698
FXUS63 KLOT 040138
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
838 PM CDT THU SEP 3 2026
KEY MESSAGES
- ANOTHER HOT AND HUMID DAY IS IN STORE FRIDAY WITH PEAK HEAT
INDEX VALUES OF 100 TO 105F, WARMEST SOUTH OF I-80.
- TORRENTIAL RAIN-PRODUCING STORMS WITH A THREAT FOR FLASH
FLOODING MAY IMPACT FAR NORTHERN ILLINOIS LATE TONIGHT INTO
EARLY FRIDAY MORNING, THOUGH CONFIDENCE IS HIGHER THAT THEY
REMAIN IN WISCONSIN.
- THERE IS ANOTHER CHANCE FOR SHOWERS AND THUNDERSTORMS FRIDAY
EVENING.
- A BREAK IN THE PATTERN WILL ARRIVE BY SUNDAY WITH LOWERING
HUMIDITY VALUES AND TEMPERATURES.
UPDATE
ISSUED AT 838 PM CDT THU SEP 3 2026
UPDATED EVENING POPS FOR ENDING OF NEAR-TERM
SHOWER/THUNDERSTORM THREAT, AND CANCELLED NORTHERNMOST TIER OF
HEAT ADVISORY A LITTLE EARLY THIS EVENING WITH HEAT INDICES
BELOW CRITERIA.
ISOLATED THUNDERSTORM THREAT HAS ENDED AS OF MID-EVENING, WITH
THE LOSS OF DIURNAL LOW-LEVEL INSTABILITY WITH SUNSET. ATTENTION
NOW TURNS TO THE UPPER MIDWEST (NORTHERN WI BACK INTO NORTHERN
MN/EASTERN ND) WHERE CLUSTERS OF THUNDERSTORMS ARE ONGOING IN
RESPONSE TO A MID-LEVEL WAVE TRACKING EAST-NORTHEAST ACROSS THAT
REGION. THIS WAVE IS EXPECTED TO TAKE ON A MORE EAST-SOUTHEAST
PROPAGATION OVERNIGHT, AS THE UPPER LEVEL RIDGE AXIS STRENGTHENS
WEST OF THE UPPER MISSISSIPPI VALLEY. A VERY WARM AND HUMID
LATE SUMMER AIR MASS REMAINS IN PLACE SOUTH OF A NEARLY
STATIONARY FRONTAL BOUNDARY STRETCHING FROM THE UPPER MIDWEST TO
THE LOWER GREAT LAKES, WITH A CORRESPONDING RESERVOIR OF
INSTABILITY ALONG/SOUTH OF THE FRONT. LATEST AVAILABLE CAM
GUIDANCE CONTINUES TO DISPLAY A BIT OF SPREAD EVEN IN RECENT HIGH-
RES INITIALIZATIONS, THOUGH THE GENERAL EXPECTATION IS THAT
ONGOING CONVECTIVE CLUSTERS OVER THE UPPER MIDWEST WILL
EVENTUALLY PROPAGATE SOUTHEAST AS ONE OF MORE MCSS ACROSS WI,
LAKE MICHIGAN AND LOWER MICHIGAN FROM LATER TONIGHT THROUGH
FRIDAY MORNING. HOW FAR SOUTHWEST CONVECTION BUILDS REMAINS
UNCERTAIN, THOUGH CERTAINLY CAN'T RULE OUT SOME OF THE SOLUTIONS
WHICH SWIPE FAR NORTHERN/NORTHEAST IL AND NORTHWEST IN TOWARD
AND AFTER DAYBREAK FRIDAY. HAVE MAINTAINED INHERITED CHANCE
(30-40%) POPS FOR THIS ROUGHLY ALONG/NORTH OF A ROCKFORD-
VALPARAISO LINE AFTER 4-5 AM. SPC'S LATEST UPDATE THROUGH 12Z
FRIDAY KEEPS A MARGINAL SEVERE RISK (LEVEL 1 OF 5) FOR WIND
ACROSS OUR NORTHEAST CWA FOR THIS POTENTIAL. MOST CURRENT
GUIDANCE DECREASES SHOWER/THUNDERSTORM CHANCES FOR THE AREA MID-
MORNING AND BEYOND.
