600
FXUS63 KLSX 200820
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
320 AM CDT SUN SEP 20 2026
KEY MESSAGES
- A COLD FRONT BEGINS SLOWLY MOVING THROUGH THE REGION TODAY
BRINGING COOLER TEMPERATURES THE REST OF THE WEEK.
- THUNDERSTORMS ALONG THE FRONT TONIGHT BRING THE BEST CHANCE
FOR RAIN ALL WEEK.
SHORT TERM /THROUGH MONDAY/
ISSUED AT 315 AM CDT SUN SEP 20 2026
SHOWERS AND THUNDERSTORMS ARE CURRENTLY MOVING NORTHEAST THROUGH
NORTHERN MO AND SOUTHEASTERN IA ALONG A COLD FRONT AND ALONG THE
NOSE OF THE LOW-LEVEL JET. AS THEY PROGRESS, THE CONVECTION IN
NORTHERN MO IS MOVING INTO A REGION WITH LESS FAVORABLE
CONDITIONS AND A WEAKENING LOW-LEVEL JET TO CONTINUE TO SUPPORT
THEIR DEVELOPMENT. AS A RESULT, THESE STORMS DON’T POSE A THREAT
TO OUR AREA ASIDE FROM A STRAY SHOWER IN OUR NORTHERN COUNTIES
DURING THE EARLY MORNING HOURS.
AN UPPER LEVEL RIDGE, WHICH HAS BEEN THE DRIVER BEHIND THE HEAT
AND HUMIDITY DURING THE LAST SEVERAL DAYS, IS BEGINNING TO BE
SUPPRESSED BY AN UPPER LEVEL TROUGH AS IT MOVES EAST. THIS
TROUGH WILL PUSH THE COLD FRONT INTO OUR AREA STARTING TODAY.
TEMPERATURES TODAY WILL STILL BE HOT AND HUMID TO THE SOUTH OF
THE FRONT AS IT BEGINS ITS SLOW PUSH THROUGH THE REGION. THIS
WILL LOWER HIGH TEMPERATURES INTO THE 70S AND 80S IN NORTHERN
MO, WHILE SOUTHEASTERN MO WILL REMAIN WELL INTO THE 90S. THANKS
TO SLIGHTLY MORE MODERATE HUMIDITY VALUES IN THE REGION,
WIDESPREAD 100+ HEAT INDEX VALUES ARE NOT EXPECTED. MONDAY’S
HIGH TEMPERATURES ARE SLATED TO BE EVEN LOWER AS THE FRONT MAKES
ITS TREK SOUTH THROUGH THE REGION. HOWEVER, THERE STILL REMAINS
SOME UNCERTAINTY IN THE EXACT HIGH TEMPERATURES FOR MONDAY.
LATEST NBM GUIDANCE’S INTERQUARTILE RANGE FOR THE HIGH
TEMPERATURES IS 6-10 DEGREES ACROSS THE REGION DUE TO THE
UNCERTAINTY IN THE FRONT’S EXACT POSITION AND SPEED. THAT BEING
SAID, BEHIND THE FRONT WE CAN EXPECT HIGH TEMPERATURES IN THE
70S AND 80S WITH SOME AREAS IN NORTHERN MO EXPERIENCING UPPER
60S FOR HIGH TEMPERATURES.
SHOWERS AND THUNDERSTORMS ARE EXPECTED ALONG AND BEHIND THE COLD
FRONT TONIGHT INTO MONDAY MORNING. WHILE THE CONFIDENCE IS LOW
ON THE EXACT TIMING AND LOCATION OF THIS CONVECTION, HREF
GUIDANCE SHOWS THERE IS A 80-100% CHANCE ACROSS THE REGION OF
MEASURABLE RAINFALL DURING THIS PERIOD, AND 30-70% CHANCE OF
SEEING 0.25 INCHES OF RAIN. THESE SHOWERS AND THUNDERSTORMS WILL
BE MULTI-CELLULAR WITH THE STRONGEST THUNDERSTORMS POSSIBLY
PRODUCING HEAVY LOCALIZED RAINFALL AND GUSTY WINDS IN REGIONS
WHERE CAPE IS SUFFICIENT (~1500-2000 J/KG). DUE TO THE LIMITED
NATURE OF THE SEVERE THREAT, THE SEVERE POTENTIAL IS NOT BEING
MESSAGED FOR THIS EVENT.
