352
FXUS61 KCTP 200911
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
411 AM EST FRI DEC 20 2024
SYNOPSIS
* LIGHT SNOW ACCUMULATION MAY RESULT IN SLIPPERY ROAD CONDITIONS
AND TRAVEL DISRUPTIONS TODAY AND TONIGHT
* BRISK AND MUCH COLDER THIS WEEKEND WITH SNOW SHOWERS SATURDAY
AND FRIGID WIND CHILLS SATURDAY NIGHT INTO EARLY SUNDAY
* SEASONABLY COLD START NEXT WEEK FOLLOWED BY A GRADUAL WARMUP
THROUGH CHRISTMAS DAY
NEAR TERM /THROUGH TONIGHT/
WEAKENING 1014MB CLIPPER SYSTEM ANALYZED OVER NORTHERN INDIANA
AT 08Z WILL YIELD TO AN AREA OF LOW PRESSURE ORGANIZING ALONG A
STRENGTHENING FRONTAL BOUNDARY OFF THE CAROLINA COAST. BROAD
UPPER LEVEL DIVERGENCE OUT AHEAD OF THE 250-500MB MEAN TROUGH
AXIS WILL SUPPLY SUFFICIENT ASCENT ALOFT IN TANDEM WITH THE
RESIDUAL MOISTURE AND LLVL WAA FROM THE CLIPPER SYSTEM TO
SUPPORT A "BURST" OF SNOW OVER CENTRAL PA BETWEEN 12-18Z. HIRES
ENSEMBLE DATA SHOWS A GRADUAL RAMP IN HOURLY SNOWFALL RATES
DURING THIS TIMEFRAME PEAKING IN THE 0.50-0.75" RANGE AS SNOW
SPREADS FROM WEST TO EAST. BLEND OF NBM/WPC/HREF DATA INDICATES
A GENERAL 1-3" ACCUMULATION NEAR AND NORTH OF I80 (HIGHEST ON
THE RIDGETOPS) WITH A COATING-1" FCST ACROSS THE LAUREL
HIGHLANDS AND MOST OF SOUTH CENTRAL PA (LOWEST AMOUNTS 0-0.5" IN
THE LOWER SUSQ VALLEY WHERE SOME RAIN/SNOW MIX IS POSSIBLE).
TIMING OF THE LIGHT TO MODERATE SNOW IS A BIT OF A CONCERN
IMPACT-WISE, AND WILL LIKELY ISSUE A SPS TO DRAW AWARENESS TO
POTENTIAL FOR SLIPPERY ROAD CONDITIONS AND TRAVEL DISRUPTIONS.
MARGINAL AIR/ROAD TEMPERATURES SUGGEST BULK OF ACCUMULATION MAY
BE ON THE GRASS OR NON-PAVED SURFACES WITH ELEVATION DEPENDENCE
PARTICULARLY NEAR AND TO THE EAST OF I99/I80 CORRIDOR. ROADS ARE
MOST LIKELY TO BE JUST WET ACROSS THE MAJORITY OF THE LOWER
SUSQUEHANAN VALLEY.
FOCUS HEADING INTO THE EVENING AND OVERNIGHT WILL BE ON A
CONVERGENCE BOUNDARY THAT CAMS ARE KEYING ON -- PROGGED TO SETUP
BENEATH THE LEFT EXIT REGION OF A MID/UPPER LEVEL JET STREAK.
HIRES DATA IS TRENDING BULLISH ON A SLOWLY PIVOTING BAND OF
DEFORMATION SNOW GRADUALLY SINKING SOUTHWARD ACROSS THE FORECAST
AREA INTO EARLY SATURDAY MORNING. THE HIGHER TERRAIN IN
SULLIVAN AND NORTHERN SCHUYLKILL COUNTIES ARE A CONCERN AND
MOST LIKELY AREAS TO PICK UP ANOTHER 1-2" BETWEEN 00-12Z SAT.
WE WOULD ALSO NOT BE SURPRISED IF THE LOWER SUSQ VALLEY PICKS
UP A 0.5" OR SO IF THIS PIVOTING BAND COMES TO FRUITION. THE
LATEST HRRR SUGGESTS LIGHT SNOW COULD CONTINUE FOR MOST OF THE
NIGHT BEFORE TAPERING OFF FROM NW TO SE BEHIND PUSH OF MUCH
COLDER/DRIER ARCTIC AIR POISED TO SURGE INTO THE AREA FOR THE
WEEKEND.
INCREASINGLY COLDER NWLY UPSLOPE FLOW ON THE BACKSIDE OF THE
SYSTEM MAY ALSO ENHANCE SNOW TOTALS OVER THE LAUREL HIGHLANDS
LATER TONIGHT INTO SATURDAY AS HURON CONNECTION BECOMES
ESTABLISHED.
SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/
THE PATTERN TURNS BRISK AND MUCH COLDER TO START WEEKEND WITH
LAKE EFFECT SNOW SHOWERS AND FLURRIES INTO SATURDAY NIGHT.
MODELS SHOW A HURON CONNECTION WITH SNOW BANDS DEVELOPING OVER
THE WESTERN MTNS. SOME PLACES COULD RECEIVE AN ADDITIONAL INCH
OR TWO WITH CAMS TARGETING SOUTHERN SOMERSET COUNTY.
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
-10F 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 /08Z 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...STEINBUGL
NEAR TERM...STEINBUGL
SHORT TERM...STEINBUGL
LONG TERM...NPB
AVIATION...NPB
CLIMATE...STEINBUGL
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