684
FXUS61 KPBZ 192348
AFDPBZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
648 PM EST THU DEC 19 2024
SYNOPSIS
WIDESPREAD SNOW IS LIKELY TOMORROW INTO SATURDAY WITH THE
APPROACH AND PASSAGE OF A LOW PRESSURE SYSTEM, WITH
ACCUMULATIONS FAVORING NORTHWEST PENNSYLVANIA AND THE WEST
VIRGINIA HIGHER TERRAIN. WELL BELOW AVERAGE TEMPERATURE WILL
DEVELOP BEHIND THE COLD FRONT BEFORE TEMPERATURE SLOWLY
MODERATES NEXT WEEK.
NEAR TERM /UNTIL 12 PM FRIDAY/
KEY MESSAGES:
- DRY WEATHER EXPECTED THROUGH 4AM FRIDAY.
- INCREASING POTENTIAL FOR LIGHT SNOWFALL ACCUMULATIONS IN
NORTHWEST PA AHEAD OF MORNING COMMUTE.
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AN APPROACHING TROUGH, FROM THE NW, IS EXPECTED TO ADVANCE INTO
THE OHIO RIVER VALLEY OVER THE NEXT 12 HOURS. THERE IS
POTENTIAL FOR ENOUGH JET ASCENT COMBINING WITH AN EMBEDDED 700MB
WAVE TO GENERATE SNOW AFTER 06Z IN THE NE QUADRANT OF THE
SYSTEM. THIS POTENTIAL IS FAVORED WITHIN THE INITIAL HI-RES
MODEL RUNS REACHING THE TIME PERIOD; SUCH DEVELOPMENT COULD
FOSTER A QUICK 1" OF SNOW AHEAD OF THE MORNING COMMUTE MAINLY
ALONG THE I-80 CORRIDOR.
OTHERWISE, ANY LIGHT PRECIPITATION THAT DEVELOPS SHOULD REMAIN
FARTHER WEST NEAR THE LOW CENTER OR ALONG THE EASTERN SLOPES OF
THE HIGHER TERRAIN WITH WEAK UPSLOPE FLOW.
SHORT TERM /FRIDAY AFTERNOON THROUGH SATURDAY NIGHT/
KEY MESSAGES:
- WIDESPREAD SNOW FAVORED FRIDAY INTO SATURDAY MORNING, THOUGH
AREAL COVERAGE MAY BE SPOTTIER AT TIMES.
- ACCUMULATIONS ARE LIKELY TO HIGHEST FOR THE WV HIGHER TERRAIN
AND ALONG THE HIGHER TERRAIN SPINE TOWARD THE I-80 CORRIDOR.
- LAKE EFFECT AND UPSLOPE SNOW POSSIBLE ON SATURDAY.
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THE UPPER TROUGH WILL TRANSITION ACROSS THE REGION THROUGH THE
DAY FRIDAY WHILE THE SURFACE MOVES INTO EASTERN OH BEFORE
PHASING WITH THE DEVELOPING COASTAL LOW. BROAD FORCING/ASCENT
PLUS COLD ADVECTION WILL FUEL MAINLY SNOW SHOWER DEVELOPMENT
THROUGH MUCH OF THE DAY INTO SATURDAY MORNING. CONFIDENCE IS
HIGH IN GENERALLY LESS THAN 1" ACCUMULATIONS FOR LOWER ELEVATION
AREAS THROUGH 6AM SATURDAY (80 TO NEAR 100% PROBABILITY) WHILE
HIGHEST AMOUNTS WILL BE TETHERED TO THE HIGHER WV TERRAIN AND
ALONG THE LAUREL HIGHLANDS TOWARD THE I-80 CORRIDOR. FOR THE
I-80 CORRIDOR TOWARD CENTRAL PA, AVAILABLE GUIDANCE SUGGESTS THE
PRIMARY ACCUMULATION WINDOW BEING ~4AM TO 2PM FRIDAY BEFORE
MOISTURE VACATES NE. FOR THE LAUREL HIGHLANDS INTO THE WV HIGH
TERRAIN, AVAILABLE GUIDANCES SUGGESTS THE PRIMARY ACCUMULATION
WINDOW BEING ~2PM FRIDAY THROUGH 6AM SATURDAY.
