936
FXUS63 KLSX 140829
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
329 AM CDT THU AUG 14 2025
KEY MESSAGES
- HEAT AND HUMIDITY RETURN TO THE REGION TOMORROW THROUGH THE
WEEKEND. WHILE NO ONE DAY WILL BE PARTICULARLY DANGEROUS,
ELEVATED HEAT INDEX VALUES MAY LAST JUST LONG ENOUGH TO PROVE
IMPACTFUL TO SENSITIVE GROUPS.
SHORT TERM
(THROUGH LATE FRIDAY AFTERNOON)
ISSUED AT 329 AM CDT THU AUG 14 2025
VALLEY FOG IS AGAIN EVIDENT ON GOES EAST NIGHTTIME MICROPHYSICS
IMAGERY, WITH LIGHT NORTHERLY WINDS AND MOSTLY-CLEAR SKIES IN
PLACE PROMOTING EFFICIENT RADIATIONAL COOLING. OTHERWISE, SURFACE
HIGH PRESSURE AND WEST-NORTHWEST FLOW ALOFT CONTINUE TO PROMOTE
TRANQUIL WEATHER ACROSS THE BI-STATE REGION. TO OUR NORTHWEST, A
DECAYING MCS IS TRAVERSING THE MID-LEVEL FLOW THROUGH THE CENTRAL
PLAINS, BUT IS LOSING STEAM AND VERY LIKELY WILL HAVE LITTLE TO NO
IMPACT TO OUR FORECAST AREA TODAY. FOG WILL CONTINUE TO GRADUALLY
DEGRADE VISIBILITES ACROSS THE LOW-LYING PARTS OF THE REGION
THROUGH SUNRISE, WHEN DAYTIME HEATING WILL SCOUR OUT THE FOG.
WE'LL ENJOY ONE MORE DAY OF MILD, LARGELY-DRY CONDITIONS TODAY AS
WE REMAIN UNDER THE INFLUENCE OF THE SURFACE RIDGE CENTERED OVER
THE UPPER MIDWEST. IT'S WORTH NOTING THAT SOME HIGH-RESOLUTION
GUIDANCE IS PERCOLATING A FEW WEAK ATTEMPTS AT CONVECTIVE
INITIATION DURING THE AFTERNOON IN SOUTHEAST MISSOURI AND
SOUTHWEST ILLINOIS, WHERE SURFACE-BASED INSTABILITY WILL BE AT A
REGIONAL MAXIMUM. THIS SOLUTION SEEMS TO BE CLOSER TO THE "WORST-
CASE" FOR RAIN CHANCES IN THAT AREA TODAY, AND WHILE THERE ARE
VERY LOW (10% OR LOWER) CHANCES THERE IN THE FORECAST, THE
POTENTIAL ITSELF IS NEAR-ZERO.
THE STALLED FRONT CURRENTLY ACROSS THE OZARKS WILL LIFT BACK NORTH
AS A WEAK WARM FRONT OVERNIGHT TONIGHT AS THE MID-LEVEL RIDGE TO OUR
WEST AMPLIFIES AND EXPANDS EASTWARD. THE WEAK BUT PERSISTENT
SOUTHERLY FLOW BEHIND THE FRONT WILL SEND HIGHS BACK INTO THE 90S
AREAWIDE FOR FRIDAY, WITH THE WARMEST TEMPERATURES EXPECTED IN THE
ST. LOUIS METROPOLITAN AREA AND ALONG THE MISSOURI RIVER VALLEY.
WHILE HUMIDITY WILL BE ON THE RISE AS WELL, MOST AVAILABLE SHORT-
RANGE GUIDANCE SUGGESTS THERE WILL BE FAIRLY STEEP 0-3KM LAPSE
RATES ACROSS THE SOUTHERN HALF OF THE REGION (INCLUDING THE I-70
CORRIDOR). FURTHER NORTH, WHERE MIXING WILL BE MORE STUNTED AND
EVAPOTRANSPIRATION WILL LOCALLY-ENHANCE HUMIDITY, TEMPERATURES
WON'T WARM AS MUCH. THE TEMPERATURE FORECAST IS FAIRLY CERTAIN,
WITH NBM INTERQUARTILE RANGES AT ANY ONE LOCATION FRIDAY OF ONLY A
FEW DEGREES AT WORST. HUMIDITY IS A BIT MORE UNCERTAIN,
ESPECIALLY NEAR THE OZARKS UP TOWARDS THE I-70 CORRIDOR. BECAUSE
OF A LACK OF EITHER SUFFICIENTLY-WARM OR SUFFICIENTLY-HUMID
CONDITIONS IN ANY ONE LOCATION, IT WILL BE DIFFICULT TO REACH
TRIPLE-DIGIT HEAT INDEX VALUES OUTSIDE OF LOCALIZED INSTANCES. IF
THAT HAPPENS, THE DURATION CRITERIA FOR ANY POTENTIAL HEAT
ADVISORY WOULD LIKELY NOT BE MET. GIVEN THIS UNCERTAINTY, NO
PRODUCTS WERE ISSUED WITH THIS FORECAST.
