292
FXUS63 KLSX 152325
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
625 PM CDT FRI AUG 15 2025
KEY MESSAGES
- DANGEROUS HEAT AND HUMIDITY IS EXPECTED TO CONTINUE THROUGH
MONDAY, AND POTENTIALLY INTO TUESDAY FOR PORTIONS OF THE AREA.
SHORT TERM
(THROUGH LATE SATURDAY AFTERNOON)
ISSUED AT 227 PM CDT FRI AUG 15 2025
THE HEAT AND HUMIDITY HAVE CERTAINLY BUILT INTO THE AREA TODAY AS
TEMPERATURES HAVE CLIMBED INTO THE LOW TO MID 90S WITH DEWPOINTS
IN THE LOW TO MID 70S. THIS COMBINATION OF HEAT AND HUMIDITY HAS
LED TO HEAT INDEX VALUES MAINLY BETWEEN 100 AND 105 DEGREES ACROSS
THE REGION, WITH A FEW SPOTS AT OR ABOVE 105F IN/AROUND AREAS
WHERE THERE IS HIGHER EVAPOTRANSPIRATION (I.E., NEAR CROPS). DUE
TO THE SLIGHTLY HIGHER HEAT INDEX VALUES BEING OBSERVED EARLY THIS
AFTERNOON, DID ELECT TO GO AHEAD AND EXPAND THE HEAT ADVISORY
ACROSS MUCH OF THE REST OF THE REGION. GIVEN HIGH CONFIDENCE IN
SEEING 3 MORE DAYS (SATURDAY - MONDAY) OF 100+ HEAT INDEX VALUES,
WE SHOULD HAVE NO TROUBLE REACHING THE 4-DAY DURATION THRESHOLD.
THERE HAVE ALSO BEEN SOME ISOLATED SHOWERS AND EVEN A FEW
THUNDERSTORMS, MAINLY IN EAST/SOUTHEAST MISSOURI NEAR THE
MISSISSIPPI RIVER. THIS ACTIVITY IS IN AN AREA OF HIGH
INSTABILITY, BUT VERY LITTLE EFFECTIVE SHEAR. SHOWERS HAVE
GENERALLY QUICKLY DISSIPATED AND THIS TREND SHOULD CONTINUE.
CLOSER TO SUNSET, SOME LOSS OF INSTABILITY AND THE BUILDING
MID/UPPER LEVEL RIDGE (INCREASING SUBSIDENCE ALOFT) SHOULD LEAD TO
DRY CONDITIONS ONCE AGAIN AREAWIDE.
DECREASING CLOUDINESS IS FORECAST OVERNIGHT WITH LIGHT SOUTHERLY
WINDS IS FORECAST. MILD CONDITIONS ARE EXPECTED, WITH LOWS ONLY
DROPPING BACK INTO THE LOW TO MID 70S. PARTS OF METROPOLITAN ST.
LOUIS SHOULD BE IN THE UPPER 70S DUE TO THE URBAN HEAT ISLAND
EFFECT.
SATURDAY AFTERNOON SHOULD BE VERY SIMILAR TO THIS AFTERNOON, THOUGH
I AM EXPECTING HIGHS TO BE ABOUT 1-2 DEGREES WARMER ON AVERAGE DUE
TO THE WARMER START TO THE DAY AND 850-HPA TEMPERATURES CLIMBING
ABOUT 1C OVER THE NEXT 24 HOURS. DRY WEATHER IS ALSO ANTICIPATED
BENEATH THE BUILDING MID/UPPER LEVEL RIDGE. A COUPLE OF THE CAMS
(NAMELY THE HRRR) ACTUALLY DEVELOP CONVECTION SATURDAY AFTERNOON.
THIS APPEARS TO BE DUE TO POOR HANDLING OF THE BOUNDARY LAYER WHERE
THE MIXED LAYER IS SO DEEP THAT IT OVERCOMES THE CAP. THIS IS A
KNOWN ISSUE WITH THE HRRR HOWEVER SO THIS SCENARIO SEEMS UNLIKELY. A
MORE PLAUSIBLE SCENARIO IS FOR CONVECTION TO DEVELOP FURTHER EAST
(EASTERN ILLINOIS/INDIANA) ON THE EDGE OF THE STRONGER CAPPING AND
ON THE PERIPHERY OF THE MID/UPPER LEVEL RIDGE WHERE A REMNANT
OUTFLOW BOUNDARY FROM AN OVERNIGHT MCS MAY RESIDE SATURDAY
AFTERNOON. DID NOT ADD ANY POPS FOR TOMORROW BUT DID ADD "SILENT
14'S" INTO THE FORECAST FOR MUCH OF EAST CENTRAL AND SOUTHEAST
MISSOURI AS WELL AS SOUTHWEST ILLINOIS.
