525
FXUS63 KLSX 270343
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
1043 PM CDT FRI JUN 26 2026
KEY MESSAGES
- SCATTERED SHOWERS AND THUNDERSTORMS CAPABLE OF PRODUCING HEAVY
RAIN RATES WILL MAINTAIN A THREAT OF FLASH FLOODING IN THE
OZARKS AND SOUTHWEST ILLINOIS THROUGH THE EVENING.
- CONFIDENCE CONTINUES TO INCREASE THAT IMPACTFUL HEAT AND
HUMIDITY WILL BUILD SUNDAY AND CONTINUE FOR MUCH OF THE WORK
WEEK.
SHORT TERM
(THROUGH LATE SATURDAY NIGHT)
ISSUED AT 232 PM CDT FRI JUN 26 2026
THE PRIMARY ITEM OF CONCERN IN THE SHORT TERM CONTINUES TO BE THE
POTENTIAL FOR THUNDERSTORMS CAPABLE OF PRODUCING HEAVY RAIN AND
FLOODING THIS AFTERNOON AND EVENING, WITH OCCASIONAL DOWNBURST WINDS
AND PERHAPS A BRIEF TORNADO OR TWO POSSIBLE AS WELL. AS OF 12:30 PM,
POCKETS OF SHOWERS AND GENERALLY WEAK THUNDERSTORMS CONTINUE TO
IMPACT VARIOUS PORTIONS OF OUR FORECAST AREA, WITH THE MOST NOTABLE
CONVECTIVE CLUSTERS ACROSS THE OZARKS AND CENTRAL MISSOURI. THUS FAR
THIS ACTIVITY IS PRIMARILY PRODUCING BURSTS OF HEAVY RAIN AND
OCCASIONAL LIGHTNING, WITH LITTLE TO SUGGEST ADDITIONAL HAZARDS AT
THIS TIME.
HOWEVER, AS THE AFTERNOON PROGRESSES, CONVECTION IS EXPECTED TO
GRADUALLY DEEPEN AS INSTABILITY SLOWLY BUILDS INTO THE AREA FROM THE
SOUTHWEST. CLOUD COVER HAS SLOWED THIS BUILDUP SO FAR TODAY, BUT A
MODEST BUT PERSISTENT SOUTHWESTERLY LOW LEVEL JET WILL CONTINUOUSLY
TRANSPORT VERY RICH MOISTURE (1.8-2.2 INCH PWAT) INTO THE OZARKS,
WHERE IT WILL INTERACT WITH A STALLED SURFACE FRONT. MODELED
THERMODYNAMIC PROFILES PROJECT VERY DEEP MOISTURE THROUGHOUT THE
COLUMN, WITH MINIMAL DRY AIR ENTRAINMENT, POOR MID LEVEL LAPSE
RATES, AND DEEP WARM CLOUD LAYERS. ALL OF THESE FACTORS SUPPORT VERY
EFFICIENT RAIN RATES AS CONVECTION DEEPENS, AND FALLING IN AN AREA
THAT HAS HAD VARIABLE BUT LOCALLY SIGNIFICANT RECENT RAINS. AS SUCH,
THE THREAT OF FLASH FLOODING WILL CONTINUE INTO THE AFTERNOON AND
EARLY EVENING, AND WE HAVE OPTED TO EXTEND A PORTION OF OUR ONGOING
FLOOD WATCH ACROSS THE OZARKS AND SOUTHWEST ILLINOIS A FEW HOURS
LATER TO ACCOUNT FOR THIS (10 PM).
MEANWHILE, THE POTENTIAL FOR A FEW DOWNBURSTS WITH SPORADIC GUSTS TO
60 MPH CONTINUES, ALTHOUGH THE WEAK INSTABILITY, POOR LAPSE RATES,
AND MINIMAL DRY AIR ENTRAINMENT IS NOT PARTICULARLY SUPPORTIVE OF
WIDESPREAD STRONG WINDS. THERE IS ALSO JUST ENOUGH LOW LEVEL WIND
SHEAR (0-1KM 15-20KT, 0-3KM 25-35KT) TO SUPPORT A BRIEF TORNADO OR
TWO, BUT CONFIDENCE IS MUCH LOWER IN EITHER OF THESE HAZARDS
MATERIALIZING LOCALLY.