HAVE CANCELLED THE HEAT ADVISORY A LITTLE EARLY THIS EVENING
FOR THE TIER OF COUNTIES FROM LEE TO WILL IN IL AND LAKE AND
PORTER IN NORTHWEST IN. THESE AREAS WERE AFFECTED BY OUTFLOW AND
ONSHORE FLOW THIS AFTERNOON, AND HEAT INDICES HAD FALLEN BELOW
ADVISORY CRITERIA. TO THE SOUTH, HEAT HEADLINES REMAIN IN
EFFECT INTO FRIDAY EVENING WITH ANOTHER DAY OF 100-105 DEGREE
PEAK HEAT INDEX READINGS EXPECTED. WITH THE POTENTIAL FOR
CONVECTION AND/OR OUTFLOW (AND POSSIBLY AN AFTERNOON LAKE
BREEZE) ACROSS THE FAR NORTH AND NORTHEAST PARTS OF THE
FORECAST AREA FRIDAY, NO HEADLINES ARE CURRENTLY IN EFFECT.
HOWEVER, IF CONVECTION OR OUTFLOW DOES NOT EXTEND AS FAR
SOUTH/SOUTHWEST INTO THE FORECAST AREA AS EXPECTED, IT IS
POSSIBLE THE ADVISORY FOOTPRINT COULD NEED TO BE EXTENDED BACK
TO THE NORTH.
RATZER
DISCUSSION
ISSUED AT 307 PM CDT THU SEP 3 2026
AN OUTFLOW BOUNDARY HAS PUSHED SOUTH TO THE I-80 CORRIDOR EARLY
THIS AFTERNOON. NORTHERLY FLOW BEHIND THE BOUNDARY HAS
MODERATED TEMPERATURES ACROSS FAR NORTHERN ILLINOIS (PRIMARILY
NORTH OF I-88), ALLOWING FOR THE EARLY CANCELLATION OF HEAT
HEADLINES. WHILE EXTREME HEAT HAS ENDED NORTH OF THE BOUNDARY,
EXPECT CONDITIONS TO REMAIN HUMID THROUGH THIS EVENING.
HEAT HEADLINES REMAIN IN EFFECT SOUTH OF I-80, WHERE AFTERNOON
TEMPERATURES ARE REACHING THE LOW TO MID-90S WITH PEAK HEAT
INDICES AROUND 105. ADDITIONALLY, THE HEAT ADVISORY HAS BEEN
EXTENDED THROUGH FRIDAY FOR AREAS GENERALLY ALONG AND SOUTH OF A
LINE FROM MENDOTA, IL, THROUGH KANKAKEE, IL, AND RENSSELAER,
IN.
THE THREAT FOR ADDITIONAL SHOWERS AND THUNDERSTORMS IN OUR AREA
THE REMAINDER OF THE DAY OVERALL LOOK LOW (20-30%), AND OF LOW
AERIAL COVERAGE, OWING TO NEBULOUS LARGE SCALE FORCING NEAR THE
MID-LEVEL RIDGE AXIS. IF ANY STORMS ARE ABLE TO DEVELOP THIS
AFTERNOON, THEIR FAVORED PLACEMENT WILL BE NEAR THE OUTFLOW
BOUNDARY FROM FAR EAST CENTRAL IL INTO NORTHERN IN. OTHERWISE,
WE ARE LOOKING TO HAVE A DRY BUT MUGGY EVENING.
TONIGHT,AN UPPER-LEVEL SHORT-WAVE TROUGH WILL TRACK FROM THE
NORTHERN PLAINS ACROSS THE UPPER MIDWEST, DRIVING A COMPLEX OF
THUNDERSTORMS ACROSS THE LOWER GREAT LAKES. WHILE THE MAIN
COMPLEX MAY STAY NORTHEAST OF OUR AREA, RENEWED CONVECTION IS
PLAUSIBLE FRIDAY MORNING ALONG ITS SOUTHWESTERN FLANK AS A
SOUTHWESTERLY LOW-LEVEL JET INTERACTS WITH A COMPOSITE OUTFLOW
BOUNDARY. THIS SETUP COULD SUPPORT STORM TRAINING AND A
LOCALIZED FLASH FLOOD THREAT, THOUGH CONFIDENCE REMAINS LOW WITH
THE HIGHEST POTENTIAL NORTH OF OUR AREA. REGARDLESS, THERE IS A
30-50% CHANCE OF SCATTERED SHOWERS AND THUNDERSTORMS ACROSS FAR
NORTHEASTERN IL AND NORTHWEST IN BETWEEN 2 AM AND 10 AM FRIDAY.
