Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182012 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 412 PM EDT Tue Apr 18 2017 .SYNOPSIS...
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A ridge of high pressure extending from the lower St. Lawarence Valley southwest to the mid- Atlantic Coast tonight will shift offshore and gradually weaken through Wednesday and Thursday. Low pressure will move east across the Great Lakes and into the Northeast on Thursday and Friday, with a trailing cold front to cross the mid-Atlantic on Friday. Weak high pressure on Saturday will gave way by Sunday to a storm system approaching from the Ohio Valley. Although the storm center is forecast to remain to our south, it will bring a good chance of rain for Sunday and Monday of next week.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Subjective surface analysis at 18Z indicates a sea breeze beginning to shift northwestward through eastern New Jersey, with temperatures a solid 5-10 degrees cooler after its passage. High-resolution models depict the sea breeze moving past the I-95 corridor later this afternoon, with winds switching to southeast and increasing to around 10 to 15 mph along with the decrease in temperatures. On the larger scale, a broad surface high was moving east through far eastern Canada this afternoon with a surface ridge axis crossing the Mid-Atlantic coastal region at this time. Winds will continue to veer to a more easterly/southeasterly direction across the area overnight as the ridge axis departs. Transient midlevel ridging will progress through the Northeast this evening with an upstream vorticity maximum expected to move rapidly into New England by tomorrow morning. Weak ascent on the southern fringes of the vorticity maximum should allow for an increase in mid and high clouds late tonight across the northern CWA, with skies generally remaining clear(er) to the south/east. With nocturnal cooling and weak moist advection from the increasingly onshore flow, there is a chance of low stratus or shallow fog development late tonight, though conditions are not ideal for its formation. Lows are expected to be near 40 in the southern Poconos and in cooler valley/rural locations and in the mid 40s in the urban corridor and near the coast.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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Weak perturbations in otherwise quasi-zonal midlevel flow will move through the Northeast on Wednesday. Our region will generally remain on the southern fringes of these vorticity maxima as broad subtropical ridging continues to influence the region. A larger- scale shortwave trough will be moving through southeast Canada, but the strongest ascent will remain to the north of the area. However, with proximity to these perturbations and the larger-scale shortwave trough increasing, expect a general increase in cloud cover through the day with gradually increasing chances for light rain showers/sprinkles in the far north/west CWA by afternoon. With onshore/southeast flow aiding in weak low-level ascent/upsloping in conjunction with the increasing cloud cover, temperatures may struggle to climb tomorrow. Brought temperatures down a few degrees across the area, more in line with the cooler MAV guidance - which places us right around seasonal averages. Most of the operational model suite keeps the area dry tomorrow, but some of the higher-resolution guidance get perilously close with rain showers (albeit light) moving into Delmarva and southeast Pennsylvania during the morning hours. For now, it looks as if this precipitation dissipates or shifts northeastward west of the region, but this will need to be monitored closely tonight. If trends suggest a more eastward push of the precipitation, PoPs will need to be raised accordingly.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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The long term period starts out with sfc high pres off the coast and a S to SW flow around it. There is low-level WAA over the area ahead of a weak frontal boundary approaching from the north. Weak ridging is noted aloft with weak impulses in the westerly flow. For Wed night and Thu the forecst maintains chances for showers mainly N of PHL, and also some sct t-storms Thu afternoon. Some instby may develop attm in an increasingly moist and unseasonably warm air mass. For Thu night and Fri...a fairly strong shrtwv trof moves E through the GrtLks with an assocd sfc cold front moving E through PA Thu night. The front should move through NJ and vcnty during the day on Friday, although models seem to indicate that most forcing for UVV remains N of PA. In spite of the frontal passage, max afternoon temps are fcst to reach the mid 70s to around 80, before cold advection picks up behind the front. Fair weather is expected for Saturday with weak high press building in and the frontal boundary well to the south. However clouds may increase late in the day as another low press develops over the lower MS valley. This system is expected to spread rain back into the area from the SW on Sat night and through Sun into Sun night. However the brunt of the system looks like it will remain to our south, and there should be a gradual improving trend on Mon and Tue as it slowly pulls away into the Atlantic.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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General VFR conditions expected through tomorrow morning. Light N/NE winds will veer to southeast and increase somewhat this afternoon as a sea breeze moves northeastward from the coast. A few gusts 15-20 kts possible just after sea breeze passage but speeds should primarily remain around 10 kts. Light E/SE winds tonight with a gradual increase in mid-high clouds. Mixed signals regarding patchy low stratus/sub-VFR CIG development late tonight; chances are too low for mention in TAFs at this time. Southeast winds increase tomorrow with CIGs gradually lowering to around 10 kft by afternoon. OUTLOOK... Thursday...Possible MVFR or IFR st/fog to start. Otherwise mainly VFR, but variable brief IFR conditions in scattered showers and thunderstorms. Southwest wind. Friday...Mainly VFR cigs with variable brief IFR conditions in st/fog/showers ending during midday. Becoming VFR sct-bkn aoa 5000 ft during the afternoon as a southwest wind in the morning shifts to northwest...gusts 20 kt. Friday night and Saturday...Mainly VFR conditions expected. Northwest to north winds. Saturday night and Sunday...VFR conditions Saturday evening becoming MVFR/IFR on Sunday in widespread showers and fog. East to northeast winds.
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&& .MARINE...
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With the return of buoy 44009, we can see that winds remain northeasterly over the Delaware coastal waters with southeast winds fully entrenched in Delaware Bay and the NJ coastal waters. As winds become more southeasterly with time, they may gust to around 20 kts or so through tomorrow, but winds/seas are expected to remain below advisory thresholds. Fair conditions are forecast through the period. OUTLOOK... Thursday and Thursday night...Sub-advisory conditions expected. Seas 2 to 4 feet with south to southwest winds around 10 knots. Fog may develop. Friday...Seas will build on Friday and may be near 5 feet on the ocean. Saturday...Seas may be near 5 feet on the outer edges of the ocean zones early but will be diminishing through the day. Sunday...SCA conditions are possible with east to northeast winds and seas building to 6 feet or higher.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...AMC Near Term...CMS Short Term...CMS Long Term...AMC Aviation...AMC/CMS Marine...AMC/CMS

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