640
FXUS61 KCTP 200414
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
1114 PM EST THU DEC 19 2024
SYNOPSIS
* WIDESPREAD LIGHT SNOW ACCUMULATION ON FRIDAY-FRIDAY NIGHT
BEFORE ANOTHER ARCTIC COLD SNAP ARRIVES THIS WEEKEND
* CHILLY AND DRY START NEXT WEEK FOLLOWED BY A GRADUAL WARMUP
AND MIXED PRECIPITATION POTENTIAL CHRISTMAS EVE AND CHRISTMAS
DAY
NEAR TERM /UNTIL 7 AM FRIDAY MORNING/
MESO MODELS CONTINUE TO GENERATE MORE THAN TRACE AMOUNTS OF
PRECIP BEFORE SUNRISE (JUST ABOUT AS LATE AS SUNRISE CAN GET,
NOW). HAVE BUMPED THE POPS UP INTO LIKELIES (60S) FOR ALL THE
WESTERN-MOST COUNTIES OF THE CWA BEFORE 12Z. WILL ADD JUST 0.1"
ONTO THOSE THAT DIDN'T HAVE BEFORE THEN WHICH CERTAINLY DOES NOT
BUMP ANYONE INTO A WIDESPREAD 3+" FOR THE EVENT.
PREV....
CLOUDS ALOFT ARE ALREADY SPREADING OVER THE ALLEGHENIES, ONLY
FURTHERING THE CLOUDY NARRATIVE FOR TONIGHT. BREAKS ARE POSSIBLE
IN THE S-CENT COUNTIES. DEVELOPING EASTERLY FLOW WILL CONTINUE
TO VEER OVERNIGHT, AND DRAW MOISTURE IN FROM THE SOUTH AS THE
STORM APPROACHES. BROAD-SCALE LIFT OVER PA AND SEEDING OF LOWER
CLOUD LAYER SHOULD RESULT IN LIGHT SNOW BEGINNING LATE
TONIGHT/BEFORE SUNRISE OVER THE CENTRAL MTNS AND ALLEGHENIES.
THE TEMPS SHOULD BE ABLE TO CREEP INTO THE M20S FOR MOST OF THE
AREA OVERNIGHT. TEMP/MOISTURE PROFILE ALOFT IS FAVORABLE FOR
OK BUT NOT GIANT, FLUFFY DENDRITES N OF I-80, AND SLRS ARE
PROGGED TO BE VERY CLOSE TO CLIMATOLOGY (10:1 S AND 15:1 N).
THIS IS A LITTLE LOWER THAN MOST CLIPPERS. SO, IF ANYWAY, THE
SLRS MAY GO UPWARD.
SHORT TERM /7 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/
AS FORCING CONTINUES TO INCREASE FROM THE APPROACHING CLIPPER
SYSTEM, COVERAGE OF LIGHT SNOW WILL EXPAND ACROSS CENTRAL PA BY
MID TO LATE MORNING. PARENT TROUGH FEEDS SIGNIFICANT SECONDARY
DEVELOPMENT OFF OF THE MID ATLANTIC COAST LATE FRIDAY AS THE
UPPER TROUGH TAKES ON A NEGATIVE TILT. FOCUS BY THIS TIME WILL
HINGE ON STRENGTH OF LLVL EASTERLY FLOW AND CONVERGENCE AXIS
PROGGED TO SETUP BENEATH THE LEFT EXIT REGION OF A MID/UPPER
LEVEL JET STREAK. POTENTIAL INTERACTION OF THESE FEATURES
COULD ALLOW FOR A SLOWLY PIVOTING NNW-SSE ORIENTED BAND OF
DEFORMATION SNOW (NORLUN TROUGH) TO ORGANIZE OVER THE FAR
EASTERN PART OF THE FORECAST AREA FRIDAY AFTERNOON THROUGH
FRIDAY NIGHT, POTENTIALLY FAVORING HIGHER SNOWFALL TOTALS, BUT
CERTAINTY FOR THIS REMAINS ON THE LOWER SIDE AND WILL NEED
FURTHER EVALUATION THIS EVENING AND OVERNIGHT.
