Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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063 FXUS61 KPHI 070905 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 405 AM EST Wed Dec 7 2016 .SYNOPSIS... A cold front will cross the area early today as an area of low pressure slowly drifts through southeast Canada. This low will continue to slowly drift eastward through the end of the week and move offshore of the Canadian maritimes by Friday and Saturday. With high pressure to our west, a strong west to northwest flow will remain through the end of the week. High pressure briefly affects the area Saturday night. An area of low pressure is forecast to move north of the area Sunday into Sunday night, bringing a cold front across the area. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Low pressure was positioned about 150 miles off the lower Delmarva coast early this morning. The coastal low will move farther out to sea today. Meanwhile, a weak cold front that was currently located on the eastern side of the Appalachians will progress eastward through the region later this morning and off the coast this afternoon. Low clouds and drizzle have persisted across the forecast area for much of the night. RAP/HRRR forecast soundings show a saturated layer in the lowest 10 kft before 09Z. The column will dry from top down over the next few hours as mid-level subsidence behind last night`s disturbance arrives and winds aloft veer out of the NW. As a result, the drizzle will become more patchy over the next few hours. With temperatures still hovering around 31-32F in the higher terrain (roughly above 1000 ft in elevation) and lingering patchy drizzle still be reported at MPO-FWN, we have decided to play it conservatively and keep the Winter Weather Advisory in effect through 6 AM. Do not expect much more than a trace of ice with the freezing drizzle in these colder spots. Do not anticipate the need to extend the headline as the drizzle should taper off for the morning rush. The low clouds should hang around most of the morning but there will be some breaks that start to develop in our far western zones late this morning behind the cold front. Skies should clear out faster from west to east this afternoon though there will be some lingering mid-level clouds across the area as well as stratocu in the coastal plain. High temperatures will generally be in the 45-50F range with cooler spots in the higher elevations of NE PA and NW NJ (mid 30s to lower 40s) and warmer spots in S DE and E MD (low 50s). && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... An incredible 1054 mb surface high over the northern Rockies is forecast to build southward into the Great Plains tonight. An extension from this high will ridge eastward toward the Mid-Atlantic region tonight. Meanwhile, an area of low pressure will move across James Bay and into northern Quebec tonight. This system`s cold front advances southeastward into the Great Lakes region. The pressure gradient is modest but it may still be strong enough to produce a light W-NW across most of the area. However, the sheltered valleys will likely see the boundary layer decouple and temperatures dropping rather fast after sunset. Accordingly, we`ll likely see a decent spread in temperatures tonight depending on location. High clouds associated with a strong upper-level jet streak are expected to move into the region from SW to NE overnight, but the impact on radiational cooling is questionable. A blend of the MAV/MET was used to make the min temperature forecast, but also used some of the mesoscale model guidance (RGEM, NCEP WRF ARW/NMM) to highlight the cooler valleys. && .LONG TERM /THURSDAY THROUGH TUESDAY/... On Thursday, an area of low pressure will slowly be moving across eastern Canada, before reaching the Canadian maritimes Friday into Saturday, while high pressure remains across the central part of the country. This will keep a steady west to northwest flow across the area. A broad trough aloft will remain across the east coast, with an elongated vorticity impulse moving across the area Thursday and a weaker impulse moving across the area Friday. There is not a lot of moisture across the area Thursday as it`s forecast to remain northwest of the area, but the strength of the vort max and the surface winds may help carry some flurries/sprinkles across the area during the day. On Friday, the short wave is weaker, but there is more enhanced low level moisture and steep low-mid level lapse rates during the day, so there could be even more widespread showers or flurries/sprinkles across the area. Especially across our northwestern areas. By Saturday, another short wave/vorticity impulse is forecast to move across the area as the northwest flow begins to weaken as the low lifts farther away and high pressure begins to approach from the west. There will continue to be steep low-mid level lapse rates during the daytime combined with some enhanced moisture, especially across the northern areas. Therefore there will continue to be a chance for some isolated showers/flurries across the northern half of the area. High pressure briefly affects the area Saturday night, providing dry conditions. Thursday through Saturday will be a cold and windy period, with Friday being the windiest, and Saturday being the coolest of the 3 day period. On Sunday night, an area of low pressure is forecast to move north of the area, pulling a cold front across the region. Another low pressure is forecast to develop along the boundary, although the GFS and ECMWF differ on where they develop the low. The GFS develops this secondary low north of the area, while the ECMWF develops it to our south. Sunday night looks like the most likely period for precipitation, but it could linger into Monday if the low does develop around or south of the area as the ECMWF indicates, so we`ll keep a chance of precipitation into Monday. Temperatures will be cold enough for portions of the area to experience wintry precipitation. The exact timing and placement of wintry precipitation is still to be determined, but as usual, the most likely areas for snow would be the northern half of the area. Dry conditions are currently expected for Monday night into Tuesday as high pressure is forecast to briefly build to our south and nose its way into our area. The GFS and ECMWF are showing the potential for a coastal low to develop on the old frontal boundary to our south by next Wednesday, which could bring some precipitation to the southern half of the area. With cold temperatures, it`s possible that a period of snow could develop for the southern portions of the area. && .AVIATION /09Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VSBYs have improved over the past few hours with RA and DZ tapering off. CIGs vary between 600 and 1500 ft AGL across the area early this morning. Expect low end MVFR and IFR CIGs to persist until mid morning. Then gradual improvement from SW to NE to 1500-2500 ft CIGS is expected to occur between about 15-18Z. The low clouds should scatter out later in the afternoon, giving way to VFR mid to late afternoon. Wind direction varies between NE and NW early this morning with speeds between 5-10 kt. A preferred wind direction out of the NW will develop by mid morning as a cold front moves thru. Winds speeds will be around 10 kt. VFR is expected tonight. W-NW winds will be light (under 10 kt). OUTLOOK... Thursday-Saturday...MVFR CIGS may be possible each morning before lifting to VFR during the day. Scattered showers or flurries/sprinkles are possible during the daytime of each day which may temporarily lower conditions. Gusty west to northwest winds 15- 25 knots, strongest winds Thursday and Friday. Sunday...VFR early, lowering to MVFR then IFR by the afternoon and overnight. Rain or snow possible late in the day and overnight. && .MARINE... GLW was downgraded to a SCA for the southern coastal waters and lower DE Bay. This SCA goes through the mid evening. Farther north along the NJ coast, the SCA was extended through the afternoon. We should still see wind gusts to 25 kt through about mid morning, then seas will be the hazard. Seas will eventually subside to below 5 ft late today. Also, the SCA was canceled for the upper DE Bay. OUTLOOK... Thursday-Saturday...Small Craft Advisory levels winds expected through the periods with west to northwest winds gusting 25-30 knots. Saturday night-Sunday...Winds expected to drop below Small Craft Advisory levels, but pick up again late Sunday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory until 6 AM EST early this morning for PAZ054-055. NJ...Winter Weather Advisory until 6 AM EST early this morning for NJZ001. DE...None. MD...None. MARINE...Gale Warning until 6 AM EST early this morning for ANZ452>455. Small Craft Advisory until 6 AM EST early this morning for ANZ450-451. Small Craft Advisory until 4 AM EST early this morning for ANZ430. Gale Warning until 4 AM EST early this morning for ANZ431. && $$ Synopsis...Robertson Near Term...Klein Short Term...Klein Long Term...Robertson Aviation...Klein/Robertson Marine...Klein/Robertson

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