Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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927 FXUS61 KPHI 262013 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 413 PM EDT Fri May 26 2017 .SYNOPSIS...
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Weak high pressure will briefly affect the area tonight, before building to our northeast over the weekend. 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 combine with a larger low across southern Canada as it moves into the eastern Great Lakes Sunday into Sunday night. An occluded frontal system is expected to affect the area around Monday. A series of cold fronts or surface troughs are forecast to move across the area Tuesday and Wednesday.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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Light showers have developed this afternoon in eastern Pennsylvania and New Jersey in advance of a northwest-to-southeast oriented vort max swinging through the Mid-Atlantic region. With the combination of sufficient surface heating and subtle convergence along a surface trough moving through the area today, showers should continue through late afternoon before quickly dissipating near sunset. A tight pressure gradient to the southwest of a surface low off the coast of New England has also enabled fairly strong mixing today, which has allowed for the downward transport of momentum to the surface. Gusts of 20 to 30 kts have occurred in much of the area this afternoon. Along with the shower coverage, wind gusts should diminish quickly near/after sunset. As the vort lobe moves east of the area overnight, transient subsidence upstream of the shortwave trough will allow for clearing skies. With the lowering winds, temperatures should cool fairly readily overnight. However, a stronger vorticity maximum will be approaching from the west late tonight. Increasing cloudiness should begin to occur near daybreak, which adds a little uncertainty to the temperature forecast. MOS has been relatively consistent the past few cycles, however, so stuck close to a consensus blend overnight. Generally, lows are expected to be near 60 in the urban corridor, with lower 50s in the Poconos and northwest New Jersey and the lower 60s in southern Delmarva. There is some chance of some radiation fog developing in the favored locations in the Poconos/Lehigh Valley, but increasing cloud cover late may prevent this from occurring. For now, left this chance out of the grids, but this will be something to monitor overnight.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
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The aforementioned vort max upstream will move through the region on Saturday. Models have sped up the timing of this system by quite a bit, generally on the order of 6 hours. Most operational guidance and some of the longer-range high-resolution simulations indicate precipitation moving into the area by tomorrow morning. Increased PoPs for the morning hours based on this trend. Cyclogenesis will occur east of the Appalachians (probably in Virginia) during the day, with a baroclinic zone rapidly developing to the east of the low. As a result, the warm sector will likely be south of the area as the zonally-oriented surface boundary will remain to the south. Not overly excited about thunder chances in this regime, but there are at least some indications that elevated instability will exist south of the Mason-Dixon Line as large-scale lift cools the midlevels atop the near-surface stable layer. Kept thunder chances during the afternoon hours for this area as a result. The vort max will be in the process of being sheared out in the confluent flow upstream of the upper low in the northwestern Atlantic. This will result in relatively weak (and weakening) lift as the system moves through. Thus, precipitation will generally be light, and given that the warm sector is expected to be south of the CWA, not expecting more robust thunderstorm development. However, if the warm sector sneaks into the southern CWA, then a strong storm or two is possible with the vertical shear being favorable for storm organization. This is considered to be a very low probability at this point. Temperature forecast is tricky with the approaching system and complications from cloud cover. MOS has come in colder tomorrow (by about 3-7 degrees), and have generally followed, though I did tweak temperatures a bit using higher-resolution guidance, which may have a better handle on how any precipitation affects proximity ambient temps. Forecast highs range from the low-mid 60s in the Poconos to the mid 70s in southern Delmarva - with lower than average confidence given the aforementioned complications.