GOSSELIN
LONG TERM
(SATURDAY NIGHT THROUGH NEXT FRIDAY)
ISSUED AT 227 PM CDT FRI AUG 15 2025
(SATURDAY NIGHT - MONDAY NIGHT)
THERE REMAINS VERY HIGH CONFIDENCE THAT DANGEROUS HEAT/HUMIDITY
WILL LAST THROUGH AT LEAST MONDAY. ANY CHANGES BETWEEN SATURDAY AND
MONDAY WILL BE SUBTLE WITH LITTLE/NO CHANGE IN THE UPPER AIR PATTERN
OR THE AIR MASS IN PLACE. CONCEPTUALLY, SUNDAY AND MONDAY MAY BE A
HAIR WARMER (BUT ALSO LESS HUMID) AS SOILS CONTINUE TO DRY OUT WITH
EACH DAY DRY, HOT DAY. THIS WON'T REALLY MAKE ANY DIFFERENCE THOUGH
IN TERMS OF HEAT INDEX VALUES WITH MOST LOCATIONS PEAKING IN THE 102-
108F RANGE EACH AFTERNOON. THE HIGHEST HEAT INDEX VALUES ARE MOST
LIKELY IN/AROUND METRO ST. LOUIS AND ADJACENT PARTS OF SOUTHWEST
ILLINOIS (WHERE THE HIGHEST DEWPOINTS SHOULD RESIDE).
(TUESDAY - TUESDAY NIGHT)
ENSEMBLE GUIDANCE REMAINS IN VERY GOOD AGREEMENT THAT THE MID/UPPER
LEVEL RIDGE WILL SHIFT WESTWARD MORE INTO THE INTERMOUNTAIN WEST BY
TUESDAY. TRENDS HAVE BEEN FOR A SLOWER COLD FRONTAL PASSAGE (VERY
LATE AFTERNOON/NIGHT TIME FRAME), SO PROBABILITIES OF ONE LAST DAY
OF HEAT AND HUMIDITY ARE HIGHER THAN IT LOOKED 24-48 HOURS AGO, AT
LEAST FOR SOUTHERN SECTIONS OF THE FORECAST AREA. PROBABILITIES FOR
>=100 HEAT INDEX ON TUESDAY FROM THE LREF ARE IN THE 30-70% RANGE,
BUT >60% ROUGHLY ALONG/SOUTH OF THE INTERSTATE 70 CORRIDOR. HOWEVER,
WITH THE MID/UPPER LEVEL RIDGE SHIFTING THAT FAR WEST, IT DOES LEAVE
OUR AREA SUSCEPTIBLE TO ANY MCS DROPPING SOUTHEASTWARD TUESDAY.
THERE IS NOT MUCH OF A SIGNAL FOR THAT RIGHT NOW IN DETERMINISTIC
GUIDANCE, BUT THE PATTERN DOES AT LEAST SHOW THE POSSIBILITY. SIX-
HOUR PROBABILITIES FOR MEASURABLE PRECIPITATION ON THE LREF ARE IN
THE 40-70% RANGE ENDING AT 1800 UTC (1PM) TUESDAY ALONG/NORTHWEST OF
I-44/I-70 IN MISSOURI AND ILLINOIS RESPECTIVELY, WHICH ALSO SHOWS
THAT SOME TYPE OF MCS MAY IMPACT TEMPERATURES ON TUESDAY. IN
ADDITION, EVEN A DECAYING MCS COULD SEND OUT A COMPOSITE OUTFLOW
BOUNDARY FURTHER SOUTH AND EAST, WHICH IF IT IS STRONG ENOUGH, COULD
AUGMENT THE AMBIENT AIR MASS SUFFICIENTLY TO NOT YIELD AS HOT/HUMID
CONDITIONS.