THROUGH THE EVENING THESE SHOWERS ARE EXPECTED TO DRIFT GRADUALLY
SOUTHWARD AND WEAKEN OVERNIGHT, AND ALSO DIMINISH IN COVERAGE.
HOWEVER, A FEW SHOWERS AND WEAK THUNDERSTORMS MAY PERSIST ACROSS
MAINLY THE OZARKS AND SOUTHWEST ILLINOIS THROUGH THE NIGHT AND INTO
TOMORROW MORNING. DURING THE DAY TOMORROW, THE SURFACE FRONT IS
EXPECTED TO DISSIPATE AND SOUTHERLY FLOW WILL RESUME, WHILE AN UPPER
LEVEL RIDGE BEGINS TO BUILD OVERHEAD. WHILE WE DON'T EXPECT MUCH IN
THE WAY OF ADDITIONAL SHOWERS AND THUNDERSTORMS DURING THE DAY,
THERE ARE SOME INDICATIONS THAT A SURGING LOW LEVEL JET IN THE
EVENING MAY TRIGGER RENEWED CONVECTION IN PARTS OF THE AREA LATE IN
THE EVENING AND OVERNIGHT. THIS IS MOST LIKELY IN NORTHEAST MISSOURI
AND WEST-CENTRAL ILLINOIS, AND SHOULD LARGELY EXIT THE AREA BY
SUNDAY MORNING. ONCE THIS ACTIVITY EXITS THE AREA TO THE NORTHEAST,
ATTENTION TURNS SQUARELY TO A PROLONGED PERIOD OF SUMMER HEAT AND
HUMIDITY SUNDAY ONWARD.
BRC
LONG TERM
(SUNDAY THROUGH NEXT FRIDAY)
ISSUED AT 232 PM CDT FRI JUN 26 2026
FROM SUNDAY ONWARD, CONFIDENCE CONTINUES TO INCREASE THAT WE WILL
SEE THE FIRST EXTENDED PERIOD OF IMPACTFUL HEAT OF THE SUMMER SO
FAR, WITH SEVERAL DAYS IN A ROW FEATURING HEAT INDEX VALUES OVER 100
DEGREES, AND PERHAPS AS HIGH AS 110 DEGREES AT TIMES THROUGHOUT THE
WORK WEEK. WHILE THERE IS SOME UNCERTAINTY REGARDING THE MAGNITUDE
AND DURATION, AT LEAST SOME MEASURE OF HEAT IMPACTS ARE EXPECTED AT
VARIOUS POINTS THROUGHOUT THE WEEK.
