANGED FROM THE MID TO UPPER 70S NEAR THE LAKE, TO THE MID
80S FARTHER INLAND. THIS PROMPTED US TO CANCEL THE HEAT ADVISORY
EARLIER FOR COOK, LAKE (IL), MCHENRY AND BOONE COUNTIES. IN
SPITE OF THIS, EXPECT TEMPERATURES TO SPIKE INTO THE UPPER 80S
TO LOW 90S LATE THIS AFTERNOON AS THE LOW-LEVEL FLOW GRADUALLY
VEERS BACK TO THE SOUTH-SOUTHWEST. THIS COULD RESULT IN A COUPLE
HOUR PERIOD OF HEAT INDICES REACHING 95-100 OUTSIDE THE CURRENT
HEAT ADVISORY. CONVERSELY, SLIGHTLY WARMER TEMPERATURES AND
HIGHER HUMIDITY WITHIN THE HEAT ADVISORY AREA WILL SUPPORT PEAK
HEAT INDICES AROUND (OR A BIT ABOVE) 105 DEGREES THROUGH EARLY
EVENING.
IN ADDITION TO STUNTING THE HEAT ACROSS NORTHEASTERN IL, THE
BREEZY EAST-NORTHEASTERLY OUTFLOW WINDS THIS MORNING HAVE
CREATED RATHER CHOPPY CONTINUES ALONG THE SOUTHERN LAKE MICHIGAN
SHORE. WAVE HEIGHTS AS HIGH AS 4.5' WERE REPORTED FROM A BUOY
OFF THE NORTHERN COOK COUNTY SHORE. WAVES OF THIS HEIGHT ARE
HAZARDOUS TO SMALL CRAFT AND SWIMMERS AT AREA BEACHES ACROSS
NORTHEASTERN IL. FOR THIS REASON, WE HOISTED A SMALL CRAFT
ADVISORY AND BEACH HAZARD STATEMENT THROUGH 7 PM THIS EVENING
FOR THE NORTHEASTERN IL LAKE MICHIGAN SHORE. EASING AND VEERING
FLOW SHOULD ALLOW THESE CONDITIONS TO SUBSIDE INTO THIS EVENING.
THE OUTFLOW BOUNDARY FROM THIS MORNING'S CONVECTION ACROSS
SOUTHERN WI HAS SETTLED INTO AREAS SOUTH OF THE KANKAKEE RIVER
VALLEY ACROSS EASTERN IL AND NORTHWESTERN IN EARLY THIS
AFTERNOON. HOWEVER, IT BENDS BACK NORTHWARD ACROSS FAR
NORTHWESTERN IL. THE AIRMASS SOUTH AND WEST OF THIS BOUNDARY
REMAINS QUITE UNSTABLE (PER THE 18Z DVN RAOB). THIS HAS RECENTLY
SUPPORTED THE DEVELOPMENT OF SOME SCATTERED STRONG STORMS WEST-
NORTHWEST OF THE AREA IN CLOSER PROXIMITY TO A SURFACE COLD
FRONT AND APPROACHING MID-LEVEL IMPULSE. IT IS CURRENTLY UNCLEAR
AS TO HOW LONG THIS ACTIVITY WILL HOLD TOGETHER, OWING TO
SUBOPTIMAL DEEP-LAYER SHEAR. NEVERTHELESS, IF THIS ACTIVITY IS
ABLE TO DEVELOP A FORMIDABLE COLD POOL, A CLUSTER OF
SOUTHEASTWARD SHIFT STORMS COULD APPROACH PARTS OF FAR NORTHERN
IL THIS EVENING AFTER 7 PM. CONFIDENCE IN THIS HOLDING TOGETHER
REMAINS LOW AT THIS TIME.
IF THE ACTIVITY TO NORTHWEST DISSIPATES, THERE REMAINS A PLAY
IN WHICH WE EXPERIENCE RENEWED DEVELOPMENT WITH THE FRONT MID TO
LATE THIS EVENING INTO THE OVERNIGHT AS FALLING HEIGHTS ALOFT
OVERSPREAD THE FRONTAL BOUNDARY IN RESPONSE TO THE MAIN
MID/UPPER-LEVEL RIDGE AXIS RETROGRADING WESTWARD AND BUILDING
NORTHWARD ACROSS THE MISSOURI VALLEY. IF THIS OCCURS, THE
FAVORED PLACEMENT WOULD BE ACROSS NORTHEASTERN IL INTO
NORTHWESTERN IN FROM 9PM THROUGH 3 AM. WHILE NOT LOOKING TO BE
MUCH OF A SEVERE THREAT, SOME LOCALLY STRONG WIND GUSTS COULD
ACCOMPANY THE STORMS, PARTICULARLY IF THEY FORM INTO A
SOUTHEASTWARD SHIFTING CLUSTER.
