730
FXUS63 KLSX 202312
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
612 PM CDT TUE MAY 20 2025
KEY MESSAGES
- MUCH COOLER AND DRIER WEATHER IS FORECAST THROUGH THE END OF THE
WORK WEEK.
- RAIN SHOWERS, ALONG WITH A FEW THUNDERSTORMS (NON SEVERE), ARE
EXPECTED (60-80% CHANCE) ON/OF THIS HOLIDAY WEEKEND ALONG WITH
WELL- BELOW NORMAL TEMPERATURES. THIS LIKELY WILL BE THE COOLEST
MEMORIAL DAY HOLIDAY WEEKEND SINCE AT LEAST 2003 FOR MOST
LOCATIONS.
SHORT TERM
(THROUGH LATE WEDNESDAY NIGHT)
ISSUED AT 232 PM CDT TUE MAY 20 2025
A WELL-DEFINED MCV ON GOES-16 SATELLITE IMAGERY IS CURRENTLY
MOVING INTO FAR SOUTHEAST IOWA AS OF 1900 UTC. THIS FEATURE HAS
HELPED INITIATE WIDESPREAD CONVECTION TO ITS EAST/SOUTHEAST ALONG
A COLD FRONT. FURTHER SOUTHWEST, CONVECTIVE INITIATION HAS BEEN
VERY DIFFICULT TO COME BY, IT LOOKS LIKE WEAKER SURFACE
CONVERGENCE AND THE STRONGER MIDLEVEL ASCENT STAYING NORTHEAST
IMMEDIATELY DOWNSTREAM OF THE AFOREMENTIONED MCV ARE THE CULPRITS.
VISIBLE SATELLITE IMAGERY SHOWS ONLY MODEST CUMULUS, SO IT DOES
NOT APPEAR DEVELOPMENT IS LIKELY, AT LEAST IMMINENTLY. THE WINDOW
FOR INITIATION ALSO IS CLOSING FROM WEST TO EAST AS THE COLD FRONT
PROGRESSES TO THE EAST. IN OTHER WORDS, IT APPEARS THE THREAT FOR
STRONG TO SEVERE THUNDERSTORMS HAS LESSENED. CANNOT COMPLETELY
RULE OUT A BRIEF STRONG TO SEVERE STORM ACROSS SOUTH-CENTRAL
ILLINOIS BETWEEN 2100-2300 UTC, BUT CHANCES APPEAR LOW. LARGE HAIL
LIKELY WOULD BE THE MAIN THREAT IF ANYTHING OCCURS. NEAR 0000
UTC, THE FRONT SHOULD EXIT PARTS OF SOUTH-CENTRAL ILLINOIS, ENDING
ANY THREAT FOR THUNDERSTORMS.
THIS FRONTAL PASSAGE USHERS IN A MAJOR PATTERN CHANGE ACROSS THE
CONUS WITH ANOMALOUS MID/UPPER LEVEL TROUGHING CARVING OUT ACROSS
THE GREAT LAKES. BELOW NORMAL TEMPERATURES AND DRY WEATHER WILL BE
THE MAIN THEME BEGINNING ON WEDNESDAY, AND SHOULD LAST AT LEAST INTO
EARLY NEXT WEEK. HIGHS TOMORROW ARE FORECAST TO BE IN THE MID 60S TO
LOW 70S FROM NORTH TO SOUTH, OR ABOUT 5-10 DEGREES BELOW NORMAL FOR
THE DATE. NORTHWEST WINDS WILL ALSO BE A BIT BRISK, ON THE ORDER OF
15-20 MPH WITH GUSTS IN THE 25-30 MPH RANGE. THIS WILL MAKE IT FEEL
EVEN A BIT COOLER THAN THE AMBIENT AIR TEMPERATURE.
A WEAK SURFACE RIDGE OF HIGH PRESSURE IS EXPECTED TO MOVE ACROSS THE
MISSISSIPPI VALLEY EARLY WEDNESDAY NIGHT, FOLLOWED BY A SECONDARY
COLD FRONTAL PASSAGE (AND REINFORCING SHOT OF COOLER THAN NORMAL
AIR) OVERNIGHT. CANNOT TOTALLY RULE OUT A FEW ISOLATED SHOWERS
ASSOCIATED WITH THIS BOUNDARY ACROSS THE MISSISSIPPI RIVER INTO
WESTERN ILLINOIS, BUT TIMING (OVERNIGHT) SUGGESTS THIS IS UNLIKELY.
ANOTHER AREA OF POTENTIAL SHOWER ACTIVITY (20% CHANCE) IS IN PARTS
OF SOUTHEAST MISSOURI, BUT BETTER CHANCES REMAIN WELL TO OUR
SOUTHWEST. WELL-BELOW NORMAL TEMPERATURES ARE FORECAST AS LOWS DROP
BACK INTO THE MID 40S TO LOW 50S. SIMILAR TO WEDNESDAY'S HIGHS,
THESE READINGS WOULD BE ABOUT 5-10 DEGREES BELOW NORMAL FOR MID/LATE
MAY.
