077
FXUS63 KLSX 201119
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
519 AM CST FRI DEC 20 2024
KEY MESSAGES
- FOLLOWING A LOW (20% CHANCE) FOR FLURRIES ALONG AND EAST OF THE
MISSISSIPPI RIVER THIS MORNING, DRY AND COLDER WEATHER STAYS IN
PLACE THROUGH SUNDAY
- A MORE ACTIVE WEATHER PATTERN WILL BRING WARMER TEMPERATURES AND
INCREASED RAIN CHANCES MONDAY ONWARDS, INCLUDING A 15-40%
CHANCE FOR LIGHT RAIN ON CHRISTMAS DAY.
SHORT TERM
(THROUGH LATE SATURDAY AFTERNOON)
ISSUED AT 206 AM CST FRI DEC 20 2024
AN AMPLIFIED RIDGE-TROUGH PATTERN CURRENTLY EXISTS OVER THE
CONUS, WITH SEVERAL SHORTWAVE TROUGHS EVIDENT ON GOES-EAST MID-
LEVEL WATER VAPOR IMAGERY TRACKING THROUGH THE MISSISSIPPI VALLEY.
AT THE SURFACE, A COLD FRONT IS NOW TRACKING THROUGH THE OHIO
RIVER VALLEY LEAVING OUR REGION IN BRISK, GUSTY NORTHWEST FLOW.
LOW STRATUS FORCED BY THE CYCLONIC FLOW ALOFT AND SUFFICIENT
MOISTURE IS ADVECTING SOUTH AND SHOULD ENCOMPASS THE MAJORITY OF
EASTERN MISSOURI AND NEARLY ALL OF ILLINOIS TODAY. A FEW FLURRIES
ARE POSSIBLE (20% CHANCE) IN THIS STRATUS ALONG AND EAST OF THE
MISSISSIPPI RIVER THROUGH MID-MORNING, WHERE MID-LEVEL FORCING
AND 700MB LIFT ARE EVIDENT IN THE LATEST SHORT-RANGE GUIDANCE.
THAT SAID, THE LIFT IS WEAK AND A BIT NEBULOUS WHICH WON'T LEAD TO
MANY IMPACTS BESIDES BRIEF INSTANCES OF LIGHT SNOW WITH LITTLE TO
NO ACCUMULATION. OTHERWISE, A CLOUDY, CHILLY, AND BLUSTERY DAY IS
IN STORE WITH HIGHS IN THE LOW TO MID-30S. AMIDST GUSTS OF
25-35MPH, PARTICULARLY ALONG AND EAST OF THE MISSISSIPPI RIVER,
WIND CHILLS STARTING IN THE TEENS AND TOPPING OUT IN THE LOW 20S.
COLD AIR ADVECTION FROM THE SURFACE TO 850MB WEAKENS THIS EVENING
WITH HIGH PRESSURE BUILDING FROM THE WEST AND WINDS SLACKENING.
ALONG WITH AT LEAST SOME DEGREE OF CLOUD COVER, THIS WILL ACT TO
KEEP TEMPERATURES TONIGHT FROM BEING ONLY A FEW DEGREES COLDER
THAN LAST NIGHT'S LOWS. WITH SURFACE RIDGING AND GRADUAL HEIGHT
RISES ALOFT ON SATURDAY, SKIES WILL CLEAR AND, ALONG WITH THE
WEAKENED SURFACE WINDS, HELP TAKE THE STING OUT OF THE COOL
DECEMBER AIR. TEMPERATURES SATURDAY WILL BE LARGELY SIMILAR TO, IF
NOT A HAIR WARMER THAN, TODAY'S HIGHS, LARGELY BECAUSE WEAK LOW-
LEVEL WINDS WILL NOT PROMOTE ANY NOTEWORTHY TEMPERATURE ADVECTION.
NBM INTERQUARTILE RANGES FOR HIGHS SUNDAY ARE ONLY 2-3 DEGREES,
LENDING HIGH CONFIDENCE (90+%) IN THESE NEAR-NORMAL TEMPERATURES.
