858
FXUS63 KLSX 200410 RRA
AFDLSX
AREA FORECAST DISCUSSION...DELAYED
NATIONAL WEATHER SERVICE SAINT LOUIS MO
1010 PM CST THU DEC 19 2024
KEY MESSAGES
- ASIDE FROM A STRAY FLURRY EAST OF I-55 IN ILLINOIS EARLY FRIDAY
MORNING, DRY WEATHER CONTINUES THROUGH SUNDAY. RAIN RETURNS TO
THE AREA MONDAY.
- COLDER TEMPERATURES ARRIVE TONIGHT THROUGH SUNDAY. MILDER
TEMPERATURES RETURN EARLY NEXT WEEK AND CONTINUE TO WARM THROUGH
MIDWEEK.
SHORT TERM
(THROUGH LATE FRIDAY NIGHT)
ISSUED AT 220 PM CST THU DEC 19 2024
REGIONAL SATELLITE IMAGERY SHOWS LOW PRESSURE ROTATING EAST-
SOUTHEAST INTO THE GREAT LAKES THIS AFTERNOON. A TRAILING COLD FRONT
EXTENDS WESTWARD FROM THE SYSTEM INTO THE PLAINS. THIS FRONT IS
EXPECTED TO PUSH THROUGH THE REGION FROM THE NORTH TONIGHT WITH THE
POTENTIAL FOR A FEW FLURRIES BEHIND THE FRONT OVER FAR EASTERN
SECTIONS OF THE CWA EARLY FRIDAY MORNING. SHORT RANGE GUIDANCE
VARIES ON THE DEPTH OF SATURATION, BUT IN GENERAL, CROSS SECTIONS
SHOW 90% RH RESIDING BETWEEN 950-800HPA. WEAK FRAGMENTS OF MID-LEVEL
LIFT NAVIGATE AROUND WESTERN PERIPHERY OF CYCLONIC FLOW, WHICH MAY
BE ENOUGH TO LEAD TO TRACE AMOUNTS OF PRECIPITATION FRIDAY MORNING
IN THE 09Z-13Z TIMEFRAME. DESPITE LOW CHANCES (20% OR LESS), GREATER
THAN HALF OF THE GFS/ECM/CAN ENSEMBLE MEMBERS (IN TOTAL) SHOW TRACE
AMOUNTS FROM TAYLORVILLE, IL THROUGH SALEM, IL. THEREFORE, ADDED
MENTIONABLE FLURRIES TO THESE AREAS WITH NO MEASURABLE PRECIPITATION.
STRONG COLD AIR ADVECTION ARRIVES WITH NORTHWEST FLOW FRIDAY. A
MARKEDLY COLDER DAY IS IN STORE AS NBM SPREADS ARE ABOUT AS GOOD AS
THEY GET. THERE IS ONLY ABOUT 3 DEGREES OF SEPARATION BETWEEN THE
25TH-75TH PERCENTILES WITH HIGHS IN THE LOW TO MID-30S. TO ADD TO
THE COLD, WINDS WILL INCREASE ALONG THE PRESSURE GRADIENT AT THE
BACK SIDE OF THE DEPARTING SYSTEM. BUFKIT PROFILES SHOW 30-40 KNOTS
OF WIND JUST OFF THE SURFACE AROUND 925HPA AND ALOFT FROM THERE.
BLUSTERY CONDITIONS WILL BE NOTICEABLE, ESPECIALLY ALONG AND EAST OF
THE MISSISSIPPI RIVER. COLD AIR ADVECTION AND CLOUD COVER DOESN'T
ALLOW TEMPERATURES TO MOVE MUCH DURING THE DAY. GUSTS PEAKING
BETWEEN 30-35 MPH AND COLD TEMPERATURES RESULT IN APPARENT
TEMPERATURES STARTING OUT IN THE LOW TO MID-TEENS. IT DOESN'T GET
MUCH BETTER FROM THERE AS APPARENT TEMPERATURES DON'T RISE ABOVE THE
LOW TO MID-20S.