GOSSELIN
LONG TERM
(THURSDAY THROUGH NEXT TUESDAY)
ISSUED AT 232 PM CDT TUE MAY 20 2025
(THURSDAY - FRIDAY)
THERE IS HIGH CONFIDENCE IN CONTINUED DRY WEATHER WITH WELL-BELOW
NORMAL TEMPERATURES TO CONCLUDE THE WORK WEEK AS A STRONGER SURFACE
RIDGE (~90TH PERCENTILE OF CLIMATOLOGY) OF HIGH PRESSURE GRADUALLY
WORKS ITS WAY ACROSS THE MISSISSIPPI VALLEY. HIGHS EACH OF THURSDAY
AND FRIDAY AFTERNOON ARE FORECAST TO BE IN THE MID 60S TO NEAR 70
DEGREES. GIVEN A NORTHWEST BREEZE AND VERY LOW HUMIDITY LEVELS
(DEWPOINT VALUES BELOW 10TH PERCENTILE OF CLIMATOLOGY), IT DOES NOT
GET MUCH MORE PLEASANT WEATHER WISE FOR LATE MAY. THURSDAY NIGHT
SHOULD BE DOWNRIGHT COOL BASED ON THE EXPECTED TIMING OF THE SURFACE
RIDGE. THE COMBINATION OF A CLEAR SKY, LIGHT WINDS, AND LOW
DEWPOINTS SHOULD YIELD VERY FAVORABLE CONDITIONS FOR RADIATIONAL
COOLING. LOWS ARE FORECAST TO RANGE FROM THE LOW TO UPPER 40S, AND I
WOULD NOT BE SURPRISED TO SEE A FEW ISOLATED LOCATIONS FLIRT WITH
THE UPPER 30S IN FAVORED VALLEYS.
(FRIDAY NIGHT - NEXT TUESDAY)
ENSEMBLE GUIDANCE IS IN PRETTY GOOD AGREEMENT WITH THE GENERAL
PATTERN ACROSS THE CONUS THIS UPCOMING HOLIDAY WEEKEND, DEPICTING
MORE QUASI-ZONAL FLOW OVER OUR REGION WITH A FRONTAL BOUNDARY TO OUR
SOUTHWEST. MULTIPLE MCSS MAY TRAVERSE ALONG/NORTH OF THIS BOUNDARY
THROUGH THE HOLIDAY WEEKEND, BUT THE FOCUS AT THIS TIME AT LEAST
CERTAINLY APPEARS TO BE CENTERED TO OUR SOUTHWEST. GIVEN THAT OUR
AREA FOR THE MOST PART SHOULD BE WELL INTO THE COOLER, MORE STABLE
AIR, THERE DOES NOT APPEAR TO BE MUCH IF ANY THREAT FOR
STRONG/SEVERE CONVECTION. IN FACT, THUNDER PROBABILITIES ARE
GENERALLY ON THE LOW SIDE, WITH JUST ISOLATED RUMBLES OF THUNDER
EXPECTED FOR THE MOST PART. THERE LIKELY WILL BE PERIODS OF RAIN
SHOWERS HOWEVER ON/OFF THIS HOLIDAY WEEKEND, AND PINPOINTING WHEN
AND WHERE THE BEST CHANCES WILL BE ARE STILL VERY DIFFICULT AT THIS
TIME RANGE.
THE OTHER NEAR-CERTAINTY WILL BE THE CONTINUED TREND FOR WELL-BELOW
NORMAL TEMPERATURES, THOUGH WITH A SHIFT TO MORE OF A FOCUS ON
DAYTIME HIGHS. PLENTY OF CLOUDS AND PRECIPITATION CHANCES SUGGEST
MORE MILDER NIGHTS (LOWS IN THE LOW TO MID 50S), OR ONLY SLIGHTLY
BELOW NORMAL. VERY ANOMALOUS DAYTIME TEMPERATURES ARE POSSIBLE
HOWEVER, BUT WILL DEPEND ON WHERE THE THICKEST CLOUD COVER AND MORE
CONTINUOUS RAIN SETS UP ON ANY GIVEN DAY. AS ALLUDED TO IN THE
PARAGRAPH ABOVE, THAT CANNOT BE ASCERTAINED THIS FAR OUT. HOWEVER,
EVEN THE 75TH PERCENTILE OF THE NBM IS AT LEAST 5+ DEGREES BELOW
NORMAL, WITH THE 25TH PERCENTILE NOT TOO FAR OFF FROM DAILY RECORD
LOW MAXIMUM TEMPERATURES. IT IS QUITE POSSIBLE AT LEAST SOME AREAS
DO NOT SEE A HIGH TEMPERATURE THE ENTIRE HOLIDAY WEEKEND AT OR ABOVE
THE 70 DEGREE MARK. FORECAST WISE, THIS WOULD BE THE COOLEST
MEMORIAL DAY HOLIDAY WEEKEND SINCE AT LEAST 2003 IN MANY LOCATIONS.
WHEREVER THICKER CLOUDS AND MORE CONTINUOUS RAIN SETS UP DURING ANY
DAY THIS WEEKEND, HIGHS MAY STRUGGLE TO EVEN REACH THE 60 DEGREE
MARK.
GOSSELIN
AVIATION
(FOR THE 00Z TAFS THROUGH 00Z WEDNESDAY EVENING)
ISSUED AT 606 PM CDT TUE MAY 20 2025
VFR CONDITIONS ARE EXPECTED OVER MUCH OF THE AREA WITH EXCEPTION
TO KUIN. MVFR CEILINGS ROTATE SOUTHEASTWARD DOWN THE WESTERN
PERIPHERY OF THE EXITING SYSTEM LATE TONIGHT INTO WEDNESDAY
MORNING, THEN SCATTER OUT TO RETURN TO VFR LATE MORNING INTO THE
AFTERNOON. WHILE GUSTS SUBSIDE TONIGHT SUSTAINED SURFACE FLOW
REMAINS ELEVATED WITH NORTHWESTERLIES AT 10-15 KNOTS. GUSTS ARE
REINTRODUCED AS MIXING COMMENCES MID-MORNING WEDNESDAY.
MAPLES
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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