THE PROGRESSION OF THE FRONT IS EXPECTED TO CONTINUE INTO
CENTRAL IL AND IN ON SATURDAY. VERY WARM AND HUMID CONDITIONS
AHEAD OF THIS FRONT COULD RESULT IN TEMPERATURES (HEAT INDICES)
TOPPING 90 (100) AGAIN ACROSS MY FAR SOUTHERN COUNTIES, BUT AT
THIS TIME ANOTHER HEAT ADVISORY IS NOT PLANNED FOR THE AREA.
OTHERWISE, BEHIND THE FRONT, IT WILL BE TURN SOMEWHAT COOLER AND
A BIT LESS HUMID SATURDAY AFTERNOON, BUT A PERIOD OF LOW
STRATUS MAY BE SLOW TO IMPROVE DURING THE AFTERNOON.
TEMPERATURES AND HUMIDITY WILL CONTINUE TO IMPROVE INTO SUNDAY.
A PERIOD OF MODERATELY STRONG NORTHERLY WINDS DOWN THE FULL
FETCH OF THE LAKE SHOULD RESULT IN AN INCREASE IN WAVES LATER
SATURDAY THROUGH EARLY SUNDAY. IT IS POSSIBLE THAT A BEACH
HAZARD STATEMENT MAY BE NEEDED FOR DANGEROUS SWIM CONDITIONS FOR
A PART OF THIS WEEKEND, WHICH IS THE CLOSING WEEKEND OF THE
BEACH SEASON.
DRY AND SEASONABLY WARM WEATHER IS EXPECTED LABOR DAY.
TEMPERATURES WILL WARM SOME AND HUMIDITY WILL BE ON THE INCREASE
TUESDAY INTO WEDNESDAY IN THE WARM AIR ADVECTION REGIME AHEAD
OF THE NEXT SHORTWAVE. AN ASSOCIATED SURFACE COLD FRONT COULD
BRING SOME SHOWERS AND THUNDERSTORMS WITH IT MIDWEEK BEFORE
ANOTHER BOUT OF SOMEWHAT COOLER, MORE SEASONABLE, TEMPERATURES
FILTER IN BEHIND THE FRONT TO CLOSE OUT THE SHORT WORK WEEK.
KJB
AVIATION /00Z TAFS THROUGH 06Z SUNDAY/
ISSUED AT 636 PM CDT FRI SEP 4 2026
FORECAST CONCERNS INCLUDE...
CHANCE OF THUNDERSTORMS THIS EVENING INTO THE EARLY OVERNIGHT.
WIND SHIFT TO NORTHEAST THIS EVENING.
LOW MVFR CIGS, POSSIBLE IFR CIGS SATURDAY MORNING.
POSSIBLE FOG EARLY SATURDAY MORNING.
CONVECTIVE TRENDS REMAIN UNCERTAIN THIS EVENING. SHOWERS ACROSS
FAR SOUTHERN WI CONTINUE TO WEAKEN AS DO THUNDERSTORMS ACROSS
FAR NORTHEAST IA. ADJUSTED PROB THUNDER TIMING EARLY BASED ON
SOME OF THE LATEST SHORT TERM GUIDANCE WHICH SHOWS POTENTIAL NEW
DEVELOPMENT ACROSS NORTHEAST IL THIS EVENING BUT CONFIDENCE
REMAINS LOW AND FURTHER REFINEMENT/CHANGES CAN BE EXPECTED AS
TRENDS EMERGE. THE END TIME OF ANY THUNDERSTORMS ALSO REMAINS
UNCERTAIN. THE LATER THEY DEVELOP, THEY LATER THEY MAY PERSIST.
THERE IS A LAKE BOUNDARY MOVING INLAND ACROSS EASTERN WI AND
INTO VERY FAR NORTHEAST LAKE COUNTY IL CURRENTLY. THIS IS
EXPECTED TO CONTINUE MOVING SOUTHWEST THIS EVENING, WITH A WIND
SHIFT TO NORTHEAST EXPECTED AT ORD/MDW BUT ONLY MEDIUM
CONFIDENCE FOR TIMING. MAINTAINED THE PREVIOUS 03Z TIMING, BUT
WILL ALSO NEED TO MONITOR TRENDS. WINDS MAY BECOME LIGHT AND
VARIABLE FOR A TIME OVERNIGHT, BEFORE TURNING BACK NORTHEAST AND
THEN REMAINING NORTHEAST FOR THE REST OF THE PERIOD WITH SPEEDS
10-15KT LIKELY SATURDAY AFTERNOON.
A PERIOD OF LOW MVFR CIGS IS EXPECTED IN THE PREDAWN HOURS
LASTING THROUGH MIDDAY/EARLY AFTERNOON SATURDAY. THERE MAY ALSO
BE IFR CIGS, ESPECIALLY FOR AREAS NORTH OF THE TERMINALS BUT
CONFIDENCE IS LOW. AREAS OF FOG WILL ALSO BE POSSIBLE OVERNIGHT
INTO SATURDAY MORNING, MAINLY FOR AREAS NORTH AND NORTHWEST OF
THE CHICAGO TERMINALS. CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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