478
FXUS63 KLSX 141935
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
235 PM CDT THU AUG 14 2025
KEY MESSAGES
- INCREASING HEAT AND HUMIDITY IS FORECAST FRIDAY INTO EARLY NEXT
WEEK. A HEAT ADVISORY HAS BEEN ISSUED FOR METROPOLITAN ST. LOUIS
AND PARTS OF SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS FRIDAY
THROUGH MONDAY.
SHORT TERM
(THROUGH LATE FRIDAY AFTERNOON)
ISSUED AT 235 PM CDT THU AUG 14 2025
A SURFACE RIDGE OF HIGH PRESSURE WILL GRADUALLY PUSH OFF TO THE
EAST TONIGHT, BUT THE PRESSURE GRADIENT REMAINS VERY WEAK ACROSS
OUR AREA. THE COMBINATION OF A MOSTLY CLEAR SKY AND LIGHT/VARIABLE
WINDS SHOULD ALLOW FOR TEMPERATURES TO COOL BACK INTO THE UPPER
60S TO NEAR 70 DEGREES (NEAR NORMAL). SOME MID 60S ARE POSSIBLE IN
PARTS OF NORTHEAST MISSOURI AS WELL AS FAVORED RIVER VALLEYS IN
PORTIONS OF SOUTHEAST MISSOURI.
A SUBTLE WARM FRONT IS EXPECTED TO MOVE NORTHWARD ACROSS THE AREA
EARLY FRIDAY, WITH WINDS TURNING OUT OF THE SOUTH. INCREASING LOW-
LEVEL WARM AIR ADVECTION AND A LOT OF SUNSHINE WILL LEAD TO A MUCH
WARMER DAY THAN THE PAST 48 HOURS. IN ADDITION, DEWPOINTS WILL BE
CLIMBING AS WELL BEHIND THE WARM FRONT. THE COMBINATION OF HIGH
TEMPERATURES IN THE LOW TO MID 90S AND INCREASING HUMIDITY LEVELS
ARE EXPECTED TO YIELD PEAK HEAT INDEX VALUES OF NEAR TO SLIGHTLY
ABOVE 100 DEGREES IN SOME AREAS. MORE SPECIFICALLY, THE BEST CHANCES
FOR 100+ HEAT INDEX VALUES FRIDAY AFTERNOON WILL BE IN METROPOLITAN
ST. LOUIS AS WELL AS ADJACENT SOUTHEAST MISSOURI AND SOUTHWEST
ILLINOIS. GIVEN THE HIGH CONFIDENCE IN CONTINUED 100+ HEAT INDEX
VALUES SATURDAY-MONDAY AFTERNOONS, WENT AHEAD AND ISSUED A HEAT
ADVISORY AS IT APPEARS LIKELY THESE COUNTIES WILL MEET DURATION
CRITERIA (AT LEAST 4 DAYS OF 100+ HEAT INDEX VALUES) FOR A HEAT
ADVISORY.
GOSSELIN
LONG TERM
(FRIDAY NIGHT THROUGH NEXT THURSDAY)
ISSUED AT 235 PM CDT THU AUG 14 2025
(FRIDAY NIGHT - MONDAY NIGHT)
THERE IS VERY HIGH CONFIDENCE THAT THE HOT AND HUMID WEATHER PATTERN
WILL CONTINUE THROUGH THE WEEKEND INTO EARLY NEXT WEEK AS MID/UPPER
LEVEL RIDGING BUILDS ACROSS THE MID-MISSISSIPPI VALLEY. CONCEPTUALLY
SPEAKING, THE HOTTEST TEMPERATURES ARE MOST LIKELY SUNDAY INTO
MONDAY AS 850-HPA TEMPERATURES CLIMB MORE INTO THE +21 TO +23C RANGE
(~90TH PERCENTILE OF CLIMATOLOGY) AND SOILS GRADUALLY DRY OUT. HIGHS
IN THE MID TO UPPER 90S ARE FORECAST FOR MOST LOCATIONS, WITH A HIGH
NEAR THE CENTURY MARK POSSIBLE IN METRO ST. LOUIS. THIS ALSO LIKELY
MEANS LOWER DEWPOINTS, BUT THE WARMING WILL OFFSET THE LOWER
HUMIDITY. PEAK AFTERNOON HEAT INDEX VALUES OF 100-105+ ARE EXPECTED
AREAWIDE SATURDAY THROUGH MONDAY. THE HIGHEST VALUES EACH DAY ARE
EXPECTED TO BE WITHIN THE ADVISORY AREA.