(WEDNESDAY - NEXT FRIDAY)
THE COLD FRONT IS EXPECTED TO CLEAR THE CWA BY LATE WEDNESDAY
MORNING, PUTTING A DEFINITIVE END TO THE HEAT AND HUMIDITY ACROSS
THE ENTIRE AREA. THE BEST CHANCE OF SHOWERS AND THUNDERSTORMS IS
ASSOCIATED WITH THIS FRONTAL PASSAGE, THOUGH THERE MAY BE SOME
REMNANT SHOWERS BEHIND THE FRONT AS A MIDLEVEL SHORTWAVE TROUGH
MOVES SOUTHWARD DOWN THE MISSISSIPPI VALLEY.
GRADUALLY COOLER AND LESS HUMID AIR WILL CONTINUE TO INFILTRATE THE
REGION, AIDED BY AN AREA OF SURFACE HIGH PRESSURE SLIDING INTO THE
UPPER MIDWEST. THERE REMAINS SOME UNCERTAINTY ON THE STRENGTH OF THE
MID/UPPER LEVEL TROUGHING IN SOUTHEAST CANADA AND THE ASSOCIATED AIR
MASS MOVING SOUTH INTO THE CONUS BEHIND THE FRONT. HOWEVER, FAIRLY
DEEP LOW-LEVEL NORTH/NORTHEASTERLY FLOW APPEARS LIKELY AS WINDS
BLOWING ACROSS THE GREAT LAKES IS FAVORABLE FOR BELOW NORMAL
TEMPERATURES AND HUMIDITY LEVELS ACROSS OUR NECK OF THE WOODS.
ENSEMBLE MEANS ON BOTH THE GEFS AND EPS HAVE WOBBLED BACK COOLER
COMPARED TO YESTERDAY AND NOW MORE RESEMBLE OUTPUT FROM A COUPLE OF
DAYS AGO. AT LEAST TWO OF THESE PREVIOUS PATTERN SHIFTS FROM
HEAT/HUMIDITY TO NORTHWEST FLOW HAVE TRENDED COOLER (AND LESS HUMID)
WITH THE AIR MASS COMING IN BEHIND EACH COLD FRONT, INCLUDING AROUND
8/1. AS HAS BEEN STATED THE PAST COUPLE OF DAYS, I WOULD NOT BE
SURPRISED TO SEE THAT HAPPEN ONCE AGAIN BUT TIME WILL TELL. FOR NOW,
A RETURN TO AT LEAST NEAR NORMAL TEMPERATURES APPEARS LIKELY, THOUGH
THE LOWER END OF THE INTERQUARTILE RANGE (25TH PERCENTILE) APPEARS
MORE LIKELY THAN THE HIGHER END (75TH PERCENTILE) FROM THE NBM. THE
25TH PERCENTILE FOR THURSDAY AND FRIDAY'S HIGHS ARE CLOSER TO THE
LOW TO MID 80S VS. THE MID TO UPPER 80S CURRENTLY FORECAST.
GOSSELIN
AVIATION
(FOR THE 00Z TAFS THROUGH 00Z SATURDAY EVENING)
ISSUED AT 619 PM CDT FRI AUG 15 2025
MAINLY DRY AND VFR CONDITIONS ARE EXPECTED THROUGH THE PERIOD WITH
WINDS REMAINING AROUND 7 KNOTS OR LESS.
BRITT
LSX WATCHES/WARNINGS/ADVISORIES
MO...HEAT ADVISORY UNTIL 8 PM CDT MONDAY FOR AUDRAIN MO-BOONE MO-
CALLAWAY MO-COLE MO-CRAWFORD MO-FRANKLIN MO-GASCONADE MO-
JEFFERSON MO-KNOX MO-LEWIS MO-LINCOLN MO-MARION MO-MONITEAU
MO-MONROE MO-MONTGOMERY MO-OSAGE MO-PIKE MO-RALLS MO-SAINT
CHARLES MO-SAINT FRANCOIS MO-SAINT LOUIS CITY MO-SAINT
LOUIS MO-SAINTE GENEVIEVE MO-SHELBY MO-WARREN MO-WASHINGTON
MO.
IL...HEAT ADVISORY UNTIL 8 PM CDT MONDAY FOR ADAMS IL-BOND IL-BROWN
IL-CALHOUN IL-CLINTON IL-FAYETTE IL-GREENE IL-JERSEY IL-
MACOUPIN IL-MADISON IL-MARION IL-MONROE IL-MONTGOMERY IL-
PIKE IL-RANDOLPH IL-SAINT CLAIR IL-WASHINGTON IL.
WFO LSX
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