Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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919 FXUS61 KPHI 130547 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1247 AM EST Wed Dec 13 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1230 am update: Made several changes to the grids, most notably a big uptick in wind gusts based on recent observations about 5 to 10 mph too low across the board. Model soundings and momentum transfer calculations are not really catching on to the stronger gusts this evening, so most of the amendments involved a positive adjustment of 3-7 kts greater than the most aggressive hi-res models. Current wind advisory looks good, with verification already occurring along the immediate coast of Delaware Bay. However, based on gusts farther northwest the past couple of hours, the advisory may require some expansion. This is especially true given that the peak in wind gusts will likely occur late tonight through tomorrow morning. Will monitor trends the next couple of hours and make a final decision at 3:30. Sky cover and temperatures were also adjusted considerably. Temperatures are much lower than forecast, especially where cloud cover has thinned or eroded (i.e., especially southern and western CWA). Cold air advection is winning the battle tonight over mixing, with the added bone chilling of the strong winds producing wind chills below zero in the southern Poconos. Forecast wind chills are around -10 by morning, which is not far from advisory thresholds. PoPs/Wx grids unchanged, but coverage of snow showers has been sparse at best. A perturbation later tonight will likely enhance snow showers for a time in the southern Poconos, but models have been overdoing the precipitation during the whole event and do not see much reason why this trend will not continue.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
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1230 am update: Adjusted wind grids upward considerably (3-7 kts) most of the day given current trends and a clear negative bias with the models so far. Previous discussion below... 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. Strong wind gusts will continue through the day.
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&& .LONG TERM /TONIGHT THROUGH TUESDAY/... 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. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...VFR with 270-310 degree wind directions. Speeds 10 to 20 kts with gusts 25 to 35 kts. Very high confidence. Wednesday...VFR with 260-310 degree wind directions. Speeds 12 to 25 kts with gusts frequently at or above 30 kts, especially in the morning. Very high confidence. 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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Gales are in full swing in the marine zones, with Lower Delaware Bay not far from storm criteria. Current gale warning looks fine. Did adjust wind gusts a little bit based on current trends, mostly upward about 1-3 kts through most of the day tomorrow, based on current coastal and buoy observations. A strong gale event looks to unfold for a lengthy period. 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...
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PA...None. NJ...Wind Advisory until 10 AM EST this morning for NJZ021>025. DE...Wind Advisory until 10 AM EST this morning for DEZ002>004. MD...None. MARINE...Gale Warning until midnight EST tonight for ANZ450>455. Gale Warning until 6 PM EST this evening for ANZ430-431.
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&& $$ Synopsis...MPS Near Term...CMS Short Term...CMS/O`Hara Long Term...MPS Aviation...CMS/MPS Marine...CMS/MPS

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