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&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
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An unsettled weather pattern continues for much of the extended forecast with several periods of rain possible. On Saturday night, the area of low pressure will be passing to our south and southeast as it moves eastward along the the frontal boundary to our south. Any showers associated with this low are expected to pull away from the area during the evening hours, with the remainder of the night likely staying dry. High pressure is forecast to build across the northeast Saturday night into Sunday, which would keep the frontal boundary to our south and keep an east southeasterly flow across the area. This easterly flow would keep cloudy and cool conditions. On Sunday night, the front to our south is forecast to begin lifting north toward the area, while an occluding frontal boundary approaches from the west as low pressure moves through the Great Lakes Region and combines with a larger low across southern Canada. The occluded front may not actually move into the area until Monday or Monday night, but the chance for rain will begin to increase from the west overnight Sunday into Monday. A series of cold fronts or surface troughs are forecast to move across the area basically each day from Tuesday through Friday as low pressure slowly drifts eastward across southern and eastern Canada. There will be a chance showers or thunderstorms each day, which will be focused around the time of any short wave/vorticity impulse passages. High pressure tries to build back into the area for Thursday, which would provide a dry forecast.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. BKN CIGs are hovering around 3500-6000 feet this afternoon and should continue to rise slowly before scattering out early this evening. Shower coverage is spotty, so any expected impacts this afternoon at the terminal should be minimal at best. CIGs return and lower steadily tomorrow morning, with chances of showers increasing through the day. Periods of MVFR/IFR conditions possible in proximity to showers. Stray lightning strikes are possible in the afternoon, but chances appear low at this time. West winds 10 to 15 kts with gusts 20 to 25 kts decreasing below 10 kts this evening. Winds should remain light tomorrow with primarily a southwesterly or southerly direction. OUTLOOK... Saturday night...MVFR to IFR CIGS possible. Chance of scattered showers in the evening. Sunday...MVFR to IFR CIGS in the morning, improving during the day. Isolated showers during the day. Sunday night...MVFR to IFR conditions possible with periods of low clouds and rain. Monday...MVFR to IFR conditions forecast to continue through the day, but some improvement by afternoon. Scattered showers and thunderstorms possible during the day. Monday night...Conditions may lower to MVFR or IFR with low clouds and fog. Tuesday-Wednesday...Generally improving to VFR during the day Tuesday and continuing through Wednesday. Chance of showers or thunderstorms on Tuesday and Wednesday afternoon/evening.
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&& .MARINE...
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Small craft advisory remains in effect through 6 pm for the Atlantic waters of southern New Jersey and Delaware and through 10 pm for the Atlantic waters of northern/central New Jersey, mainly for high seas. Significant wave heights have varied considerably this afternoon, generally ranging from 4 to 6 feet. Wave guidance continues to show a downward trend this evening, but given the variability in the buoy observations, not confident enough to change the expiration times or to cancel the advisory early. Winds should remain below advisory criteria through tomorrow, with directions primarily westerly tonight. Directions should switch to the southeast quadrant by tomorrow afternoon. A chance of showers (with a slight chance of thunderstorms) exists tomorrow, especially during the afternoon. RIP CURRENTS... Rough seas continue this afternoon and evening on the Atlantic coastal waters, where a moderate risk of rip currents exists. However, seas will lower tonight, and winds should generally be around 10 kts or less through most of the day tomorrow. Current expectation is for the risk of rip currents to be in the low category for tomorrow. However, steadier east winds will develop late in the day and continue through Sunday, so the risk of rip currents may approach the moderate category later this weekend. OUTLOOK... Saturday night-Wednesday...Conditions expected to remain below advisory levels, although winds may gust around 20 knots at times.
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&& .TIDES/COASTAL FLOODING...
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Astronomical tides continue to run high, and despite an offshore flow, tidal departures remain about 0.5 to 1.0 feet above astronomical tide. Therefore, minor coastal flooding is expected to occur on the coastal areas of New Jersey and Delaware and areas along Delaware Bay. As we go into the weekend, winds are expected to become onshore again, which could keep tidal departures elevated, possibly leading to additional coastal flooding with the evening high tides Saturday and Sunday, especially on Saturday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT Saturday for NJZ012>014-020>027. DE...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT Saturday for DEZ002>004. MD...None. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ452>455. Small Craft Advisory until midnight EDT tonight for ANZ450-451.
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&& $$ Synopsis...Robertson Near Term...CMS Short Term...CMS Long Term...Robertson Aviation...CMS/Robertson Marine...CMS/Robertson Tides/Coastal Flooding...

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