037
FXUS63 KLSX 151717
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
1217 PM CDT TUE SEP 15 2026
KEY MESSAGES
- UNSEASONABLY HOT AND HUMID WEATHER CONTINUES THIS WEEK WITH
HEAT INDEX VALUES REACHING 105F+ ALONG AND SOUTH OF I-70
TODAY.
- SCATTERED SHOWERS AND THUNDERSTORMS (30-70%) ARE FORECAST
TODAY AND TONIGHT, WITH THE MAIN THREATS BEING LOCALIZED
DAMAGING WINDS AND HEAVY RAINFALL.
SHORT TERM /THROUGH WEDNESDAY/
ISSUED AT 337 AM CDT TUE SEP 15 2026
THE LATEST MID-LEVEL GOES-19 WATER VAPOR IMAGERY REVEALS A LONG
FETCH OF MOISTURE BEING ADVECTED NORTHEASTWARD INTO THE MIDWEST FROM
THE PACIFIC/BAJA COAST. THIS FEED OF MOISTURE IS BEING DRIVEN BY AN
ANOMALOUSLY STRONG MID-LEVEL RIDGE (HEIGHTS ABOVE CLIMATOLOGICAL
MAXIMUMS) CENTERED NEAR THE TENNESSEE RIVER VALLEY. AT THE SURFACE,
AN ANTICYCLONE IS POSITIONED ACROSS NEW ENGLAND WITH A ROBUST MID-
LATITUDE CYCLONE TRAVERSING EASTWARD JUST NORTH OF THE US-CANADIAN
BORDER. THE ATTENDANT COLD FRONT HAS MADE ITS WAY INTO IA EARLY THIS
MORNING WITH A BROAD REGION OF CONVECTION ALONG THIS BOUNDARY BEING
AIDED BY THE NOCTURNAL LLJ ENHANCING MOISTURE CONVERGENCE. THIS
BOUNDARY AND ASSOCIATED PRECIPITATION IS FORECAST TO REMAIN JUST
NORTH OF THE AREA (ACROSS IA) THROUGH THE EARLY AFTERNOON TODAY.
ANOMALOUSLY WARM 850MB TEMPS (>20C), CONTINUED SOUTHERLY FLOW, AND
ABUNDANT SOLAR INSOLATION WILL COMBINE TO CREATE NEAR RECORD HEAT
ACROSS MOST OF THE REGION TODAY. HIGHS ALONG AND SOUTH OF I-70 ARE
EXPECTED TO LARGELY REACH THE UPPER 90S WITH A FEW ISOLATED
LOCATIONS POTENTIALLY TOPPING 100F. THESE NEAR RECORD TEMPS WITH
DEWPOINTS IN THE LOW 70S (NEAR RECORD VALUES) WILL LEAD TO HEAT
INDEX VALUES OF 105-110 ACROSS PORTIONS OF THE AREA ALONG AND SOUTH
OF I-70 WHERE A HEAT ADVISORY IS IN EFFECT.
BY MID-LATE AFTERNOON, FORECAST SOUNDINGS INDICATE SOME LOCATIONS
WILL REACH THE CONVECTIVE TEMPERATURE, RESULTING IN AN UNCAPPED
ENVIRONMENT. AT THE SAME TIME, THE EFFECTIVE COLD FRONT AND ANY
REMNANT OUTFLOW BOUNDARIES FROM CONVECTION TO THE NORTH THIS
MORNING, SHOULD PROVIDE ENOUGH LIFT TO INITIATE SCATTERED
THUNDERSTORM DEVELOPMENT ACROSS PORTIONS OF THE AREA. THE BEST DEEP-
LAYER SHEAR WILL BE ALONG/POLEWARD OF THE EFFECTIVE COLD FRONT,
PLACING MOST OF THE AREA IN MARGINAL SHEAR, GENERALLY 15-20KTS. AS A
RESULT, THE STORM MODE IS FORECAST TO BE PULSE CONVECTION WITH THE
MAIN THREAT BEING LOCALIZED DOWNBURSTS POTENTIALLY LEADING TO
DAMAGING WIND GUSTS. A SOUTHWESTERLY ORIENTED NOCTURNAL LLJ (15-
25KTS) IS EXPECTED TO FORM TONIGHT, WITH THE NOSE MOSTLY FOCUSED
ACROSS PORTIONS OF NORTHWESTERN/NORTH-CENTRAL MO. WITH THE EFFECTIVE