(TUESDAY - NEXT THURSDAY)
FORECAST UNCERTAINTY INCREASES BEGINNING ON TUESDAY AS THE ANOMALOUS
MID/UPPER LEVEL RIDGE SHIFTS WESTWARD. ENSEMBLE GUIDANCE ALMOST
UNIFORMLY SHOW THIS SCENARIO, WHICH SHOULD ALLOW FOR A COLD FRONTAL
PASSAGE BY MIDWEEK. THE EXACT TIMING OF THIS FRONTAL PASSAGE HOWEVER
SHOWS QUITE A BIT OF SPREAD THIS FAR OUT, RANGING FROM EARLY TUESDAY
TO WEDNESDAY. THIS FRONT WILL BRING THE NEXT CHANCE OF SHOWERS AND
THUNDERSTORMS ALONG WITH A DEFINITIVE END TO THIS ROUND OF DANGEROUS
HEAT. IT IS POSSIBLE THAT THE ADVISORY WILL NEED TO BE EXTENDED INTO
TUESDAY, BUT AT THIS TIME THERE WAS ENOUGH UNCERTAINTY TO ONLY GO
THROUGH EARLY MONDAY EVENING.
THERE HAS BEEN A SHIFT TOWARD A SLIGHTLY WEAKER MID/UPPER LEVEL
TROUGH ACROSS SOUTHEAST CANADA MID/LATE NEXT WEEK. THIS HAS LED TO A
SHIFT MORE TOWARD NEAR NORMAL TEMPERATURES BEHIND THE COLD FRONT.
NOT SURE IF THIS IS A BLIP OR THE START OF A TREND GIVEN PREVIOUS
TRANSITIONS TO NORTHWEST FLOW HAVE ACTUALLY TRENDED COOLER FOR THE
MOST PART SINCE JULY. SO WHILE COOLER TEMPERATURES WILL OCCUR BEHIND
THE COLD FRONT, THERE REMAINS QUITE A BIT OF UNCERTAINTY WITH HOW
"COOL" THEY ULTIMATELY WILL BE. THE SPREAD ON THE 13Z NBM BETWEEN
THE 25/75TH PERCENTILES SHOWS THIS UNCERTAINTY QUITE WELL, WITH
ROUGHLY A 6-10 DEGREE SPREAD FOR HIGH TEMPERATURES WEDNESDAY AND
THURSDAY.
GOSSELIN
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)
ISSUED AT 1217 PM CDT THU AUG 14 2025
A VERY BRIEF SPRINKLE OR SHOWER IS POSSIBLE THIS AFTERNOON, BUT
THE PROBABILITY IS TOO LOW THAT ANY METRO TERMINAL WILL BE
IMPACTED. LEFT TAFS DRY AS A RESULT. THERE MAY BE SOME RIVER
VALLEY FOG LATE TONIGHT INTO EARLY FRIDAY MORNING, BUT THE BETTER
SIGNAL IS IN SOUTHEAST MISSOURI. THERE MAY BE SOME FOG AT
KSUS/KJEF/KCPS, BUT CONFIDENCE WAS TOO LOW TO INTRODUCE ANY AT
THIS TIME. OUTSIDE OF ANY FOG DEVELOPMENT, DRY/VFR CONDITIONS WITH
LIGHT WINDS ARE EXPECTED.
GOSSELIN
LSX WATCHES/WARNINGS/ADVISORIES
MO...HEAT ADVISORY FROM NOON FRIDAY TO 8 PM CDT MONDAY FOR FRANKLIN
MO-JEFFERSON MO-SAINT CHARLES MO-SAINT LOUIS CITY MO-SAINT
LOUIS MO-SAINTE GENEVIEVE MO.
IL...HEAT ADVISORY FROM NOON FRIDAY TO 8 PM CDT MONDAY FOR CLINTON IL-
MADISON IL-MONROE IL-RANDOLPH IL-SAINT CLAIR IL-WASHINGTON
IL.
WFO LSX
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