687
FXUS63 KLSX 132337
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
637 PM CDT SUN SEP 13 2026
KEY MESSAGES
- SUMMERTIME HEAT AND HUMIDITY RETURNS TUESDAY.
- CHANCES OF SHOWERS AND THUNDERSTORMS EACH DAY ARE MAINLY
CONFINED TO NORTHEAST MISSOURI AND WEST-CENTRAL ILLINOIS.
KSUS 132321Z 1400/1424 03007KT P6SM SCT150
SHORT TERM /THROUGH MONDAY/
ISSUED AT 220 PM CDT SUN SEP 13 2026
THE SURFACE COLD FRONT WHICH MOVED THROUGH ON SATURDAY IS
LARGELY WASHED OUT, BUT LOOKS TO BE LOCATED FROM THE MISSOURI-
ARKANSAS BORDER NORTHEAST INTO SOUTHWEST INDIANA AS OF 1800 UTC.
THIS BOUNDARY MAY HELP SPURN A FEW SHOWERS AND THUNDERSTORMS
NEAR OR JUST SOUTH OF OUR SOUTHEAST MISSOURI COUNTIES LATE THIS
AFTERNOON/EARLY THIS EVENING. LOSS OF DIURNAL INSTABILITY SHOULD
QUICKLY LEAD TO THE CESSATION OF THIS ACTIVITY. DRY WEATHER IS
THEN FORECAST THROUGH THE REMAINDER OF THE EVENING INTO THE
EARLY OVERNIGHT HOURS. WIDESPREAD SHOWERS AND THUNDERSTORMS ARE
EXPECTED TO DEVELOP AFTER MIDNIGHT NEAR THE MISSOURI-KANSAS
BORDER IN RESPONSE TO INCREASE MOISTURE CONVERGENCE ON THE NOSE
OF A ~30 KNOT SOUTHERLY LOW-LEVEL JET. THESE STORMS SHOULD
EAST/NORTHEAST WITH TIME ALONG/POLEWARD OF A RETREATING SURFACE
WARM FRONT. SCATTERED SHOWERS AND A FEW THUNDERSTORMS ARE
EXPECTED ACROSS NORTHEAST MISSOURI AND WEST- CENTRAL ILLINOIS
MONDAY MORNING. AS THE LOW-LEVEL JET ABATES AND SHIFTS MORE TO
THE NORTHEAST, DRY WEATHER IS THEN FORECAST AREAWIDE FOR MONDAY
AFTERNOON. TEMPERATURES (AND HUMIDITY LEVELS) WILL INCREASE
BEHIND THE SURFACE WARM FRONT. THIS WILL BE MOSTLY FELT FROM
CENTRAL INTO SOUTHEAST MISSOURI WHERE HIGHS ARE EXPECTED TO TOP
OUT NEAR 90 DEGREES WITH PEAK HEAT INDEX VALUES ABOVE 95F.
LONG TERM /MONDAY NIGHT THROUGH SUNDAY/
ISSUED AT 220 PM CDT SUN SEP 13 2026
AN AREA OF SURFACE LOW PRESSURE IS EXPECTED TO MOVE ACROSS FAR
SOUTHERN CANADA MONDAY NIGHT INTO EARLY TUESDAY, BRINGING A COLD
FRONT DRAPED TO ITS SOUTHWEST TOWARD NORTHEAST MISSOURI AND WEST-
CENTRAL ILLINOIS. THIS BOUNDARY WILL BE OF HIGH IMPORTANCE THROUGH
NEXT WEEK, WITH ITS EXACT LOCATION AND EVOLUTION UNCERTAIN FOR MUCH
OF NEXT WEEK. CONCEPTUALLY SPEAKING, I DO THINK ITS MORE LIKELY THAN
NOT FOR THE FRONT TO OOZE SOUTHWARD DUE TO A COUPLE OF REASONS: 1)
REPEATED ROUNDS OF CONVECTION ALONG/NORTH OF THE BOUNDARY AND 2)
STRENGTHENING SURFACE HIGH PRESSURE MOVING INTO THE UPPER MIDWEST.
CONVECTION TENDS TO HELP SHIFT THE BOUNDARY (OR EFFECTIVE BOUNDARY)
EQUATORWARD VIA COLD POOLS. THE INCOMING SURFACE HIGH IN THE UPPER
MIDWEST ALSO LOOKS QUITE STRONG FOR THE MIDDLE OF SEPTEMBER (~1026
HPA; >95TH PERCENTILE OF CLIMATOLOGY). THIS EVOLUTION THEN WOULD
LEAD TO GRADUALLY "COOLER" TEMPERATURES ACROSS THE REGION, THOUGH
THE SPECIFICS ARE PRETTY UNCERTAIN ESPECIALLY WEDNESDAY ONWARD.
