Area Forecast Discussion
Issued by NWS

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000 FXUS61 KPBZ 240120 AFDPBZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 920 PM EDT THU APR 23 2015 .SYNOPSIS...
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HIGH PRESSURE WILL BUILD IN FRIDAY BEFORE LOW PRESSURE TRACKS ACROSS THE OHIO VALLEY...BRINGING SHOWER CHANCES FOR THE WEEKEND.
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&& .NEAR TERM /THROUGH FRIDAY/...
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CLEARING SHOULD CONTINUE TO WORK IN FROM THE WEST AS SUBSIDENCE INCREASES UNDER BUILDING HIGH PRESSURE. MAINLY SUNNY SKIES ARE EXPECTED FRIDAY UNDER THE HIGH WITH CU RULE INDICATING ONLY A FEW DIURNAL CU. NUDGED LOWS DOWN TONIGHT NEAR THE LATEST GFS NUMBERS...WITH TEMPERATURES 10 TO 15 DEGREES BELOW THE SEASONAL AVERAGE THROUGH FRIDAY.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
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GIVEN THE NON-PROGRESSIVE PATTERN IN PLACE...STRONG CLOSED LOW PRESSURE AT 500 MB WILL REMAIN RELATIVELY STATIONARY OVER THE NORTHEASTERN CONUS THROUGH THE WEEKEND...WITH SEVERAL SHORTWAVE TROUGHS ROTATING AROUND THE LOW. NWLY FLOW WILL DOMINATE THIS PERIOD AS THE REGION REMAINS BETWEEN THIS LOW AND A RIDGE TO THE WEST...BUT THIS FLOW WILL BE SUPPLANTED BRIEFLY ON SATURDAY NIGHT AS ONE SUCH SHORTWAVE TROUGH APPROACHES FROM THE NORTHERN PLAINS AND PHASES INTO THE STRONG UPPER LOW. PRECIPITATION WILL BEGIN TO SPREAD NORTHEASTWARD SATURDAY MORNING AS WARM AIR ADVECTION INCREASES...BUT WILL HAVE CONSIDERABLE LOW-LEVEL DRY AIR TO OVERCOME BEFORE MEASURABLE RAIN IS REALIZED. AS A RESULT...ONSET OF PRECIPITATION WAS SLOWED FOR SATURDAY MORNING. MODELS ARE IN GENERAL AGREEMENT THAT THE CENTER OF THE APPROACHING SHORTWAVE TROUGH WILL PASS JUST SOUTH OF THE FORECAST AREA SATURDAY NIGHT AND SUNDAY. SUCH A TRACK WILL KEEP THE FORECAST AREA ALONG OR JUST NORTH OF THE ASSOCIATED WARM FRONT...AND IN PRIME TERRITORY FOR A BAND OF MODERATE RAIN OWING TO DEFORMATION AND MID-LEVEL WARM AIR ADVECTION. THE SYSTEM WILL EMERGE RAPIDLY INTO THE ATLANTIC SUNDAY AFTERNOON...SPELLING A RAPID END TO PRECIPITATION. GIVEN THIS EXPECTED STORM TRACK...PRECIPITATION CHANCES SATURDAY NIGHT AND SUNDAY MORNING SEEM INCREASINGLY LIKELY. ALTHOUGH A SHARP NORTHERN PRECIPITATION GRADIENT IS LIKELY TO EXIST...IT IS UNCLEAR EXACTLY HOW FAR NORTH PRECIPITATION WILL REACH. NEVERTHELESS...POPS WERE INCREASED TO CATEGORICAL LEVELS IN THE SOUTH AND TO LOW-END CHANCE LEVELS IN THE NORTH...WHICH DEPICTS A MILD WEAKENING OF THE GRADIENT FROM THE PREVIOUS FORECAST GIVEN UNCERTAINTY IN NORTHWARD EXTENT OF RAIN. TEMPERATURES PRIOR TO THE WEEKEND RAIN WILL BE TRICKY ON FRIDAY NIGHT. INITIAL CONDITIONS OF DRY AIR...LIGHT WIND AND CLEAR SKY WILL BE QUITE FAVORABLE FOR RADIATIONAL COOLING...BUT CIRRUS IS EXPECTED TO BECOME INCREASINGLY ABUNDANT DURING THE NIGHT AND WILL LIMIT COOLING LATER. IT IS POSSIBLE ALL AREAS WILL DROP BELOW FREEZING...BUT THE MOST LIKELY LOCATIONS WILL BE THE TYPICAL NORTHERN ZONES. APART FROM THE DISTURBANCE SATURDAY NIGHT AND SUNDAY...NO MAJOR PRECIPITATION IS EXPECTED. DAYTIME MAXIMA WILL REACH THE UPPER 50S AND MINIMA WILL BE IN THE MID-UPPER 30S AS CLOUD COVER AND MODEST WARM AIR ADVECTION OFFSET DIURNAL COOLING.