FOLLOWING MORNING ACTIVITY, CAPPING SHOULD MAINTAIN DRY
CONDITIONS THROUGH FRIDAY AFTERNOON. WE ARE MONITORING FOR
SCATTERED THUNDERSTORMS DEVELOPING ACROSS SOUTHERN WISCONSIN
LATE FRIDAY AFTERNOON OR EVENING ALONG A COLD FRONT, WHICH WOULD
LIKELY SHIFT SOUTHEAST INTO OUR AREA FRIDAY NIGHT. THE SPC HAS
INCLUDED OUR AREA IN A LEVEL 1 OF 5 SEVERE THREAT DUE TO THE
POTENTIAL FOR LOCALLY STRONG WIND GUSTS.
SURFACE HIGH PRESSURE BUILDING INTO THE GREAT LAKES THIS WEEKEND
WILL END OUR HOT AND HUMID SPELL, AT LEAST FOR A FEW DAYS.
TEMPERATURES WILL TREND TOWARD THE UPPER 70S NEAR THE LAKE TO
THE LOW 80S INLAND BY SUNDAY. A MODERATE TO HIGH SWIM RISK IS
POSSIBLE AT LAKE MICHIGAN BEACHES LATE SATURDAY THROUGH SUNDAY
DUE TO PERSISTENT ONSHORE FLOW AND CHOPPY CONDITIONS.
KJB
AVIATION /00Z TAFS THROUGH 06Z SATURDAY/
ISSUED AT 640 PM CDT THU SEP 3 2026
FORECAST CONCERNS INCLUDE...
CHANCE OF THUNDERSTORMS FRIDAY MORNING.
WIND SHIFT TO NORTHEAST FRIDAY MORNING.
GUSTY SOUTHWEST WINDS FRIDAY AFTERNOON.
DRY WEATHER IS EXPECTED THROUGH AT LEAST THE EARLY OVERNIGHT
HOURS WITH ANY ISOLATED SHOWERS AND THUNDERSTORMS THIS EVENING
REMAINING WELL SOUTH OF THE TERMINALS. ATTENTION TURNS TO
CONVECTION ACROSS NORTHERN MN AND NORTHWEST WI. THIS ACTIVITY IS
EXPECTED TO MOVE SOUTHEAST TONIGHT AND POSSIBLY REACH NORTHEAST
IL AROUND DAYBREAK. CONFIDENCE IS LOW FOR HOW WIDESPREAD THIS
ACTIVITY WILL BE ACROSS NORTHERN IL/NORTHWEST IN AND THE
TERMINALS. IT IS POSSIBLE THE BULK OF THIS ACTIVITY MAY PASS TO
THE NORTHEAST OF THE LOCAL AREA. CURRENT PROB THUNDER MENTION/
TIMING HAS THE POTENTIAL HANDLED WELL AND MADE NO CHANGES. EVEN
IF THE CONVECTION REMAINS NORTHEAST OF THE TERMINALS, AN
OUTFLOW BOUNDARY WILL STILL BE LIKELY, SHIFTING WINDS TO THE
NORTHEAST SOMETIME AFTER DAYBREAK FRIDAY MORNING. WHILE THE
FORECAST REMAINS UNCHANGED, CHANGES ARE POSSIBLE AS TRENDS
EMERGE LATER TONIGHT.
THERE MAY BE ADDITIONAL CHANCES FOR THUNDERSTORMS, POSSIBLY
EARLY/MID FRIDAY EVENING, FOR THE TERMINALS, BUT CONFIDENCE IS
TOO LOW TO INCLUDE ANY MENTION WITH THIS FORECAST.
A LAKE BREEZE HAS MOVED WEST OF ORD/MDW AND WILL DISSIPATE WITH
SUNSET. EAST/SOUTHEAST WINDS BEHIND THIS BOUNDARY WILL BECOME
SOUTHERLY THIS EVENING AND THEN SOUTH/SOUTHWEST FOR THE ENTIRE
AREA OVERNIGHT. AS NOTED ABOVE, A PERIOD OF NORTHEAST WINDS IS
EXPECTED FRIDAY MORNING, THAT ARE THEN EXPECTED TO SHIFT TO THE
SOUTHWEST BY EARLY FRIDAY AFTERNOON WITH GUSTS INTO THE 20KT
RANGE POSSIBLE, WHICH WILL DIMINISH WITH SUNSET FRIDAY EVENING.
CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...HEAT ADVISORY UNTIL 7 PM CDT FRIDAY FOR ILZ019-ILZ021-ILZ023-
ILZ032-ILZ033-ILZ039.
IN...HEAT ADVISORY UNTIL 7 PM CDT /8 PM EDT/ FRIDAY FOR INZ010-
INZ011-INZ019.
LM...NONE.
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