925
FXUS61 KPBZ 201141
AFDPBZ
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
641 AM EST FRI DEC 20 2024
SYNOPSIS
WIDESPREAD SNOW SHOWERS ARE LIKELY TODAY INTO SATURDAY WITH THE
APPROACH AND PASSAGE OF LOW PRESSURE. WELL BELOW NORMAL
TEMPERATURE WILL SWEEP IN BEHIND THE COLD FRONT BEFORE
TEMPERATURE SLOWLY MODERATES NEXT WEEK.
NEAR TERM /THROUGH TONIGHT/
KEY MESSAGES:
- SNOW OVERSPREADS THE REGION TODAY.
- HIGHEST ACCUMULATIONS EXPECTED OVER NORTHWEST PA AND THE
RIDGES.
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THE UPPER TROUGH WILL TRANSITION ACROSS THE REGION TODAY WHILE
THE SURFACE LOW MOVES INTO EASTERN OH BEFORE PHASING WITH A
DEVELOPING COASTAL LOW THIS AFTERNOON. BROAD FORCING/ASCENT,
PLUS COLD ADVECTION, WILL PRODUCE MAINLY SNOW SHOWERS TODAY.
CONFIDENCE IS HIGH FOR LESS THAN 1" ACCUMULATIONS FOR LOWER
ELEVATION AREAS THROUGH 6AM SATURDAY (90 TO 100% PROBABILITY)
WHILE HIGHEST AMOUNTS WILL BE TETHERED TO THE RIDGES AND THE
I-80 CORRIDOR.
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. THE LATEST NBM PROBS FOR > 3+" FOR WV HIGH TERRAIN IS
CLOSE TO 20%, RESULTING IN NO ACTION ON WINTER HEADLINES AT
THIS TIME.
SHORT TERM /SATURDAY THROUGH SUNDAY/
KEY MESSAGES:
- WIDESPREAD SNOW EXPECTED INTO SATURDAY MORNING, AREAL
COVERAGE MAY BE SPOTTIER AT TIMES.
- ACCUMULATIONS ARE LIKELY HIGHEST FOR THE RIDGES AND PORTIONS
OF THE I-80 CORRIDOR.
- LAKE EFFECT AND UPSLOPE SNOW POSSIBLE SATURDAY.
- COLD AND DRY SUNDAY.
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TRANSITION OF THE UPPER TROUGH AXIS EAST SATURDAY WILL ENABLE
STRONGER 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 WHILE OTHER AREAS REMAIN SNOW FREE.
GLOBAL AND HI-RES MODELS REMAIN BULLISH ON THIS FEATURE, 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 PRODUCE 1-2" OF SNOWFALL ACCUMULATION, BUT
FURTHER ANALYSIS WILL BE REQUIRED AS HI-RES MODEL DATA BEGINS TO
COVER THE BANDS.
HEIGHT RISES AND STRONG SUBSIDENCE OF THE MEAGER MOISTURE
COLUMN SHOULD END ALL SNOWFALL BY LATE SATURDAY NIGHT.
THERE IS HIGH CONFIDENCE IN DRY WEATHER WITH BELOW AVERAGE
TEMPERATURE SUNDAY UNDER THE INFLUENCE OF HIGH PRESSURE WHILE
HEIGHT RISES CONTINUES ALOFT.
LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/
KEY MESSAGES:
- COLD DRY WEATHER CONTINUES INTO MONDAY.
- INCREASING LIKELIHOOD FOR PRECIPITATION, INCLUDING SNOW, FOR
NORTHERN AREAS CHRISTMAS EVE.
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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 /12Z FRIDAY THROUGH TUESDAY/
AN INITIAL WAVE OF LIGHT SNOW ASSOCIATED WITH WEAK 700MB WARM
ADVECTION AHEAD OF THE LOW CENTER WILL TRAVERSE NE THIS
MORNING, EXITING MUCH OF THE AREA BY 18Z. EXPECT VARIABILITY IN
CEILINGS/VISIBILITIES DURING THIS TIMEFRAME WITH RESTRICTION
DROPS IN SNOW BUT 1-2 CATEGORICAL IMPROVEMENTS OUTSIDE OF THE
SNOW BAND.
THE SURFACE LOW CENTER AND UPPER TROUGH AXIS WILL CROSS THIS
AFTERNOON AND EVENING TO POTENTIAL DRIVE WIDESPREAD SNOW CHANCES,
BUT FORCING MAY MAKE COVERAGE FAIRLY SPOTTY AND GENERALLY
FAVORING THE HIGHER TERRAIN. WEAK MOISTURE CONVERGENCE SHOULD
SUPPORT MORE UNIFORMITY IN MVFR DECKS, WITH DROPS LIKELY TIED TO
FALLING SNOW/VISIBILITY.
BRIEF SUBSIDENCE AND A DRIER POCKET OF AIR MAY DEVELOP AFTER
00Z AS THE SYSTEM QUICKLY MOVES EAST, RESULTING IN A DOWNWARD
TREND IN SNOW COVERAGE AND POTENTIAL FOR CIG IMPROVEMENTS
(INCLUDING LOW PROBABILITY OF DECK SCATTERING). NARROW BANDS OF
LAKE ENHANCED SNOW SHOWERS WILL BE THE PRIMARY
CLOUD/RESTRICTION DRIVER AT THE END AND AFTER THE TAF PERIOD
SATURDAY AMID NW FLOW AND COLD ADVECTION.
OUTLOOK...
LOW VFR TO MVFR RESTRICTIONS ARE FAVORED SATURDAY AMID COLDER NW
FLOW WITH NARROW BANDS OF LAKE ENHANCED SNOW SHOWERS THAT COULD
FURTHER REDUCE RESTRICTIONS. TERMINALS IN WEST CENTRAL PA TO
NORTHWEST PA ARE THE MOST LIKELY TO SEE IMPACTS FROM THOSE
SHOWERS THROUGH 06Z SUNDAY.
VFR UNDER THE INFLUENCE OF HIGH PRESSURE IS FAVORED SUNDAY INTO
MONDAY BEFORE THE NEXT UPPER LEVEL SYSTEM CROSSES THEN REGION
LATE MONDAY INTO TUESDAY, BRINGING ANOTHER ROUND OF LIGHT SNOW
AND AREAWIDE RESTRICTIONS.
PBZ WATCHES/WARNINGS/ADVISORIES
PA...NONE.
OH...NONE.
WV...NONE.
SYNOPSIS...22/FRAZIER
NEAR TERM...22/FRAZIER
SHORT TERM...22/FRAZIER
LONG TERM...FRAZIER
AVIATION...FRAZIER
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