THE BIGGEST UNCERTAINTY LAYS IN TOTAL QPF AVAILABLE AS SHOWERS
GENERATE AS WELL AS SHORTWAVE PLACEMENT ON THE SW EDGE OF THE
TROUGH AND ITS UPSLOPE WIND COMPONENT AS IT REACHES THE HIGH
TERRAIN. CIPS AND GLOBAL GUIDANCE ARE MORE BULLISH FOR
ADVISORY-LEVEL ACCUMULATIONS IN THE WV HIGH TERRAIN (AND LESSER
FOR NW PA CORRIDOR) WHEREAS INITIAL HI-RES MODELS PUSH THE WAVE
FARTHER SOUTH, LIMITING THE UPSLOPE COMPONENT AND KEEPING
MOISTURE DEPTHS ON THE LOWER END OF THE DGZ. THESE LATEST TRENDS
LOWERS THE PROBABILITY OF 3+" FOR WV HIGH TERRAIN (20-50%),
RESULTING IN NO ACTION ON WINTER HEADLINES AT THIS TIME.
TRANSITION OF THE UPPER TROUGH AXIS EAST SATURDAY WILL ENABLE
GREATER COLD ADVECTION WITHIN NORTHERLY TO NORTHWESTERLY
BOUNDARY-LAYER FLOW. THIS FLOW OVER THE WARM LAKE WATERS WILL
FOSTER NARROW BANDS OF LAKE ENHANCED SNOW SHOWERS FOR LOCALIZED
PORTIONS OF THE REGION (FAVORING 15Z/10AM THROUGH 03Z/10PM)
WHILE SNOW ENDS FOR OTHER AREAS. GLOBAL AND INITIAL HI- RES
MODELS HAVE REMAINED BULLISH ON THIS FEATURE DEVELOPMENT, THOUGH
VARIATION REMAINS IN BAND INTENSITY/ORIENTATION/LOCATION BUT
SHOULD GENERALLY START AS A FETCH OF LAKE HURON AND MOVE TOWARD
THE OH/PA BORDER NW OF PITTSBURGH. IT STILL APPEARS THAT THESE
BANDS WILL ONLY ADD AN 1-2" OF SNOWFALL ACCUMULATION TO THE SUB-
ADVISORY AMOUNTS IN THE AREA, BUT FURTHER ANALYSIS AS MORE HI-
RES MODELS REACH THIS TIMEFRAME IS NEEDED TO ASSESS POTENTIAL
FOR GREATER IMPACTS/ACCUMULATIONS.
HEIGHT RISES AND STRONG SUBSIDENCE OF THE MEAGER MOISTURE
COLUMN SHOULD END ALL SNOWFALL BY SATURDAY NIGHT.
LONG TERM /SUNDAY THROUGH THURSDAY/
KEY MESSAGES:
- COLD BUT DRY WEATHER LIKELY SUNDAY INTO MONDAY.
- INCREASING LIKELIHOOD FOR PRECIPITATION, INCLUDING SNOW, FOR
NORTHERN AREAS CHRISTMAS EVE.
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THERE IS HIGH CONFIDENCE IN DRY WEATHER WITH BELOW AVERAGE
TEMPERATURE SUNDAY UNDER THE INFLUENCE OF HIGH PRESSURE WHILE
HEIGHT RISES CONTINUES ALOFT.
ENSEMBLES FAVOR SHORTWAVE RIDGING TO QUICKLY TRAVERSE THE AREA
AHEAD OF THE NEXT WEAKER BUT LIKELY SWIFT MOVING SHORTWAVE
ARRIVES LATE MONDAY INTO TUESDAY. EXPECT TEMPERATURE MODERATION
MONDAY WITH THE DEGREE OF MODERATION DEPENDENT ON TIMING OF MID
AND HIGH LEVEL CLOUD ARRIVALS.
