416
FXUS63 KLOT 310711
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
211 AM CDT MON AUG 31 2026
KEY MESSAGES
- PERIODIC SHOWER AND STORMS PERSIST THIS WEEK, THOUGH THERE
WILL AMPLE DRY PERIODS.
- HOTTER AND MORE HUMID CONDITIONS BUILD MIDWEEK.
DISCUSSION
ISSUED AT 200 AM CDT MON AUG 31 2026
THUNDERSTORMS HAVE FIZZLED OUT, BUT ISOLATED SHOWERS FESTER OVER
THE FORECAST AREA EARLY THIS MORNING. SIMILAR TO LAST NIGHT, A
SOUTHWEST TO NORTHEAST LOW LEVEL JET (LLJ) IS EXPECTED TO
AMPLIFY PRIOR TO DAYBREAK AND HELP INDUCE ADDITIONAL SHOWER AND
STORM COVERAGE. THERE IS LOWER CONFIDENCE ON THE EXACT
COVERAGE; BUT CONSIDERING HOW IMPRESSIVE LAST NIGHT/YESTERDAY
MORNING'S COVERAGE ENDED UP BEING, WIDELY SCATTERED SHOWERS AND
STORMS CAN BE EXPECTED THROUGH DAYBREAK BEFORE GRADUALLY
DIMINISHING (AS THE LLJ WEAKENS) AND MOVES EAST. FORCING BECOMES
FAIRLY NEBULOUS BEHIND THE LLJ. MOST AREAS SHOULD RECEIVE A
PERIOD OF DRY CONDITIONS AROUND MIDDAY, BUT MAINTAINED SLIGHT
CHANCE POPS FOR A STRAY DIURNAL SHOWER THAT MIGHT POP-UP THIS
AFTERNOON, MAINLY NORTH OF INTERSTATE 88.
STEPPING BACK AND ZOOMING OUT FOR A SECOND, GOES IR AND WATER
VAPOR SATELLITE IMAGERY CURRENTLY SHOWS BROAD CLOUD COVER
SWEEPING ACROSS THE CONUS FROM THE FOUR CORNERS, TO THE
MINNESOTA ARROWHEAD, DOWN TO OUR NATION'S CAPITAL. THEIR
MOVEMENT IS RIDING OVER AN UPPER LEVEL RIDGE THAT HAS DOMINATED
THE FORECAST FOR THE LAST SEVERAL DAYS. IR IS SHOWING SHOWERS
AND STORMS OVER EASTERN COLORADO/NORTHWESTERN KANSAS THAT IS
COLLOCATED WITH AN UPPER LEVEL SHORT WAVE EMBEDDED IN THE
RIDGE. THIS WAVE IS EXPECTED TO QUICKLY MOVE ACROSS THE GREAT
PLAINS AND HEAD TOWARD SOUTHWESTERN WISCONSIN TODAY. MODEL
SOUNDINGS ARE SHOWING A FAIRLY STRONG CAP AND A ROBUST DRY AIR
OVER MUCH OF THE AREA THIS AFTERNOON AND EARLY EVENING WITH
DCAPE VALUES AROUND 1000 J/KG IN NORTHWEST ILLINOIS. THE CAP
MIGHT BE ENOUGH TO INHIBIT SHOWER AND STORM DEVELOPMENT, BUT
LAPSE RATES ARE EXPECTED TO STEEPEN ALONG WITH MODEST DEEP LAYER
SHEAR WITH THE ARRIVAL OF THE SHORT WAVE. ANY ELEVATED STORM
THAT DEVELOPS UPSTREAM STRENGTHEN AND POSSIBLY PRODUCE SOME
HAIL UP TO THE SIZE OF A QUARTER OR POTENTIALLY LOCALIZED
DAMAGING WIND GUSTS, MAINLY FOR AREAS NORTH OF INTERSTATE 88.
THE MAIN BIT OF UNCERTAINTY IS THE EXACT TRACK OF THE WAVE. IF
IT FOLLOWS THE RIDGE, IT COULD MOVE RIGHT OVER NORTHERN
ILLINOIS, WITH FAVORED AREAS CLOSER TO THE WISCONSIN STATELINE.
