151
FXUS63 KLOT 310505
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1205 AM CDT MON AUG 31 2026
KEY MESSAGES
- PERIODIC SHOWER AND STORMS THROUGH THE EARLY PART OF THE WEEK,
THOUGH THERE WILL AMPLE DRY PERIODS.
- HOTTER AND MORE HUMID CONDITIONS BUILD THROUGH MIDWEEK.
DISCUSSION
ISSUED AT 231 PM CDT SUN AUG 30 2026
A SHORTWAVE DISTURBANCE OVER NORTHEASTERN IOWA WILL CONTINUE TO
DRIVE SHOWERS AND EMBEDDED THUNDERSTORMS EASTWARD THROUGH EARLY
THIS EVENING, WITH THE GREATEST COVERAGE EXPECTED NORTH OF
I-80. WHILE A 20% CHANCE FOR LIGHTNING REMAINS, POOR MID-LEVEL
LAPSE RATES ACROSS NORTHERN ILLINOIS SHOULD LIMIT THUNDERSTORMS
TO ISOLATED COVERAGE.
FOLLOWING THE EASTWARD DEPARTURE OF THIS SHORT-WAVE IMPULSE
THIS EVENING, A SOUTHWESTERLY LOW-LEVEL JET WILL RAMP UP AGAIN
OVERNIGHT, WITH THE CORE EXPECTED TO RESIDE OVER IA
NORTHEASTWARD INTO SOUTHERN WI. ASSOCIATED ISENTROPIC UPGLIDE
(WARM AIR ADVECTION) MAY THUS FOSTER MORE ISOLATED TO SCATTERED
SHOWERS AND THUNDERSTORMS LATE TONIGHT INTO EARLY MONDAY
MORNING. THEREAFTER, CAPPING IS LIKELY TO CURTAIL THE THREAT OF
CONVECTION FOR MUCH OF THE AREA INTO MONDAY AFTERNOON. HOWEVER,
ANOTHER SHORT-WAVE IMPULSE IS PROGGED TO TRANSLATE EASTWARD
TOWARDS NORTHWESTERN IL LATE IN THE DAY, AND THUS MAY ACT AS THE
IMPETUS FOR ISOLATED TO WIDELY SCATTERED AFTERNOON DEVELOPMENT
ACROSS NORTHWESTERN IL AFTER 2 PM. SHOULD THIS ACTIVITY OCCUR,
~40 KT OF MID-LEVEL WESTERLY FLOW COULD SUPPORT A FEW STRONGER
STORMS ACROSS NORTHWESTERN IL LATE IN THE AFTERNOON. CONFIDENCE
REMAINS LOW WITH THE EXTENT OF THIS THREAT.
CONVECTIVE TRENDS CONTINUE TO BE OF LOW CONFIDENCE MONDAY NIGHT
INTO TUESDAY MORNING. NEVERTHELESS, LOW (20-30%)CHANCES FOR
STORMS CONTINUE DUE TO THE PERSISTENT WARM-AIR ADVECTION PATTERN
IN COMBINATION WITH ADDITIONAL SMALLER SCALE PERTURBATIONS
LIKELY TO TRANSITION EASTWARD ACROSS SOUTHERN WI THROUGH THE
MID-LEVEL RIDGE AXIS. THE BETTER FOCUS FOR STORMS MAY REMAIN IN
SOUTHERN WI, BUT AT THIS TIME, CANNOT RULE OUT STORMS AS FAR
SOUTH AS THE I-80 CORRIDOR. THE SPC DAY 3 SEVERE OUTLOOK (FOR
TUESDAY) PLACES MOST AREAS NORTH OF I-80 IN A LEVEL 1 OF 5
THREAT. SO, WHILE CONFIDENCE IN THE COVERAGE OF STORMS REMAINS
LOW INTO TUESDAY MORNING, THE METEOROLOGICAL ENVIRONMENT WILL BE
SUPPORTIVE OF A CONDITIONAL THREAT FOR STRONG STORMS.
OUTSIDE OF THE THREAT OF STORMS THROUGH TUESDAY MORNING,
TEMPERATURES AND HUMIDITY LEVELS WILL BE ON THE RISE THROUGH
MID-WEEK AS THE AIRMASS UNDER THE EXPANSIVE MID/UPPER-LEVEL
RIDGE CONTINUES TO WARM. WE ANTICIPATE HIGH TEMPERATURES IN THE
UPPER 80S MONDAY, THE LOW 90S TUESDAY AND THE LOW TO MID 90S
WEDNESDAY. IN SPITE OF US BEING PAST THE PEAK TIMING FOR
EVAPOTRANSPIRATION, LIMITED MIXING UNDER AN INVERSION CENTERED
NEAR 850 MB SHOULD KEEP DEW POINTS IN THE LOW TO MID 70S DURING
PEAK HEATING, PARTICULARLY ON TUESDAY AND WEDNESDAY, RESULTING
IN PEAK HEAT INDICES IN THE 100 TO 105 RANGE.
RELIEF FROM THE HEAT LOOKS PLAUSIBLE LATER IN THE WEEK AS A
BACKDOOR COLD FRONT SAGS SOUTHWARD INTO THE AREA. HOWEVER, THIS
LOOKS TO COME AT THE EXPENSE OF A PERIOD OF MORE ACTIVE WEATHER
(A PERIOD OR TWO OF THUNDERSTORMS, SOME POSSIBLY STRONG) AT THE
END OF THE WEEK. THE SPECIFICS SUCH AS THE TIMING OF THE FRONT
AND BEST POTENTIAL FOR THUNDERSTORMS REMAIN A BIT UNCLEAR AT
THIS POINT, BUT STORM CHANCES BEGIN TO RAMP UP ACROSS OUR AREA
LATE WEDNESDAY NIGHT INTO THURSDAY.
KJB
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.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page