829
FXUS63 KLSX 182034
AFDLSX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAINT LOUIS MO
234 PM CST WED DEC 18 2024
KEY MESSAGES
- DRY WEATHER IS EXPECTED THROUGH THE WEEKEND BEFORE AN ACTIVE
PERIOD BEGINS ON MONDAY, WITH PERIODS OF RAIN EXPECTED (80-90%)
EARLY NEXT WEEK.
- TEMPERATURES WILL BECOME COLDER THROUGH SATURDAY BEFORE A
WARMING TREND BEGINS ON SUNDAY.
SHORT TERM
(THROUGH LATE THURSDAY NIGHT)
ISSUED AT 230 PM CST WED DEC 18 2024
A DEEP UPPER-LEVEL TROUGH IS MOVING THROUGH THE CENTRAL U.S. PER
AFTERNOON WATER VAPOR IMAGERY. AS THIS TROUGH SLIDES OUT OF THE
REGION, HIGH PRESSURE AT THE SURFACE IS MOVING INTO THE MIDDLE
MISSISSIPPI VALLEY. DEEP NORTHWESTERLY FLOW IS MOVING COLDER AIR
INTO THE CWA, AND WITH THE CLOUD COVER OVER MUCH OF THE AREA, IS
CAUSING TEMPERATURES FOR MOST LOCATIONS TO STALL OR DROP AS OF EARLY
THIS AFTERNOON. COLD AIR WILL CONTINUE TO FILTER INTO THE CWA
TONIGHT. WITH HIGH PRESSURE IN THE VICINITY, WINDS WILL BE LIGHT,
BUT SOME DEGREE OF CLOUD COVER SHOULD LIMIT ROBUST RADIATIVE
COOLING. THIS SUPPORTS LOWS BOTTOMING OUT IN THE UPPER 20S FOR MOST
LOCATIONS.
THE UPPER-LEVEL PATTERN QUICKLY EVOLVES THROUGH TOMORROW, WITH A
SHORTWAVE IN THE WAKE OF THE TROUGH DIVING SOUTHEASTWARD INTO THE
MIDWEST. GUIDANCE CONSENSUS REMAINS STRONG THAT ITS SURFACE
REFLECTION WILL PASS JUST NORTH OF THE CWA, FAVORING WINDS TO TURN
SOUTHERLY EARLY TOMORROW AND ADVECT WARM AIR INTO THE AREA. WINDS
WILL BECOME INCREASINGLY SOUTHWESTERLY INTO THE AFTERNOON,
SUPPORTING ADIABATIC WARMING AS AIR TRAVELS DOWN OFF THE OZARKS.
THEREFORE, TEMPERATURES FOR MOST LOCATIONS TOMORROW AFTERNOON ARE
FORECAST TO AT LEAST BE A DEGREE OR TWO WARMER THAN TODAY, WITH
PORTIONS OF CENTRAL AND SOUTHEASTERN MISSOURI SEEING THE GREATEST
BOOST DUE TO THE DOWNSLOPING EFFECTS OF THE OZARKS.
DESPITE THE LOW SWINGING A COLD FRONT THROUGH THE CWA THURSDAY
EVENING AND NIGHT, FORCING AND LOW-LEVEL MOISTURE IS NOT ROBUST
ENOUGH TO SUPPORT PRECIPITATION. ADDITIONALLY, THIS SYSTEM IS
CLIPPER-LIKE IN NATURE, WHICH CLIMATOLOGICALLY FAVORS THE MAIN
PRECIPITATION SHIELD TO BE LOCATED NORTH-NORTHEAST OF THE LOW AS
REFLECTED IN THE CURRENT FORECAST, LEADING TO HIGH CONFIDENCE (+90%)
THAT THE CWA WILL REMAIN DRY THROUGH THIS SYSTEM.
ELMORE
LONG TERM
(FRIDAY THROUGH NEXT WEDNESDAY)
ISSUED AT 230 PM CST WED DEC 18 2024
THE MIDDLE MISSISSIPPI VALLEY WILL BE ON THE BACKSIDE OF A DEPARTING
UPPER-LEVEL TROUGH AND SHORTWAVE AT THE START OF THE EXTENDED
FORECAST PERIOD PER GUIDANCE CONSENSUS. THE ASSOCIATED SURFACE LOW
WILL BE ENTERING THE OHIO VALLEY FROM THE NORTHWEST. THE
PRECIPITATION SHIELD ASSOCIATED THIS LOW IS EXPECTED TO REMAIN
NORTHEAST OF THE CWA AS IT DEPARTS THE MIDWEST, WITH ONLY 5% OF
GLOBAL ENSEMBLE MEMBERS HAVING MEASURABLE PRECIPITATION IN THE FAR
NORTHEASTERN PORTIONS OF OUR CWA ACROSS WEST-CENTRAL AND SOUTH-
CENTRAL ILLINOIS. DEEP NORTHWESTERLY FLOW WILL OVERSPREAD THE REGION
EARLY ON FRIDAY, PUSHING COLD AIR INTO THE CWA. ENSEMBLE GUIDANCE IS
TIGHTLY CLUSTERED AROUND HIGHS IN THE LOW TO MID 30S FOR FRIDAY -
WHICH WILL MAKE THE DAY THE COLDEST OF THE FORECAST PERIOD. WHILE
THIS NORTHWESTERLY FLOW WILL PERSIST INTO SATURDAY, THE MAJORITY OF
ENSEMBLE GUIDANCE SUPPORTS TEMPERATURES WARMING SLIGHTLY, AS THE
APEX OF A MID-LEVEL RIDGE NOSES INTO THE MIDDLE MISSISSIPPI VALLEY
AND BUMPS MID-LEVEL TEMPERATURES UP BY A DEGREE OR TWO.