MRB
LONG TERM
(FRIDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 329 AM CDT THU AUG 14 2025
THE MID-LEVEL RIDGE BECOMES MORE FIRMLY ESTABLISHED OVERHEAD FOR THE
WEEKEND, WHEN OUR HOTTEST TEMPERATURES ARE LIKELY TO OCCUR. THE
SIGNAL FOR ANOMALOUSLY-HIGH 500MB HEIGHTS REMAINS UNCHANGED FROM
PREVIOUS FORECASTS, WHICH IS A KEY FEATURE OF OUR LOCAL CONCEPTUAL
MODEL FOR 100-104F HEAT INDEX VALUES. THE RESULTANT MID-90S HIGH
TEMPERATURES (PERHAPS UPPER 90S IN THE URBANIZED AREAS) WILL COUPLE
WITH SOMEWHAT HUMID CONDITIONS TO SEND HEAT INDEX VALUES INTO THE
LOW 100S SATURDAY AND SUNDAY FOR MOST OF THE REGION. BY MONDAY,
NEARLY ALL GUIDANCE CONTINUES TO SHOW THE RIDGE ERODING SOMEWHAT,
WHICH WOULD OPEN THE BI-STATE TO SLIGHTLY-HIGHER THUNDERSTORM
CHANCES. THAT SAID, IT STILL LOOKS QUITE HOT TO START THE NEXT WORK
WEEK. THE HOTTEST TEMPERATURES AND HIGHEST HUMIDITY STILL LOOK
SOMEWHAT SPATIALLY DISPLACED, AGAIN DUE TO STRONGER MIXING POTENTIAL
OVER THE SOUTHERN HALF OF THE REGION. IF WE MANAGE TRIPLE-DIGIT HEAT
INDEX VALUES ON FRIDAY, BY MONDAY WE WILL REACH THE FOUR-DAY
DURATION THRESHOLD OF 100F OR WARMER HEAT INDEX VALUES TO WARRANT A
HEAT ADVISORY. AT THIS POINT, THE EVENT AS A WHOLE APPEARS TO BE ON
THE LOWER END IN TERMS OF POTENTIAL IMPACTS, ESPECIALLY AMIDST THE
HEAT WAVE WE JUST RECENTLY BROKE. THAT SAID, IMPACTS TO SENSITIVE
AND VULNERABLE POPULATIONS ARE CERTAINLY IN THE CARDS.
BY TUESDAY, UNCERTAINTY INFILTRATES THE TEMPERATURE FORECAST AS THE
RIDGE ALOFT RETROGRADES AND AMPLIFIES OVER THE WESTERN CONUS. WHILE
SOME SOLUTIONS KEEP THE RIDGE OVERHEAD LONG ENOUGH TO PROLONG HOT
AND HUMID CONDITIONS FOR SOME, OTHERS ERODE THE RIDGE AND PAINT
INCREASED RAIN CHANCES AND COOLER TEMPERATURES OVER US. THE LATTER
WOULD LIKELY BE THE RESULT OF DECAYING MESOSCALE SYSTEMS DIVING
SOUTH FROM THE UPPER MISSISSIPPI VALLEY, AS IS DEPICTED IN SOME
DETERMINISTIC GUIDANCE. IT IS FAIRLY LIKELY (80% CHANCE) THAT THIS
WILL BE THE LAST GASP OF THE HEAT BEFORE ANOTHER COOLDOWN BRINGS US
RELIEF BY MID-WEEK. THE MAGNITUDE OF THIS RESPITE DOES VARY FAIRLY
NOTABLY IN THE AVAILABLE ENSEMBLE SUITE, WITH HIGH TEMPERATURE
INTERQUARTILE RANGES OF 10F+ ON WEDNESDAY. REGARDLESS OF THE
MAGNITUDE, THE ENTIRE DISTRIBUTION COOLS COMPARED TO THE DAY BEFORE
AND STAYS SEASONABLY MILD INTO THE WEEKEND.
MRB
AVIATION
(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)
ISSUED AT 1036 PM CDT WED AUG 13 2025
FOG IS LIKELY TO DEVELOP BETWEEN 07-09Z WITH WITH IFR/LIFR
VISIBILITES POSSIBLE AT VALLEY AIRPORTS BETWEEN 09-14Z INCLUDING
JEF/SUS/CPS. CONFIDENCE HAS INCREASED THAT SOME MVFR VISIBILITIES
IN FOG MAY OCCUR AT STL, SO HAVE ALSO INCLUDED A TEMPO GROUP
BETWEEN 09-13Z AT THAT TERMINAL. THE FOG IS THEN EXPECTED TO
DISSIPATE AFTER 14Z LEAVING DRY AND VFR CONDITIONS WITH GENERALLY
LIGHT AND VARIABLE WINDS.
BRITT
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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