197
FXUS63 KLSX 181051
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
551 AM CDT FRI JUL 18 2025
KEY MESSAGES
- ISOLATED TO WIDELY SCATTERED THUNDERSTORMS (20-30% CHANCE) ARE
POSSIBLE THIS AFTERNOON, WITH OCCASIONAL STRONG DOWNBURST WINDS
AND HEAVY RAIN RATES POSSIBLE.
- ADDITIONAL THUNDERSTORMS ARE POSSIBLE OVERNIGHT TONIGHT THROUGH
TOMORROW AFTERNOON ACROSS MAINLY NORTHERN MISSOURI AND ILLINOIS,
AGAIN WITH A LIMITED GUSTY WIND AND HEAVY RAIN THREAT. CHANCES
FOR REPEATED ROUNDS OF THUNDERSTORMS ARE POSSIBLE AGAIN SATURDAY
NIGHT AND SUNDAY NIGHT IN THE SAME AREAS.
- CONFIDENCE IN WIDESPREAD DANGEROUS HEAT IS HIGHEST MONDAY
ONWARD, BUT IN SOME AREAS IT MAY BEGIN AS EARLY AS SATURDAY.
EXACTLY WHEN AND WHERE DANGEROUS HEAT BEGINS WILL LARGELY DEPEND
ON HOW THUNDERSTORMS EVOLVE OVER THE WEEKEND.
SHORT TERM
(THROUGH LATE SATURDAY AFTERNOON)
ISSUED AT 344 AM CDT FRI JUL 18 2025
WHILE THE LOOMING PROSPECT OF ANOTHER ROUND OF PROLONGED HEAT IS
JUST AROUND THE CORNER, THE PRIMARY CONCERN OVER THE NEXT COUPLE OF
DAYS REMAINS THE POTENTIAL FOR ISOLATED STRONG/SEVERE THUNDERSTORMS
AND LOCALLY HEAVY RAIN.
AS THE DAY BEGINS, CONVECTION HAS LARGELY DISSIPATED ACROSS THE AREA
AS A WEAK SURFACE FRONT HAS SETTLED JUST TO OUR SOUTH. OVER THE
COURSE OF THE DAY THOUGH, THIS BOUNDARY WILL WASH OUT AND WINDS WILL
BECOME SOUTHERLY ONCE AGAIN, WHICH WILL GRADUALLY TRANSPORT VERY
MOISTURE-RICH AIR TO THE NORTH. FORECAST AFTERNOON DEWPOINTS REACH
WELL INTO THE MID TO UPPER 70S ACROSS A BROAD AREA, WITH AFTERNOON
TEMPERATURES GENERALLY IN THE MID TO UPPER 80S. IN ADDITION TO
CREATING RATHER UNCOMFORTABLE LEVELS OF HUMIDITY, THIS IS ALSO
LIKELY TO PRODUCE AROUND 1500 TO 2500 J/KG OF SBCAPE BY MID
AFTERNOON, ALONG WITH PRECIPITABLE WATER VALUES RANGING FROM 1.8 TO
2.2 INCHES. WIND SHEAR CONTINUES TO BE RATHER LIMITED, BUT A MODEST
ENHANCEMENT IN MID-LEVEL FLOW DOES PRODUCE AROUND 20 TO 25KT OF 0-
6KM BULK SHEAR ACROSS NORTHERN MISSOURI DURING THE AFTERNOON.
