Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 311204 AFDPHI AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MOUNT HOLLY NJ 804 AM EDT TUE MAR 31 2015 .SYNOPSIS... LOW PRESSURE WILL QUICKLY TRACK SOUTHEASTWARD FROM THE GREAT LAKES REGION, CROSSING OUR AREA LATER TODAY INTO THIS EVENING. HIGH PRESSURE WILL THEN BUILD INTO OUR REGION WEDNESDAY BEFORE QUICKLY SHIFTING OFFSHORE WEDNESDAY NIGHT INTO THURSDAY. THE NEXT COLD FRONT IS SCHEDULED TO ARRIVE LATE FRIDAY, WITH A SECONDARY COLD FRONT MOVING THROUGH LATE SATURDAY NIGHT AND SUNDAY MORNING. A HIGH PRESSURE SYSTEM IS THEN NEARBY FOR MONDAY. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 8:00 AM: FORECAST STILL LOOKS ON TRACK FOR A LIGHT ACCUMULATING SNOWFALL IN THE HIGHER TERRAIN OF SUSSEX NJ AND THE POCONOS TODAY WITH A PERIOD OF RAIN OR RAIN/SNOW MIX ELSEWHERE. SLOWED THE INCREASE OF CLOUDS SOMEWHAT OVER THE NEXT FEW HOURS THOUGH. THE PRECIPITATION ONSET TIME MAY NEED TO BE PUSHED BACK BY 1-2 HOURS AS WELL BASED ON THE LATEST RAP AND HRRR RUNS. ATTM DID NOT MAKE THIS ADJUSTMENT WITH THE POSSIBILITY STILL PRESENT OF RAIN/ MIX ENTERING THE WESTERN FRINGE OF THE CWA BY 16-17Z. IN THE POCONOS AND NW NJ, THE PRECIPITATION MAY START AS A MIX BEFORE CHANGING TO SNOW WITH EVAPORATIVE COOLING OCCURRING. PREVIOUS: THE GFS INITIALIZATION LOOKED A LITTLE WEAK WITH THE APPROACHING TROF AND MAYBE WHY ITS TIMING IS THE FASTEST. ITS SLIGHTLY WARMER AT 925MB UPSTREAM THAN THE MODELS THINK. OVERALL THE MODELS ARE MUCH CLOSER THAN THEY WERE YESTERDAY, BUT THERE REMAINS DIFFERENCES AS TO THE NORTHERN EXTENT OF HEAVIER PCPN. THEY HAVE TRENDED COLDER AND OVERALL WETTER. MODEL PREDICTED PWATS HOWEVER ARE AROUND HALF AN INCH AND GIVEN THIS IS ABOUT A 6 TO 12 HR EVENT, BELIEVE QPF ABOVE HALF AN INCH ARE TOO ROBUST UNLESS A FGEN BAND CONCENTRATES IN A NARROW AREA FOR A PROTRACTED PERIOD. OUR FORECAST SOLUTION IS CLOSEST TO A COMBINATION OF THE UKMET`S QPF FIELD, GFS PATH (COUPLE OF HOURS SLOWER) AND WRF-NMM`S BETTER LLVL THERMALS AND SFC TEMPS FOR THE NORTHERN SNOWIER AREAS. WINTER WEATHER ADVISORY FOR OUR THREE NORTHERN COUNTIES. IF THIS WAS NIGHTTIME THIS WOULD BE MORE OF A CONFIDENT SNOWFALL FCST, BUT GIVEN THIS IS A TIMING REPEAT OF MARCH 20TH, THERE REMAINS UNCERTAINTY AS TO HOW MUCH SNOW CAN ACCUMULATE. MARCH 20TH SHOWED INTENSITY CAN TRUMP THE LATE MARCH SUN. THE HOURLY RATES AT PEAK TIME DURING THIS AFTERNOON IS NEARING .1 WATER EQUIVALENT PER HOUR. THIS SHOULD BE ENOUGH TO START ACCUMULATING. BUT THIS OVERALL WINDOW IS SMALLER THAN THAT PREVIOUS EVENT. ALSO THE INITIAL BOUNDARY LAYER CAN WARM THIS MORNING AND THIS WILL PROLONG THE CHANGE TO