THAT CHANGES STARTING SUNDAY, WHEN A GRADUAL PATTERN CHANGE
BEGINS TO TAKE FORM.
MRB
LONG TERM
(SATURDAY NIGHT THROUGH THURSDAY)
ISSUED AT 206 AM CST FRI DEC 20 2024
CONFIDENCE REMAINS HIGH (80-90% CHANCE) THAT THE SURFACE HIGH
(NOW CENTERED OVER THE GREAT LAKES) AND STRONGER SOUTHERLY WINDS
WILL SEND TEMPERATURES INTO THE 40S AREAWIDE SUNDAY AHEAD OF AN
APPROACHING COLD FRONT. TEMPERATURES WARM FURTHER, AS WARM AS THE
LOW 50S IN CENTRAL MISSOURI, ON MONDAY AHEAD OF THE FRONT, WHICH
INTRODUCES OUR FIRST OF SEVERAL CHANCES FOR PRECIPITATION THROUGH
LATE-WEEK. THE FRONT INITIALLY ENTER THE BI-STATE REGION DRY, BUT
AS IT FINALLY TAPS INTO MOISTURE CLOSER TO THE I-70 CORRIDOR,
RAIN CHANCES INCREASE TO 40- 60%, WITH THE HIGHER END OF THIS
RANGE FURTHER SOUTH. ENSEMBLE GUIDANCE DIFFERS ON EXACTLY WHERE
RAIN BEGINS, BUT UNIFORMLY SHOW THAT IT WILL REMAIN LIGHT OWING
TO WEAK LOW-LEVEL CONVERGENCE AND UPPER-LEVEL LIFT. CHANCES OF
SEEING OVER 0.25 INCHES OF RAIN ALONG THE FRONT BY LATE MONDAY ARE
AROUND 10% AND CENTERED SOUTH OF THE I-44 CORRIDOR NEAR THE
OZARKS, WHERE THE FRONT WILL SLOW AND EVENTUALLY STALL. DESPITE
THE FRONT MAKING IT LARGELY THROUGH THE REGION, COLD AIR STAYS LOCKED
FAR TO OUR NORTH AND TEMPERATURES WILL REMAIN IN THE UPPER 40S TO
LOW 50S. TEMPERATURE SPREAD GRADUALLY INCREASES, SUGGESTING
SLIGHTLY LESS CERTAINTY, DUE IN LARGE PART TO THE VARYING
SOLUTIONS OF WHERE THE FRONT STALLS.
RAIN CHANCES REMAIN HIGHEST ALONG THE STALLED FRONT MONDAY NIGHT
INTO TUESDAY, BUT EXACT CHANCES HINGE ON THE FRONT'S LOCATION.
WHILE ANY ONE LOCATION ALONG AND SOUTH OF THE I-44 (MO)/I-64 (IL)
CORRIDOR HAS A 50-70% CHANCE FOR RAIN AT THIS POINT, THERE IS
NEAR-CERTAINTY THAT AREAS SOUTH OF I-70 WILL SEE AT LEAST SOME
RAIN INTO CHRISTMAS EVE ALONG AND SURROUNDING THE FRONT. UPPER-
LEVEL FORCING AND LIFT BEGINS TO INTENSIFY AHEAD OF A SHORTWAVE
THAT DROPS INTO THE SOUTHERN GREAT PLAINS DURING THE DAY TUESDAY.
EXACTLY HOW AMPLIFIED THAT WAVE IS, AND WHERE IT DEVELOPS, WILL
AFFECT THE RESULTANT CYCLOGENESIS AND EVOLUTION OF RAIN CHANCES
LATE CHRISTMAS EVE INTO CHRISTMAS DAY.
