080
FXUS63 KLSX 171737
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
1237 PM CDT SUN AUG 17 2025
KEY MESSAGES
- DANGEROUS HEAT AND HUMIDITY WILL CONTINUE THROUGH MONDAY, AND
POTENTIALLY INTO TUESDAY FOR PORTIONS OF THE AREA.
- SCATTERED THUNDERSTORMS ARE FORECAST THIS AFTERNOON AND MONDAY
AFTERNOON. MORE WIDESPREAD RAIN CHANCES (30-50%) WILL BE
ASSOCIATED WITH A COLD FRONT TUESDAY NIGHT THROUGH WEDNESDAY.
SHORT TERM
(THROUGH LATE MONDAY AFTERNOON)
ISSUED AT 325 AM CDT SUN AUG 17 2025
LATEST ANALYSIS SHOWS MID TO UPPER LEVEL RIDGING IN CONTROL OF THE
MID-MISSISSIPPI VALLEY. THIS HAS MAINTAINED THE FOCUS FOR BETTER
CONVECTIVE POTENTIAL ACROSS THE NORTHERN PLAINS AND UPPER
MISSISSIPPI VALLEY, JUST SOUTH OF A STRONG SURFACE RIDGE THAT
EXPANDS ACROSS MANITOBA AND ONTARIO PROVINCES. MORE WIDESPREAD
CONVECTION IS EXPECTED TO REMAIN TO THE NORTH AS THE UPPER LEVEL
RIDGE IS PARKED OVER THE CENTRAL U.S., BUT WILL BE A FACTOR IN
SURFACE FEATURES WITH REFERENCE TO OUTFLOW EACH CONVECTIVE ROUND
SENDS SOUTHWARD INTO THE LOCAL AREA.
DESPITE WHAT HAS CONSISTENTLY BEEN A 594-596 DAM UPPER HIGH OVER THE
AREA THE LAST COUPLE OF DAYS, LEADING TO SOME OF THE SEASON'S
WARMEST TEMPERATURES/HEAT INDEX VALUES, IT HASN'T PREVENTED ISOLATED
TO SCATTERED SHOWERS AND THUNDERSTORMS. THE SMALL SCALE FEATURES
HAVE BEEN BETTER RESOLVED BY HI-RES GUIDANCE, WHILE MEDIUM AND LONG
RANGE (LOWER RESOLUTION) GUIDANCE HAS HAD DIFFICULTY GRASPING THESE
FEATURES UNTIL THEY BECOME MORE APPARENT. THIS CONSIDERED, IT IS
LOOKING MORE LIKELY THAT TODAY AND MONDAY WILL BE A REPEAT WITH
ADDITIONAL OUTFLOW PUSHING SOUTH FROM THE CONVECTIVE ACTIVITY TO THE
NORTH. ONLY MINOR DIFFERENCES EXIST BETWEEN THE TWO DAYS, LARGELY
LEANING ON THE EXACT POSITION OF THE MID-LEVEL RIDGE AND WEAK
CAPPING EAST OF THE MISSISSIPPI RIVER. THE HIGHEST PROBABILITIES (40-
50%) WILL BE ALONG AND SOUTH OF I-70 IN MISSOURI TODAY. THOUGHT
MONDAY'S PROBABILITIES (20-25%) LOOK LOW, I WOULDN'T BE SURPRISED IF
THESE WERE INCREASED TO SIMILAR PROBABILITIES AND CONFIDENCE IN
GEOGRAPHICAL PLACEMENT INCREASES. THE PERSISTENCE IN THE TRACK AND
TIMING OF OUTFLOW MANAGES TO FAVOR THIS REGION EACH AFTERNOON AS
MLCAPE CLIMBS TO 2000-4000+ J/KG. NOT SURPRISINGLY, SHEAR CONTINUES
TO BE WEAK (<15 KTS) WITH A FEW SAMPLES OF RAP SOUNDINGS SHOWING
DCAPE AT OR ABOVE 1,000 J/KG AND STEEP LAPSE RATES >8C. SLOW-MOVING,
SHORT-LIVED SHOWERS AND THUNDERSTORMS, MOST FAVORED BETWEEN 19Z-00Z,
MAY PRODUCE A LOCALIZED BOUT OF GUSTY WINDS AND BOUTS OF MODERATE TO
HEAVY RAINFALL.