SNOW AT LOWER TERRAIN LOCATIONS. THE HIGHER TERRAIN OF THE POCONOS AND WESTERN SUSSEX COUNTY NJ IT WILL BE PRETTY MUCH COLD ENOUGH FOR SNOW. THE UNCERTAINTY IS TO WILL THE HEAVIER PCPN REACH THAT FAR NORTH. THE GENERAL CONSENSUS IS YES, BUT THERE IS NOT UNIFORMITY. THE GFS DIAGS DO SHOW FGEN BANDING IN THAT AREA, BUT THE EXIT REGION OF THE 250MB JET AND BEST MID LEVEL QVEC CONVERGENCE IS A BIT FTHR TO THE SOUTH. THERE WILL BE A 45 KT LLJ FEEDING ACROSS THIS BOUNDARY AND THERE IS A BROAD 200MB OF LIFT PREDICTED AT 290K ACROSS OUR CWA THIS AFTERNOON. HERE GOES, PCPN MOVES IN NORTH LATE THIS MORNING AND EARLY THIS AFTERNOON AND LATER IN THE AFTERNOON THE REST OF OUR CWA. THE SOUNDINGS ARE DRY ABOVE THE SFC AND EVAP COOLING SHOULD CHANGE ANY BRIEF START OF RAIN IN THE HIGHER TERRAIN OF THE POCONOS TO SNOW. ELSEWHERE ITS THE LESS EFFICIENT DYNAMIC COOLING THAT IS EXPECTED TO DO THE TRICK. OBVIOUSLY ELEVATION HELPS AS THE GFS PREDICTED FREEZING LEVEL AT ABE DROPPING TO 800 FEET SHOWS. ALL THE MODELS LOWER SFC TEMPS AND WBZS TO AROUND FREEZING IN THE POCONOS, BUT STRUGGLE ELSEWHERE. SO THERE IS GREATER UNCERTAINTY FARTHER EAST IN SUSSEX COUNTY NJ. THE NEXT TIER SOUTHWARD, THERE COULD BE SOME HIGHER TERRAIN LOCATIONS THAT DO GET A COUPLE OF INCHES, NOT AS WIDESPREAD AS THE ADVISORY AREA WE BELIEVE. WE WILL HAVE TO WATCH IF THE FGEN BANDING ESTABLISHES ITSELF IN THIS LOCATION LATER TODAY, BUT CONFIDENCE IS NOT HIGH ENOUGH TO ISSUE AN ADVISORY. WE WILL KEEP MENTION IN THE HWO. TEMPERATURES ARE ALSO PROBLEMATICAL AS THE GFS LOWER TEMPS ARE BANKING ON A FASTER PCPN ARRIVAL TIME. GIVE ITS ARRIVAL UNTIL THE AFTERNOON AND NAM MOS MAY BE RIGHT FOR THE WRONG REASON. WE COMPROMISED BETWEEN THE DATASETS. 06Z MODELS PRETTY MUCH HELD THEIR OWN. E.G. ABE FREEZING LEVEL LATE THIS AFTERNOON AT 800 TO 900 FEET. 06Z MODEL QPF VERIFICATION GEOGRAPHICALLY WAS GOOD WITH THE GFS AND NAM A BIT WET. GFS ENSEMBLE MEAN WAS SLIGHTLY SLOWER THAN THE OP. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... OPTING FOR THE SLOWER MODEL TIMING, WE CARRIED PCPN INTO THIS EVENING, ESPECIALLY IN NJ. EVENT SHOULD BE DONE BY ABOUT MIDNIGHT. THE PREDICTED VORT MAX OVERNIGHT BECOMES QUITE CHANNELIZED AND MODELS PREDICTED MOISTURE OVERALL MEAGER. WE CLEARED SKIES OVERNIGHT. WE OPTED FOR A STAT GUIDANCE COMPROMISE FOR MINS. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... THE OVERALL SYNOPTIC SETUP IS AN ACTIVE ONE BUT ALSO COMPRISED OF A FAST FLOW REGIME. AN UPPER-LEVEL TROUGH ACROSS THE NORTHEAST TO START WEDNESDAY WILL SHIFT EAST WITH A RIDGE BUILDING IN THURSDAY AS SHORT WAVE ENERGY MOVES ACROSS THE MIDWEST. THIS SHORT WAVE ENERGY WILL HELP TO AMPLIFY AN UPPER-LEVEL TROUGH ACROSS THE GREAT LAKES AND OHIO VALLEY LATE FRIDAY, THEN INTO THE EAST SATURDAY. IT APPEARS THIS TROUGH REMAINS THROUGH MONDAY, ALTHOUGH IT SHOULD START TO RETREAT NORTHWARD WITH TIME. WE USED A MODEL BLEND FOR WEDNESDAY THROUGH FRIDAY, THEN INCORPORATED THE 00Z WPC GUIDANCE IN WITH CONTINUITY THEREAFTER. SOME ADJUSTMENTS WERE THEN MADE FOLLOWING ADDITIONAL COLLABORATION WITH OUR NEIGHBORING OFFICES. FOR WEDNESDAY AND THURSDAY...AN UPPER-LEVEL TROUGH AXIS IS FORECAST TO PIVOT ACROSS THE AREA WEDNESDAY. A SHORT WAVE ROUNDING THE BASE OF THIS TROUGH MAY HAVE SOME SHOWERS WITH IT TO OUR NORTHWEST, HOWEVER IT MAY END UP DAMPENING OUT AS IT TRACKS SOUTHEAST AND EAST. THEREFORE, WE WILL CONTINUE WITH A DRY FORECAST. SOME CAA AT LEAST DURING THE FIRST HALF OF THE DAY WILL RESULT IN A COOLER DAY. HIGH PRESSURE SHIFTS OFFSHORE WEDNESDAY NIGHT INTO THURSDAY ALLOWING FOR A RETURN FLOW. AS THIS OCCURS, WAA WILL INCREASE AHEAD OF OUR NEXT SYSTEM. THIS WILL BE IN THE FORM OF A COLD FRONT THAT WILL BE APPROACHING LATE THURSDAY NIGHT. AHEAD OF THIS, A NOTABLE INCREASE IN AFTERNOON TEMPERATURES ARE EXPECTED THURSDAY /ALTHOUGH PROBABLY COOLER ALONG THE COAST DUE TO A SOUTHERLY FLOW OFF THE COLDER OCEAN/. IT APPEARS THAT A LEAD SHORT WAVE GENERATES SOME SHOWERS WELL OUT AHEAD OF THE COLD FRONT THURSDAY AND THESE MAY START TO MOVE INTO OUR WESTERN ZONES LATER THURSDAY NIGHT. IT APPEARS THAT IT WILL TAKE AWHILE FARTHER EAST THEREFORE WE SLOWED DOWN THE POP INCREASE AT NIGHT FROM WEST TO EAST. IN ADDITION, THE FORECAST SOUNDINGS GENERALLY SHOW A DEEPENING MOIST PROFILE AS A LOW-LEVEL JET INCREASES THE THETA-E ADVECTION. THIS RESULTS IN A DECREASE IN THE SHOWALTER VALUES, HOWEVER NO REAL INSTABILITY LOOKS TO BE PRESENT. AS A RESULT THERE IS A QUESTION WHETHER OR NOT CHARGE SEPARATION CAN TAKE PLACE AND THEREFORE WE REMOVED THE SLIGHT CHC OF THUNDER ATTM. AS MIXING INCREASES THURSDAY, A BREEZY AFTERNOON SHOULD RESULT. FOR FRIDAY AND SATURDAY...A COLD FRONT WILL GRADUALLY SETTLE SOUTHEAST ACROSS OUR AREA LATER FRIDAY INTO SATURDAY. A WARM SOUTHWESTERLY FLOW WILL BE IN PLACE AND THIS COMBINED WITH LARGE SCALE LIFT FROM THE INCOMING COLD FRONT, UPPER-LEVEL TROUGH AND AN UPPER-LEVEL JET SHOULD RESULT IN SHOWERS OR EVEN BANDS OF SHOWERS. IF ENOUGH INSTABILITY CAN BE REALIZED, SOME THUNDER COULD OCCUR ON FRIDAY AHEAD OF THE COLD FRONT. WE WILL MAINTAIN CONTINUITY AND INCLUDE A SLIGHT CHC OF THUNDER IN THE FORECAST DATABASE. LOW PRESSURE SHOULD BE TRACKING TO OUR NORTH AND NORTHEAST FRIDAY NIGHT, PULLING THE COLD FRONT TO OUR EAST EARLY SATURDAY. AS SOME ENERGY ROTATES AROUND THE BASE