LONG TERM /MONDAY NIGHT THROUGH SATURDAY/
ISSUED AT 315 AM CDT SUN SEP 20 2026
THE END OF THE RELENTLESS HEAT IS IN SIGHT. BEHIND THE FRONT
TEMPERATURES ARE EXPECTED TO BE MUCH COOLER. FOR TUESDAY AND
WEDNESDAY, NBM GUIDANCE SHOWS THAT HIGH TEMPERATURES WILL BE IN
THE LOW TO MID 70S, WITH THE NORTHERN PORTIONS OF THE AREA
POSSIBLY STAYING IN THE UPPER 60S FOR THEIR HIGH TEMPERATURES
(60-80% CHANCE). LOW TEMPERATURES WILL ALSO DIP INTO THE 50S.
THIS AIR MASS WILL CONTINUE TO DOMINATE FOR THE REST OF THE WEEK
WITH A GRADUAL WARMING THROUGH THE 70S, REACHING THE LOW 80S BY
NEXT WEEKEND.
THE GFS AND ECMWF BOTH SHOW A MID-LEVEL RIDGE BUILDING OVER THE
GREAT LAKES AND INTO THE SOUTHEASTERN CANADA REGION WITH A
CENTER OF LOW PRESSURE OVER THE OHIO VALLEY BY MID WEEK. THE
RIDGE SHOULD REMAIN FAIRLY STATIONARY OVER SOUTHEASTERN CANADA,
WHILE THE LOW PRESSURE CENTER WILL SLOWLY MOVE TO THE EAST COAST
OF THE CONUS. THIS PATTERN WILL ALLOW FOR CONTINUED WESTERLY AND
NORTHWESTERLY FLOW TO HELP MODERATE OUR TEMPERATURES INTO NEXT
WEEKEND AND LIMIT THE MOISTURE TRANSPORTED INTO THE REGION,
PREVENTING ANOTHER RETURN OF THE HEAT AND HUMIDITY THIS WEEK.
AVIATION /00Z TAFS THROUGH 06Z MONDAY/
ISSUED AT 1047 PM CDT SAT SEP 19 2026
AREA OF SHOWERS AND A FEW THUNDERSTORMS OVER NORTH CENTRAL
MISSOURI IS EXPECTED TO MOVE JUST NORTH OF UIN OVER THE NEXT
SEVERAL HOURS, HOWEVER STILL EXPECT THE CHANCES FOR SHOWERS
WILL INCREASE AT UIN LATE TONIGHT INTO SUNDAY WITH A CHANCE
(30-40%) BETWEEN 09-14Z. THERE IS AN ADDITIONAL CHANCE AT UIN,
AFTER THAT BUT CONFIDENCE IS TOO LOW IN TIMING TO INCLUDE IN
THE TAF AT THIS TIME. EXPECT MVFR CEILINGS TO MOVE INTO UIN BY
MID-MORNING REMAIN THROUGH THE REST OF THE PERIOD WITH A CHANCE
(30-40%) FOR IFR CEILINGS AFTER 00Z. THERE ARE ALSO LOW CHANCES
(20-30%) FOR SHOWERS AT COU/JEF BETWEEN 12-15Z. CHANCES FOR
SHOWERS REMAIN LOW AT THE ST. LOUIS METRO TERMINALS UNTIL AFTER
00Z ON SUNDAY EVENING WHEN I HAVE CONTINUED A PROB30 GROUP
STARTING AT 02Z. I HAVE ONLY INCLUDED SHOWERS AT THIS POINT AS
THE COVERAGE OF THUNDERSTORMS IS EXPECTED TO BE LIMITED, THOUGH
I CANNOT RULE OUT THUNDERSTORMS AT ANY OF THE SITES DURING THE
TIME I HAVE INCLUDED RAIN CHANCES. CEILINGS WILL DROP TO MVFR
AFTER 06Z ON SATURDAY NIGHT. WINDS WILL GENERALLY BE LIGHT,
PARTICULARLY OUTSIDE OF THUNDERSTORMS.
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
SHORT TERM...ASEL/KIMBLE
LONG TERM...ASEL/KIMBLE
AVIATION...BRITT
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