THE ISOLATED TO SCATTERED NATURE OF THUNDERSTORMS WILL NOT BRING
ANYTHING MORE THAN LOCALIZED COOLING TO IMPACTED LOCATIONS. IF
ANYTHING, LOCALIZED AFFECTED BY THUNDERSTORMS WILL FEEL MORE HUMID,
ESPECIALLY THE SHORTER THE DURATION OF RAINFALL. HREF PROBABILITIES
FOR 0.01" ARE GENERALLY LOW (<30%) WITH SPLOTCHES NEAR 60%. 850 MB
TEMPERATURES OF 20-22C SHROUD THE REGION THIS AFTERNOON, SUGGESTING
TEMPERATURES WILL REACH SIMILAR TERRITORY (MID/UPPER 90S) AS
YESTERDAY. RAP GUIDANCE SHOWS A SLIGHT BUMP IN MID-LEVEL
TEMPERATURES AS THE MID-LEVEL RIDGE SUBTLY SHIFTS SOUTHEAST. THIS
VEERS WINDS FROM SOUTH TO SOUTHWEST, WARMING THE H8 PLAIN TO 21-23C
WEST OF THE MISSISSIPPI RIVER. DETERMINISTIC CROSS SECTIONS SHOW A
SHALLOW LAYER OF RELATIVELY HIGHER RH WITHIN A DEEP MIXING LAYER
BELOW THE 700 MB PLAIN. THIS MAY STUNT THE POTENTIAL TO WARM EVEN
FURTHER (TRIPLE DIGITS), ASIDE FROM THE LOCALIZED EFFECTS FROM
RAINFALL.
FORECAST HIGHS IN THE MID TO UPPER 90S LOOK SOLID WITH HEAT INDEX
VALUES IN THE 100-108 RANGE BEING WELL REPRESENTED. LOCALIZED VALUES
COULD REACH 110 DEGREES WHERE IS REMAINS DRY. WITHOUT A BROAD SPREAD
IN THESE VALUES AND LOW CONFIDENCE IN SUCH, THERE IS NO CHANGED TO
THE CURRENT HEAT HEADLINES.
MAPLES
LONG TERM
(MONDAY NIGHT THROUGH SATURDAY)
ISSUED AT 325 AM CDT SUN AUG 17 2025
THE UPPER LEVEL RIDGE THAT HAS ALLOWED FOR A PERSISTENT TEMPERATURE
FORECAST OVER THE LAST FEW DAYS WILL BEGIN TO BUILD WESTWARD LATE
MONDAY INTO TUESDAY. LREF'S 850 MB MEAN HEIGHTS ALSO SHOW THE
NORTHERN PERIPHERY OF THE MID-LEVEL RIDGE FLATTENING AS IT TAKES ON
AN ELONGATED ORIENTATION AND SHIFTS SOUTHWEST IN THE ARKANSAS,
OKLAHOMA AND TEXAS. THERE HAS BEEN HIGH CONFIDENCE THAT HEAT
CONTINUES TUESDAY OVER PARTS OF THE AREA AND WHILE THAT IS STILL THE
CASE OVER SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS, THE LATEST
INFORMATION EXPANDS SOME OF THE UNCERTAINTY TO THE I-70 CORRIDOR.
THE PATTERN SHIFT WILL ALLOW A COLD FRONT TO SHIFT SOUTHWARD LATE
MONDAY NIGHT INTO TUESDAY MORNING. LREF HAS CONSISTENTLY SHOWN SOME
OF THE GREATEST SPREAD IN DATA ACROSS SECTIONS OF SOUTHERN IOWA AND
FAR NORTHERN MISSOURI INTO WEST-CENTRAL ILLINOIS. DETERMINISTIC
GUIDANCE SHOWS THE FRONT REMAIN NORTH OF THE MISSOURI/IOWA BORDER BY
12Z TUESDAY. HOWEVER, CONVECTION IGNITES ALONG THE BOUNDARY MONDAY
NIGHT WHICH IS LATER INVIGORATED BY AN UPPER LEVEL SHORTWAVE
(POTENTIAL MCS) THAT APPROACHES FROM THE NORTHERN PLAINS. AT THE
VERY LEAST, CLOUD COVER COULD IMPACT TEMPERATURES OVER NORTHEAST
MISSOURI AND WEST-CENTRAL ILLINOIS, WHERE LREF SHOWS >50% OF THE
MEMBERS SHOWING SKY COVER AT 50% OR GREATER. THIS IS ALSO THE REGION
THAT STANDS THE BEST CHANCE AT RECEIVING RAINFALL AS A RESULT OF
DECAYING CONVECTION WITH 30-50% OF THE MEMBERS DEPICTING MEASURABLE
PRECIPITATION (>0.01"). CONSIDERING THE PERSISTENCE IN THESE
SIGNALS, I'D BE MORE INCLINED TO SIDE WITH COOLER TEMPERATURES.