FOR NOW WE'RE STAYING CLOSE TO NBM/WPC QPF/SNOW OUTPUT AND SLRS
WITH OVERALL AMOUNTS RANGING FROM 1-3" IN THE WESTERN AND
NORTHERN MTNS TO A COATING-1" FOR MOST AREAS ALONG AND TO THE
SOUTHEAST OF THE I99/I80 CORRIDOR FROM THE SOUTH CENTRAL
ALLEGHENIES THROUGH THE MID AND LOWER SUSQUEHANNA VALLEY.
INCREASINGLY COLDER NWLY UPSLOPE FLOW ON THE BACKSIDE OF THE
SYSTEM MAY ALSO ENHANCE SNOW TOTALS OVER THE LAUREL HIGHLANDS
THROUGH FRIDAY NIGHT. WPC PROBS ARE SLIGHTLY BETTER THAN 50/50
FOR >4" OF SNOW IN THE TYPICALLY FAVORED WINDWARD SLOPES, SKI
AREAS, AND SUMMITS ABOVE 2000FT PARTICULARLY IN WESTERN SOMERSET
COUNTY NEAR INTERSECTION OF WESTMORELAND/FAYETTE LINE. SOME
DISCUSSION ABOUT POTENTIAL HEADLINES FOR THIS WITH NEIGHBORS FOR
LATER FRIDAY, BUT CONSENSUS WAS TO HOLD OFF AND SPS THE MORNING
ARRIVAL OF SNOWFALL IF NECESSARY FRIDAY MORNING.
THE PATTERN TURNS BRISK AND MUCH COLDER TO START WEEKEND WITH
LAKE EFFECT SNOW SHOWERS AND FLURRIES INTO SATURDAY NIGHT.
MODELS SHOW A POTENTIAL HURON CONNECTION OVER THE WESTERN MTNS
WITH AN ADDITIONAL INCH OR SO POSSIBLE IN THE HIGHER TERRAIN.
HIGHS ON SATURDAY WILL BE 10 TO 15 DEGREES BELOW THE HISTORICAL
AVERAGE WITH MAX TEMPS IN THE 20-30F RANGE. A BRISK AND GUSTY
NW WIND WILL MAKE IT FEEL LIKE THE SINGLE DIGITS/TEENS TO
AROUND 20F. SATURDAY NIGHT WILL BE QUITE COLD WITH MIN TEMPS IN
THE +SINGLE DIGITS TO UPPER TEENS WITH MINIMUM WIND CHILLS FROM
-5F TO +10F INTO EARLY SUNDAY MORNING.
LONG TERM /SUNDAY THROUGH THURSDAY/
MODEL GUIDANCE CONTINUES TO INDICATE NORTHWESTERLY FLOW SUNDAY
MORNING, WITH THE HIGHEST CHANCES OF LINGERING LAKE EFFECT SNOW
SHOWERS ACROSS NW PA DURING THE PRE-DAWN HOURS. NORTHWESTERLY
FLOW WILL CONTINUE TO PROMOTE CHANCES THROUGH THE LATE MORNING
HOURS ON SUNDAY BEFORE CHANCES DECREASE. ANOTHER ASPECT OF THE
FORECAST ON SUNDAY WILL BE SEASONABLY COOL TEMPERATURES WITH
BREEZY WINDS THAT WILL PROMOTE SUB-ZERO WIND CHILLS ACROSS
NORTHEASTERN PORTIONS OF THE AREA AT SUNRISE SUNDAY MORNING.
