Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 122000 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 300 PM EST Tue Dec 12 2017 .SYNOPSIS...
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A cold front will move across the region this evening, and then high pressure passes south of the region on Wednesday. Several weak low pressure systems will impact the area through the end of the work week. High pressure returns for the weekend, and then another system will affect the East Coast early in the new week.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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A cold front will cross the area through the evening. Widely sct showers will accompany the front, but overall the present depiction on radar is not well defined and weak. Much colder air arrives behind the front on gusty W to NW winds. Gusts will be 30 to 40 mph overnight. Temperatures will drop through the 30s overnight and bottom out in the teens across the far N/W areas and low 20s over Philadelphia and Delmarva areas. Wind chill values will be near zero far N/W and 5 to 10 degrees most other areas.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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The core of the cold air will begin moving away Wednesday. The ridge will cross the region late in the day. A mostly sunny and cold day is expected across all areas. Highs will reach the mid/upper 20s north and low 30s elsewhere. Winds will remain gusty through much of the day, but then begin to taper off late.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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A fairly active pattern is in place for the Long Term as several weak clipper systems pass through the region through the end of the work week, and then again for the start of the new week. Each system will bring a reinforcing shot of cold air that will keep temperatures below normal into the weekend. Temperatures may then moderate a bit back closer to normal for the start of the new week. The first clipper system approaches Wednesday evening and then slowly works its way across the region before moving offshore by Thursday afternoon. The surface low tracks from the Great Lakes and Ohio Valley Wednesday evening and moves off the Mid- Atlantic coast late Thursday morning. Fairly potent H5 shortwave passes through southeast PA and NJ and 125 KT jet streak passes south of the Delmarva Thursday morning. With overnight lows in the teens and 20s, there will be plenty of cold air in place to support an all snow event starting in the evening, and then really getting going from around midnight through daybreak or so Thursday. QPF amounts will be light, generally around 1/10", but temps will be cold enough to support around 1", but in some cases, up to 2" of new snow Wednesday night through Thursday morning. Could make for a slippery commute in most areas. Precip tapers off Thursday afternoon, possibly mixing with a brief period of rain mainly south of I-195 before ending. Dry weather on tap for Thursday night as surface high pressure passes through the region. Cold night with lows in the teens across most of the area, and in the 20s from around Philly to Wilmington, and into the Delmarva. The next clipper system approaches on Friday and slowly passes well north of the region Friday night. There is some uncertainty with this one as to where everything sets up. 12Z/12 GFS and 12Z/12 ECMWF keep the weak primary low across Great Lakes, into northern NY State, and then through northern New England and offshore by Saturday morning. However, some strong mid-level shortwave energy passes through the region Friday afternoon and Friday night, while a 175 KT jet at 300 MB passes through the Mid-Atlantic and Southeast U.S. This develops a fairly strong coastal low Friday night, but latest guidance seems to keep the precip offshore. 12Z/12 CMC-GDPS indicates some possible moderate banding right over NJ, but that is because it is farther north with the developing secondary low than the GFS and ECMWF. For now, will run with chance PoPs on Friday for the whole region, and chance PoPs for most of NJ and down into Delaware for Friday night. This would probably be a mostly snow event, but may start out as rain over southern Delaware Friday afternoon. Low confidence forecast for Friday-Friday night. High pressure builds into the region Saturday and moves offshore on Sunday. Yet another system is gearing up to impact the region for the start of the new week, but with upper level ridging passing through the East Coast, this allows for more of a southwest flow that would usher milder air into the region, which would allow for a period of rain sometime Sunday afternoon and night. Another upper level low could then affect the region on Monday, and then dry weather looks to return on Tuesday.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR. Gusty west to northwest winds between 30 and 35 knots. Snow showers with restrictions far north/west. Clouds this evening then decreasing overnight. Wednesday...VFR. Gusty west to northwest winds up to 40 knots. Few snow showers possible early across the far N/W. Outlook... Wednesday night through Thursday morning...MVFR or lower in snow showers. Potential for 1-2" snow accumulation on runways by Thursday morning. West winds 10-15 KT become LGT/VRB late Wednesday night, then turn NW 5-10 KT Thursday morning. Moderate confidence. Thursday afternoon through Thursday night...VFR. Light NW winds. Moderate confidence. Friday through Friday night...Primarily VFR conditions, but there is the potential for MVFR or lower conditions in snow showers. Low confidence. Saturday...Mainly VFR conditions expected. West to southwest winds around 10 knots. Confidence: Moderate Sunday...VFR initially, then sub-VFR conditions possible in snow and/or rain showers. Low confidence.
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&& .MARINE...
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Raised the Gale flag at noon, with the fropa occurring later this afternoon and Gale gusts expected right behind it. Sct showers will accompany the front, but other than that, fair weather is expected. Seas increasing to 6 to 9 ft out on the ocean by Wednesday. Outlook... Wednesday night...Gale force gusts come to an end on the ocean and Small Craft Advisory conditions come to an end on DE Bay in the evening. SCA conditions persist on the ocean. VSBY restrictions in snow possible late Wednesday night. Thursday...Morning snow showers taper off in the afternoon. SCA conditions will continue. Thursday night and Friday...Sub-SCA conditions expected. Snow may develop Friday afternoon. Friday night and Saturday...SCA conditions look to develop again. Snow possible Friday night. Sunday...Sub-SCA conditions expected.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning until midnight EST Wednesday night for ANZ450>455. Gale Warning until 6 PM EST Wednesday for ANZ430-431. && $$ Synopsis...MPS Near Term...O`Hara Short Term...O`Hara Long Term...MPS Aviation...MPS/O`Hara Marine...MPS/O`Hara

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