A WEAK UPPER SHORTWAVE QUIETLY PASSES OVERHEAD FRIDAY NIGHT INTO
SATURDAY WITH A LOT OF DRY AIR BELOW 8K FEET. SOME MID TO HIGH
CLOUDS LINGER, WHICH MAY KEEP US FALLING ANY FURTHER THAN THE 20S
SATURDAY MORNING. WHILE IT REMAINS CHILLY, HIGH PRESSURE LEADS TO
CLEARING SKIES AND LIGHT SURFACE FLOW. THIS WILL CERTAINLY FEEL
BETTER THAN FRIDAY WITH HIGHS INCHING UP BY A FEW DEGREES.
MAPLES
LONG TERM
(SATURDAY THROUGH NEXT THURSDAY)
ISSUED AT 220 PM CST THU DEC 19 2024
SURFACE HIGH PRESSURE KEEPS SKIES MOSTLY CLEAR SATURDAY NIGHT WITH
LIGHT EASTERLY FLOW. IT'S EXPECTED TO BE THE LAST OF THE COLD AIR
WITH LOWS IN THE LOW TO MID-20S SUNDAY MORNING. RETURN FLOW SETS UP
SUNDAY AS THE SURFACE HIGH MOVES INTO THE OHIO VALLEY AND MID-LEVEL
RIDGING AMPLIFIES FROM THE GULF OF MEXICO OVER THE EASTERN CONUS.
SOUTHWESTERLY FLOW ALOFT WILL STRENGTH WARM AIR ADVECTION THROUGH
THE DAY, MARKING THE BEGINNING OF A WARMING TREND THAT WILL CONTINUE
THROUGH MIDWEEK.
IMPACTS HOLD OFF UNTIL MONDAY WHEN RAIN CHANCES INCREASE OVER THE
REGION. A COLD FRONT IS EXPECTED TO EXTENDED SOUTHWEST OF LOW
PRESSURE THAT TRACKS WEST TO EAST ACROSS THE UPPER MIDWEST AT THE
BEGINNING OF THE WEEK. MEANWHILE, A MID-LEVEL SHORTWAVE ROUNDS THE
BASE OF AN AMPLIFIED UPPER LEVEL TROUGH EXTENDING NORTH TO SOUTH
THROUGH THE PLAINS. THIS INTRODUCES LOBES OF VORTICITY THAT RIDE THE
FRONTAL COMPONENTS FROM WEST TO EAST THROUGH MIDWEEK. THE NBM SHOWS
A RAPID INCREASE IN SPREADS WITH TIME. THIS CAN BE ATTRIBUTED TO
QUESTIONS WITH THE SOUTHWARD PROGRESS AND EVENTUAL PLACEMENT OF THE
SURFACE FRONT AND THE EVOLUTION OF THE MID-LEVEL RIDGE OVER THE
SOUTHEASTERN U.S. WHILE THERE CERTAINLY IS MOISTURE RETURN,
SATURATION LOOKS DEEPEST 12Z MONDAY THROUGH 18Z TUESDAY, WHEN
SATURATION IS PRESENT BELOW 700HPA. CONVERGENCE AND FORCING OVER THE
FRONTAL BOUNDARIES WILL PLAY A KEY ROLE IN THE DURATION AND
PLACEMENT OF PRECIPITATION. IT'S POSSIBLE NBM PROBABILITIES ARE TOO
SPREAD OUT, THANKS IN PART TO THE DIFFERING SOLUTIONS OF THE LONG
RANGE GUIDANCE. GFS AND ECM DEPICT THIS WELL AS THE GFS CONFINES
RAIN CHANGES TO THE BACK EDGE OF THE MID-LEVEL RIDGE, WHILE THE ECM
LIFTS A WARM FRONT ACROSS THE REGION THAT RESULTS IN WIDER
DISPERSION OF POPS. THIS ALL LEADS TO A HIGH PROBABILITY (GREATER
THAN 60%), A REASONABLY LOW QPF EVENT. LREF GUIDANCE SHOWS
PROBABILITIES FOR GREATER THAN 0.25 INCHES AT OR BELOW 40% NORTH
OF I-44 IN MO AND NORTH OF I-64 IN IL WITH PROBABILITIES PEAKING
AT 55% TO THE SOUTH.
