917
FXUS63 KLSX 131954
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
254 PM CDT WED MAY 13 2026
KEY MESSAGES
- A WARMER AND WETTER WEATHER PATTERN WILL BEGIN EARLY FRIDAY
CONTINUING INTO EARLY NEXT WEEK, WITH HIGHEST CHANCES OF SHOWERS
AND THUNDERSTORMS FRIDAY MORNING, SATURDAY, AND EITHER LATE
MONDAY OR TUESDAY.
SHORT TERM
(THROUGH LATE THURSDAY NIGHT)
ISSUED AT 253 PM CDT WED MAY 13 2026
A SURFACE HIGH PRESSURE CENTER WILL SHIFT INTO THE MID-MISSISSIPPI
RIVER VALLEY TONIGHT, PROMOTING EFFICIENT RADIATIONAL COOLING WHEN
COMBINED WITH CLEAR SKIES. AS A RESULT, LOW TEMPERATURES WILL COOL
TO AROUND 40 TO THE MID-40S F, BUT A FEW RIVER VALLEY LOCATIONS
DIPPING INTO THE UPPER 30S F CANNOT BE RULED OUT. ALTHOUGH IT IS
UNLIKELY THAT CROSSOVER TEMPERATURES WILL BE REACHED, AIR-WATER
TEMPERATURE DIFFERENCES COULD ALLOW DEVELOPMENT OF PATCHY RIVER
VALLEY FOG EARLY THURSDAY MORNING.
THE HIGH PRESSURE CENTER WILL GRADUALLY DEPART THE CWA ON THURSDAY
WITH LOW-LEVEL FLOW BECOMING MORE SOUTHEASTERLY, BUT ADVECTION WILL
REMAIN WEAK THROUGH THE DAY. WITH ANOTHER DAY OF STRONG INSOLATION,
HIGH TEMPERATURES ON THURSDAY WILL BE SIMILAR TO TODAY.
PFAHLER
LONG TERM
(FRIDAY THROUGH NEXT WEDNESDAY)
ISSUED AT 253 PM CDT WED MAY 13 2026
THURSDAY NIGHT, A WARM FRONT LIFTING NORTHEASTWARD THROUGH MO AND A
40 TO 50 KT SOUTHWESTERLY LLJ WILL RESULT IN STRONG LOW-LEVEL WAA
AND MOISTURE TRANSPORT EXTENDING INTO THE MID-MISSISSIPPI RIVER
VALLEY. CAMS ARE IN GENERAL AGREEMENT THAT SHOWERS AND THUNDERSTORMS
WILL FORM OVERNIGHT THURSDAY NIGHT AT THE NOSE OF THE LLJ ACROSS
NORTHERN MO, POTENTIALLY GROWING UPSCALE INTO A MULTICELL CLUSTER OR
MCS THAT PROPAGATES SOUTHEASTWARD INTO LOCATIONS ALONG AND NORTH OF
I-70 THROUGH FRIDAY MORNING. HREF MEAN MUCAPE REACHES 500 TO 1000
J/KG ALONG/WEST OF THE MISSISSIPPI RIVER AND DETERMINISTIC GUIDANCE
GENERALLY DEPICTS EFFECTIVE WIND SHEAR AROUND 20 TO 30 KT,
SUPPORTING A THREAT OF SMALL HAIL WITH ANY STRONGER THUNDERSTORMS.
HOWEVER, GIVEN MARGINAL WIND SHEAR AND AN UNFAVORABLE PREFERENTIAL
MULTICELL/LINEAR STORM MODE, CONFIDENCE IS CURRENTLY LOW IN ANY
THUNDERSTORMS PRODUCING SEVERE HAIL.
AS SHOWERS AND THUNDERSTORMS OUTRUN THE RESERVOIR OF INSTABILITY AND
THE LLJ WEAKENS, THEY SHOULD WEAKEN AND DISSIPATE THROUGH THE
MORNING. SLIGHT MID-LEVEL HEIGHT RISES AND A RESIDUAL CAPPING
INVERSION CASTS DOUBT IN ANYTHING MORE THAN ISOLATED REDEVELOPMENT
OF SHOWERS AND THUNDERSTORMS FRIDAY AFTERNOON AND EVENING. IN FACT,
ENSEMBLE MODEL PROBABILITIES OF MEASURABLE RAINFALL LOWER TO ONLY 10
TO 20 PERCENT BY THAT TIME. WARMER TEMPERATURES ARE FORECAST ON
FRIDAY WITH THE NORTHEASTWARD-ADVANCING WARM FRONT LEAVING THE CWA
IN THE WARM SECTOR. HIGH TEMPERATURES ARE, THEREFORE, ANTICIPATED TO
REACH THE 80S F WITH THE EXCEPTION OF COOLER TEMPERATURES ACROSS
SOUTH-CENTRAL/SOUTHWESTERN IL WHERE CLOUD DEBRIS COULD LINGER LONGER.
