Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182005 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 305 PM EST Sat Nov 18 2017 .SYNOPSIS...
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Low pressure over the Ohio Valley will lift to the north and east and into southeast Canada tonight, and then a strong cold front will move across the region Sunday morning. High pressure then builds in from the west and moves off the Mid-Atlantic on Tuesday. A cold front moves through the region Wednesday night. Low pressure develops along that front and may impact the region on Thursday. High pressure returns on Friday, followed by low pressure passing north of the area on Saturday.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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An area of low pressure will move across the eastern Great Lakes region and toward the northeast tonight. A strong cold front associated with this low will approach the area overnight tonight and toward Sunday morning. Southwest flow aloft will continue across the area, with several short wave/vorticity impulses moving across the area. A couple rounds of showers are expected tonight across the are. The first will exit the are this evening as the initial short wave moves past the area. Then a brief dry period may occur for a time this evening. However, a second short wave will approach the area around/after midnight, leading to a second round of showers. Finally, as the cold front approaches, the final round of showers will move into the area toward daybreak Sunday. There is not much instability, although the front and short wave/vorticity impulse are fairly potent. So it would not be surprising to see an isolated lightning strike or rumble of thunder, but the chances are pretty small so we did not include them in the forecast. 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. Another concern overnight is the increasing winds. Winds will be increasing through the night and wind gusts of 25-35 mph are possible through the night. 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. Winds have the potential to gust to around 50 mph as the front moves through around daybreak, so we have issued a Wind Advisory across the area.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
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The cold front pushes through the area around and shortly after daybreak Sunday, and the rain showers will be ending as the front moves through. The main concern for the daylight hours Sunday will be the winds. Right as the front moves through, a surge of wind is possible to occur, then the winds could continue to gust 45-50 mph through the morning and into the early afternoon hours. Therefore the Wind Advisory will continue into the afternoon hours Sunday across the entire area.
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&& .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
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Strong NW flow continues night as low pressure lifts into eastern Canada and high pressure builds into the TN valley and Gulf Coast states. Winds will gust to 25-30 mph in the evening, and will gradually diminish after midnight Sunday night. Though precip will be over for the vast majority of the forecast area, some Lake Effect Snow streamers may make it into the Pocono Mountains, and could result in some spots picking up 1/4" snow. Cold night in store with lows dropping into the 20s for the Poconos and NW NJ, otherwise in the low 30s for the Lehigh Valley, southern NJ, and the Delmarva. Wind chills will range from the teens up north to around 30. High pressure just north of the Gulf Coast will build east on Monday, and will gradually move off the Mid-Atlantic coast on Tuesday. Chilly conditions on Monday with highs in the 30s up north and in the 40s elsewhere will moderate back into the 50s to near 60 on Tuesday as return flow sets up. From there, low pressure develops off the Southeast U.S. coast and lifts to the north along the coast as a cold front approaches from the west. That front passes through the region Tuesday night or Wednesday. Confidence is low for this timeframe, as there is uncertainty with how far west that low will track and how much precip will fall. For now, will follow a blend of guidance and carry chance PoPs Tuesday night- Wednesday. High pressure builds in from the west on Thursday, but models are indicating another coastal low passing south and east of the area during this time, and depending on how close it tracks, more precip is possible for the area. Will carry slight chance PoPs for southeast portions of the forecast area, but it will take a couple of days or so before the details are ironed out with this potential low. Dry conditions on tap for Friday. Low pressure passes north of the area Saturday and drags a cold front through the region. Some snow is possible up north Friday night, and then precip changes to rain on Saturday.
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&& .AVIATION /20Z SATURDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions are in place across the area this afternoon and evening. Temporary near MVFR conditions are possible as showers move across the area late this afternoon, but should remain VFR through most of the evening. As showers increase overnight, conditions are expected to lower to MVFR. This is especially true as a cold front moves across the area around daybreak Sunday. Winds will increase through the night, and could gust 15-25 knots for much the night, then increase after midnight, and especially through daybreak as the cold front moves through to around 25-35 knots. Sunday...MVFR conditions are expected right around daybreak as the showers move through the area, then a return to VFR is expected through the morning hours into the afternoon. Winds are expected to be gusting 30-40 knots through much of the day. Outlook... Sunday night...VFR. Some flurries possible at KRDG/KABE. NW winds 15- 20 KT with gusts up to 30 KT, diminishing to 10-15 KT late. High confidence. Monday through Tuesday...VFR. West winds 10-15 KT on Monday become S 5-10 KT Monday night, and then 10-15 KT with gusts up to 20 KT on Tuesday. High confidence. Tuesday night through Thursday...Potential for rain Tuesday night/Wednesday, then again on Thursday with sub-VFR conditions. Low confidence forecast.
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&& .MARINE...
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A Gale Warning is in effect for all areas through Sunday night. The Delaware Bay does not begin until 1 am, but the Gale Warning for the Atlantic coastal waters is already in effect. OUTLOOK... Sunday night...NW Gales continue Sunday night, but may end just after midnight on DE Bay. Monday through Monday night...NW winds gusting to 25 KT on the ocean waters, along with 4-6 ft seas. Tuesday through Tuesday night...SW winds 15-20 KT with 25-30 KT gusts on the ocean; SW winds gusting to 25 KT on DE Bay. Winds diminish after midnight Tuesday night. Wednesday...Winds and seas subside to sub-SCA conditions. Thursday...Low confidence forecast with the potential for SCA conditions on the ocean.
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&& .TIDES/COASTAL FLOODING... There is a potential for blow out tides on Sunday, especially in the Delaware Bay and the tidal portion of the Delaware River, where strong W-NW winds will effectively drain water out of the bay. The low tide Sunday afternoon/evening seems to be the most susceptible for low water concerns that could impact navigation in the area. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Wind Advisory from 5 AM to 4 PM EST Sunday for PAZ054-055- 060>062-070-071-101>106. NJ...Wind Advisory from 5 AM to 4 PM EST Sunday for NJZ001-007>010- 012>027. DE...Wind Advisory from 5 AM to 4 PM EST Sunday for DEZ001>004. MD...Wind Advisory from 5 AM to 4 PM EST Sunday for MDZ008-012-015- 019-020. MARINE...Gale Warning until 5 AM EST Monday for ANZ450>455. Gale Warning from 1 AM Sunday to 5 AM EST Monday for ANZ430- 431.
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&& $$ Synopsis...MPS Near Term...Robertson Short Term...Robertson Long Term...MPS Aviation...Robertson/MPS Marine...Robertson/MPS Tides/Coastal Flooding...

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