323
FXUS63 KLOT 040802
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
302 AM CDT FRI SEP 4 2026
KEY MESSAGES
- TODAY WILL BE ANOTHER UNSEASONABLY HOT AND HUMID DAY WITH PEAK
AFTERNOON HEAT INDICES OF 100 TO 110 DEGREES.
- 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.
DISCUSSION
ISSUED AT 301 AM CDT FRI SEP 4 2026
AN UNUSUALLY STRONG LATE SEASON MID-UPPER LEVEL RIDGE OF HIGH
PRESSURE CONTINUES TO DOMINATE THE WEATHER ACROSS MUCH OF THE
CENTRAL AND SOUTHERN PORTIONS OF THE COUNTRY EAST OF THE
ROCKIES. SHORTWAVE TROUGHS ROTATING AROUND THE PERIPHERY OF THIS
UPPER RIDGE CONTINUE TO SUPPORT MCS ACTIVITY IN A "RING OF
FIRE" TYPE PATTERN MORE REMINISCENT OF MID-SUMMER THAN EARLY
SEPTEMBER.
ONE SUCH SHORTWAVE IS NOTED ON WATER VAPOR IMAGERY EARLY THIS
MORNING JUST NORTH OF INTERNATIONAL FALLS. DCVA IN ADVANCE OF
THIS SHORTWAVE COMBINED WITH ISENTROPIC ASCENT ASSOCIATED WITH A
MODEST 25-30 KT LOW LEVEL JET HAS LED TO A COUPLE OF SMALL
MCSS. THE ONE OF MOST IMMEDIATE CONCERN LOCALLY IS OVER CENTRAL
WI, NEAR THE NOSE OF THE WEST-SOUTHWESTERLY LOW LEVEL JET. AN HP
SUPERCELL SOUTHWEST OF GREEN BAY HAS DEVELOPED A BIT OF A COLD
POOL AND IS SHOWING SIGNS OF ORGANIZING INTO A SMALL BOWING
CONVECTIVE LINE. FORWARD PROPAGATING CORFIDI VECTORS, 850-300 MB
THICKNESS, AND THE INSTABILITY GRADIENT WOULD ALL SUGGEST THIS
DEVELOPING SMALL BOW SHOULD REMAIN WELL NORTH/NORTHEAST OF OUR
CWA. HOWEVER, AN OUTFLOW BOUNDARY FROM THIS LINE COULD
ACCELERATE DOWN THE LAKE AND RESULT IN A WIND SHIFT TO
NORTHEASTERLY ACROSS PORTIONS OF NORTHEAST IL EARLY THIS
MORNING. THIS HAS BEEN DEPICTED TO VARYING DEGREES BY MANY CAM
SOLUTIONS OVER THE PAST 24 HOURS.
ABSENT ANY SIGNIFICANT INCREASE IN THE SIZE AND STRENGTH OF THE
ATTENDANT COLD POOL WITH THE EASTERN WI MCS, IT SEEMS LIKELY
THAT THE LAKE ENHANCED OUTFLOW SHOULD MIX OUT BY MID-LATE
MORNING WITH WINDS TURNING SOUTHWESTERLY, EVEN RIGHT UP TO THE
LAKEFRONT BY EARLY AFTERNOON. THIS SHOULD SET THE STAGE FOR
ANOTHER DAY OF UNSEASONABLY HOT AND HUMID CONDITIONS. GIVEN
925MB TEMPS AT NEAR RECORD LEVELS IN THE MID-UPPER 20S CELCIUS,
HIGH TEMPS SOLIDLY INTO THE LOW TO MID 90S APPEAR LIKELY TODAY.
POOLING DEWPOINTS NEAR AND SOUTH OF THE SYNOPTIC WARM FRONT,
ENHANCED BY LATE SEASON EVAPOTRANSPIRATION SHOULD RESULT IN
DEWPOINTS IN THE 70S ONCE AGAIN THIS AFTERNOON. PEAK AFTERNOON
HEAT INDICES OF 100 TO 107 DEGREES ARE LIKELY THIS AFTERNOON,
WITH THE TYPICALLY HIGHER DEWPOINT MORE AGRICULTURAL AREAS
LIKELY SEEING PEAK HEAT INDICES AROUND 110 DEGREES AGAIN THIS
AFTERNOON.
FOR THIS REASON, HAVE EXPANDED THE HEAT ADVISORY NORTH TO
ENCOMPASS OUR ENTIRE CWA AGAIN TODAY. HEAT INDICES IN CHICAGO
FAILED TO REACH 100 DEGREES YESTERDAY, SO THE 3+ CONSECUTIVE DAY
100+ HEAT INDEX CRITERIA FOR AN EXTREME HEAT WARNING FOR COOK
COUNTY ISN'T GOING TO BE MET. FOR OUR SOUTHERN CWA, THIS WILL BE
THE 4TH CONSECUTIVE DAY WITH ADVISORY LEVEL HEAT INDICES, WHICH
COULD JUSTIFY AN UPGRADE TO AN EXCESSIVE HEAT WARNING. HOWEVER,
MOST NIGHTS HAVE HAD LOW TEMPERATURES DIPPING INTO THE LOWER
70S WITH MANY HOURS OF RELIEF FROM THE HEAT AT NIGHT, SO OPTED
NOT TO UPGRADE TO AN EXTREME HEAT WARNING FOR THESE AREAS. GIVEN
THE STRONG INSTABILITY PRESENT AND LOW LEVEL JET VEERING TOWARD
THE IL/WI BORDER TOWARD AND JUST AFTER SUNRISE, CERTAINLY
CANNOT RULE OUT THE POSSIBILITY OF SOME ISOLATED TO WIDELY
SCATTERED SHOWERS/STORMS DEVELOPING NEAR AND BEHIND THE LAKE
ENHANCED OUTFLOW BOUNDARY EARLY THIS MORNING INTO NORTHEAST IL.
