Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 041737 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1237 PM EST Sun Dec 4 2016 .SYNOPSIS... High pressure over the region today will progress offshore tonight. A weak disturbance will pass through the area tonight into early Monday. High pressure builds back into the Mid-Atlantic states Monday afternoon and night. An area of low pressure over the Gulf of Mexico tracks northeastward and toward the region Tuesday. This low is forecast to pass to our south Tuesday afternoon and evening and then move off the Mid-Atlantic coast late Tuesday night and Wednesday. A strong cold front likely moves through our area Thursday as another area of low pressure tracks across the Northeast states. This low will strengthen over the Canadian Maritimes Friday and Saturday while high pressure builds in from the central U.S. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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With skies clearing out over much of the CWA, temps have risen by several degrees, so I went ahead and bumped up max temps by a few degrees. Most areas south of I-95 could come close to 50, and some parts of the Delmarva may get into the low 50s. Otherwise, highs generally in the 40s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... A fast moving shortwave trof and associated surface pres trof will cross the area tonight. Winds become southerly with some decent low- level warm advection and upward VV ahead of the trof. Some increased moisture should be available as clouds are streaming NE from the NW GulfMex already. Our forecast area should see a quick shot of fairly light precip between midnight and dawn on Monday. Based on the forecast temperature and moisture profiles, it looks like p-type will be mostly snow in the Poconos and NW NJ, a mix of rain and snow N/W of PHL and north-central NJ, and mostly rain elsewhere. Some snow accumulation, up to around and inch, is possible where the precip remains mostly snow. && .LONG TERM /MONDAY THROUGH SATURDAY/... A northern-stream shortwave disturbance and associated surface trough is forecast to quickly swing through the region early Monday morning. Models have trended toward a more progressive system, which would equate to a faster ending time of precipitation Monday morning. This trend is good news for the Monday morning commute in northeastern PA and northwestern NJ as any snowfall accumulations after sunrise should be minor. We may still see up to an additional half inch of snowfall in the higher elevations along and north of I- 80 before it all ends later that morning. High pressure briefly builds into the region Monday afternoon and night. Expect skies to clear from southwest to northeast during the afternoon. High temperatures Monday afternoon range from the upper 30s in the Poconos to the lower 50s from Philadelphia and points south/east. A favorable radiational cooling pattern will initially be in place Monday night with a break in clouds and calm winds underneath the high. High clouds will start to overspread the area from the southwest late tomorrow night as low pressure approaches from the Gulf Coast states; which may curb cooling across the south. Low temperatures near or slightly below freezing are forecast for all of northeastern PA/northwestern NJ, and rural locations west of the Phila metro in southeast PA and in the NJ Pine Barrens. Models advertise a double-barrel low approaching the Mid-Atlantic region from the south and west on Tuesday. The system is predicted to feature a primary surface low that cuts into the Ohio Valley and a secondary low forming on the lee side of the southern Appalachians Tuesday morning. This secondary coastal low will likely track up the Southeast coast before passing to our south late in the day Tuesday and offshore sometime Tuesday night. Precip from this system will spread in from the southwest, arriving the Delmarva during the morning rush. The onset of precip across our northern-most zones may be delayed until afternoon where a residual dry layer near the surface will be more apparent. Thermal profiles look warm enough for the ptype to be rain for most of the region. However, precip at the onset may briefly start out as wintry mix west of the Fall Line before transitioning to liquid as warm as a warm nose aloft becomes increasingly established. The cold outlier model solutions show surface temperatures hovering in the mid 30s (too close to freezing for comfort) in our coldest spots north of I-80 (southern Poconos in PA, Sussex County in NJ). It`s too soon to rule out the outside possibility of freezing rain if temperatures in these coldest spots wind up verifying just a few degrees lower than currently forecast. Evaporative cooling, particularly early on in the event, could be a mechanism to keep temperatures near freezing in these colder spots if models are underdoing the magnitude of the residual dry layer in the boundary layer. Since freezing rain is not the most likely outcome, kept snow/ice accumulations out of the forecast for Tuesday and Tuesday night. The secondary low is forecast to have moved off the Mid-Atlantic coast by Wednesday morning. The back edge of the rain should taper off from southwest to northeast during the day as the low tracks farther offshore. A shortwave disturbance rounding the base of the large-scale upper trough will progress through the region on Thursday. Light precip may move back into the area as early as Wednesday night and continue into Thursday. Partial thicknesses from the GFS/ECMWF/CMC indicate a potential for snow or a rain/snow mix in the Poconos and rain elsewhere. A strong cold front will move through later on Thursday (probably either afternoon or night). Northwest winds behind the front will usher in the coldest air of the season thus far. With daytime highs only in the 30s, Friday and Saturday could be the coldest days since last February. A tight pressure gradient resulting from an impressive 1040 mb high over the central CONUS and a deepening low over the Canadian Maritimes will yield breezy conditions on Friday. Wind chills in the teens are possible area wide with single-digit wind chills in the Poconos Thursday night and Friday night. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions through around 06Z Monday. BKN CIGs of 4000 FT will develop later this evening and going into tonight. NW winds less than 10 KT will become LGT/VRB this evening. MVFR conditions develop in rain for most terminals, but snow at KABE/KRDG. Snow will eventually mix with and change to plain rain by daybreak Monday, and up to an inch or so of accumulation is possible during that time. By daybreak, IFR CIGs likely. Could be some spotty IFR VSBYs, but confidence too low to include in TAF at this time. Wind become SE 5 KT or less by daybreak, and behind departing low, winds shift to the W-NW and increase to 5-10 KT. Precip tapers off from west to east between 15Z-18Z, and conditions improve to VFR. OUTLOOK... Monday night...VFR. Tuesday and Tuesday night...RA moves back into the area from southwest to northeast on Tuesday. Conditions will likely lower to MVFR shortly after precip onset and then IFR later Tuesday and Tuesday night. Wednesday and Wednesday night...Steady rain tapers off, but lower clouds may linger. Thursday...Generally VFR, but scattered showers are possible during the day which may temporarily lower conditions. Gusty northwest winds 25-30 knots develop behind a cold front during the afternoon.
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&& .MARINE... Sub SCA conditions are expected through today and tonight. NW winds should remain a bit gusty for a while today but will diminish by this evening as a ridge of high pressure moves overhead. Seas will gradually subside along with the wind. Winds overnight will become light and variable, however visibility may be reduces in areas of rain and fog after midnight. OUTLOOK... Monday and Monday night...Winds and seas below SCA criteria. Tuesday through Wednesday...Onshore flow strengthens on Tuesday. SCA may be needed for winds by Tuesday afternoon. SCA for winds and seas likely Tuesday night and Wednesday. Winds may reach gale force for a period late in the day Tuesday, particularly across the coastal waters of DE and southern NJ as well as the lower DE Bay. Wednesday night...Winds likely drop below SCA levels, but seas could remain above 5 feet. Thursday and Thursday night...Northwest winds increase behind a cold front. A SCA will likely be needed. Gust near gale force are possible beginning Thursday night. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...AMC/MPS Short Term...AMC Long Term...Klein Aviation...AMC/Klein Marine...AMC/Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.