THERE IS MORE CONFIDENCE THAT THE FRONT WILL BE MOSTLY TO THE NORTH
OF THE AREA (WITH NOTABLE EXCEPTION OF NORTHEAST MISSOURI AND WEST-
CENTRAL ILLINOIS) ON TUESDAY. WITHIN THE WARM SECTOR, THE EXPECTED
AIR MASS REMAINS PRETTY EXTRAORDINARY FOR THIS TIME OF YEAR.
TEMPERATURES AT 850 HPA ABOVE +20C WOULD BE ABOVE THE 99TH
PERCENTILE OF CLIMATOLOGY, WITH THE STRENGTH OF THE MIDLEVEL RIDGE
AT 500 HPA RECORD-BREAKING FOR MID SEPTEMBER. IN OTHER WORDS, IN THE
ABSENCE OF CLOUD COVER, NEAR-RECORD BREAKING HIGH TEMPERATURES
APPEAR POSSIBLE AT BOTH KSTL AND KCOU FOR MIDWEEK. PLEASE SEE THE
CLIMATE SECTION BELOW FOR MORE DETAILS. SPEAKING OF HISTORICAL
CLIMATE INFORMATION, ONE MORE TIDBIT: SINCE 1930, ST. LOUIS HAS
ONLY REACHED A HEAT INDEX OF 105F+ TWO TIMES ON OR AFTER 9/15
(105F ON 9/17/1931 AND 106F ON 9/19/1972). WE VERY WELL MAY GET
A THIRD ON TUESDAY 9/15 DUE TO TEMPERATURES IN THE MID TO UPPER
90S AND DEWPOINTS IN THE LOW 70S. NEAR-RECORD HIGH TEMPERATURES
AND DEWPOINT VALUES ALSO NEAR DAILY RECORDS ARE THE COMBINATION
REQUIRED TO YIELD 100-105F+ HEAT INDEX VALUES THIS TIME OF YEAR,
AND THIS DOES LOOK LIKELY WITHIN THE WARM SECTOR. BOTH
MAGNITUDE (105F+) AND/OR DURATION (100F+) HEAT ADVISORY CRITERIA
MAY BE REACHED, WITH CHANCES INCREASING THE FURTHER TO THE
SOUTH YOU TRAVEL (LESSER CHANCES OF STORMS AND CLOUD DEBRIS).
GIVEN THE UNCERTAINTY IN THE EXACT PLACEMENT AND EVOLUTION OF
THE FRONT THOUGH THIS WEEK, WE HAVE ELECTED TO HOLD OFF ON
ISSUING A HEAT ADVISORY AT THIS TIME.
CHANCES OF SHOWERS AND THUNDERSTORMS SHOULD TEND TO FOCUS
ALONG/NORTH OF THE BOUNDARY THIS WEEK. THEREFORE, THE BEST CHANCES
(50-80%) ARE IN NORTHEAST MISSOURI AND WEST-CENTRAL ILLINOIS TUESDAY
NIGHT THROUGH WEDNESDAY. SOME OF THESE HIGHER POPS MAY TEND TO MOVE
SOUTHWARD FOR THE REASONS DESCRIBED ABOVE, BUT THIS IS NOT CERTAIN.
A THREAT FOR FLASH FLOODING MAY EXIST IN NORTHERN SECTIONS OF THE
AREA IF THEY ARE IMPACTED BY MULTIPLE ROUNDS OF CONVECTION. THE
PRECIPITATION ALSO MAY TEND TO BE EFFICIENT GIVEN DEEP WARM CLOUD
DEPTHS EXPECTED ALONG WITH PRECIPITABLE WATER VALUES APPROACHING 2"
(~99TH PERCENTILE OF CLIMATOLOGY).
AVIATION /00Z TAFS THROUGH 06Z TUESDAY/
ISSUED AT 636 PM CDT SUN SEP 13 2026
VFR CONDITIONS ARE EXPECTED TO PREVAIL FOR THE TAF PERIOD FOR
ALL TERMINALS. THERE ARE CURRENTLY SOME SHOWERS MOVING THROUGH
NORTHEASTERN MO AND MAY BRIEFLY IMPACT UIN, BUT ARE UNLIKELY TO
CAUSE A VISIBILITY RESTRICTION. UIN HAS ANOTHER CHANCE OF
SHOWERS AND THUNDERSTORMS MONDAY MORNING, BUT CURRENT GUIDANCE
HAS BEEN INDICATING THAT THIS CONVECTION WILL WEAKEN AS IT
APPROACHES UIN GIVING UNCERTAINTY IN THE INTENSITY.
CLIMATE
ISSUED AT 220 PM CDT SUN SEP 13 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
KSTL 101/1939 98/1954 97/1931
KCOU 101/1893 99/1954 98/1895
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
SHORT TERM...GOSSELIN
LONG TERM...GOSSELIN
AVIATION...ASEL/KIMBLE
CLIMATE...GOSSELIN
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