941
FXUS61 KCTP 201931
AFDCTP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
231 PM EST FRI DEC 20 2024
SYNOPSIS
LIGHT SNOW ACCUMULATION MAY RESULT IN SLIPPERY ROAD CONDITIONS
AND TRAVEL DISRUPTIONS TODAY AND TONIGHT. WINDY AND MUCH COLDER
CONDITIONS ARRIVE TONIGHT AND SATURDAY, WITH COLD WEATHER
LASTING THROUGH SUNDAY AND MONDAY, FOLLOWED BY A GRADUAL
WARMUP THROUGH CHRISTMAS DAY
NEAR TERM /THROUGH TONIGHT/
FORECAST ON TRACK THIS MORNING WITH WAA INDUCED LIGHT SNOW
COVERING MUCH OF CENTRAL PA THIS MORNING. BEST WAA AND VERTICAL
MOTION TO SUPPORT STEADY SNOW LIFTS NORTHEAST OF THE REGION BY
MID AFTERNOON, WITH FOCUS SHIFTING ON CAD AND LAKE EFFECT FOR
THE ALLEGHENIES AND DEFORMATION-FORCED SNOWFALL ACROSS EAST
CENTRAL AND SOUTHEAST PARTS OF PA TONIGHT. NO SIG CHANGES AT
THIS TIME.
PREVIOUS DISCUSSION...
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 CONTINUE TO HIGHLIGHT POTENTIAL FOR
SLIPPERY ROAD CONDITIONS AND TRAVEL DISRUPTIONS VIA THE HWO AND
SOCIAL MEDIA. MARGINAL AIR/ROAD TEMPERATURES AND OVERALL LIGHT
INTENSITY RATES 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 SNOW COVERED IN THE NORTHERN TIER WHILE JUST
WET ACROSS THE MAJORITY OF THE LOWER SUSQUEHANNA VALLEY.
FOCUS HEADING INTO THE EVENING AND OVERNIGHT WILL BE ON A
CONVERGENCE BOUNDARY THAT CAMS HAVE BEEN 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 SUNDAY 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.
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.
LONG TERM /MONDAY THROUGH THURSDAY/
MODEL CONSENSUS INDICATES SURFACE HIGH PRESSURE DRIFTING OFF
THE EAST COAST MONDAY, SUPPORTING GENERALLY CLEAR SKIES ON
SUNDAY THROUGH THE FIRST HALF OF MONDAY.
STRONG RADIATION COOLING UNDER CLEAR SKIES AND WEAK GRADIENT
WILL LEAD TO THE COLDEST TEMPS OF THIS OUTBREAK WITH DAYBREAK
READINGS ON MONDAY WITHIN A FEW DEG F EITHER SIDE OF ZERO ACROSS
THE NORTH AND 5-10F ACROSS MUCH OF CENTRAL AND SOUTHERN PA.
WIND CHILLS WILL BE LESS OF A CONCERN MONDAY MORNING WITH JUST A
LIGHT NORTH TO EASTERLY BREEZE AT LESS THAN 5 MPH.
LOW-MID LEVEL WARM ADVECTION FROM THE WEST MONDAY LEADS TO AN
AREA OF GENERALLY NON-PRECIPITATING MID CLOUDS SPREADING ACROSS
PENNSYLVANIA FOLLOWING BY LOWERING CLOUD BASES AND SOME LIGHT
SNOW LATE MONDAY INTO TUESDAY AS A FAIRLY FAST MOVING, NORTHERN
STREAM SHORTWAVE CROSSES PA. BEST CHANCE FOR -SN AND A LIGHT
ACCUM OF A COATING TO 2 INCHES WILL BE ACROSS THE NORTHERN MTNS
OF THE STATE.
GFS AND ITS ENSEMBLES ARE FURTHER NORTH WITH THE TRACK OF THE
MID AND UPPER LEVEL TROUGH, BUT QPF ALONG THE TRAILING COLD
FRONT LEADS TO A SIMILAR OUTCOME TO THE EC WITH RESPECT TO SNOW
PROBS/AMOUNTS.
DAY TO DAY MAX TEMP RISES FROM SUNDAY TO TUESDAY WILL BE ABOUT 7
DEG F.
MORE OF A BLOCKY/SPLIT FLOW PATTERN DEVELOPS ACROSS THE ENTIRE
CONUS AS WE HEAD THROUGH CHRISTMAS EVE AND CHRISTMAS DAY. MODELS
TO INDICATE A GENERAL DRY AND CHILLY CHRISTMAS DAY WITH MAX TEMPS
RANGING FROM THE LOW TO MID 30S NE TO THE LOW 40S IN THE
METRO AREAS OF SOUTHERN PA.
A STALLED OUT FRONTAL BOUNDARY TO OUR SOUTH COULD LIFT INTO
SOUTHERN PA AND BRING A LOW PROBABILITY FOR A LIGHT WINTRY MIX
LATER CHRISTMAS DAY INTO THURSDAY.
THE MAIN CHANGE FROM THE PREVIOUS FORECAST PACKAGES WAS TO TRIM
POPS BY ABOUT 20 PERCENT ACROSS THE BOARD WHERE MODELS SHOW A
FAIRLY EXPANSIVE SFC HIGH FROM NRN NEW ENGLAND TO THE CENTRAL
APPALACHIANS.
AVIATION /19Z FRIDAY THROUGH WEDNESDAY/
PERIODS OF LIGHT SNOW WITH WIDESPREAD IFR TO LOW END MVFR
CONDITIONS TO CONTINUE THROUGH THIS EVENING WITH SLOW
IMPROVEMENT AS WE HEAD INTO EARLY SATURDAY.
CONDITIONS FOR MOST SITES AFTER 05Z SATURDAY WILL BE
BORDERLINE MVFR. A FEW LINGERING SNOW SHOWERS ARE POSSIBLE,
ESPECIALLY AT BFD WHERE SOME LAKE EFFECT SNOW COULD OCCUR.
VFR CONDITIONS WILL BE LIKELY ACROSS THE CENTRAL RIDGE AND
VALLEY REGION AFTER 11Z SAT, WHILE PA WESTERN MTN AIRFIELDS
SHOULD STAY MVFR TO HIGH END IFR IN OCCASIONAL SNOW SHOWERS IN
BANDS STREAMING SE OFF THE GREAT LAKES.
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...DEVOIR/STEINBUGL
SHORT TERM...DEVOIR/STEINBUGL
LONG TERM...LAMBERT/NPB
AVIATION...LAMBERT/NPB/BOWEN
CLIMATE...STEINBUGL
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