WHILE THE THERMODYNAMIC ENVIRONMENT DOES SEEM TO SUPPORT THE
POSSIBILITY OF THUNDERSTORMS, THE LACK OF A CLEAR FORCING MECHANISM
AND LIMITED REPRESENTATION IN HIGH-RESOLUTION MODEL GUIDANCE
SIGNIFICANTLY LIMITS CONFIDENCE IN STORM COVERAGE. THE HIGHEST
PROBABILITY OF STORMS APPEARS TO BE ALONG AND SOUTH OF I-70 ALONG
WHAT'S LEFT OF THE REMNANT SURFACE BOUNDARY, BUT A FEW STORMS ARE
ALSO POSSIBLE FARTHER NORTH AS WELL, AND THERE ARE NO AREAS THAT WE
CAN CONFIDENTLY SAY WILL BE COMPLETELY DRY. HOWEVER, IT'S DIFFICULT
TO SEE A SCENARIO THAT RESULTS IN MORE THAN WIDELY SCATTERED
COVERAGE AT ANY TIME DURING THE DAY, AND MOST CAMS MAINTAIN A RATHER
LARGE GAP IN STORM COVERAGE ACROSS MUCH OF THE AREA. WHATEVER STORMS
DO DEVELOP WILL HAVE PLENTY OF MOISTURE AND INSTABILITY TO WORK
WITH, THOUGH, AND WOULD LIKELY BE CAPABLE OF PRODUCING INTENSE RAIN
RATES AND OCCASIONAL DOWNBURST WINDS OF 30 TO 50 MPH.
MORE LIKELY, THOUGH, IS THE POTENTIAL FOR A ROUND OF DECAYING
THUNDERSTORMS TO MOVE INTO THE AREA FROM THE NORTH OVERNIGHT AND
EARLY TOMORROW MORNING. A SHORTWAVE IS LIKELY TO TRIGGER A LINE OF
THUNDERSTORMS ACROSS IOWA LATE IN THE DAY, WHICH WILL LIKELY MOVE
SOUTHEAST AND INTO NORTHEAST MISSOURI / WEST-CENTRAL ILLINOIS VERY
LATE, AND PERHAPS CLOSE TO SUNRISE SATURDAY MORNING. WHILE
CONFIDENCE IS LOW THAT THIS ACTIVITY WILL BE STRONG OR SEVERE BY THE
TIME IT ARRIVES, THERE WILL BE ENOUGH MOISTURE AND LINGERING
INSTABILITY AROUND THAT WE CAN'T RULE OUT A FEW STRONG WIND GUSTS
AND INTENSE RAIN RATES. THERE WILL ALSO LIKELY BE A MODEST BOOST IN
WIND SHEAR AS THE SHORTWAVE MOVES THROUGH THE AREA, AND THAT COULD
HELP TO MAINTAIN THIS LINE JUST LONG ENOUGH FOR SOME MINOR IMPACTS.
AS THE DAY PROGRESSES SATURDAY, REMNANTS OF THIS ACTIVITY ARE LIKELY
TO CONTINUE TO DRIFT SOUTHEASTWARD, AND CONFIDENCE IN ITS EVOLUTION
IS SOMEWHAT LOW. THERE IS AT LEAST SOME POTENTIAL FOR THIS STORM TO
CONTINUE TO BACK-BUILD AS IT MOVES THROUGH OUR AREA, AND POSSIBLY
STRENGTHEN AGAIN AS THE AFTERNOON WEARS ON AND INSTABILITY BUILDS.
AS SUCH, WE CAN'T RULE OUT A FEW STRONG WIND GUSTS THROUGHOUT THE
DAY SATURDAY, PARTICULARLY EAST OF THE MISSISSIPPI RIVER. NOT ONLY
THIS, BUT WITH VERY RICH MOISTURE REMAINING IN PLACE, WE WILL ALSO
NEED TO MONITOR THE POTENTIAL FOR LOCALIZED FLOODING DUE TO INTENSE
RAIN RATES, SLOW STORM MOTIONS, AND THE AFOREMENTIONED BACK-BUILDING
POTENTIAL.
