901
FXUS63 KLSX 151935
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
235 PM CDT THU MAY 15 2025
KEY MESSAGES
- THERE REMAINS A VERY CONDITIONAL THREAT OF A COUPLE OF SEVERE THUNDERSTORMS
THIS EVENING ACROSS SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS.
LARGE HAIL IS THE PRIMARY THREAT.
- MORE WIDESPREAD SEVERE THUNDERSTORMS ARE LIKELY FRIDAY AFTERNOON
AND EARLY EVENING WITH ALL HAZARDS, INCLUDING VERY LARGE HAIL
(2.75+"), DAMAGING WINDS, AND A LOW CHANCE OF A STRONG TORNADO.
THE GREATEST SEVERE RISK IS ALONG/SOUTH OF I-44 IN MO AND I-70
IN IL.
SHORT TERM
(THROUGH LATE FRIDAY AFTERNOON)
ISSUED AT 221 PM CDT THU MAY 15 2025
THE PACIFIC COLD FRONT CONTINUES ITS EASTWARD MARCH ACROSS THE
CWA EARLY THIS AFTERNOON. AS OF 1900 UTC, IT WAS LOCATED JUST WEST
OF A KVIH>>KPPQ LINE. THE BOUNDARY IS MOST EVIDENT LOOKING AT
DEWPOINT VALUES, AS THERE IS VERY LITTLE WIND SHIFT AND NOT MUCH
OF A TEMPERATURE GRADIENT ASSOCIATED WITH THIS FRONT. THE LIMITING
FACTORS DISCUSSED THE PAST FEW DAYS ARE ALL IN PLAY, WITH FAIRLY
STRONG CAPPING EVIDENT BETWEEN 800 AND 900 HPA ON ACARS
SOUNDINGS. TEMPERATURES HOWEVER TOWARD THE TOP OF THE INVERSION
ALOFT ARE BEGINNING TO COOL FROM WEST TO EAST, SO THE STRENGTH OF
THE CAP SHOULD BE WEAKENING OVER THE NEXT FEW HOURS. THE BIGGER
PROBLEM FROM A CONVECTIVE INITIATION STANDPOINT IS FORCING.
SURFACE CONVERGENCE ALONG THE BOUNDARY IN OUR NECK OF THE WOODS IS
NEARLY NON EXISTENT, WITH MAYBE 20-40 DEGREES OF VEER AS YOU
TRAVEL FROM EAST TO WEST ACROSS THE BOUNDARY. THE MAIN THRUST FOR
MID/UPPER LEVEL FORCING FOR ASCENT IS ALSO WELL TO THE NORTH OF
THE AREA. THERE IS SOME CONVECTION ACROSS NORTHERN ARKANSAS AND
FAR SOUTHEAST MISSOURI WHERE SURFACE CONVERGENCE ALONG THE
BOUNDARY IS A BIT STRONGER. PERHAPS THIS IS ASSOCIATED BY A WEAK,
BUT SUBTLE MIDLEVEL IMPULSE AS WELL BUT NEITHER SHORT-TERM
GUIDANCE NOR WATER VAPOR IMAGERY IS REALLY PICKING UP ON MUCH OF ANYTHING.
THE TRAJECTORY OF THESE STORMS HOWEVER SHOULD KEEP THEM SOUTHEAST
OF OUR CWA. SOME CAMS HAVE SUGGESTED A BRIEF ISOLATED SUPERCELL
OR TWO OUT AHEAD OF THIS AREA IN SOUTHWEST/SOUTH CENTRAL ILLINOIS
BETWEEN ~23 AND 02Z. MAYBE THIS IS DUE TO A SUBTLE MIDLEVEL
IMPULSE AND SOME COOLING ADVECTING IN ALOFT. THIS IS A LONG WAY OF
SAYING THAT ANY SEVERE THREAT THIS EVENING REMAINS HIGHLY
CONDITIONAL. THE THREAT IS ALSO STILL VERY CONFINED BOTH
TEMPORALLY AND SPATIALLY AS DETAILED ABOVE.
THE PACIFIC COLD FRONT FINISHES MOVING THROUGH THE AREA AROUND
0300 OR 0400 UTC, WITH DRY WEATHER FAVORED FOR THE REMAINDER OF
THE NIGHT. CANNOT TOTALLY RULE OUT A STRAY SHOWER OR WEAK
THUNDERSTORM OVERNIGHT IN FAR SOUTHEAST MISSOURI, BUT CHANCES
APPEAR TO BE AT OR BELOW 10%.
CONFIDENCE HAS CONTINUED TO INCREASE WITH RESPECT TO CONVECTIVE
TRENDS ON FRIDAY AHEAD OF A STRONGER COLD FRONT THAT WILL MOVE
THROUGH THE CWA. SOUTHERLY WINDS SHOULD INCREASE QUICKLY FRIDAY
MORNING, ALLOWING FOR WARMER AND UNSTABLE AIR TO SURGE BACK NORTHWARD.