CURRENTLY, ENSEMBLE GUIDANCE RANGES FROM TRACKING THE SURFACE LOW
DUE EAST AND MISSING US (LITTLE TO NO RAIN) TO DRIFTING IT INTO
SOUTHEAST MISSOURI AND SOUTHWEST ILLINOIS (AT LEAST SOME LIGHT
RAIN) ON WEDNESDAY. THAT SAID, MOST OF THE AVAILABLE GUIDANCE DOES
PAINT AT LEAST SOME RAIN GENERALLY ALONG AND SOUTH OF I-70, WHERE
THE STALLED FRONT DRIFTS NORTH AS A WARM FRONT, WITH SOME
EXTENDING THOSE CHANCES EVEN FURTHER NORTH AND WEST. WITH PRETTY
VARIED SOLUTIONS, THE NBM PRECIPITATION PROBABILITIES (15-40%)
WERE USED IN THIS FORECAST. WHILE THE LOCATION AND CHANCES FOR
PRECIPITATION ARE UNCERTAIN, WARM AIR ALOFT BRINGS NEAR-CERTAINTY
(95% CHANCE) THAT ANY PRECIPITATION STAYS LIQUID. WHILE IT MAY BE
A LITTLE DAMP FOR SANTA'S MIDNIGHT RIDE, AND A WHITE CHRISTMAS IS
NOT IN THE CARDS, THERE AREN'T MANY INDICATIONS OF A WASHOUT FOR
THE HOLIDAY. THE BEST-CASE SCENARIO IS THAT THE WAVE REMAINS WEAK
AND PROGRESSIVE, EXITING BY WEDNESDAY AFTERNOON. IF THE WAVE
CLOSES OFF AND SLOWS DOWN AS SOME GUIDANCE SUGGESTS, RAIN WOULD
LINGER LONGER INTO CHRISTMAS AND PERHAPS SPREAD FURTHER NORTH AND
WEST. NEITHER OF THESE SCENARIOS BODE WELL FOR HEAVY RAIN, WITH
NEAR- ZERO INSTABILITY AND WEAK MOISTURE CONVERGENCE ALONG THE
WARM FRONT. CHANCES FOR GREATER THAN 0.25 INCHES OF RAIN TOP OUT
AROUND 25% SOUTH OF I-44, WITH CHANCES DROPPING OFF QUICKLY
FURTHER NORTH AND WEST. WHILE TEMPERATURES WILL VARY ACROSS THE
REGION FROM NORTH TO SOUTH, HIGHS GENERALLY IN THE 40S TO LOW 50S
ARE FORECAST FOR THE HOLIDAY. THE UNSETTLED PATTERN CONTINUES INTO
THE LATTER HALF OF NEXT WEEK, WITH SEVERAL WAVES EVIDENT IN
ENSEMBLE GUIDANCE LEADING TO A 15-35% CHANCE FOR RAIN LINGERING
ACROSS THE REGION.
MRB
AVIATION
(FOR THE 12Z TAFS THROUGH 12Z SATURDAY MORNING)
ISSUED AT 502 AM CST FRI DEC 20 2024
MVFR STRATUS IS NOW IMPACTING ALL TAF SITES IN THE REGION BEHIND A
POTENT COLD FRONT THAT CROSSED THROUGH THE REGION OVERNIGHT. WINDS
ARE ALSO GUSTING CONSISTENTLY AT 20-30KTS BEHIND THE FRONT OUT OF
THE NORTHWEST. GUIDANCE CONTINUES TO EXTEND THE RESIDENCE TIME OF
THE STRATUS AND DOES NOT IMPROVE CONDITIONS THROUGH MOST OF THE
DAY. WITH STRONG NORTHWEST FLOW ALOFT, WHICH TYPICALLY PROMOTES
THESE CONDITIONS, THE TAFS NOW HOLD ONTO MVFR CONDITIONS A LITTLE
BIT LONGER. WE MAY EVEN SEE A RETURN OF MVFR THIS EVENING,
PARTICULARLY IN CENTRAL MISSOURI AND KUIN, BUT IT'S NOT CLEAR
ENOUGH IF A CEILING WILL EXIST TO INCLUDE IT IN THE TAF YET.
SUSTAINED WINDS AND GUSTS ARE FORECAST TO GRADUALLY WEAKEN TODAY
AS THE SURFACE PRESSURE GRADIENT RELAXES AND HIGH SURFACE PRESSURE
GRADUALLY BUILDS FROM THE WEST.
MRB
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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