BOUNDARY LIKELY ACROSS CENTRAL/NORTHERN PORTIONS OF THE AREA, THIS
WILL LEAD TO MOISTURE CONVERGENCE THAT SHOULD SUSTAIN ONGOING
SHOWERS AND THUNDERSTORMS. IT APPEARS THAT THE ORIENTATION OF THE
LLJ AND THE POTENTIAL LOCATION OF THE LOW-LEVEL BOUNDARY FAVORS
CONTINUED CONVECTION TONIGHT ACROSS PORTIONS OF NORTHEAST MO/WEST-
CENTRAL IL. FORECAST SOUNDINGS REVEAL THAT PWATS NEAR 95TH
PERCENTILES (2"), DEEP WARM CLOUD LAYERS (>12,000FT), AND UNIFORM
CAPE PROFILES WILL FAVOR WARM RAIN PROCESSES LEADING TO EFFICIENT
RAINFALL PRODUCTION. AS A RESULT, RAINFALL RATES OF 1-2" PER HOUR
WILL BE POSSIBLE IN LOCALIZED INSTANCES ACROSS PORTIONS OF THE AREA
TONIGHT. CURRENTLY, THE MOST UNCERTAIN VARIABLE REGARDING HEAVY
RAINFALL TONIGHT IN OUR AREA IS THE COVERAGE OF CONVECTION, AS THERE
IS NO FOCUSED AREA OF FORCING. THIS WOULD RESULT IN SHARP RAINFALL
GRADIENTS WITH SOME AREAS RECEIVING NO RAINFALL WHILE LOCALIZED
POCKETS OF 2-3" ARE POSSIBLE, GENERALLY ACROSS PORTIONS OF
NORTHERN MO. ALL THAT TO SAY THE POTENTIAL FOR HEAVY RAINFALL
AND FLASH FLOODING ACROSS PORTIONS OF NORTHEAST MO/WEST-CENTRAL
IL IS THERE AND WILL NEED TO BE MONITORED INTO THIS EVENING.
AS THE NOCTURNAL LLJ SLOWLY WANES AND THE EFFECTIVE BOUNDARY SLOWLY
LIFTS POLEWARD WEDNESDAY MORNING, A GRADUAL REDUCTION IN THE
COVERAGE OF CONVECTION SHOULD COMMENCE. UNCERTAINTY REMAINS WITH HOW
QUICKLY THIS HAPPENS, WITH POTENTIAL FOR LINGERING CONVECTION INTO
THE LATE MORNING/EARLY AFTERNOON THE HIGHEST TO THE NORTH ACROSS
NORTHEAST MO/WEST-CENTRAL IL. REMNANT CLOUD COVER/SCATTERED SHOWERS
FURTHER SOUTH MAY THROW A WRENCH IN THE TEMPERATURE FORECAST, WITH
POTENTIAL FOR WEDNESDAY TO BE ONE OF THE 'COOLEST' DAYS THIS WEEK.
ANY LOCATIONS THAT CLEAR OUT WILL QUICKLY WARM UP INTO THE MID 90S
LEADING TO ANOTHER DAY OF NEAR-RECORD MID SEPTEMBER HEAT. AS OF NOW,
THE BEST CHANCE FOR HEAT INDEX VALUES OF 100-105F+ IS SOUTH OF I-70,
BUT COULD EXPAND NORTH IF THE FRONT LIFTS POLEWARD FASTER. AS THINGS
BECOME MORE CLEAR OVER THE NEXT DAY, AN EXTENSION OF THE HEAT
ADVISORY WILL NEED TO BE CONSIDERED FOR PORTIONS OF THE AREA
WEDNESDAY.
LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/
ISSUED AT 337 AM CDT TUE SEP 15 2026
AS WE MOVE TOWARDS THE MID-LATE PART OF THE WEEK, GUIDANCE INDICATES
THE ANOMALOUS MID-LEVEL RIDGE NUDGES NORTHWEST, ENCOMPASSING MORE OF
THE REGION. THERE IS GOOD CONSENSUS THAT THE BOUNDARY RETREATS
POLEWARD INTO NE AND IA, PLACING OUR AREA FURTHER INTO THE WARM
SECTOR AWAY FROM RAIN CHANCES. EXTREMELY WARM 850MB TEMPS AROUND 20C
MEAN THAT CONFIDENCE IS HIGH IN THE CONTINUATION OF NEAR RECORD HEAT
INTO THE WEEKEND WITH HIGHS IN THE 90S ACROSS MOST OF THE AREA.
