Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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384 FXUS61 KPHI 260741 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 341 AM EDT Thu Apr 26 2018 .SYNOPSIS...
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Low pressure will continue to move across New England and into the Canadian Maritimes today, while weak high pressure builds across the Ohio Valley. Low pressure then moves across our area Friday, with a cold front moving through our region on Saturday. High pressure across the Great Lakes and Ohio Valley Sunday builds to our south Monday, then remains centered just off the Mid-Atlantic coast Tuesday and Wednesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Fair weather is expected today after a few pre-dawn clouds exit srn NJ and Delmarva. A ridge of high pressure with drier air is expected to settle across the region. Skies will be mostly sunny with the descending air associated with the high. It will be a seasonably mild day, with highs a few degrees above normal. Winds will be mostly from the Northwest at 10 to 12 mph with some gusts up to 20 mph during the late morning and early afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
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Good weather should continue into the evening before clouds and greater chances for showers arrive towards dawn. The showers will probably only affect Delmarva and perhaps srn NJ or SE Pennsylvania. It will remain mild tonight with lows in the upper 40s to low 50s S and E while it will be cool up north where it will drop into the low 40s. Winds will be light tonight mostly from the south.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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Summary...Showers at times into the start of the weekend, then a significant warm-up looking likely during the first half of next week. Synoptic Overview...An upper-level trough from the Midwest and Great Lakes to the East Coast initially has several strong short wave troughs embedded within, however these merge into one main trough in the East which then lingers through Sunday. Strong energy then rolling through the western U.S. allows for a ridge to build in the Plains during the weekend which then shifts eastward early next week. Despite initially multiple short wave troughs within the larger trough, the timing looks to be pretty good. The upper-level trough though for the weekend is trending deeper and a little slower. There are two systems that will produce showers across our area, one moves through during Friday in the form of a shearing short wave trough (accompanying surface low) then the main trough arrives Saturday which produces more showers. It is the latter where the details are a little less certain. The evolving pattern favors warming (Friday and Sunday are the coolest days), then as we transition to an incoming ridge during the first half of next week the development of significant warmth is looking likely. For Friday...As the main upper-level trough becomes more consolidated, a short wave trough tracks up its east side and across the Mid-Atlantic and Northeast. This feature looks to open up and start to shear out as it tracks northeastward, however plenty of lift with it as it takes on some initial negative tilt should produce widespread showers especially through early afternoon. A weak surface low accompanies it and this may help maintain some showers into early Friday evening. There could be enough instability developing Friday afternoon for isolated thunder, however opted to leave it out for now given low confidence and the main forcing shifting to our northeast in the afternoon. Widespread low clouds accompanying the showers should limit the overall heating potential, therefore went a little cooler for the high temperatures. There should be some improving conditions at night, although light enough winds and not much dry air advection could lead to some fog. For Saturday and Sunday...The upper-level trough sharpens to a closed low as it slides eastward across the Northeast and northern Mid-Atlantic region during this time frame. Meanwhile, a ridge shifts eastward from the Plains. The trend is for the evolving closed low to be deeper and a little slower. As this arrives Saturday, forcing for ascent along the leading edge of the more pronounced height falls and positive vorticity advection results is some showers arriving from the west. There may be enough instability Saturday, especially in the afternoon with the arrival of a cold front, to result in some low-topped convective elements. Given the uncertainty regarding the amount of instability and the overall placement of the stronger forcing, opted to not include thunder at this time. Despite the incoming upper-level trough/closed low, mild air should hang on during Saturday then cooling arrives through Sunday as the closed low traverses the region. Sunday is expected to be dry, although some showers may be close to our northern areas, with a northwesterly gusty breeze. Given the stronger system aloft, went a little cooler for Sunday and increased the cloud cover some especially the northern areas. If the winds drop off and the sky clears, there could be some frost in parts of the region Sunday night. For Monday through Wednesday...As the pattern shifts to a trough out West, a ridge builds eastward and this places surface high pressure to our south Monday then just offshore on Tuesday into Wednesday. The presence of the building ridge will result in warm air advection and therefore the likely start of a significant warm-up Monday and especially Tuesday, followed by even warmer air on Wednesday. Given the presence of surface high pressure with a ridge aloft, dry conditions are forecast.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR conditions are expected today with a ridge of high pressure settling across the area. Some pre-dawn cloudiness across NJ will exit off the coast with mostly clear skies expected after that. Winds today will be mostly Northwest at 10 to 12 knots with some gusts close to 20 knots during the late morning and afternoon. Tonight...The ridge of high pressure will begin to move away while clouds and rains associated with the next system approach. VFR conditions expected thru the night with lowering clouds across Delmarva, southern New Jersey and SE Pennsylvania after midnight. A few showers may arrive at KILG or KMIV by 12Z. OUTLOOK... Friday...MVFR/IFR due to low clouds and showers. The showers taper off by late day, with improving conditions overall occurring during the night. Saturday...Some showers during the day into the evening, which should result in times of sub-VFR conditions. Sunday and Monday...VFR. West to northwest winds 10-18 knots with gusts up to 25 knots, diminishing some Sunday night and Monday.
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&& .MARINE...
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No changes to SCA flags today. Higher seas and a few wind gusts close to 25 kts expected today and into tonight. Fair weather today and most of tonight then a few showers by dawn Friday over Delaware Bay and the Atlantic coastal waters adjacent to Delaware and Southern New Jersey. OUTLOOK... Friday...The winds are expected to be below Small Craft Advisory criteria, however seas on the Atlantic coastal waters may be around 5 feet for a time. Saturday...The wind and seas overall are anticipated to be below Small Craft Advisory criteria. Sunday and Monday...West-northwest winds may gust to 25 knots at times especially nearshore, and seas are anticipated to be below 5 feet.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Friday for ANZ450>455. && $$ Synopsis...Gorse Near Term...O`Hara Short Term...O`Hara Long Term...Gorse Aviation...Gorse/O`Hara Marine...Gorse/O`Hara

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