AROUND 60 TO 80 PERCENT OF ENSEMBLE MODEL MEMBERSHIP HAS ANOTHER
ROUND OF SHOWERS AND THUNDERSTORMS DEVELOPING SATURDAY MORNING,
COINCIDENT WITH THE MAXIMUM IN NOCTURNAL LLJ STRENGTH, BUT ONE OR
MORE PASSING PERTURBATIONS WITHIN QUASI-ZONAL UPPER-LEVEL FLOW WILL
SUPPORT A CONTINUATION OF SHOWERS AND THUNDERSTORMS AT TIMES THROUGH
THE DAY. AN INCREASINGLY MOIST AND UNSTABLE AIRMASS WILL BE IN PLACE
BY SATURDAY, BUT THE VAST MAJORITY OF MODEL GUIDANCE HAS WEAK DEEP-
LAYER WIND SHEAR (<20 KT) IN PLACE LIMITING CONFIDENCE AT THIS RANGE
IN THUNDERSTORMS BECOMING SEVERE. HOWEVER, THESE TYPES OF PATTERNS
CAN SOMETIMES YIELD MCVS THAT LEAD TO LOCALLY HIGHER WIND SHEAR, BUT
THESE DETAILS ARE NEARLY IMPOSSIBLE TO DETERMINE AT THIS RANGE. WITH
THE PRESENCE OF POTENTIAL CLOUDS AND PRECIPITATION, HIGH
TEMPERATURES ARE RELATIVELY MORE UNCERTAIN ON SATURDAY WITH ENSEMBLE
MODEL GUIDANCE INTERQUARTILE RANGES SPANNING 5 TO 8 F, TRANSLATING
TO ANYWHERE FROM THE MID-70S TO MID-80S F. THE CURRENT FORECAST
REFLECTS THE HIGHER END OF THIS SPAN OF TEMPERATURE OUTCOMES.
CONTRASTINGLY, CONFIDENCE IS INCREASING IN SUNDAY BEING A MAINLY DRY
DAY ACROSS THE CWA WITH ANY ADDITIONAL SHOWERS AND THUNDERSTORMS
LIFTING TO THE NORTH/NORTHEAST AND MID-LEVEL HEIGHTS RISING AS THE
UPPER-LEVEL FLOW PATTERN AMPLIFIES ACROSS THE CONUS, ALLOWING HIGH
TEMPERATURES TO MORE FIRMLY WARM INTO THE 80S F CWA-WIDE.
GLOBAL MODEL GUIDANCE IS IN AGREEMENT THAT UPPER-LEVEL TROUGHING
WILL TAKE PLACE ACROSS THE ROCKY MOUNTAINS SUNDAY INTO EARLY NEXT
WEEK, WITH UNCERTAINTY IN THE EJECTION OF ONE OR MORE SHORTWAVE
TROUGHS THROUGH SOUTHWESTERLY FLOW EXTENDING INTO THE MID-
MISSISSIPPI RIVER VALLEY. DUE TO TIMING AND TRACK DIFFERENCES ACROSS
MODEL GUIDANCE ON WHEN THE MAIN SHORTWAVE TROUGH EJECTS, THERE IS
UNCERTAINTY WHEN AN ASSOCIATED COLD FRONT WILL APPROACH AND PASS
THROUGH THE CWA, RANGING FROM LATE MONDAY EVENING/NIGHT TO TUESDAY
NIGHT, DETERMINING WHEN CHANCES OF ADDITIONAL SHOWERS AND
THUNDERSTORMS ARE MAXIMIZED. WITH STRONGER WIND SHEAR AND LARGE-
SCALE FORCING, THE POTENTIAL FOR SEVERE THUNDERSTORMS WILL ALSO NEED
TO BE MONITORED IF THE FRONT IS FAVORABLY IN PHASE WITH THE DIURNAL
MAXIMUM IN INSTABILITY (E.G., AFTERNOON AND EVENING), BUT AT THIS
JUNCTURE THERE IS A PREFERENCE IN ENSEMBLE AND MACHINE-LEARNING
MODEL GUIDANCE FOR THE SEVERE THREAT DURING THIS PERIOD TO BE
GREATEST ON MONDAY TO THE WEST OF THE CWA. AHEAD OF THE FRONT, WARM,
ABOVE AVERAGE TEMPERATURES WILL PERSIST WITH HIGHS IN THE 80S TO
NEAR 90 F.
PFAHLER
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 1215 PM CDT WED MAY 13 2026
DRY AND VFR FLIGHT CONDITIONS WILL PERSIST THROUGH THE TAF PERIOD,
ASIDE FROM PATCHY RIVER STEAM FOG EARLY THURSDAY MORNING. HOWEVER,
CONFIDENCE IS LOW AT THIS TIME THAT RIVER VALLEY TERMINALS WILL BE
IMPACTED. GUSTY NORTHWESTERLY WINDS WILL QUICKLY SLACKEN AND BECOME
LIGHT AND VARIABLE AROUND SUNSET AS A SURFACE HIGH PRESSURE CENTER
ARRIVES. WINDS WILL THEN BECOME SOUTHEASTERLY ON THURSDAY.
PFAHLER
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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