RECORD HIGHS MAY BE THREATENED AT STL AND COU THURSDAY AND FRIDAY
(PLEASE REFER TO THE CLIMATE SECTION BELOW). TEMPERATURE IQR SPREADS
ON THE LREF REMAIN AROUND ONLY 5 DEGREES THROUGH FRIDAY,
ILLUSTRATING THAT EXTREMELY HOT CONDITIONS WILL CONTINUE TO IMPACT
THE AREA INTO THE WEEKEND.
THERE CONTINUES TO BE INCREASING SIGNAL AMONG LONG-RANGE GUIDANCE
THAT THE STRONG MID-LEVEL RIDGE BREAKS DOWN AS A LONGWAVE TROUGH
SWINGS ACROSS THE INTERMOUNTAIN WEST ON SATURDAY. IT APPEARS THAT
SATURDAY WILL REMAIN HOT FOR A MAJORITY OF THE AREA WITH IQR
TEMPERATURE SPREADS INCREASING TO 15-20 DEGREES ON SUNDAY,
INDICATING THE POTENTIAL FOR A COLD FRONT BRINGING COOLER AIR INTO
THE REGION. BY NEXT WEEK, IQR TEMPERATURE SPREADS DECREASE AGAIN TO
5-10 DEGREES, BUT THIS TIME CENTERED ON VALUES NEAR CLIMATOLOGICAL
NORMALS. THE 25TH PERCENTILES INDICATES THE POTENTIAL FOR HIGHS IN
THE 70S WITH THE 75TH PERCENTILE REPRESENTING VALUES THAT ARE IN THE
UPPER 70S/LOWER 80S. ALL OF THIS REVEALS THE SIGNAL IS THERE FOR AN
END TO THE HEAT BY THE START OF NEXT WEEK.
AVIATION /18Z TAFS THROUGH 00Z THURSDAY/
ISSUED AT 1211 PM CDT TUE SEP 15 2026
A COLD FRONT WILL SLOWLY MOVE SOUTHEAST THIS AFTERNOON/TONIGHT
AND STALL OUT ACROSS THE AREA, LIKELY NEAR THE ST. LOUIS METRO.
THIS FRONT AND POINTS NORTHWARD IS WHERE THE BEST CHANCES OF
SHOWERS AND THUNDERSTORMS ARE TONIGHT INTO WEDNESDAY MORNING.
KUIN SHOULD BE MOST HEAVILY IMPACTED, WITH MORE PERSISTENT
SHOWERS AND ON/OFF THUNDERSTORMS. ALSO BEHIND THE FRONT, LOOK
FOR LOW MVFR CEILINGS. THE BEST CHANCES FOR THESE LOW CEILINGS
ARE ALSO AT KUIN, FOLLOWED BY KCOU AND THEN KJEF. THE METRO
SITES MAY TEND TO STAY JUST FAR ENOUGH SOUTHEAST. HOWEVER, I DID
ADD A SCT015 GROUP TO ACCOUNT FOR THIS POSSIBILITY.
CLIMATE
ISSUED AT 231 PM CDT MON SEP 14 2026
DAILY RECORD HIGH TEMPERATURES MAY BE THREATENED FOR BOTH ST. LOUIS
(KSTL) AND COLUMBIA (KCOU) THIS WEEK. NOTE: RECORDS AT QUINCY (KUIN)
ARE NOT SHOWN BELOW DUE TO COOLER TEMPERATURES FORECAST:
9/15 9/16 9/17 9/18 9/19
KSTL 101/1939 98/1954 97/1931 97/1925 96/1955
KCOU 101/1893 99/1954 98/1895 98/1893 98/2022
LSX WATCHES/WARNINGS/ADVISORIES
MO...HEAT ADVISORY UNTIL 8 PM CDT FRIDAY FOR MOZ047>049-059>065-
072>075-084-085-099.
IL...HEAT ADVISORY UNTIL 8 PM CDT FRIDAY FOR ILZ064-069-070-074-079-
100>102.
SHORT TERM...PEINE
LONG TERM...PEINE
AVIATION...GOSSELIN
CLIMATE...GOSSELIN
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