Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 270040 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 840 PM EDT Fri May 26 2017 .SYNOPSIS... Weak high pressure will briefly affect the area tonight. A weak low is forecast to move along a developing stationary front to our south Saturday into Saturday night. Developing low pressure from the upper Great Lakes into southern Canada Sunday and Monday will direct a frontal system east across the mid Atlantic states. Weak cold fronts are forecast to move across the area Tuesday and Wednesday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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930 PM ESTF issuing early for showers e PA. Showers/sprinkles possible this evening eastern PA otherwise bands of cloudiness should thin and diminish during the night. Winds will continue to diminish.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... 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. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... 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. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR sct-bkn aoa 5000 ft. northwest wind diminishing. Saturday...VFR CIGS developing during the morning with a band or two of showers passing east or east southeast around 30 kt across the TAFS sites, generally 15z-21z. Small chance MVFR conds develop in the showers. At this time no thunder forecast. light wind. 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. && .MARINE...
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Winds and seas slowly subside and by midnight the NNJ SCA should be cancelled or expired. No further SCA anticipated Saturday with variable winds probably trending east in the afternoon. RIP CURRENTS... Tomorrow and Sunday: as it stands now...we should have a low risk for the formation of dangerous rip currents. Low does not mean NO!risk. Weak swimmer should always be within sight of a lifeguard. A 2-3 ft se swell with 7 second period both days. A somewhat stronger east wind is outlooked for Sunday. Even Monday looks to be a low risk day, however thunderstorms could put a damper on beach activities on Memorial day. Water temps are sort of coolish...60-65F. 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... Astronomical tides continue to run high, and despite an offshore flow, tidal departures remain about 0.5 to 0.8 feet above astronomical tide. Therefore, minor coastal flooding will occur on the coastal areas of New Jersey and Delaware and areas along Delaware Bay this evening. 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. && .CLIMATE...
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ACY 2nd wettest May on record so far with 7.11 inches. The record 8.80. PHL #11 so far with 6.12 inches.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory until 2 AM EDT Saturday for NJZ012>014- 020>027. DE...Coastal Flood Advisory until 2 AM EDT Saturday for DEZ002>004. MD...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ450-451. && $$ Synopsis...Robertson Near Term...Drag Short Term...CMS Long Term...Robertson Aviation...Drag/Robertson Marine...Drag/Robertson Tides/Coastal Flooding... Climate...

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