BUT SOME MODELS HAVE A SLIGHTLY MORE NORTHERN TRACK INTO
WISCONSIN WHICH WOULD LIMIT SHOWER/STORM COVERAGE.
EVEN AS THE WAVE PASSES TO THE NORTHEAST, BROAD WARM AIR
ADVECTION BEHIND THE WAVE COULD ALLOW FOR FESTERING SHOWERS ONCE
AGAIN OVERNIGHT INTO EARLY TUESDAY MORNING. FOR NOW, KEPT
"SILENT" POPS (10 TO 14 PERCENT CHANCE) WITH NO FORMAL FORECAST
MENTION, BUT TRENDS WILL BE MONITORED. AT THE RISK OF SOUNDING
LIKE A BROKEN RECORD, BUT MODELS ARE SUGGESTING YET ANOTHER
WAVE EMBEDDED WITHIN THE RIDGE IS EXPECTED TO PASS NEARBY ON
TUESDAY MORNING/EARLY AFTERNOON. HOWEVER, MODELS ARE SHOWING A
LOT OF DISAGREEMENT WITH THIS WAVE. THE NAM CURRENTLY HAS THE
NORTHERNMOST SOLUTION THAT MAY KEEP MOST OF THE FORECAST AREA
DRY, WHEREAS THE LATEST 00Z GUIDANCE IS STARTING TO BRING THAT
WAVE CLOSER DOWN INTO NORTHERN ILLINOIS. WITH INCREASING
INSTABILITY AHEAD OF THE WAVE, IT PROVIDES ANOTHER CHANCE WHERE
STORMS COULD BECOME STRONG TO SEVERE. THIS ALSO JUST HIGHLIGHTS
THE PATTERN THAT THE REGION CURRENTLY FINDS ITSELF IN WITH
PERSISTENT WAVES WITHIN THE RIDGE PATTERN THAT KEEPS PERIODIC
SHOWER/STORM CHANCES IN THE FORECAST. HOWEVER, SHOWER/STORM
CHANCES ON TUESDAY COULD IMPACT THE TEMPERATURE FORECAST.
AS THE UPPER LEVEL RIDGE SLOWLY AMBLES EAST THROUGH MIDWEEK,
SOUTHWEST WINDS ARE EXPECTED TO ADVECT IN A WARMER AIR MASS AND
HIGHER DEWPOINTS INTO THE REGION. HIGH TEMPERATURES ARE EXPECTED
TO CLIMB INTO THE LOW 90S WITH DEW POINT TEMPERATURES INTO THE
LOW TO POSSIBLY MID 70S. MODELS HAVE BEEN FAIRLY CONSISTENT IN
SUGGESTING WEDNESDAY COULD BE THE HOTTEST DAY OF THE WEEK WHERE
HEAT INDEX VALUES COULD REACH THE MID TO UPPER 90S FOR MUCH OF
THE FORECAST AREA, AND BETTER CHANCES THAT THEY COULD MEET OR
EXCEED 100 SOUTH OF INTERSTATE 80. CONFIDENCE LOWERS IN EXACT
HEAT VALUES FROM SOUTH TO NORTH ACROSS THE FORECAST AREA WITH
GREATEST UNCERTAINTY CLOSER TO THE TOP OF THE RIDGE (NORTH OF
INTERSTATE 80) GIVEN THE BETTER CHANCES FOR SHOWERS AND STORMS
THERE.
MODELS ARE SUGGESTING THAT THE RIDGE COULD BREAK DOWN BY THE
END OF THE WORK WEEK. THAT WILL HELP THE HOTTEST OF TEMPERATURES
COOL OFF A BIT, BUT IT WILL RETURN THE UPPER LEVEL FLOW PATTERN
TO THE NORTHWEST AND THUS BRING WITH IT ADDITIONAL CHANCES FOR
SHOWERS/STORMS TOWARD AND OVER THE WEEKEND.