HOWEVER, FORCING ALONG THIS BOUNDARY IS EXPECTED TO BE PRETTY
SHALLOW AND LOW LEVEL JET LIKELY TO WEAKEN DIURNALLY, SO THREAT
OF ANY ISOLATED SHOWERS/STORMS SHOULD WRAP UP BY MID-LATE
MORNING. THE REMAINDER OF TODAY SHOULD BE DRY WITH NO FORCING
MECHANISMS FOR ANY STORMS THIS AFTERNOON.
THE UPPER RIDGE IS EXPECTED TO BEGIN TO RETROGRADE WESTWARD
TONIGHT WITH MID-UPPER LEVEL HEIGHT FALLS LIKELY TO ALLOW A WEAK
COLD FRONT OVER THE UPPER GREAT LAKES TO BEGIN MOVING
SOUTHWARD. THE TREND IN GUIDANCE HAS GENERALLY BEEN TOWARD A
SLOWER SOUTHWARD PROGRESSION, WHICH WOULD MAKE SENSE GIVEN THE
EXPECTATION OF NOT MUCH CONVECTION TO OUR NORTH TO AID ITS
SOUTHWARD PUSH. THERE IS STILL SOME SIGNAL IN HREF GUIDANCE FOR
SOME CONVECTION TONIGHT AHEAD OF THIS BOUNDARY, BUT WITH PRETTY
MUTED FORCING, OPTED TO GO WITH NOTHING MORE THAN 20-30%
CHANCES.
IF THE PROGRESSION OF THE FRONT CONTINUES TO SLOW IN GUIDANCE,
THEN IT IS PROBABLE THAT WE WILL NEED TO EXTEND THE HEAT
ADVISORY INTO SATURDAY FOR PORTIONS OF OUR SOUTHERN CWA. GIVEN
THAT CONFIDENCE ISN'T ALL THAT HIGH YET ON FRONTAL TIMING, OPTED
TO DEFER ANY ADVISORY EXTENSION FOR THE DAY SHIFT TO CONSIDER.
BEHIND THE FRONT, IT WILL BE SOMEWHAT COOLER AND A BIT LESS
HUMID, BUT PROBABLY STILL A BIT ABOVE AVERAGE FOR SEPTEMBER.
TEMPERATURES AND HUMIDITY WILL CONTINUE TO MODERATE ON 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.
- IZZI
AVIATION /06Z TAFS THROUGH 12Z SATURDAY/
ISSUED AT 1215 AM CDT FRI SEP 4 2026
KEY MESSAGES FOR THE 06Z TAF PERIOD:
* POSSIBILITY OF SHOWERS AND THUNDERSTORMS FRIDAY MORNING
* UNCERTAINTY IN WIND TRENDS FRIDAY MORNING
THUNDERSTORMS ARE EXPANDING IN COVERAGE ACROSS PORTIONS OF
CENTRAL WI AS OF THIS WRITING. A FEW SCATTERED SHOWERS AND
STORMS MAY ATTEMPT TO SPREAD SOUTH ACROSS NORTHERN ILLINOIS
DURING THE MORNING. PROB30S WERE MAINTAINED FOR THIS POTENTIAL
AT ALL TAF SITES, ALTHOUGH LATEST GUIDANCE HAS BEEN LARGELY
DIALING BACK ON COVERAGE, ESPECIALLY THUNDER COVERAGE.
A PASSING SHOWER OR STORM OR TWO WILL ALSO BE POSSIBLE DURING
THE LATE AFTERNOON AND EVENING. CONFIDENCE WAS TOO LOW AT THIS
TIME FOR A MENTION IN TAFS.
LIGHT SSW WINDS WILL PREVAIL THROUGH THE NIGHT. EARLY FRIDAY
MORNING, OUTFLOW FROM THUNDERSTORMS TO THE NORTH MAY SHIFT WINDS
TO NE AT LARGELY BELOW 10 KT FOR A PERIOD DURING THE MORNING AT
THE CHICAGOLAND SITES. THIS LATEST TAF PACKAGE REFLECTS THIS
WIND SHIFT, ALTHOUGH CONFIDENCE ISN'T TERRIBLY HIGH. DIRECTION
MAY DECIDE TO STAY SW THROUGH THE MORNING HOURS, BUT IF NOT, SW
SHOULD RETURN BY EARLY AFTERNOON. EXPECT A SHIFT TO N/NNE EARLY
SATURDAY MORNING NEAR THE END OF THE 30-HR TAF PERIOD AS A COLD
FRONT DRIFTS SOUTH ACROSS THE AREA.
DOOM
LOT WATCHES/WARNINGS/ADVISORIES
IL...HEAT ADVISORY UNTIL 10 PM CDT THIS EVENING FOR ILZ003-ILZ004-
ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-
ILZ105-ILZ106-ILZ107-ILZ108.
IN...HEAT ADVISORY UNTIL 10 PM CDT /11 PM EDT/ THIS EVENING FOR
INZ001-INZ002-INZ010-INZ011-INZ019.
LM...NONE.
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