FINALLY, WHILE FORECAST NBM HEAT INDEX VALUES SATURDAY DO APPROACH
ADVISORY LEVELS (AND WARNING LEVELS IF IT CONTINUES FOR SEVERAL MORE
DAYS) ALONG AND SOUTH OF I-70, CONFIDENCE IS VERY LOW IN THIS
MATERIALIZING DUE TO THE UNCERTAINTIES ASSOCIATED WITH THE
PREVIOUSLY DISCUSSED CONVECTION. A CONTINUOUSLY BUILDING
THUNDERSTORM COMPLEX AND SURGING OUTFLOW WOULD LIKELY LIMIT THE
MOST SIGNIFICANT HEAT TO AREAS SOUTH OF I-70, AND LATEST FORECASTS
HAVE REDUCED TEMPERATURES A BIT. STILL, WE CAN'T RULE OUT HEAT
HEADLINES AS SOON AS SATURDAY, EITHER AS AN ADVISORY OR PERHAPS
EVEN AN EXTREME HEAT WATCH WHICH EXTENDS SEVERAL DAY BEYOND
SATURDAY. WHILE THIS MAY BE NEEDED IN SHORT ORDER, CONFIDENCE IS
JUST NOT HIGH ENOUGH REGARDING SATURDAY'S TEMPERATURES TO ISSUE
HEADLINES JUST YET.
BRC
LONG TERM
(SATURDAY NIGHT THROUGH THURSDAY)
ISSUED AT 344 AM CDT FRI JUL 18 2025
WHILE WE MAY REMAIN IN RANGE FOR RIDGE-RUNNING THUNDERSTORM
COMPLEXES SUNDAY AND MONDAY, BUILDING DANGEROUS HEAT IS EXPECTED TO
TAKE CENTER-STAGE AS WE MOVE INTO THE WORK WEEK.
VERY LITTLE CHANGE IS EXPECTED IN THE OVERALL FLOW PATTERN SUNDAY,
AS WE WILL REMAIN ON THE NORTHERN PERIPHERY OF A FLAT UPPER RIDGE,
AND WITHIN RANGE OF PASSING SHORTWAVES TO OUR NORTH. WE WILL ALSO
CONTINUE TO HOLD ON TO A VERY HUMID AIRMASS, PROVIDING PLENTY OF
INSTABILITY AND FUEL FOR NOCTURNAL THUNDERSTORM COMPLEXES THAT CAN
MAINTAIN THEMSELVES AS THEY ARRIVE FROM THE NORTHWEST. AS SUCH, IT'S
POSSIBLE THAT WE COULD SEE A SIMILAR SCENARIO UNFOLD AGAIN SATURDAY
NIGHT, AND POSSIBLY EVEN AGAIN SUNDAY NIGHT, WHERE A DECAYING
COMPLEX OF THUNDERSTORMS DRIFTS INTO NORTHERN MISSOURI AND INTO
ILLINOIS LATE AT NIGHT AND LINGERS ACROSS OUR EASTERN HALF OVER THE
COURSE OF THE DAY. CONFIDENCE IN THE PRECISE CONVECTIVE EVOLUTION
EACH DAY IS RATHER LOW, BUT WITH ONLY MINOR CHANGES IN THE OVERALL
PATTERN EXPECTED THROUGH SUNDAY NIGHT, IT'S NOT HARD TO ENVISION A
REPEATING PATTERN SUCH AS THIS.