DK
AVIATION /06Z TAFS THROUGH 12Z TUESDAY/
ISSUED AT 1204 PM CDT MON AUG 31 2026
PER RECENT REGIONAL VAD WIND PROFILES, A SOUTHWESTERLY LOW-
LEVEL HAS REDEVELOPED TONIGHT ACROSS THE MIDDLE MISSISSIPPI
RIVER VALLEY AND IS POINTED TOWARD THE LOWER GREAT LAKES. WHILE
THE DIFFERENCE BETWEEN THE SURFACE AND 2000 FOOT WIND SPEEDS IS
AROUND 30 KT, THE CHANGE IN WIND DIRECTION IS 40 TO 70 DEGREES,
SO OPTED TO TOSS IN A MENTION LLWS AT RFD (THE WIND AT 2000 FEET
IS COMPARATIVELY WEAKER TOWARD LAKE MICHIGAN). BROKEN CLOUDS
BASED BETWEEN 7000 AND 15000 FEET WILL PREVAIL THROUGH THE
NIGHTTIME HOURS.
SIMILAR TO YESTERDAY, AM ANTICIPATING THE DEVELOPMENT OF
SCATTERED SHOWERS AND THUNDERSTORMS TOWARD DAYBREAK AS THE LOW-
LEVEL JET VEERS LEADING TO INCREASING ISENTROPIC ASCENT ACROSS
THE LOWER GREAT LAKES. WILL MAINTAIN THE INHERITED PROB30
GROUPS FOR THUNDER AT ALL TERMINALS GENERALLY BETWEEN 11 AND 15Z
AND CONSIDER AN UPGRADE TO A TEMPO WITH THE 12Z TAF SET AS
ACTIVITY IS PRESUMABLY DEVELOPING (BY THEN, SHOULD HAVE A BETTER
HANDLE ON COVERAGE). SOUTHWEST WINDS WILL INCREASE WITH GUSTS
UP TO ABOUT 20KT FROM LATE MORNING THROUGH EARLY AFTERNOON.
ALSO SIMILAR TO YESTERDAY, AN UPPER-LEVEL WAVE CURRENTLY MOVING
ACROSS NEBRASKA WILL ARRIVE IN THE IOWA/ILLINOIS/WISCONSIN
BORDER VICINITY THIS AFTERNOON LEADING TO AN UPTICK IN SHOWERS
AND THUNDERSTORMS. AM SUSPICIOUS OF GUIDANCE DEPICTING MINIMAL
COVERAGE IN AN OTHERWISE LARGELY UNCAPPED AND UNSTABLE
ATMOSPHERE, THOUGH DO THINK COVERAGE MAY BE HIGHEST NEAR THE
WISCONSIN STATE LINE CLOSER TO THE TRACK OF THE WAVE.
ACCORDINGLY, WILL MAINTAIN THE INHERITED PROB30 GROUPS FOR
THUNDER GENERALLY IN THE 21-24Z WINDOW THIS AFTERNOON. WITH A
DRY SUB-CLOUD LAYER, CANNOT RULE OUT LOCALLY GUSTY WINDS UP TO
AROUND 30KT WHERE THUNDERSTORMS OCCUR.
SHOWERS MAY LINGER BEYOND SUNSET ACROSS THE AREA, ESPECIALLY AS
BROAD BUT GENERALLY UNFOCUSED ISENTROPIC ASCENT FESTERS ACROSS
THE LOWER GREAT LAKES REGION. WILL LEAVE THE TAFS DRY OVERNIGHT
FOR NOW, BUT WOULD NOT BE SURPRISED IN THE LEAST TO HAVE TO
INTRODUCE -SHRA OR -TSRA IN A LATER TAF PACKAGE. FINALLY, AM
NOTING A SIGNAL FOR A WEAK FRONTAL BOUNDARY TO WASH OUT OVER THE
TERMINALS AFTER SUNSET. SHOULD THAT OCCUR, WINDS MAY BECOME
LIGHT AND VARIABLE THROUGH THE LAST 6 TO 12 HOURS OF THE TAF.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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