SURFACE HIGH PRESSURE IS PROGGED BY THE BULK OF MODEL GUIDANCE
TO CONTINUE DRIFT OFF THE EAST COAST SUNDAY EVENING INTO MONDAY,
PROMOTING GENERALLY CLEAR SKIES ON SUNDAY AND INTO THE FIRST
HALF OF MONDAY. THIS WILL ALLOW FOR PEAK WARMING DURING THE
DAYTIME HOURS AND RADIATIONAL COOLING SUNDAY EVENING INTO
MONDAY, THUS HAVE UNDERCUT NBM MINTS IN THIS TIMEFRAME. THE
INFLUENCE OF HIGH PRESSURE WILL KEEP WINDS LIGHTER, SO WIND
CHILLS WILL BE LESS ON A CONCERN MONDAY MORNING.
CLOUDS BEING TO INCREASE ACROSS WESTERN PENNSYLVANIA BY MIDDAY
ON MONDAY AND INCREASING AREA WIDE THROUGH MONDAY NIGHT. THERE
REMAINS CONSIDERABLE SPREAD IN MODEL GUIDANCE IN THE TUESDAY TO
WEDNESDAY TIMEFRAME WITH GFS MODEL GUIDANCE OUTLINING LOW
PRESSURE MOVING TO THE NORTH AND BRINING HIGHER SNOWFALL CHANCES
WHILE THE ECMWF OUTLINES A WEAKER DISTURBANCE WITH SNOWFALL
CONFINED TO THE NORTHERN TIER, MAINLY ON TUESDAY. THE MAIN
DEVIATION FROM NBM GUIDANCE IN THIS TIMEFRAME WAS TO TRIM POPS
ACROSS THE LOWER SUSQUEHANNA VALLEY ON WEDNESDAY MORNING, WHERE
MODEL GUIDANCE DOES INDICATE SOME DRIER AIR POTENTIALLY LIMITING
PRECIPITATION CHANCES. THE UNSETTLED PATTERN IS EXPECTED TO
CONTINUE INTO THURSDAY, WITH LITTLE DEVIATION FROM NBM GUIDANCE
DUE TO THE AFOREMENTIONED UNCERTAINTY IN MODEL GUIDANCE TOWARDS
THE END OF THE LONG-TERM FORECAST PERIOD.
AVIATION /04Z FRIDAY THROUGH TUESDAY/
03Z/10PM UPDATE: RECENT NAM/RAP MODEL GUIDANCE HAS INDICATED
THAT SNOWFALL COULD COME +1-2HRS COMPARED TO PREVIOUSLY
FORECAST, THUS HAVE PUSHED RESTRICTIONS BASED ON SNOWFALL UP IN
TIMING. IT IS OF NOTE THAT GLAMP GUIDANCE CONTINUES TO HAVE
SIMILAR ASPECTS ON TIMING, SO THIS PUSH UP IS WITH MODERATE
(~40-50%) CONFIDENCE.
PREVIOUS DISCUSSION, ISSUED 705 PM EDT DEC 19 2024:
RECENT OBSERVATIONS OUTLINE VFR PREVAILING ACROSS MUCH OF THE
FORECAST AREA OUTSIDE OF THE WESTERN (BFD/JST) AIRFIELDS. AT
JST, RECENT GLAMP/RAP MODEL SOUNDINGS OUTLINE SOME POTENTIAL FOR
SCATTERING OF THE LOW-LEVEL DECK WHICH REASONABLY (50-60%
CONFIDENCE) WILL ALLOW FOR A BRIEF PERIOD OF VFR CONDITIONS IN
THE 03-06Z WINDOW. ALTERNATIVE SOLUTIONS ARE THAT LOW-LEVEL
CLOUDS SCATTER OUT SLIGHTLY EARLIER AND ALLOW FOR A LONGER
PERIOD OF VFR FLIGHT CONDITIONS WITH THE LATEST REASONABLE TIME
FOR CONDITIONS TO DIP DOWN TOWARDS MVFR ~08Z FRIDAY WITH
ENHANCED LOW-LEVEL MOISTURE AND CLOUD COVER MAKING WAY INTO THE
AIRFIELD AHEAD OF PRECIPITATION. CONSENSUS OF RECENT MODEL
GUIDANCE KEEPS BORDERLINE MVFR-TO-IFR CONDITIONS AT BFD
THROUGHOUT THE ENTIRE TAF PERIOD, SO HAVE OUTLINED CEILINGS
BELOW IFR THRESHOLDS THROUGHOUT THE MAJORITY OF THE TAF PERIOD
WITH MODERATE-TO-HIGH (70-80%) CONFIDENCE OUTSIDE OF A BRIEF
WINDOW OF MVFR CEILINGS IN BETWEEN 03-06Z PROGGED BY MODEL
SOUNDINGS AND GLAMP GUIDANCE, WITH THE LATEST REASONABLE TIMING
OF MVFR CEILINGS RESOLVING TOWARDS MVFR ~09Z FRIDAY.