MEANWHILE, HEAT IS LIKELY TO STEADILY BUILD IN AREAS THAT REMAIN
FREE OF CONVECTIVE OUTFLOW, AND PEAK ACROSS MUCH OF THE AREA
SOMETIME IN THE EARLY TO MID-WEEK TIMEFRAME AS THE UPPER RIDGE
AMPLIFIES AND PUSHES THE WAVE TRAIN FARTHER TO OUR NORTH. WHILE THE
POTENTIAL FOR MULTIPLE ROUNDS OF SHOWERS AND THUNDERSTORMS LIMITS
THE CONFIDENCE OF THE ONSET OF DANGEROUS HEAT IN MANY AREAS OVER THE
WEEKEND, CONFIDENCE REMAINS HIGH THAT BY EARLY NEXT WEEK (MONDAY
ONWARD) DANGEROUS HEAT WILL SPREAD ACROSS MUCH OF THE REGION AND
PERSIST FOR SEVERAL DAYS. LREF PROBABILITIES FOR HEAT INDEX VALUES
OVER 100 DEGREES CLIMB TO BETWEEN 50 AND 80 PERCENT MONDAY AND THIS
CONTINUES THROUGH THE WORK WEEK AND BEYOND. THIS MAY ALSO BE
UNDERSELLING THINGS A BIT, AS THE NBM IS EVEN MORE BULLISH ON
FORECAST TEMPERATURES EVEN IF THE LATEST OPERATIONAL RUN WAS
SLIGHTLY COOLER THAN THE PREVIOUS ITERATION.
AS SUCH, IT IS VERY LIKELY THAT HEAT HEADLINES OF SOME VARIETY WILL
BE NEEDED SOON. HOWEVER, THE PRIMARY HOLD-UP CONTINUES TO BE
UNCERTAINTY ON-THE FRONT END OF THE EVENT, AS THE AFOREMENTIONED
THUNDERSTORM COMPLEXES MAY DELAY THE ONSET OF HEAT IN MANY AREAS.
THIS IS ESPECIALLY TRUE NORTH OF I-70, WHERE WE MAY NOT REACH
ADVISORY LEVELS OF HEAT UNTIL AS LATE AS MONDAY OR EVEN TUESDAY. SO
WHILE SOME AREAS MAY DANGEROUS HEAT BEGINNING AS EARLY AS SATURDAY
AFTERNOON, WE'VE OPTED TO HOLD OFF ON ISSUING HEADLINES UNTIL WE CAN
GET A LITTLE MORE CONFIDENCE ON HOW THUNDERSTORMS WILL EVOLVE
OVERNIGHT TONIGHT THROUGH TOMORROW AFTERNOON.
BRC
AVIATION
(FOR THE 12Z TAFS THROUGH 12Z SATURDAY MORNING)
ISSUED AT 545 AM CDT FRI JUL 18 2025
THE PRIMARY CONCERNS DURING THE 1Z TAF PERIOD WILL BE LOW CEILINGS
AND PATCHY FOG EARLY THIS MORNING, FOLLOWED BY A MODEST CHANCES
FOR SHOWERS AND THUNDERSTORMS A VARIOUS TIMES THIS AFTERNOON AND
AGAIN OVERNIGHT. IFR TO LIFR CEILINGS HAVE BEEN OBSERVED TO
VARYING DEGREES AT ALL TERMINALS THIS MORNING, WITH SOME DENSE
FOG OBSERVED AT CPS AS WELL. CEILINGS ARE EXPECTED TO GRADUALLY
LIFT THROUGH THE MORNING, LEADING TO VFR CEILINGS BY MID-DAY. FOG
WILL ALSO DISSIPATE SHORTLY AFTER SUNRISE.
WIDELY SCATTERED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE AT
I-70 TERMINALS THIS AFTERNOON, ALTHOUGH CONFIDENCE IS LOW THAT ANY
INDIVIDUAL TERMINAL WILL BE IMPACTED. IF A THUNDERSTORM DOES MOVE
OVERHEAD, INTENSE RAIN RATES AND BRIEF GUSTY WINDS WILL BE
POSSIBLE, ALONG WITH CEILING AND VISIBILITY REDUCTIONS. VFR
CONDITIONS ARE LIKELY TO DEVELOP SHORTLY AFTERWARD.
OVERNIGHT, ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS MAY IMPACT
UIN AND LATER ST. LOUIS AREA TERMINALS. THIS IS LIKELY TO OCCUR
NEAR THE VERY END OF THE PERIOD, WITH GUSTY WINDS AND HEAVY RAIN
POSSIBLE ONCE AGAIN.
BRC
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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