Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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066 FXUS61 KPHI 172021 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 321 PM EST Fri Nov 17 2017 .SYNOPSIS...
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High pressure will weaken and move to our south tonight. An area of low pressure will move into the eastern Great Lakes later Saturday, then into New England Saturday night into Sunday. A warm front will lift across the area later Saturday, followed by a strong cold front overnight Saturday into early Sunday morning. High pressure will build to our south Monday into Tuesday, before a cold front moves across the area Tuesday night. High pressure then builds toward he area later Wednesday into Thursday, before weakening and shifting southward Friday.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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High pressure west of the Appalachians will build to the south and east tonight and move off the Mid-Atlantic coast by daybreak Saturday. Winds diminish fairly quickly this evening with the loss of diurnal heating, and become light and variable to calm tonight. Cirrus clouds increasing during the overnight hours will inhibit radiational cooling, but this is a chilly airmass, so it will be cold anyway tonight. Lows drop into the mid and upper 20s across much of NJ and into the Lehigh Valley, and will be in the lower 30s across the Delmarva. The urban corridor of Trenton to Philly will be several degrees warmer, in the low to mid 30s.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
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High pressure off the Mid-Atlantic coast continues to move offshore on Saturday. Ahead of low pressure moving through the Great Lakes and into the Ohio Valley on Saturday, a warm front will lift north through the region during the late morning hours. Southerly flow develops in the afternoon, and with strong low level jet passing through the region, expecting 25-30 mph wind gusts across much of the Delmarva and southern NJ, while winds will gust to 20 mph across northern NJ and southeast PA. Showers associated with this system will move into the Lehigh Valley and northern NJ by late morning, but the bulk of the activity should hold off until the afternoon. Best chances will be north and west of the I-95 corridor.
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&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
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Gusty and rainy conditions start the long term period, then drier weather returns to the forecast. As we move into Saturday night, a strong cold front is expected to move across the area. Ahead of this front, gusty southwest winds will continue through the night with gusts of 25-35 mph possible. It will likely not be gusty everywhere overnight as mixing will not be very deep everywhere during the night time period. However, as any showers move through, and as the cold front approaches, gusts have a better chance of mixing down. As the front moves through during the overnight, shower chances will be increasing across the area, and move from west to east fairly quickly. There is not much instability forecast, so we do not have thunder in the forecast at this time. Although it would not be surprising for an isolated lightning strike or rumble of thunder with the strength of the short wave. However, PW values continue to be forecast to reach 1.00-1.25 inches, so there could be some moderate, to occasionally heavy rainfall at times. The cold front is forecast to have moved through the area, and either be offshore, or just along the coastal areas of New Jersey and Delaware by around daybreak Sunday. Showers will come to an end behind the cold front, with the exception of possible continued showers in the Poconos as some lake effect streamers may develop. As the strong cold front moves across the area, winds will continue to be gusty, but become west to northwest as it shifts eastward. Wind gusts could reach 35-45 mph at times as the front crosses the area early Sunday, before slowly diminishing during the day. It is possible that a Wind Advisory may be needed in the future as winds increase abruptly early Sunday morning. As we go into Sunday night, winds could remain gusty around 25-30 mph and precipitation is expected to have ceased. The only exception is the possibility for some flurries in the Poconos Sunday evening if any lake effect streamers remain. High pressure builds to our south Monday into Tuesday. Dry weather is expected Monday through Tuesday, but winds will likely remain gusty around 20-25 mph. A cold front is forecast to move across the area Tuesday night into early Wednesday. The GFS remains dry, but the ECMWF has the front ingesting some moisture associated with an area of low pressure located to our south offshore of the east coast. With the uncertainty, we introduced a slight chance of showers. Dry weather is expected to continue through the end of the week as high pressure is forecast to build to our west then south with our area on the eastern and northern side.
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&& .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR. Cirrus clouds above 20000 ft will thicken towards daybreak. NW winds 10-15 KT with 20-23 KT this afternoon will quickly diminish to less than 10 KT, and then will become LGT/VRB this evening. Saturday...Predominantly VFR. Some showers possible at KRDG/KABE after 18Z, but bulk of showers will hold off until after 21Z. Conditions may lower to MVFR after 21Z. South winds increase to around 10 KT in the late morning, and then to 10-15 KT with gusts up to 20 KT after 18Z. OUTLOOK... Saturday night...Conditions lowering to MVFR and IFR overnight, with periods of showers associated with a cold frontal passage. Gusty southwest winds 25-30 knots possible. Moderate to high confidence. Sunday...Improving to MVFR, then VFR behind the cold frontal passage. Gusty west-northwest winds 30-35 knots. Moderate to high confidence. Sunday night...Generally VFR. Gusty northwest winds 25-30 knots. Moderate to high confidence. Monday...Generally VFR. Gusty west-northwest winds 20-25 knots. Moderate to high confidence. Monday night-Tuesday night...VFR. Winds may gust 15-20 knots during the afternoon Tuesday. Moderate ti high confidence. Wednesday...Slight chance of showers, though CIGS likely to remain VFR.
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&& .MARINE...
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Tonight...SCA remains up for the ocean waters through this evening, but as winds diminish fairly rapidly with loss of daytime heating, the SCA may be cancelled prior to expiration. For DE Bay, will cancel SCA that was up through this evening. Saturday...SCA conditions develop in the late morning as southerly flow increases ahead of low pressure moving through the Great Lakes and Ohio Valley. Gales develop on the ocean waters in the late afternoon. Will upgrade Gale Watch to Gale Warning for the ocean. For DE Bay, looks as if gales will not develop during the day, but wind gusts of 25-30 KT will develop late Saturday morning. Will go ahead and hoist a SCA from late morning through Saturday evening. OUTLOOK... Saturday night-Sunday night...An extended period of gale force wind gusts are expected starting Saturday afternoon, then continuing Saturday night through early Sunday night. Conditions likely to lower to Small Craft Advisory levels overnight. Monday-Tuesday night...Small Craft Advisory conditions expected to continue into Monday and Tuesday. A brief lull may be possible late Monday into Monday evening. Wednesday...Conditions likely to fall below advisory levels.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ450>453. Gale Warning from 1 PM Saturday to 6 PM EST Sunday for ANZ450>455. Small Craft Advisory until 6 PM EST this evening for ANZ454- 455. Gale Watch from Saturday evening through Sunday afternoon for ANZ430-431. Small Craft Advisory from 9 AM to 6 PM EST Saturday for ANZ430- 431.
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&& $$ Synopsis...Robertson Near Term...MPS Short Term...MPS Long Term...Robertson Aviation...Robertson/MPS Marine...Robertson/MPS

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