THERE REMAINS SOME DEGREE OF UNCERTAINTY ON EXACTLY HOW FAR NORTH
THE RICHER DEWPOINTS (AND ERGO, INSTABILITY) ADVECTS BY FRIDAY
AFTERNOON, BUT IT APPEARS TO BE A SLAM DUNK FOR SOUTHEAST MISSOURI
AND SOUTHWEST ILLINOIS. PARTS OF EAST-CENTRAL MISSOURI AND
WESTERN ILLINOIS ARE MORE ON THE NORTHWESTERN EDGE OF WHERE
CONVECTION IS LIKELY TO DEVELOP OR WHERE LEFT MOVING SUPERCELLS
(AFTER SPLITS) MAY MOVE INTO. THE CONVECTIVE MODE SHOULD BE
PREDOMINANTLY SUPERCELLS, GIVEN A STRONG ORTHOGONAL COMPONENT OF
THE DEEP-LAYER SHEAR VECTOR TO THE ORIENTATION OF THE COLD FRONT.
HODOGRAPHS SHOW EITHER STRAIGHT HODOGRAPHS OR VEER-BACK-VEER
PROFILES, BOTH OF WHICH TEND TO FAVOR AT LEAST SOME SPLITTING OF
SUPERCELLS. LEFT MOVERS ARE TYPICALLY SHORTER LIVED AND ALMOST
EXCLUSIVELY A LARGE HAIL THREAT WHILE RIGHT MOVERS ARE BOTH HAIL
AND TORNADIC THREATS. BECAUSE OF THESE SPLITS, THERE ALSO MAY BE
SOME MERGERS. THESE CAN BE DESTRUCTIVE OR CONSTRUCTIVE IN NATURE,
BUT THESE MERGERS MAY ALSO LEAD TO SOME SMALL BOWING SEGMENTS.
THIS IS WHERE DAMAGING WINDS WOULD BE MOST FAVORED, AS THE CURRENT
THINKING IS THAT ANY FORMATION OF A WELL-DEFINED QLCS WILL NOT
OCCUR UNTIL AFTER IT EXITS THE CWA. OVERALL, THE MAIN CONCERNS
CONTINUE TO BE ON THE POTENTIAL FOR VERY LARGE HAIL (2.75+") GIVEN
SUPERCELLS AS A PRIMARY CONVECTIVE MODE AND VERY LARGE CAPE
WITHIN THE -10 TO -30C LAYER/HAIL GROWTH ZONE. ANY LONG-TRACKED
SUPERCELL WOULD HAVE THE CAPABILITY OF PRODUCING A SWATH OF VERY
LARGE HAIL AS WELL. THERE CERTAINLY IS TORNADO POTENTIAL,
ESPECIALLY WITH MORE SOUTHERN EXTENT. LOW-LEVEL SHEAR/HELICITY
INCREASES AS YOU GO MORE INTO SOUTHEAST MISSOURI AND SOUTHWEST
ILLINOIS. STORMS WILL ALSO NOT EXIT THIS AREA UNTIL CLOSER TO
23-0Z, WHICH MEANS SOME INCREASE POTENTIALLY IN LOW-LEVEL
CYCLONIC CURVATURE OF HODOGRAPHS AS THE NOCTURNAL LOW-LEVEL JET
BEGINS TO INCREASE. IN TERMS OF TIMING, THERE IS MORE CONFIDENCE
ON THE END OF THE WINDOW FOR SEVERE THUNDERSTORMS THAN THE
BEGINNING. STORMS SHOULD EXIT THE AREA ROUGHLY BETWEEN 23 AND 0Z,
BUT THE ONSET COULD BE AS EARLY AS ~17Z OR AS LATE AS ~20Z. THE
LARGE DIFFERENCE ON THE FRONT END IS SIMPLY DUE TO DIFFERENCES IN
EXACTLY WHEN CONVECTIVE INITIALIZATION OCCURS TOMORROW.
GOSSELIN
LONG TERM
(FRIDAY NIGHT THROUGH NEXT THURSDAY)
ISSUED AT 221 PM CDT THU MAY 15 2025
(FRIDAY NIGHT - SATURDAY NIGHT)
AFTER THE PASSAGE OF THE COLD FRONT EARLY FRIDAY EVENING, A BRIEF
PERIOD OF COOLER AND DRIER WEATHER IS LIKELY TO START OFF THE
WEEKEND. TEMPERATURE TRENDS WITH RESPECT TO SATURDAY CONTINUE TO
COOL AS GUIDANCE HAS TRENDED MORE TOWARD THE COOLER GFS/GEFS
SOLUTION OF THE PAST COUPLE OF DAYS. HIGHS ON SATURDAY ARE FORECAST
TO BE MAINLY IN THE MID TO UPPER 70S, OR RIGHT AROUND NORMAL FOR
THIS TIME OF YEAR. LOWS EACH NIGHT ARE EXPECTED TO DROP BACK INTO
THE 50S, WHICH WOULD ALSO BE NEAR SEASONAL NORMALS.