THOUGH TIMING AND STRENGTH OF THE AFOREMENTIONED SHORTWAVE
REMAINS VARIABLE, ITS CROSSING SHOULD GENERATE AN AREA OF
PRECIPITATION FAVORING NORTHWEST PA (DUE TO CLOSER PROXIMITY TO
THE WAVE). LATEST GUIDANCE SUGGESTS A 30-70% CHANCE FOR
ACCUMULATING SNOWFALL NORTH OF THE MASON-DIXON LINE THROUGH
TUESDAY EVENING/CHRISTMAS EVE, BUT THE LACK OF DEEPER MOISTURE
AND COLDER AIR LIKELY WILL PREVENT NOTABLE ACCUMULATIONS (ONLY
NORTHWEST PA SEES 20-30% PROBABILITY FOR GREATER THAN 1" OF
SNOW). OTHERWISE, THE WARMING TREND IS LIKELY TO CONTINUE INTO
WEDNESDAY/CHRISTMAS DAY WITH GRADUAL HEIGHT RISES THAT LIKELY
WILL RESULT IN SLIGHTLY ABOVE NORMAL TEMPERATURE.
AVIATION /00Z FRIDAY THROUGH TUESDAY/
MANY TERMINALS ARE STILL SEEING CIGS STRADDLE THE MVFR/VFR LINE BUT
AS EXPECTED MORE VFR IS PREVAILING ACROSS THE REGION. THIS IS
EXPECTED TO CONTINUE THROUGH THE EVENING AND INTO THE OVERNIGHT
PERIOD. HI-RES GUIDANCE SHOWS A SHARP RISE IN THE PROBABILITY
OF MVFR CIGS BETWEEN 06Z-08Z AS THE MOISTURE ADVECTION AHEAD OF
OUR NEXT LOW PRESSURE SYSTEM BEGINS TO PICK UP.
THE FIRST FLAKES LIKELY BEGIN TO FLY SHORTLY AFTER THIS IN
FKL/DUJ/LBE AS SNOW SHOWERS RIDE WARM ADVECTION AND GENTLE UPSLOPING
NORTH OF PIT AND ALONG THE PA/WV RIDGES. LIGHT SNOW MAY BREAK OUT
NEAR/AFTER 12Z FOR OTHER AREA TERMINALS. VCSH WAS USED AT THE
MAJORITY OF TAF SITES DUE TO THE LACK WIDESPREAD ORGANIZED SNOW
EXPECTED OUTSIDE OF THE NORTH AND RIDGES.
PROBABILITIES OF IFR CIGS RISE SHARPLY AFTER 12Z FOR ALL TERMINALS
BUT ESPECIALLY FKL/DUJ/LBE. MODEL SOUNDINGS HOWEVER ARE LESS
SUPPORTIVE OF IFR CONDITIONS AT OTHER AREA TERMINALS AND THERE
REMAINS LOW CONFIDENCE ON JUST HOW WIDESPREAD IFR CIGS CAN
BECOME.
OUTLOOK...
MORE WIDESPREAD RESTRICTIONS ARE LIKELY FRIDAY NIGHT AS
LOW PRESSURE CROSSES THE REGION. OCCASIONAL RESTRICTIONS MAY THEN
CONTINUE THROUGH SATURDAY AS COLD ADVECTION UNDER NORTHWEST FLOW
PROMOTES THE CONTINUATION OF LAKE-EFFECT CLOUDS AND SCATTERED SNOW
SHOWERS. THE NEXT CHANCE FOR VFR LOOKS TO BE LATER SATURDAY NIGHT OR
SUNDAY.
PBZ WATCHES/WARNINGS/ADVISORIES
PA...NONE.
OH...NONE.
WV...NONE.
SYNOPSIS...FRAZIER
NEAR TERM...HEFFERAN/FRAZIER
SHORT TERM...FRAZIER
LONG TERM...FRAZIER
AVIATION...CL/AK
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