809
FXUS63 KLSX 170412
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
1112 PM CDT WED SEP 16 2026
KEY MESSAGES
- A LATE SEASON HEAT WAVE CONTINUES THROUGH AT LEAST SATURDAY
WITH PEAK HEAT INDEX VALUES OF 100-105F+ ALONG/SOUTH OF I-70.
AREAS JUST NORTH OF I-70 HAVE BEEN ADDED TO THE HEAT ADVISORY,
AND THE REST OF THE ADVISORY HAS BEEN EXTENDED TO SATURDAY.
- ISOLATED TO SCATTERED THUNDERSTORMS ARE EXPECTED THIS
AFTERNOON. THESE STORMS ARE NOT EXPECTED TO STRONG ENOUGH TO
PRODUCE SEVERE WEATHER, THOUGH SMALL HAIL, GUSTY WINDS, AND
BRIEF HEAVY DOWNPOURS ARE POSSIBLE.
SHORT TERM /THROUGH THURSDAY/
ISSUED AT 340 AM CDT WED SEP 16 2026
A ROBUST UPPER LEVEL RIDGE REMAINS IN PLACE OVER MUCH OF THE
SOUTHEASTERN CONUS, WITH MID-AND UPPER LEVEL HEIGHTS APPROACHING
CLIMATOLOGICAL MAXIMUMS ACROSS THAT PORTION OF THE COUNTRY. THERE IS
A BAROCLINIC ZONE ON THE NORTHWESTERN FLANK OF THE RIDGE, CURRENTLY
DRIFTING NORTH OUT OF THE MID-MISSISSIPPI VALLEY. THIS FEATURE
WILL REMAIN OUTSIDE OF OUR AREA AND WILL BE THE PRIMARY FOCUS
FOR CONTINUED PRECIPITATION AND CLOUD COVERAGE OVER THE NEXT
SEVERAL DAYS AS THE RIDGE AMPLIFIES AND DRIFTS WESTWARD.
TODAY, LINGERING CLOUD COVERAGE ASSOCIATED WITH THE ABOVE FRONT
ARE LIMITING TEMPERATURES IN NORTHERN MISSOURI AND CENTRAL IL,
BUT THE MUCH OF REST OF THE FORECAST AREA IS EXPECTED TO BAKE
UNDER MOSTLY SUNNY SKIES. MOISTURE WILL ALSO BE ANOMALOUSLY
HIGH, WHICH SHOULD PUSH HEAT INDEX READINGS ABOVE 100 DEGREES
FOR AREAS ALONG AND SOUTH OF I-70. WHILE DEEP-LAYER MOISTURE
WILL BE HIGHEST ALONG THE FRONT, THERE APPEARS TO BE A SECONDARY
BAND OF EXTREME PRECIPITABLE WATER CURRENTLY ARCING FROM THE
ARKLATEX, THROUGH SW MO, AND INTO THE OHIO RIVER VALLEY. THIS
FEATURE IS ALSO EXPECTED TO DRIFT NORTH TODAY, AND MOST MODEL
GUIDANCE HAS SHOWN ALL MORNING THAT THIS WILL LEAD TO ISOLATED
TO SCATTERED THUNDERSTORMS ALONG THE I-44 CORRIDOR DURING PEAK
HEATING THIS AFTERNOON. INDEED THIS CONVECTION WAS UNDERWAY
AROUND 1PM, AND SHOULD CONTINUE THROUGH AROUND SUNDOWN. GIVEN
RISING HEIGHTS AND WEAK DEEP-LAYER SHEAR, THIS CONVECTION IS NOT
EXPECTED TO BE VERY STRONG, BUT IT MAY AT LEAST BRING A BRIEF
BREAK TO THE HEAT FOR SOME AREAS.
TOMORROW LOOKS LIKE IT WILL BE SIMILAR TO TODAY, WITH JUST A FEW
DIFFERENCES. FIRST OF ALL, THE RIDGE WILL BE CENTERED OVER THE LOWER
MISSISSIPPI VALLEY, A BIT FURTHER WEST THAN TODAY. ADDITIONALLY, THE
FRONT WILL BE STALLED FURTHER NORTH, LIKELY OVER IOWA. THIS SHOULD
RESULT IN HOT TEMPERATURES ACROSS THE VAST MAJORITY OF THE FORECAST
AREA. HUMIDITY IS EXPECTED TO REMAIN HIGH, PUSHING THE HEAT INDEX
TOWARD OR ABOVE 100 FOR MOST LOCATIONS. THERE ARE ALSO SOME
INDICATIONS THAT THE SECONDARY BAND OF HIGH PWS MAY KICK OFF ANOTHER
ROUND OF WEAK, ISOLATED CONVECTION, ROUGHLY ALONG THE I-44 CORRIDOR.
LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/
ISSUED AT 340 AM CDT WED SEP 16 2026
THE RIDGE WILL REMAIN IN PLACE THROUGH A LEAST SATURDAY, THOUGH IT
IS EXPECTED TO START WEAKENING ON FRIDAY. DESPITE THE GRADUALLY
FALLING HEIGHTS, PERSISTENT DEEP SOUTHWESTERLY FLOW WILL CONTINUE TO
ADVECT WARMER TEMPERATURES INTO THE REGION, WITH MEAN 850MB
TEMPERATURES WELL ABOVE 20C ON BOTH DAYS. THAT SOUTHWESTERLY
FLOW WILL ALSO KEEP THE BAROCLINIC ZONE TO OUR NORTH, LIMITING
PRECIP CHANCES AND CLOUD COVERAGE. THE ONLY SAVING GRACE FOR
FRIDAY AND SATURDAY IS THAT OUR HUMIDITY WILL BE A BIT LOWER,
PARTICULARLY ON SATURDAY. NEVERTHELESS, OUR HEAT INDEX READING
WILL TOP OUT RIGHT AROUND 100 ON BOTH DAYS, ESPECIALLY FOR AREAS
ALONG AND SOUTH OF I-70. WITH THAT, WE'VE DECIDED TO EXTEND THE
HEAT ADVISORY THROUGH SATURDAY FOR THOSE LOCATIONS. WE'VE ALSO
ADDED A TIER OF COUNTIES NORTH OF I-70 TO THE ADVISORY AS IT
LOOKS LIKE THEY WILL NOW REACH CRITERIA, THOUGH THAT PORTION OF
THE ADVISORY WILL END ON FRIDAY.
FROM SUNDAY INTO EARLY NEXT WEEK, A BROAD, SLOW MOVING UPPER TROUGH
WILL PUSH THROUGH THE NORTHERN PLAINS AND UPPER MIDWEST, FINALLY
BREAKING THE GRIP OF THE RIDGE (AT LEAST TEMPORARILY). AN
ASSOCIATED COLD FRONT WILL SLIDE INTO THE REGION, LIKELY LATE
SUNDAY OR ON MONDAY, BRINGING SOME RELIEF TO THE HEAT. WHILE
THERE IS LITTLE DOUBT THE START OF NEXT WEEK WILL BE COOLER THAN
THIS WEEK, JUST HOW COOL IT WILL GET REMAINS AN OPEN QUESTION.
ENSEMBLE MEAN HIGH TEMPERATURES ARE RIGHT AROUND SEASONAL
NORMALS, BUT THERE IS NOTABLE SPREAD IN THE GUIDANCE, WITH
REASONABLE OUTCOMES RANGING FROM HIGHS IN THE UPPER 80 TO LOW
70S. ALONG WITH COOLER TEMPERATURES, THAT FRONT IS LIKELY BRING
AT LEAST A LITTLE RAIN TO THE REGION, THOUGH AT THIS TIME THERE
IS LITTLE SIGNAL FOR IMPACTFUL PRECIPITATION.
AVIATION /00Z TAFS THROUGH 06Z FRIDAY/
ISSUED AT 1108 PM CDT WED SEP 16 2026
VFR FLIGHT CONDITIONS ARE EXPECTED TO PREVAIL THROUGH THE
PERIOD. LIGHT AND VARIABLE SURFACE FLOW WILL TURN TO THE WEST-
SOUTHWEST THURSDAY MORNING ACROSS MUCH OF THE AREA, THEN A WEAK
COLD FRONT WILL DIP INTO NORTHERN MISSOURI AND CENTRAL ILLINOIS.
THERE ARE SOME INDICATIONS THAT WIDELY SCATTERED THUNDERSTORMS
WILL DEVELOP IN THE AFTERNOON AGAIN SOUTH OF THE FRONT...MAINLY
ALONG AND SOUTH OF I-70. CONFIDENCE IN DEVELOPMENT AND COVERAGE
IS LOW AT THIS TIME SO HAVE LEFT MENTION OF STORMS OUT OF THE
TERMINAL FORECASTS.
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 MOZ041-050>052.
HEAT ADVISORY UNTIL 8 PM CDT SATURDAY FOR MOZ047>049-059>065-
072>075-084-085-099.
IL...HEAT ADVISORY UNTIL 8 PM CDT FRIDAY FOR ILZ058>060-065-098-099.
HEAT ADVISORY UNTIL 8 PM CDT SATURDAY FOR ILZ064-069-070-074-
079-100>102.
SHORT TERM...BSH
LONG TERM...BSH
AVIATION...CARNEY
CLIMATE...GOSSELIN
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