OF THE TROUGH, A COUPLE OF WAVES OF LOW PRESSURE SHOULD DEVELOP ALONG THE COLD FRONT. THE FIRST ONE SHOULD DELAY THE COLD FRONTAL PASSAGE ACROSS OUR AREA AND THEN THE SECOND ONE TRACKS WELL TO OUR SOUTH AND EAST AFTER THE FRONT CLEARS OUR AREA EARLY SATURDAY. THE TIMING AND POSITIONING OF THE COLD FRONT FRIDAY WILL IMPACT TEMPERATURES, HOWEVER OUR ENTIRE CWA SHOULD REMAIN IN THE WARM SECTOR. WE THEN KEPT LOW CHC POPS FOR SHOWERS IN FOR SATURDAY THEN DRYING OVERTAKES THE AREA. A SECONDARY COLD FRONT LOOKS TO DROP INTO THE AREA LATE SATURDAY NIGHT. FOR SUNDAY AND MONDAY...A COLD FRONT SHOULD MOVE ACROSS THE AREA SUNDAY MORNING, HOWEVER THIS LOOKS MOISTURE STARVED. HIGH PRESSURE THEN SLIDES TO OUR EAST LATE SUNDAY INTO MONDAY, ALTHOUGH THE TAILEND OF THE AFOREMENTIONED COLD FRONT MAY REMAIN FROM THE OHIO VALLEY WESTWARD TO THE NORTHERN PLAINS. FOR NOW, KEPT THIS TIMEFRAME DRY WITH MODERATION IN TEMPERATURES ON MONDAY. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY, AND SURROUNDING AREAS. THE 12Z TAFS STILL BRING ALL OF OUR SITES TO LOW MVFR OR IFR CONDITIONS THIS AFTERNOON WITH A HIGH DEGREE OF UNCERTAINTY ABOUT THE WIND DIRECTIONS THIS AFTERNOON AND EARLY EVENING. THIS MORNING A MID LEVEL CIG ARRIVING, SOME RAIN POSSIBLE LATE AT KABE AND KRDG. SOUTH TO SOUTH WEST WINDS AVERAGING 5 TO 10 KNOTS. THIS AFTERNOON AT KABE AND KRDG CIGS AND VSBYS LOWERING TO IFR. RAIN MAY MIX WITH OR CHANGE TO SNOW LATE. ESP KABE. TARMAC ACCUMS NOT EXPECTED. LOCAL KPHL AIRPORTS RAIN ARRIVES AND CIGS AND VSBYS LOWERING TO THE BORDER BETWEEN MVFR AND IFR CONDITIONS. KMIV AND KACY SIMILAR EVOLUTION, BUT LATER IN THE AFTERNOON. WINDS WILL BE TRICKY AS IT DEPENDS UPON THE TRACK OF THE LOW. WE ARE EXPECTING IT TO PASS TO THE SOUTH OF ALL THE TERMINALS ACROSS DELMARVA. THERE WILL BE A SLOW BACKING OF THE WINDS FROM SOUTH TO NORTHEAST AS THE AFTERNOON PROGRESSES. SPEEDS SHOULD BE AROUND 10 KNOTS. THIS EVENING, CONDITIONS BOTTOMING EAST OT IFR EARLY AND THEN IMPROVING CONDITIONS THE REST OF THE EVENING. PCPN SHOULD END BY 04Z ALL TERMINALS. WINDS WILL CONTINUE TO BACK TOWARD THE NORTH AND THEN NORTHWEST WITH AVERAGE SPEEDS AROUND 10 KNOTS, BUT SOME GUSTINESS TO AROUND 20 KNOTS FROM THE KPHL AIRPORTS SOUTHEASTWARD. OVERNIGHT IMPROVING CONDITIONS CONTINUING WITH A VFR CIG BECOMING CLEAR AND NORTHWEST WINDS AVERAGING AROUND 10 TO 15 KNOTS. OUTLOOK... WEDNESDAY AND THURSDAY...VFR OVERALL. AN INCREASE IN CLOUDS SHOULD OCCUR LATE THURSDAY AHEAD OF A COLD FRONT WITH SHOWERS LURKING NOT TO FAR TO OUR WEST LATE AT NIGHT. GENERALLY LIGHT WINDS WEDNESDAY DUE TO HIGH PRESSURE IN PLACE, THEN A SOUTHERLY FLOW DEVELOPS WEDNESDAY NIGHT AND