AROUND 10Z FRIDAY, SNOWFALL IS PROGGED BY THE BULK OF MODEL
GUIDANCE TO ENTER THE WESTERN AIRFIELDS WITH BFD/JST THE FIRST
AIRFIELDS TO REMAIN/RESOLVE TOWARDS IFR THRESHOLDS BASED ON
VISIBILITY. RESTRICTIONS WILL CONTINUE TO SPREAD TOWARDS ALL
AIRFIELDS OF CENTRAL PENNSYLVANIA, REACHING MDT/LNS CLOSER TO
THE 14Z FRIDAY TIMEFRAME. THERE REMAINS SOME UNCERTAINTY WITH
REGARDS TO THE INITIAL FALL IN CEILINGS/VISIBILITIES AT MDT/LNS
WITH GLAMP GUIDANCE OUTLINING AN INITIAL FALL TOWARDS MVFR
CEILINGS BEFORE IFR PREVAILS 1-2 HOURS LATER WITH RAP MODEL
SOUNDINGS OUTLINING A MORE DRASTIC FALL IN VISIBILITIES. FOR
THIS TAF PACKAGE, HAVE VEERED CLOSER TO THE GLAMP SOLUTION BASED
ON SOME UNCERTAINTY ON INITIAL LOW-LEVEL MOISTURE AT MDT/LNS;
HOWEVER, COULD REALISTICALLY SEE A MORE DRASTIC DROP IN
VISIBILITIES IF SNOW SHOWERS MAKES WAY INTO THESE AIRFIELDS
CLOSER TO THE 15-16Z FRIDAY TIMEFRAME. SOME HEAVIER BANDS OF
SNOW COULD BRING LOCALIZED DROPS TOWARDS LIFR, WITH RECENT HREF
PROBABILITIES OUTLINING THE BEST CHANCES AT BFD/JST BUT
CONFIDENCE REMAINS TOO LOW (~30%) TO INCLUDE OUTRIGHT IN THE
FORM OF A FM GROUP; THUS, HAVE INCLUDED MENTIONS WITH A PROB30.
OUTLOOK...
SAT...LINGERING SHSN NW/SW PA; GUSTY WINDS ~25-30KTS ACROSS THE
HIGHER ELEVATIONS DURING THE LATE AM/PM HOURS.
SUN...SHSN FAR NW PA; OTHERWISE, NO SIG WX.
MON...NO SIG WX.
TUE...SNOW ENTERS W PA AM, OVERSPREADS AREA BY AFTERNOON.
CLIMATE
ASTRONOMICAL WINTER (SOLSTICE) BEGINS ON SATURDAY, DECEMBER 21ST
AT 4:19 A.M. EST.
CTP WATCHES/WARNINGS/ADVISORIES
NONE.
SYNOPSIS...DEVOIR/STEINBUGL
NEAR TERM...DANGELO
SHORT TERM...DEVOIR/STEINBUGL
LONG TERM...NPB
AVIATION...NPB
CLIMATE...STEINBUGL
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