FORTUNATELY, TEMPERATURES LOOK TOO WARM TO SUPPORT WINTER
PRECIPITATION WITH ABOVE-FREEZING AIR THROUGHOUT MUCH OF THE LONG
TERM PERIOD. THE ONLY TIME IT COMES CLOSE IS MONDAY MORNING, BUT LOW
TEMPERATURES MAY OCCUR EARLIER IN THE NIGHT AS WARM AIR ADVECTION
TAKES OVER. BY THE TIME LOW AND MID LEVELS SATURATION OCCURS,
TEMPERATURES RISE INTO THE MID AND UPPER 30S.
DESPITE BEING A DIVERGING DATASET THE FURTHER WE GET INTO THE LONG
TERM PERIOD, CHRISTMAS DAY IS TRENDING DRIER THROUGH THE DAY. LREF
PROBABILITIES FOR MEASURABLE RAINFALL (0.01" OR GREATER) START THE
DAY BETWEEN 30-50% AND TREND DOWNWARD THROUGH THE LATE MORNING.
BUT AGAIN, THIS WILL ALL DEPENDENT ON THE AFOREMENTIONED PATTERN
IN THE PRIOR PARAGRAPHS. THE GENERAL TREND LOOKS LIKE THE NORTHERN
2/3RDS OF THE FORECAST AREA COULD VERY WELL BE DRY, WHILE RAIN
CHANCES LINGER ALONG THE NORTHERN PERIPHERY OF THE MID-LEVEL RIDGE
OVER SOUTHERN SECTIONS OF THE CWA. REGARDLESS OF THE
PRECIPITATION CHANCES, IT SHOULD BE MORE OF A NUISANCE THAN AN
IMPACT AS IT ALL REMAINS LIQUID WITH TEMPERATURES RANGING FROM THE
MID-40S TO MID- 50S.
MAPLES
AVIATION
(FOR THE 06Z TAFS THROUGH 06Z FRIDAY NIGHT)
ISSUED AT 1004 PM CST THU DEC 19 2024
THE PRIMARY CONCERNS DURING THE 06Z TAF PERIOD CONTINUE TO BE
LOWERING CEILINGS AND GUSTY NORTHWESTERLY WINDS FOLLOWING A COLD
FRONTAL PASSAGE THIS EVENING. THIS COLD FRONT HAS ALREADY REACHED
COU AND UIN AS OF 04Z (10PM CST) AND HAS BROUGHT GUSTY SURFACE
WINDS TO THESE TERMINALS ALREADY. WINDS WILL REMAIN GUSTY THROUGH
THE NIGHT AND FOR MUCH OF DAYLIGHT HOURS TOMORROW BEFORE
DIMINISHING LATE IN THE AFTERNOON OR NEAR SUNSET.
LOW CLOUDS HAVE LAGGED THE INITIAL WIND SHIFT BY A COUPLE OF
HOURS, BUT SURFACE AND SATELLITE OBSERVATIONS INDICATE THAT
WIDESPREAD MVFR STRATOCUMULUS IS NEARING UIN AND IS LIKELY TO
SPREAD OVER ALL LOCAL TERMINALS OVER THE FIRST SEVERAL HOURS OF
THE TAF PERIOD, AND PERSIST FOR MUCH OF THE DAY TOMORROW. THERE
REMAINS ONLY LOW CONFIDENCE REGARDING WHEN THESE CLOUDS WILL
DIMINISH TOMORROW AFTERNOON, AND THEY MAY LINGER UNTIL SUNSET AT
ST. LOUIS TERMINALS AND UIN. A FEW NON-IMPACTFUL SNOW FLURRIES
WILL ALSO BE POSSIBLE OVERNIGHT THROUGH TOMORROW MORNING.
BRC
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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