IN THE FORTUNATE SENSE, THE SOUTHWARD EXPANSION IN THE DATA'S SPREAD
GIVES SOME HOPE THAT COOLER TEMPERATURES MAY BE FELT FURTHER SOUTH.
PROBABILITIES FOR HEAT INDICES AT OR ABOVE 100 DEGREES HAS DROPPED
SLIGHTLY ALONG I-70 WITH LESS CERTAINTY IN SKY COVER AND
PRECIPITATION TRENDS THROUGH LATE TUESDAY MORNING. WE'VE BEEN
PROVIDED A COUPLE OF EXAMPLES THIS WEEK OF HOW OUTFLOW CAN PROVIDE A
MECHANISM FOR THUNDERSTORM POTENTIAL, EVEN UNDERNEATH SUBSIDENCE
ALOFT. CONSIDERING THE TRANSITION OF UPPER/MID-LEVEL RIDGING, THE
APPROACHING COLD FRONT AND ANY SOUTHWARD REINFORCEMENT OF COOLER AIR
DUE TO CONVECTION, TRENDS COULD PROVIDE BETTER INSIGHT INTO HEAT
POTENTIAL TUESDAY AFTERNOON. ANOTHER THING TO CONSIDER IN THESE
SCENARIOS (SLOW-MOVING SUMMER FRONTS) IS HOW MOISTURE CAN POOL ALONG
AND JUST SOUTH OF THE BOUNDARY, MAKING EVEN 90S FEEL OPPRESSIVE. I
STILL HAVE HIGH CONFIDENCE IN SOUTHEAST MISSOURI AND SOUTHWEST
ILLINOIS REACHING HEAT INDICES OF 100-105 DEGREES (LREF 60-75%
PROBS.), BUT HAVE BEEN PROVIDED MORE QUESTION THAN CERTAINTY
ALONG I-70 AND POINTS NORTH, WHERE THESE PROBABILITIES HAVE
DROPPED TO 50% OR LESS. FOR THIS REASON, NO DECISION HAS BEEN MADE
TO EXTEND THE CURRENT HEAT HEADLINES.
FORTUNATELY, THERE IS BETTER AGREEMENT IN THE LONG RANGE PERIOD FROM
WEDNESDAY THROUGH SATURDAY. THE POTENTIAL FOR SHOWERS AND
THUNDERSTORMS SHIFT SOUTHWARD, BUT NOT BEFORE ANOTHER FLARE UP OVER
SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS WEDNESDAY AFTERNOON. THEN
AGAIN, THE CURRENT FORECAST PROBABILITIES (30-40%) WILL CERTAINLY
ADJUST AS THE FRONT'S SPEED AND PLACEMENT BECOME MORE CLEAR. IT DOES
LOOK MORE EVIDENT THAT THE UPPER RIDGE BECOMES CENTERED OVER THE
WESTERN CONUS WITH NORTHWEST FLOW ALOFT. SURFACE RIDGING EXTENDS
FROM SOUTHERN CANADA THROUGH THE GREAT LAKES AND INTO THE UPPER
MISSISSIPPI VALLEY, LEADING TO NORTHEASTERLY SURFACE FLOW. THIS WILL
DRY THINGS OUT AND BRING COOLER TEMPERATURES AND DRIER AIR THAT
PERSISTS THROUGH THE END OF THE WEEK.
MAPLES
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)
ISSUED AT 1230 PM CDT SUN AUG 17 2025
SCATTERED SHOWERS AND THUNDERSTORMS EXIST AGAIN THIS AFTERNOON
INTO EARLY THIS EVENING, MAINLY IN CENTRAL AND EASTERN MISSOURI.
THE COVERAGE SUGGESTS PROB30S ARE STILL THE WAY TO GO, WITH MOST
AREAS STAYING DRY. HOWEVER, IF ANY TERMINAL DOES SEE A DIRECT HIT
FROM A THUNDERSTORM, BRIEF HEAVY DOWNPOURS MAY REDUCE VISIBILITY
TO IFR/MVFR ALONG WITH THE POTENTIAL AT LEAST FOR SOME GUSTY
WINDS. THE OVERNIGHT PERIOD INTO MONDAY MORNING SHOULD BE DRY.
CANNOT RULE OUT A BRIEF PERIOD OF MVFR FOG AT KSUS/KJEF, BUT
CONFIDENCE WAS TOO LOW TO ADD ANY MENTION.
GOSSELIN
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-
IRON MO-JEFFERSON MO-KNOX MO-LEWIS MO-LINCOLN MO-MADISON MO-
MARION MO-MONITEAU MO-MONROE MO-MONTGOMERY MO-OSAGE MO-PIKE
MO-RALLS MO-REYNOLDS 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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