(SUNDAY - NEXT THURSDAY)
A RETURN TO A MORE ACTIVE PATTERN REMAINS IN THE OFFING LATE THIS
WEEKEND AND INTO EARLY NEXT WEEK. NOT MUCH HAS CHANGED OVER THE
PAST COUPLE OF DAYS, WITH AN MCS EXPECTED BOTH SUNDAY NIGHT AND
MONDAY NIGHT ALONG/NORTH OF A RETREATING WARM FRONT. THIS BOUNDARY
SHOULD PROGRESS NORTHWARD IN TIME, WHICH MEANS IT IS UNLIKELY
THAT THE SUBSEQUENT NOCTURNAL MCS MOVES OVER THE SAME REGION EACH
NIGHT. TRYING TO PINPOINT WHERE EACH OF THESE MCSS WILL TRACK THIS
FAR OUT IS A FOOL'S ERRAND, BUT IT DOES APPEAR LIKELY THAT
WIDESPREAD RAINFALL WILL ACCOMPANY AT LEAST PORTIONS OF THE CWA
WITH EACH ROUND, THE QUESTION IS EXACTLY WHERE. HEAVY RAINFALL
(AND POSSIBLE FLASH FLOODING) COULD BECOME MORE OF A CONCERN IF
MONDAY NIGHT'S ROUND TRACKS OVER THE SAME AREAS AS SUNDAY NIGHT'S,
BUT AS ALL SPELLED OUT ABOVE, THAT DOES NOT SEEM LIKELY AT THIS
POINT IN TIME. TOTAL PRECIPITATION PROBABILITIES FOR 2+" ON THE
LATEST LREF PEAK OUT AT ABOUT 40-50%, BUT DROPS TO LESS THAN 10%
FOR 4+". WHILE APRIL WAS VERY WET, SOIL MOISTURE HAS LARGELY RESET
BACK CLOSER TO NORMAL LEVELS DUE TO MOSTLY NEAR OR SLIGHTLY BELOW
NORMAL RAINFALL SO FAR DURING THE MONTH OF MAY. IN FACT, A GOOD
PORTION OF NORTHEAST MISSOURI AND WEST-CENTRAL ILLINOIS HAS BEEN
DRIER THAN NORMAL AND CAN REALLY USE 2-3" OF RAINFALL.
THERE ALSO REMAINS QUITE A BIT OF UNCERTAINTY WITH HOW
TUESDAY/TUESDAY NIGHT WILL EVOLVE AS CLUSTERS OF THE 500-HPA HEIGHT
PATTERN DIVERGE QUITE A BIT WITH RESPECT TO THE TIMING, TRACK, AND
STRENGTH OF THE MID/UPPER LEVEL LOW COMING OUT OF THE DESERT
SOUTHWEST. THERE MAY BE A CONDITIONAL THREAT FOR SEVERE WEATHER, BUT
THIS WILL LIKELY BE DEPENDENT UPON AVAILABILITY OF SUFFICIENT
INSTABILITY. GIVEN THAT AN MCS IS EXPECTED MONDAY NIGHT, ONGOING
SHOWERS/THUNDERSTORMS AND CLOUD DEBRIS COULD REALLY HELP LIMIT
DAYTIME INSTABILITY ON TUESDAY.
WHILE THE EXACT TIMING IS STILL UNCLEAR, ENSEMBLE GUIDANCE IS IN
GOOD AGREEMENT AT A PRETTY SIGNIFICANT PATTERN CHANGE FOR MID/LATE
NEXT WEEK. A SLOW-MOVING MID/UPPER LEVEL CLOSED LOW IS LIKELY GOING
TO BE TO OUR NORTHEAST, WITH WELL-BELOW NORMAL TEMPERATURES
ADVECTING IN BEHIND IT. BOTH THE EPS/GEFS SHOW STRONG 850-HPA
TEMPERATURE ANOMALIES (-5 TO -8C). THAT IS NOTABLE FOR LATE
SPRING/ALMOST EARLY SUMMER. HIGHS IN THE 60S TO NEAR 70 DEGREES ARE
LIKELY WITH LOWS DROPPING INTO THE 40S TO LOW 50S.
GOSSELIN
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z FRIDAY AFTERNOON)
ISSUED AT 1224 PM CDT THU MAY 15 2025
A WEAK COLD FRONT IS FORECAST TO MOVE THROUGH THIS AFTERNOON, WITH
WINDS VEERING A BIT MORE TO THE SOUTHWEST. AN ISOLATED
THUNDERSTORM IS POSSIBLE ALONG THIS BOUNDARY, BUT MAINLY TO THE
EAST OF THE TERMINALS SO KEPT FORECASTS DRY. WIND GUSTS WILL
DIMINISH THIS EVENING AND BACK SLIGHTLY TO THE SOUTH BEFORE
VEERING AND INCREASING IN SPEED AHEAD OF A STRONGER COLD FRONT ON
FRIDAY.
GOSSELIN
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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