Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 200746 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 346 AM EDT Fri Oct 20 2017 .SYNOPSIS...
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A dry frontal boundary will move offshore of the northeast this morning. Then high pressure builds across the area today into Saturday. The high builds offshore Saturday night into Sunday, and remains offshore through Monday. A frontal boundary is forecast to move across the area Tuesday into Tuesday night, bringing a return of unsettled weather to the area.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Any lingering patchy ground fog will burn off around 8 am. Weak cold front across central NY/PA will track to the south and east this morning, and will pass through northern zones by 8 am, then through the Delaware Valley and southern NJ by noon. The front will not be through the Delmarva until later today. Winds turn to the NW behind the passage of the front. There is not much CAA behind the front, so highs today will be right around what they were on Thursday. There will be a drier airmass building south, though, as surface dewpoints will fall from the mid/upper 40s to the lower 40s late this morning. With abundant sunshine and NW winds backing to the W, there should be some mixing to around 900 mb, but there is not much wind aloft, so winds should average 10 MPH with gusts to 20 MPH. This will also result in some drier air to mix down, resulting in surface dewpoints generally in the upper 30s across SE PA and NJ, but possibly as low as in the lower 30s across southern NJ. With the front slow to cross the Delmarva, winds should range from 5- 10 MPH, and dewpoints will be in the lower 40s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
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The front continues to sag south tonight. Dewpoints rebound a bit from the afternoon mixing, but will still be in the low 40s across NJ/SE PA, and in the mid 40s in the Delmarva. Dewpoints will drop into the mid and upper 30s in the Poconos. Lows tonight will be in the low 50s in the Philly metro area, and in the low to mid 40s across the rest of SE PA and NJ, and in the mid to upper 40s in the Delmarva.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/...
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Quiet weather is expected for the weekend as high pressure will be in control of the weather through Saturday, before moving offshore Saturday night into Sunday and remaining offshore through Monday. Ridging aloft remains in place across the area this weekend then builds offshore Sunday into Monday. This will keep temperatures well above normal over the weekend into Monday. A cold front is expected to move across the area Tuesday as an area of low pressure moves through the Great Lakes region. Showers could begin as early as Monday night as moisture and lift begin to move into the area ahead of the approaching storm system. The rain chances are highest during the day Tuesday as the front moves across the area. There is also some weak instability forecast during the day Tuesday, so there is a slight chance of isolated thunderstorms as well. Winds may also become quite gusty in the southwest flow ahead of the cold front during the day Tuesday. Wind gusts of 30 mph or greater are possible. The front is forecast to slowly move to the east and northeast into Wednesday and Thursday. Also, an area of low pressure is forecast to develop to our east along this boundary and move into northern New England. Meanwhile, a cold mid/upper trough is forecast to move across the area for Wednesday into Thursday. This could help create some scattered showers across the area Wednesday into Thursday, in addition to bringing cooler conditions.
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&& .AVIATION /08Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Any patchy ground fog at the TAF sites this morning will burn off by 13Z. Otherwise, VFR with SKC-FEW250 through the TAF period. LGT/VRB winds through 14-15Z, and then NW winds increase to 5-7 KT. by this afternoon, winds back to the W-NW at 10 KT with gusts to 15- 18 KT. Winds diminish to 5 KT or less tonight. OUTLOOK... Saturday-Monday...VFR conditions expected. Winds may gust around 15 knots Friday, Sunday, and Monday. Monday night-Tuesday...Conditions lower overnight Monday into Tuesday as a cold front moves through and showers move across the area. Winds could gust at least 25-30 knots on Tuesday.
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&& .MARINE...
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SW winds 10-15 KT will shift to the NW later this morning as a cold front slowly works its way south across the waters. Gusts to 20 KT possible in the immediate passage of the front, then winds gradually decrease later this afternoon, and then N-NW winds will range from 5- 10 KT tonight. Seas will average 3-4 ft on the ocean, generally in a 9 second period, and 2 ft or less on DE Bay. OUTLOOK... Saturday-Monday...Conditions expected to remain below Small Craft Advisory levels through the period. Monday night-Tuesday...Conditions likely to increase to Small Craft Advisory levels, possibly gale force, late Monday night into Tuesday.
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&& .FIRE WEATHER...
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NW winds later today will bring a drier airmass to the region, and winds will range from 10-15 MPH with gusts up to 20 MPH, especially across SE PA and most of NJ. RH values across most of NJ and PA will range from 25-30 percent. Additionally, fine fuels are dry (below 10 percent) across the region. These conditions could potentially support an enhanced risk for the spread of wildfires by the late morning or afternoon. A Special Weather Statement will be issued for NJ and PA for this risk.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Robertson Near Term...MPS Short Term...MPS Long Term...Robertson Aviation...Robertson/MPS Marine...Robertson/MPS Fire Weather...MPS

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