DETERMINISTIC MODELS AND ENSEMBLE CLUSTERS ARE WELL ALIGNED WITH THE
PHASING OF AN UPPER-LEVEL RIDGE THAT WILL BEGIN EDGING INTO THE
MIDWEST ON SUNDAY AND PUSH HIGH PRESSURE AT THE SURFACE EASTWARD.
IN TURN, LOW-LEVEL FLOW WILL BECOME SOUTHERLY AND PUMP WARM AIR INTO
THE CWA, WITH ENSEMBLE MEANS SUPPORTING RISING TEMPERATURES THROUGH
CHRISTMAS. HOWEVER, THE EXACT VALUES DAY TO DAY ARE UNCERTAIN, AS
SPREAD WITHIN THE INTERQUARTILE RANGE OF ENSEMBLE GUIDANCE IS
ROUGHLY 10 DEGREES DURING THIS PORTION OF THE PERIOD. THIS SPREAD IS
DUE AT LEAST IN PART TO DIFFERENCES IN PHASING OF SHORTWAVES SET TO
PASS THROUGH THE MIDWEST, BUT ALSO CLOUD COVER. THAT BEING SAID,
CONFIDENCE IS HIGH THAT TEMPERATURES WILL BE AT LEAST ABOVE NORMAL
(GENERALLY MID 40S FOR HIGHS, UPPER 20S FOR LOWS), AS ENSEMBLE-BASED
PROBABILITIES ARE 60% FOR AT LEAST SUCH VALUES BY MONDAY AND 90%+ BY
TUESDAY AND BEYOND.
THIS WARM, ACTIVE PATTERN WILL ALSO INCREASE THE CHANCES FOR
PRECIPITATION AS EARLY AS MONDAY. AS OF RIGHT NOW, FROM MONDAY
THROUGH WEDNESDAY/CHRISTMAS DAY, ENSEMBLE-BASED PROBABILITIES TOP
OUT AT 80-90% FOR MEASURABLE RAINFALL ACROSS PORTIONS OF THE CWA.
THEN, GUIDANCE HAS THE MOST PRONOUNCED SHORTWAVE/TROUGH OF THE
PERIOD PHASING THROUGH THE REGION. THE RELATIVELY LOWER POPS (50-60%
AT MOST) IN THE CURRENT FORECAST ARE DUE TO DIFFERENCES IN THE
TIMING OF THE SYSTEM AMONG GUIDANCE, LEADING TO CHANCES BEING SPREAD
OVER A GREATER WINDOW OF TIME (48 HOURS) VS WHAT WILL LIKELY BE
REALITY (24 HOURS OR LESS).
OF GREAT INTEREST THIS TIME OF YEAR IS THE CHANCE OF A WHITE
CHRISTMAS. NOW THAT THE HOLIDAY HAS COME INTO RANGE OF THE EXTENDED
FORECAST PERIOD, CURRENT DATA SUPPORTS THAT THE CHANCE OF SNOW FOR
CHRISTMAS IS ABOUT AS CLOSE TO 0% AS POSSIBLE ACROSS THE CWA, AS ONE
TO TWO ENSEMBLE MEMBERS OF THE 100 TOTAL PRODUCE MEASURABLE SNOWFALL
ACROSS FAR NORTHEASTERN MISSOURI AND WEST-CENTRAL ILLINOIS EARLY
TUESDAY MORNING. THEREFORE, CONFIDENCE IS HIGH IN PRECIPITATION
FALLING AS RAIN THROUGH THE CHRISTMAS HOLIDAY.
ELMORE
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 1104 AM CST WED DEC 18 2024
THE MAIN SOURCE OF IMPACTS AT ALL LOCAL TERMINALS THIS FORECAST
PERIOD IS AN AREA OF OVERCAST TO BROKEN STRATUS MOVING SOUTHWARD
ACROSS THE REGION. UPSTREAM OBSERVATIONS SHOW LARGER BREAKS IN
THIS STRATUS DECK EXPAND AND CONTRAST WITH TIME, LEADING TO
UNCERTAINTY IN HOW CONSISTENTLY IMPACTS WILL PREVAIL LATER IN THE
AFTERNOON AND EARLY THIS EVENING. A COUPLE HOURS OF VFR FLIGHT
CONDITIONS ARE POSSIBLE AT ALL LOCAL TERMINALS LATER THIS
AFTERNOON AND EARLY THIS EVENING IF THESE BREAKS IN THE STRATUS
ARE ABLE TO MAKE IT IN TO PORTIONS OF THE REGION. EVEN IF STRATUS
IS ABLE TO CLEAR FOR SOME TERMINALS THIS EVENING, COOLING
TEMPERATURES OVERNIGHT WILL LIKELY ALLOW FOR STRATUS TO REDEVELOP
OR EXPAND THROUGH THE EARLY MORNING HOURS. VFR FLIGHT CONDITIONS
ARE EXPECTED TO RETURN TOMORROW MORNING AS WINDS BECOME
INCREASINGLY SOUTHERLY.
ELMORE
LSX WATCHES/WARNINGS/ADVISORIES
MO...NONE.
IL...NONE.
WFO LSX
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