Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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116 FXUS61 KPHI 202310 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 710 PM EDT Fri Oct 20 2017 .SYNOPSIS... High pressure centered over the upper Ohio River Valley this afternoon will slide to the east. The center of the air mass is forecast to pass off the coast on Saturday before moving out to sea. The high should continue to influence our weather into Monday. A cold front approaching from the west is expected to arrive on Tuesday night. Low pressure is anticipated to develop over New England and its coastal waters late in the week as another area of high pressure noses into our region from the southwest and south. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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High pressure centered over the central Appalachians early this evening will build eastward toward the mid-Atlantic coast tonight. Winds have already started to diminish around sunset in response to the loss of daytime mixing and to the pressure gradient weakening over the region as the high builds into the area. There is still enough of a residual gradient over the area to maintain a light breeze this evening, particularly outside of sheltered locations. Accordingly, the rate of cooling over the next few hours will be considerably different across the region (i.e., cool much faster in sheltered areas owing to boundary-layer decoupling), resulting in a relatively large spread in temperatures from hour to hour and from location to location. There will be a low floor for temperatures to bottom out at given the ideal radiational cooling conditions (especially overnight as the high shifts overhead) and the dry airmass in place. Under this setup, felt it was prudent to adjust min temperatures downward from stat guidance. Forecast lows range from the upper 30s in the the valleys of NE PA/NW NJ to the lower 50s in downtown Philadelphia and along the coast. The dry airmass should inhibit radiational fog outside of the typical sheltered river valleys in rural NE PA and NW NJ.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... With high pressure over the region another sunny day is expected. Temperatures will warm fairly rapidly into the 70`s across the region with a few spots pushing 80. With a dry ground, it should be another day where temperatures slightly outperform MET/MAV guidance. Winds in the afternoon are likely to pick up a bit, staying under 10 mph from the southeast as the high pressure system moves offshore. Hope you can get outside and enjoy it! && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... The axis of the mid level ridge is forecast to pass overhead on Saturday night. Meanwhile, a sharp trough is expected to be located over the middle part of the country during the weekend. A mid level low is anticipated to develop over the south central states on Sunday. A re-enforcing mid level trough is expected to drop over the eastern states for the mid week period. The trough is forecast to pull the mid level low east then northeastward and over our region on Tuesday. The long wave trough should begin to lift to our northeast late in the week. At the surface, high pressure moving out to sea should maintain some influence over our weather for Sunday and Monday with no precipitation expected. However, there should be an increase in cloud cover during the period. A return southerly flow may result in the development of low clouds and perhaps some fog from late Sunday night into Monday morning. The end of our current dry spell is anticipated to arrive on Monday night. Showers ahead of an approaching cold front should begin to move into our region at that time. We are expecting an increasing chance of showers on Tuesday. As the surface cold front nears our area and as the remnants of the mid level low pass overhead there should be added lift. As a result, there may be a period of moderate and perhaps briefly heavy rain along with isolated thunderstorms on Tuesday afternoon and into Tuesday evening. There is the potential for a half inch to an inch of rain in spots. The cold front and the precipitation associated with it should pass off the coast on Tuesday night. Low pressure is forecast to develop along the departing front off the Middle Atlantic coast. The low is expected to lift over New England and its coastal waters late in the week. The system may push clouds and showers back into our region from the northeast for the period from Wednesday night into Thursday night. Otherwise, surface high pressure will likely try to build into our region from the southwest and south. Daytime temperatures will remain about 10 degrees above normal on Sunday, Monday and Tuesday. Readings are forecast to drop back to near normal levels for late October on Thursday, Friday and Saturday. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR tonight and Saturday. Light W-NW winds under 10 kt early this evening will become light/variable by midnight and persist thru the morning. These light winds will eventually acquire a more southerly direction (3-6 kt) during the afternoon. OUTLOOK... Saturday night and Sunday...Mainly VFR. Sunday night and Monday...Low clouds and fog are possible from late Sunday night into Monday morning. Otherwise...mainly VFR. Monday night through Tuesday night...Conditions lowering to MVFR and IFR in showers. Isolated thunderstorms and moderate to heavy rain are possible on Tuesday afternoon and Tuesday evening, along with a south wind gusting around 25 to 30 MPH. Wednesday...Conditions improving to VFR.
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&& .MARINE...
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Winds and seas well below SCA criteria tonight and Saturday. Light W-NW winds 5-10 kt over the waters this evening will become light and variable later tonight. Winds will eventually become more S-SEly and increase to 5-10 kt late in the day Saturday. Seas in our coastal waters of 2-3 ft this evening will settle closer 2 ft overnight and Saturday. OUTLOOK... Saturday night through Monday night...No marine headlines are anticipated. Tuesday and Tuesday evening...A Small Craft Advisory may be need for southerly wind gusts of 25 to 30 knots. Waves on our ocean waters may build to 6 to 8 feet. Late Tuesday night and Wednesday...A Small Craft Advisory may be needed on our ocean waters for wave heights of 5 to 6 feet, even as the wind becomes westerly and diminishes to 10 to 15 knots.
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&& .FIRE WEATHER...
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A precipitous rise in RH have occurred early this evening as temperatures drop and dewpoints rise. There is no fire weather concerns for tonight. RH values are likely to drop to around 30% again Saturday afternoon but light winds will limit the risk for the spread of wildfires.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Iovino Near Term...Klein Short Term...Gaines Long Term...Iovino Aviation...Klein/Iovino Marine...Klein/Iovino Fire Weather...Klein

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