Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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429 FXUS61 KPHI 190135 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 935 PM EDT Sun Mar 18 2018 .SYNOPSIS... High pressure over the Midwest will gradually track to the south and east tonight and then will move off the Mid-Atlantic coast by Monday evening. Canadian high pressure then re- establishes itself north of the Great Lakes. Meanwhile, a complex area of low pressure will move east emerging over the Mid-Atlantic Monday night into Tuesday. This system may linger off the coast through Wednesday before moving off to the north and east by Thursday. High pressure returns from the north to close out the work week. Low pressure then approaches next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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Quiet conditions expected overnight tonight across the area. High pressure builds to the south of the area through tonight, while a weak area of low pressure pushes a backdoor cold front across the area. This back door cold front will be a dry frontal passage, and not bring any precipitation. It likely will not bring much cloud cover with it either, except across the far northern portions of the area. Winds will become light and variable, and another night of below normal temperatures is expected. The hourly temperatures and dew points were adjusted in a few spots to account for some local variations. Winds have become light in most spots.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... Quiet conditions continue through the daytime hours Monday as high pressure noses in from the north, then builds offshore during the day and keeps dry weather in the forecast. Clouds are expected to increase from south to north late in the day in advance of the next approaching system, but no precipitation is expected during the daylight hours. Highs on Monday are expected to remain below normal. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Coastal storm on track to impact the region Monday night through Tuesday, and then a secondary coastal storm will impact the region Tuesday night and Wednesday. Models are coming into better agreement in terms of timing and placement of the upcoming storm system, but there remain differences which keep this a low confidence forecast. Low pressure emerges southwest of the Delmarva Peninsula Tuesday morning, and tracks just offshore by Tuesday evening before stalling off the Mid-Atlantic Coast. An upper trough then follows behind it Tuesday night and Wednesday, and then the low re-intensifies on Wednesday just offshore before lifting to the north and east and towards the Canadian Maritimes Wednesday night. Models continue to indicate a fairly sharp cutoff with how far north the precip gets. GFS keeps pretty much all of the area north of I-195 dry Tuesday and Wednesday with the passage of both systems. The ECMWF is a bit farther north with the extent of the heavier precip, but generally keeps the southern Poconos dry. The NAM is similar to the GFS in terms of the cutoff. The Canadian is similar to the ECMWF in terms of placement of the QPF, but is much wetter. As a result, capped PoPs at slight chance for far northern zones, and otherwise slight chance/low- end chance for areas generally north of I-195. For the rest of the region, PoPs ramp up to likely/categorical for the Delmarva and southern NJ Monday night and Tuesday, and likely PoPs for Tuesday night and Wednesday with the passage of the secondary low. PType remains an issue. For area north of I-195, where PoPs are the lowest, temperatures and thermal profiles will remain cold enough to support all snow. But with the bulk of the precip staying to the south, not expecting much more than an inch or two with the two systems. For southern portions of the Delmarva and into extreme southeast NJ, there will be enough of an onshore flow to keep temperatures warm enough to support at least a mix, and snow mainly at night. This keeps accumulations down to less than an inch for the coast, and maybe 1-2" inland. The big question lies for the I-95 corridor, from generally Trenton to Wilmington, and then south towards Georgetown. At night, temperatures will be cold enough for all snow. But the heaviest of the precip falls during the day. And during the day, there should be a wintry mix of snow, sleet, and rain. This keeps accumulation down, even with the passage of the secondary storm on Wednesday. For this area, generally going with 1-3", but there may be some spotty areas of up to 4". After running the snow probability graphics, the high-end has come down from this morning, and now there is generally the potential for 5-7". A Winter Storm Watch is not necessary at this time for this event, but there is the potential a Winter Weather Advisory may be needed if these accumulations hold. In addition to the precip, strong northeast winds develop on Tuesday, generally 15-20 mph with gusts up to 30 mph, and stronger right along the coast. These winds continue into Wednesday, and may even strengthen a bit. Behind the departing system, winds turn to the northwest on Thursday. High pressure builds in from the north and west. Temperatures will be well below normal levels through this week. Yet another storm system is gearing to make a run at us next weekend, but it is way too soon to determine impacts at this time. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions continue tonight. Northwest winds diminish after sunset and become light and variable for most places. Monday...VFR conditions continue, although clouds increase from south to north during the day. Light and variable winds early become northwest, then northeast during the day, with speeds around 5-10 knots. OUTLOOK... Monday night and Tuesday...Onset of precip still looks to be late evening over the Delmarva as rain. By late Monday night and Tuesday morning, widespread MVFR/IFR conditions likely from KPHL/KPNE southward. There is high uncertainty as you go farther north toward ABE regarding if snow will make it that far north with a sharp cutoff in precip expected. For MIV and ACY, a considerable amount of mixing of rain, snow and perhaps sleet is likely while snow is favored farther northward toward PHL. NE winds 10-20 kt with gusts 25-35 kt (higher end of range near the coast). Moderate confidence. Tuesday night through Wednesday night...Additional precip is possible, mainly snow (except along the coast). Confidence in precip occurring is moderate near the coast and decreases farther inland. MVFR or IFR conditions would be possible if snow reaches the terminals but confidence is low at this point. Gusty N-NE winds expected, especially near the coast. Thursday...VFR. NW winds gradually relax. High confidence. Friday...VFR. NW winds. Moderate confidence. && .MARINE... Conditions expected to remain below advisory levels for tonight through Monday, although winds may gust around 20 knots at times. OUTLOOK... Monday night through Tuesday night...A Gale Watch is now in effect for the northernmost NJ ocean waters Tuesday night, and a Gale Watch remains in effect for all other ocean waters for Tuesday and Tuesday night. The strongest winds are expected over southern ocean waters where gusts could reach 40-45 kt. Wave heights in our southern coastal waters are forecast to build to 8-12 ft and to 6 to 10 ft for northern coastal ocean waters by late Tuesday. Conditions should remain below gale on DE Bay, but gusts to 35 kt are possible at the mouth of Lower DE Bay. Wednesday...The potential for gales exists but it speeds will depend on how close a second coastal low tracks to our area and how fast it deepens which is still uncertain. Adjusted wave heights 2 to 3 feet above WaveWatch guidance with the potential for northeasterly gales to continue. Wednesday night and Thursday...SCA conditions likely. Friday...Sub-SCA conditions expected. && .TIDES/COASTAL FLOODING... An extended period of strong northeasterly winds are expected to occur from late Monday night through Wednesday with two coastal storms tracking south and east of the region. Positive tidal anomalies will increase with each successive high tide as water piles up along the coast. The threat for coastal flooding looks to develop as early as the Tuesday evening high tide along the NJ-DE coast and continue through the Wednesday night high tide. The degree of coastal flooding will depend on the track and strength of both coastal lows, which is still uncertain especially with with the second one Tuesday night into Wednesday. ETSS, ESTOFS and other tidal prediction guidance from Stevens Flood Advisory System indicate minor to possibly low- end moderate coastal flooding is most likely. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Tuesday morning through late Tuesday night for ANZ451>455. Gale Watch from Tuesday evening through late Tuesday night for ANZ450. && $$ Synopsis...MPS Near Term...Robertson/PO Short Term...Robertson Long Term...MPS Aviation...Robertson/MPS Marine...Robertson/MPS Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.