Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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046 FXUS61 KPHI 252358 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 758 PM EDT Thu May 25 2017 .SYNOPSIS... An area of low pressure will move across the area tonight, then move away to our northeast through Friday. A weak surface trough is expected to cross the area during the day Friday. Weak high pressure may briefly move across the area Friday night into early Saturday. A weak low is forecast to move along a developing stationary front to our south Saturday into Saturday night. Another area of low pressure is forecast to weaken as it moves into the eastern Great Lakes Sunday into Sunday night, then an occluded frontal system is expected to affect the area Sunday night into early Monday. A series of cold fronts are forecast to move across the area Monday through Wednesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Convection underway. Showers and thunderstorms will continue to develop and lift to the north and east through the region. Surface low pressure lies over SE NJ, and and a potent H5 shortwave will lift through the region this evening. Associated cool air aloft will enable sufficient (though marginal) instability to develop to allow convection to continue northeastward into the CWA this evening. The shear profiles, particularly in the low levels, are not particularly strong, and an unfavorable veer-back-veer directional profile will likely develop this evening. Nevertheless, the strong dynamics of the approaching system and the steep low-level lapse rates may allow for some "loose" organization, the latter of which via the development/merging of strong cold pools. Convection-allowing model guidance is fairly aggressive in developing a fairly broad region of strong convection to the southwest, lifting it through the area this evening (generally through midnight). The main threats with these storms should be small to marginally severe hail, especially if convection can remain somewhat discrete, gusty winds given the dry midlevels and steep low-level lapse rates, and heavy rainfall. The main limiting factors at this time for a more robust threat include the marginal nature of CAPE and the aforementioned limitations of the vertical wind profile. Fast storm motions and dry midlevel air will likely preclude a widespread flash flood threat, but there is some concern that convective cells may train from south to north should more widespread development occur. Although storms are producing locally heavy rain, they seem to be moving fast enough to prevent widespread flooding from developing. Will have to see how this continues as secondary wave of showers and thunderstorms over the Delmarva lift to the northeast and into areas in SE PA and SW NJ where locally heavy rain already fell. Once the main convection exits northeast late this evening, the pivoting nature of the approaching upper low suggests wraparound showers remain a possibility for the overnight hours, especially north of the Mason-Dixon Line. Kept higher PoPs in this area through 6 am, though kept thunder chances to slight after 2 am, when the main show should be done. Low-confidence temp/wind forecast, with several updates likely overnight. Winds should transition to westerly by late tonight. Lows generally in the 50s, maybe near or slightly above 60 near the coast and in Delmarva.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... The main upper low shifts into New England on Friday, with westerly flow prevailing across the area during the day. Several perturbations in the flow are likely to move through the area during the day, especially in the morning. This should keep skies partially to mostly cloudy most of the day, and may allow for a sprinkle or light shower to occur, especially north of the Mason-Dixon Line. The 18Z NAM has a few showers farther to the south as a vort max rotates southeastward through the Mid- Atlantic around midday, but there is not a lot of support from higher-resolution guidance at the moment. As such, kept PoPs low or non-mentionable for the most part south of the I-78/I-195 corridor. Main sensible weather impact tomorrow may be the winds, with some stronger mixing likely to occur during the afternoon hours. Winds above the boundary layer are not particularly strong (20-30 kts), but gusts of 20-30 mph are likely by afternoon across most of the area. Forecast max temperatures range from the mid 60s in the Poconos to the mid 70s in the urban corridor to around 80 in southern Delmarva. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... An unsettled weather pattern continues for much of the extended forecast with several periods of rain possible. Friday night through the early part of Saturday is expected to be mostly dry as weak high pressure quickly moves across the area. However, later in the day, a weak area of low pressure will be approaching from the west but passing to our south along a developing frontal boundary to our south. Also, a short wave/vorticity impulse is expected to move across the area during the day, so there will be a chance of an isolated shower during the day Saturday. On Saturday night, the frontal boundary is expected to remain to our south, but should begin lifting northward toward the area Sunday. It will likely not make its way into the area until Sunday night. There will be a chance of showers overnight Saturday into Sunday, but the best chance of showers is expected to be Sunday night as the occluded front moves into the area. A series of cold fronts are forecast to move across the area Monday through Wednesday of next week as low pressure remain nearly stationary across southern Canada just north of the Great Lakes region.As any short wave/vorticity impulses slide across the area, they could lead to additional showers or thunderstorms early next week. High pressure tries to build back into the area for Thursday, which would provide a dry forecast. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Low confidence forecast for the TAF period through 12Z Friday. VFR conditions south of the warm front, generally at KACY and KMIV. Otherwise, widespread IFR CIGs, through some showers and thunderstorms may break those low CIGs to VFR/MVFR briefly. Showers and thunderstorms likely for most of the airspace this evening, generally through around 04Z. Lingering showers possible through 09Z, especially in NJ. Slow improvement in CIGs this evening/overnight, with generally VFR conditions by daybreak Friday. Winds 10 kts or less through tonight, but becoming west/gusty tomorrow. OUTLOOK... Friday night...Generally VFR. Saturday...Generally VFR. Showers moving into the area later in the day, which could lead to lowering ceilings. Saturday night-Sunday...MVFR CIGS possible. Chance of scattered showers. Sunday night...MVFR to IFR conditions possible with periods of low clouds and rain. Monday...Generally improving to VFR during the morning. Scattered showers and thunderstorms possible during the day. Monday night-Tuesday...Generally VFR. Chance of showers or thunderstorms on Tuesday.
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&& .MARINE... Small craft advisory continues, mainly for high seas, through Friday afternoon. Seas at the buoys have increased to 5-6 feet. Gusts to 25 KT will develop tonight, and then gusty W-NW winds could continue to gust to 25 KT on Friday. Showers and thunderstorms likely on the waters tonight. Gusty winds, small hail, and frequent cloud-to-ground lightning are all threats from these storms. Locally strong winds/rough seas are likely in vicinity of these storms. Winds become westerly on Friday. A light shower or two may occur, especially off the New Jersey coast. RIP CURRENTS... A moderate risk of rip currents continues through this evening given the easterly-to-southerly flow in place. However, as winds become westerly tonight, conditions will become less favorable for the development of dangerous rip currents. For now, think chances remain in the low category tomorrow. OUTLOOK... Friday night-Tuesday...Conditions expected to remain below advisory levels, although winds may gust around 20 knots at times. && .TIDES/COASTAL FLOODING...
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High tide has passed for Cambridge, MD, and with water levels receding, will go ahead and cancel the Coastal Flood Advisory that was in effect until 8 PM for MD. Otherwise, Coastal Flood Advisory remains in effect for this evening for coastal counties of Delaware and New Jersey, as well as counties along the Delaware and Raritan Bays. The astronomical tides with the new moon are particularly high with the evening high tide cycle, and the persistent onshore flow has lead to a 1-2 foot tidal departure. It is possible that additional spotty flooding may occur with the evening high tides Friday and Saturday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...Coastal Flood Advisory until 1 AM EDT Friday for NJZ012>014- 020>027. DE...Coastal Flood Advisory until 1 AM EDT Friday for DEZ002>004. MD...None. MARINE...Small Craft Advisory until 6 PM EDT Friday for ANZ450>455.
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&& $$ Synopsis...Robertson Near Term...CMS/MPS Short Term...CMS Long Term...Robertson Aviation...CMS/Robertson/MPS Marine...CMS/Robertson/MPS Tides/Coastal Flooding...MPS

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