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&& .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... THE UPPER-LEVEL LOW WILL SIT AND SPIN OVER NEW ENGLAND SUNDAY NIGHT INTO MONDAY NIGHT. MOISTURE WILL SPILL INTO WESTERN PENNSYLVANIA BY MONDAY MORNING...AND A SHORTWAVE TROUGH PIVOTING AROUND THE UPPER LOW WILL BRING AT LEAST A SLIGHT CHANCE OF PRECIPITATION FOR PENNSYLVANIA COUNTIES MONDAY INTO MONDAY NIGHT. CANNOT RULE OUT A FEW SNOWFLAKES MIXING IN RIGHT AROUND SUNRISE FOR FOREST...CLARION...AND JEFFERSON COUNTIES BOTH MONDAY AND TUESDAY MORNINGS. BY THE TIME THE UPPER LOW MOVES OFFSHORE TUESDAY...A SURFACE LOW WILL BEGIN DEVELOPING ALONG THE GULF COAST AND MOVE TO THE NORTHEAST. THE ECMWF KEEPS THE LOW WELL EAST OF THE REGION WITH NO RAIN LOCALLY...BUT THE GFS...UKMET AND THE NAEFS KEEP THE LOW CLOSER TO THE COAST AND SPREAD RAIN INTO THE REGION. HAVE BUMPED UP POPS WEDNESDAY...ALTHOUGH FUTURE MODEL RUNS WILL BE NEEDED TO DETERMINE THE EXACT TRACK OF THE LOW. WITH SOME SORT OF NORTHERLY FLOW EXPECTED THROUGH THE EXTENDED FORECAST AND MUCH CLOUD COVER WITH THE TWO MAIN SYSTEMS... TEMPERATURES WILL CONTINUE TO BE BELOW NORMAL BUT WILL RISE CLOSER TO CLIMATOLOGICAL VALUES BY THE END OF THE PERIOD. THE EASTERN CONUS TROUGH AND RESULTING NORTHWESTERLY WIND WILL CONTINUE TO HOLD TEMPERATURES UNDER THE AVERAGES. HAVE TRENDED TOWARDS WPC NUMBERS...WHICH KEEP AT LEAST SOME CHANCE OF PRECIPITATION IN THE FORECAST. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... VFR CONDITIONS WILL CONTINUE THROUGH THE TAF PERIOD WITH DECREASING CLOUD COVERAGE AND WINDS BECOMING GUSTY AGAIN BY MID MORNING. .OUTLOOK.../FRIDAY THROUGH TUESDAY/... RESTRICTIONS WILL BE POSSIBLE SATURDAY AFTERNOON THROUGH SUNDAY AFTERNOON AS LOW PRESSURE TRACKS ACROSS THE OHIO VALLEY. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...NONE. OH...NONE. PA...NONE. WV...NONE. && $$

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