Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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879 FXUS61 KPHI 250718 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 318 AM EDT Wed Apr 25 2018 .SYNOPSIS...
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Low pressure will move across our area today then into New England tonight and Thursday. The next low pressure system moves across the East Coast during Friday, then a cold front moves through on Saturday. High pressure builds into our region Monday before gradually shifting just off the Mid-Atlantic coast Tuesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Low pressure over western North Carolina will lift to the north and east, and should be over the Delmarva Peninsula later this morning, and continues its northern track towards NYC by early this evening. Meanwhile, an upper trough over the Great Lakes tracks east through the day and moves into western NY/PA by this evening as well. Deep easterly flow ahead of the low will usher abundant low level moisture into the region. PWATs will build to 1.3", and this will be enough moisture to result in locally heavy rain mainly during the morning commute. 00Z NAM has the bulk of the precip across the Coastal Plain of NJ, while the 00Z RGEM has the slug of heavy precip farther west, generally over southeast PA. Heavy rain lifts to the north later in the morning as the low lifts to the north. Models indicate a dry slot lifting north through the region during the afternoon, but it is during this time that the upper trough begins to dig into the Northeast U.S. This results in scattered showers for the afternoon and early evening. There may be enough elevated instability for some isolated thunderstorms, mainly across southeast NJ and into the Delmarva. Will go ahead and add isolated thunderstorms for those areas, but not expecting much convection. The rain and clouds will keep northern portions of the forecast area relatively colder than the rest of the region, and highs will top off in the 50s to low 60s north and west of the Fall Line. for southern NJ, southeast PA, and the Delmarva, there should be a break in the precip and possibly a break in the clouds that result in warmer temps, and highs will get into the mid and upper 60s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
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Upper trough passes north of the region tonight, and several strong shortwaves will lift through the Mid-Atlantic and Northeast. This will keep some showers in the region, with the best chances in northern NJ and the southern Poconos. Showers taper off after midnight as this system departs. Winds shift to the northwest, and conditions will dry out towards daybreak Thursday. Lows range from the 40s to low 50s.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/...
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Summary...Some showers at times into the weekend, then a significant warm-up probable starting early next week. Synoptic Overview...An upper-level trough from the Plains to the East Coast initially has several strong short wave troughs embedded within, however with time this consolidates into one main trough in the East. This is forecast to take place through the weekend, then strong energy 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. Due to the initial multiple short wave troughs within the larger trough, timing can be less certain as well as the strength of the surface features which impacts the details. The overall pattern favors milder air overall (Sunday the coolest day), then as we transition to an incoming ridge early next week the development of significant warmth is probable. For Thursday...A strong short wave trough lifts up across the Northeast with northwesterly flow in its wake across our area. The main synoptic ascent is forecast to be to our northeast, therefore a dry forecast is carried for during the daytime. There will be a northwest breeze and with this downsloping especially into the coastal plain results in mild temperatures. A period of dry air advection should help to erode the clouds, however additional clouds will be on the increase at night as a short wave trough rotating around the main trough aloft approaches from the southwest. Some showers with this may arrive into our southwestern areas late at night. 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 showers especially through early afternoon. A weak surface low may accompany it and this may help maintain some showers into early Friday evening. It is not out of the question that enough instability develops Friday afternoon for some thunder, however opted to leave it out for now. Widespread low clouds accompanying the showers may limit the overall heating potential, therefore leaned toward the cooler guidance for high temperatures. There should be some improving conditions at night. For Saturday and Sunday...The upper-level trough sharpens some across the Northeast and northern Mid-Atlantic region Saturday as a ridge shifts eastward from the Plains. There may be enough instability Saturday (especially the afternoon) with strong short wave energy glancing the area and a cold front moving through to produce some showers, especially across the northern half of the area. Despite the incoming upper-level trough, mild air should hang on during Saturday then cooling arrives at night and Sunday as the trough axis crosses our area. Sunday is expected to be dry with a northwest breeze as high pressure starts to build in from the west. For Monday and Tuesday...As the pattern shifts to a trough out West, a ridge shifts eastward and this drives surface high pressure over the Mid-Atlantic region Monday then just offshore on Tuesday. The presence of the building ridge will result in warm air advection and therefore a probable significant warm-up starting during this time frame. Given the presence of surface high pressure with a ridge aloft, dry conditions are forecast.
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&& .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Moderate to locally heavy rain this morning results in IFR and possibly LIFR VSBYs. Rain tapers off from south to north by early afternoon, and then additional showers are possible this afternoon. Generally IFR CIGs, lifting to MVFR late this afternoon. Cannot rule out a thunderstorm this afternoon, mainly at KACY. East winds 10-15 kt, becoming LGT/VRB late this afternoon. Tonight...Scattered showers through midnight or so. IFR CIGs lift to VFR after midnight. LGT/VRB winds in the evening become NW 10 kt or less by daybreak Thursday. OUTLOOK... Thursday...VFR during the day, then MVFR (locally IFR) ceilings may develop as showers arrive by late night. West-northwest winds 10-15 knots, then diminishing in the evening. Low confidence with onset of sub-VFR conditions. Friday...MVFR (locally IFR) with showers, then conditions should improve to VFR especially at night. Saturday...Mostly VFR, with a few daytime showers possible. Sunday...VFR. Northwest winds may gust up to 25 knots, then diminish toward late day and in the evening.
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&& .MARINE...
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Rain and patchy fog resulting in reduced VSBYs on the waters through late morning, then scattered showers this afternoon. A few thunderstorms are possible on the waters late this afternoon and early this evening. On DE Bay, wind gusts of 25-30 kt will continue through this morning. For the ocean, wind gusts of 25-30 kt through this afternoon, but seas remain elevated through tonight. OUTLOOK... Thursday...Small Craft Advisory for the Atlantic coastal waters due to elevated seas. The seas however should subside some at night. Friday and Saturday...A Small Craft Advisory may be needed for a time for the Atlantic coastal waters. The winds should be below advisory criteria, however southeasterly flow ahead of a system may be enough to build the seas to around 5 feet. Sunday...The conditions are anticipated to be below Small Craft Advisory criteria, although wind gusts to around 20 knots may occur nearshore during the day.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EDT Thursday for ANZ450>455. Small Craft Advisory until 7 AM EDT this morning for ANZ430- 431.
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&& $$ Synopsis...Gorse Near Term...MPS Short Term...MPS Long Term...Gorse Aviation...Gorse/MPS Marine...Gorse/MPS

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