THURSDAY WITH SOME GUSTINESS DURING THURSDAY. FRIDAY AND SATURDAY...MVFR TO POTENTIALLY IFR CONDITIONS WITH SHOWERS. A THUNDERSTORM CANNOT BE RULED OUT AHEAD OF A COLD FRONT FRIDAY. POTENTIALLY GUSTY SOUTHWESTERLY WINDS FRIDAY, SHIFTING TO THE NORTHWEST FRIDAY NIGHT INTO SATURDAY. SHOWERS SHOULD END SATURDAY MORNING FOLLOWED BY CONDITIONS RETURNING TO VFR. THERE IS LOWER CONFIDENCE REGARDING THE TIMING OF THE CLEARING SATURDAY. && .MARINE... WE STARTED THE SCA FOR THE OCEAN FROM THE GET GO WITH THIS PACKAGE. SOME SEAS ARE STILL CLOSE TO 5 FEET ON THE OCEAN WATERS. IT IS A BIT DISJOINTED. THE GRADIENT IS CURRENTLY WEAK, BUT IS PREDICTED TO INCREASE TODAY AS THE LOW PRESSURE SYSTEM APPROACHES. THEN WINDS SHOULD RELAX FOR A PERIOD THIS AFTERNOON AS THE AIR TEMPS RISE OVER THE WATERS, BUT SWELLS FROM THE MORNING WINDS SHOULD REACH CRITERIA. THEN THERE SHOULD BE A SCA LEVEL NORTHWEST WIND SURGE TONIGHT AS THE CLIPPER EXITS. THE ADVISORY IS NOT IN EFFECT FOR DELAWARE BAY, ALTHOUGH THERE COULD BE A FEW GUSTS TO 25 KNOTS THIS MORNING AND AGAIN LATER TONIGHT WITH THE DOWN THE BAY FLOW. WE WILL REACCESS THE BAY LATER TODAY. OUTLOOK... WEDNESDAY...HIGH PRESSURE BUILDS OVER THE AREA BUT THEN BEGINS TO SHIFT OFFSHORE AT NIGHT. AS A RESULT, THE WINDS AND SEAS WILL DECREASE AND BE BELOW ADVISORY CRITERIA. THURSDAY AND FRIDAY...A SOUTH TO SOUTHWEST FLOW WILL INCREASE AS THE PRESSURE GRADIENT TIGHTENS AHEAD OF A COLD FRONT. A MUCH WARMER AIRMASS HOWEVER WILL BE MOVING OVER THE COLDER WATERS, AND THIS WILL LIMIT THE MIXING. AS A RESULT, THE HIGHEST GUSTS WILL PROBABLY BE CLOSER TO THE COAST. THE INCREASING SOUTHERLY FLOW WILL BUILD THE SEAS AND THEREFORE A SMALL CRAFT ADVISORY LOOKS TO BE NEEDED. THE WAVEWATCH GUIDANCE IS PROBABLY OVERDONE SOME WITH THE SEAS AS THE GFS IS A BIT TO STRONG WITH THE SURFACE WINDS, THEREFORE WE STAYED CLOSER TO CONTINUITY WHICH HAD THIS FACTORED IN. SATURDAY...A COLD FRONT SHOULD BE EAST OF OUR AREA WITH A WIND SHIFT TO THE NORTHWEST, AND THE WINDS COULD BE A BIT GUSTY AS SOME COOLING TAKES PLACE. THE SEAS SHOULD BE AT OR ABOVE 5 FEET ON THE OCEAN, BUT TENDING TO SUBSIDE WITH THE OFFSHORE FLOW. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...WINTER WEATHER ADVISORY FROM NOON TODAY TO 10 PM EDT THIS EVENING FOR PAZ054-055. NJ...WINTER WEATHER ADVISORY FROM NOON TODAY TO 10 PM EDT THIS EVENING FOR NJZ001. DE...NONE. MD...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EDT WEDNESDAY FOR ANZ450>455. && $$ SYNOPSIS...GORSE NEAR TERM...GAINES/GIGI SHORT TERM...GIGI LONG TERM...GORSE AVIATION...GIGI/GORSE MARINE...GIGI/GORSE

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