THE PRIMARY DRIVERS OF THE RAPIDLY INCREASING TEMPERATURES WILL BE
THE DEVELOPMENT OF A STOUT UPPER LEVEL RIDGE AND PERSISTENT
SOUTHERLY LOW LEVEL FLOW, WHICH ARE BOTH EXPECTED TO LINGER OVERHEAD
OR NEARBY FOR MOST OF THE WEEK, AND POSSIBLY AS LATE AS NEXT
WEEKEND. THE UPPER RIDGE IS EXPECTED TO BUILD DIRECTLY OVERHEAD
SUNDAY AND SLIDE JUST EAST OF THE AREA INTO THE OHIO RIVER VALLEY
AND GREAT LAKES MONDAY ONWARD, MAINTAINING ANOMALOUSLY WARM
TEMPERATURES ALOFT FOR SEVERAL DAYS (500 MB NEAR THE 90TH
PERCENTILE, 850 MB NEAR THE 95TH PERCENTILE). MEANWHILE, SOUTHERLY
RETURN FLOW WILL ALSO MAINTAIN HIGH DEWPOINTS THROUGHOUT THIS
PERIOD, ADDING A HUMIDITY COMPONENT THAT WILL DRIVE HIGH HEAT INDEX
VALUES. CURRENT LREF AND NBM PROJECTIONS SUGGEST ACTUAL TEMPERATURES
REACHING 90 TO 95 DEGREES BY SUNDAY, AND A FEW DEGREES HIGHER YET
MONDAY THROUGH THURSDAY, WITH MINIMAL RAIN CHANCES THROUGHOUT THIS
PERIOD GIVEN THE POSITION OF THE UPPER RIDGE. AS SUCH, CONFIDENCE IS
VERY HIGH THAT WE WILL SEE SEVERAL CONSECUTIVE DAYS OF HEAT INDEX
VALUES EXCEEDING 100 DEGREES, WITH THE POTENTIAL TO REACH 110
DEGREES AT TIMES IN OUR WARMEST AREAS. OVERNIGHT LOWS ARE ALSO
EXPECTED TO REMAIN IN THE 70S, AND LIKELY THE MID TO UPPER 70S BY
MID WEEK. BY NOW, THE MAIN SOURCE OF UNCERTAINTY IS REGARDING THE
ONSET OF THE WORST HEAT, AND ALSO THE DURATION LATE IN THE WEEK AS
THE RIDGE BEGINS TO SLOWLY DEAMPLIFY. WHILE OUR OFFICIAL FORECAST
ONLY EXTENDS TO THURSDAY AT THIS POINT, LONG RANGE ENSEMBLE GUIDANCE
DOES SUGGEST THAT THIS RIDGE MAY BE SLOW TO DE-AMPLIFY, AND
MAINTAINS THE POTENTIAL FOR HEAT TO CONTINUE BEYOND THURSDAY AS
WELL. DUE TO LOW CONFIDENCE IN THE START OF THE WORST HEAT SUNDAY
(SOME AREAS MAY BE SLOWER TO REACH THE 100 DEGREE HEAT INDEX
THRESHOLD FOR A HEADLINE), WE HAVE OPTED TO HOLD OFF ON ISSUING A
HEAT WATCH FOR THE TIME BEING.
BRC
AVIATION
(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)
ISSUED AT 1036 PM CDT FRI JUN 26 2026
LIGHT SHOWERS AND SOME LIGHT DRIZZLE WILL REMAIN POSSIBLE
THROUGH 09Z, PARTICULARLY AT UIN AND THE ST. LOUIS AREA
AIRPORTS. OTHERWISE CEILINGS ARE EXPECTED TO DROP TO IFR
OR LIFR BY 08-09Z WITH VISIBILITIES BETWEEN 1-4 MILES IN BR.
THERE WILL BE SOME IMPROVEMENT BY 16-19Z WITH VISIBILITIES
IMPROVING TO VFR AND CEILINGS IMPROVING TO MVFR/VFR. THERE IS SOME
CHANCE FOR SCATTERED SHOWER OR THUNDERSTORM DEVELOPMENT AGAIN ON
SATURDAY, BUT CHANCES ARE TOO LOW (<30%) AT THIS POINT TO INCLUDE
IN ANY OF THE TAFS.
BRITT
LSX WATCHES/WARNINGS/ADVISORIES
MO...FLOOD WATCH UNTIL 7 AM CDT SATURDAY FOR CRAWFORD MO-IRON MO-
JEFFERSON MO-MADISON MO-REYNOLDS MO-SAINT FRANCOIS MO-
SAINTE GENEVIEVE MO-WASHINGTON MO.
IL...FLOOD WATCH UNTIL 7 AM CDT SATURDAY FOR BOND IL-CLINTON IL-
FAYETTE IL-MADISON IL-MARION IL-MONROE IL-RANDOLPH IL-SAINT
CLAIR IL-WASHINGTON IL.
WFO LSX
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