Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 250424 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1224 AM EDT Thu May 25 2017 .SYNOPSIS... As an area of low pressure moves into the Ohio Valley tonight into Thursday, an occluded front and warm front will lift toward our region. A triple point low will develop and take over as the dominant low as it moves across our area Thursday night. This low will move away from the area Friday, with a weak frontal boundary or surface trough crossing the area during the day. Weak high pressure may briefly move across the area Friday night into early Saturday. A weak low may move along a developing warm front to our south Saturday into Saturday night. An occluded frontal system is expected to affect the area Sunday into Monday, followed by another frontal boundary on Tuesday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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We have made some adjustments to the timing of the initial band of rain associated with the approaching warm front. The leading edge of the rain was into eastern Maryland and southern Delaware around midnight. It should reach a Reading- Philadelphia-Atlantic City line around 2:00 AM and it is expected to arrive in the Poconos and far northern New Jersey around 4:00 AM. The rain is forecast to become moderate and locally heavy at times. We are anticipating maybe about 0.40 to 0.70 inches through 8:00 AM in much of southeastern Pennsylvania, southern New Jersey, Delaware and northeastern Maryland. Amounts in the Lehigh Valley, the Poconos, and central and northern New Jersey should range from around 0.10 to 0.40 inches. The wind is forecast to remain out of the east around 8 to 14 MPH. The wind direction may begin to veer toward the southeast in parts of northeastern Maryland and Delaware around daybreak with the arrival of the warm front. The onshore flow is anticipated to keep temperatures mainly in the 50s for the balance of the night.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 AM FRIDAY/... Low pressure is expected to slowly lift northward through the Midwest/Ohio Valley on Thursday. Meanwhile, its attendant warm front will also move northward through the Delmarva region before stalling in eastern PA-NJ (likely near the Philadelphia metro). Models are indicating the development of a secondary low during the afternoon on the lee side of the Blue Ridge Mountains. This low would then approach the forecast area late in the afternoon. PoPs are highest initially in the morning hours when the southeasterly low-level jet interacts with the approaching warm front. This band of steady precip will eventually lift north of the region late in the morning. Coverage of showers will generally be more widely scattered during the afternoon. Breaks in the cloud cover will promote strong heating south of the warm front across Delmarva and perhaps southeastern PA/southern NJ. The environment will be conducive for thunderstorms south of the warm front later in the afternoon as the boundary-layer destabilizes and strong lift arrives ahead of the negatively-tilted trough. A few storms in these southern zones could potentially become severe with damaging winds being the primary threats. These storms will also pose a risk for localized heavy rainfall that could lead to urban/poor drainage flooding. && .LONG TERM /FRIDAY THROUGH THURSDAY/...
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An unsettled weather pattern continues for much of the extended forecast with several periods of rain possible. By Thursday night, the triple point low will begin moving across the area, pulling an occluded frontal system across the area as well. Showers will likely be ongoing during the evening and continue into the overnight hours. By Friday morning, the low will continue to move to our northeast and offshore of New England, and showers associated with the low will move to our northeast as well. However, a weak frontal boundary or surface trough is forecast to move across the area during the day. With help from the trough aloft and any short wave/vorticity impulses, there will be another chance of scattered showers during the afternoon. Friday night through most of Saturday is expected to be dry as weak high pressure quickly moves across the area. However, a warm front is expected to develop to the south of the area, while a weak area of low pressure moves along this boundary during the day. Also, a short wave/vorticity impulse may 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 warm front to the south will begin lifting northward, but not likely make its way into the area until Sunday. A triple point may form near the area by Sunday night as well, which could slow the progression of the frontal system as it begins to occlude. The exact timing of these features is still a little uncertain, but Sunday into Sunday night look to have the best chance of showers, and possible thunderstorms if enough instability builds on Sunday. The actual cold front is forecast to move across the area Monday, with another cold front possible on Tuesday. This could lead to additional showers or thunderstorms, especially during the day Monday and Tuesday.
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&& .AVIATION /04Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. CIGs will continue to lower overnight. Rain is expected to arrive from S to N between 05Z and 08Z. Expect IFR conditions shortly after the rain arrives with CIGs dropping below 1 kft. The steadier rain ends from S to N between approximately 11Z and 14Z Thursday morning. However, CIGs may lower to LIFR during the morning hours with the probability of an extended period of LIFR higher for northern terminals (TTN-ABE-RDG). Farther south, CIGs will try to slowly improve as a warm front lift northward. E-SE winds this afternoon around 10 kt with occasional gusts 15-20 kt thru sunset. Winds look to become more easterly tonight with speeds increasing to 10-15 kt and gusts 20-25 kt late tonight and early Thursday morning. A wind shift out of the S-SE is possible during the afternoon from about PHL southward assuming a warm front makes it this far north. OUTLOOK... Thursday night...MVFR to IFR conditions possible with low clouds and fog/drizzle possible. Friday-Friday night...Improving to VFR during the day and into the night, scattered showers possible during the daytime. Gusty northwest winds 20-25 knots. Saturday...Generally VFR. Showers moving into the area later in the day, which will lead to lowering ceilings. Saturday night-VFR conditions early, possibly lowering overnight. Sunday-Sunday night...MVFR to IFR conditions possible with periods of low clouds and rain. Thunderstorms possible Sunday. Monday...Generally improving to VFR during the morning. Scattered showers and thunderstorms possible during the day.
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&& .MARINE...
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SCA in effect for the coastal waters for tonight and Thursday. Although E-NE winds are generally 10-20 kt this afternoon, seas have been building to around 5 ft at our offshore buoys. Winds may reach SCA criteria of 25 kt early Thursday morning when the pressure gradient increases ahead of a warm front. Seas will remain in the 5-6 ft range through the day on Thursday. There is a moderate risk for the development of rip currents at ocean beaches on Thursday. OUTLOOK... Thursday night...Small Craft Advisory in effect. Friday...Winds may drop below advisory levels, but seas may remain above 5 feet into Friday. Friday night-Monday...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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Some of the highest astronomical tides of the year are occuring through the end of the week. As a result of that and onshore flow, minor coastal flooding is expected with the high tide again on Thursday evening. One source of guidance shows water levels along the northern NJ shore approaching moderate flooding thresholds with the Thursday evening high tide, but this seems uncertain at this time. Minor coastal flooding may again be possible with the Friday evening high tide.
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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 Friday for NJZ012>014-020>027. Coastal Flood Advisory until 1 AM EDT early this morning for NJZ016. DE...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT Friday for DEZ002>004. Coastal Flood Advisory until 1 AM EDT early this morning for DEZ001. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Friday for ANZ450>455.
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&& $$ Synopsis...Robertson Near Term...Iovino Short Term...Klein Long Term...Robertson Aviation...Klein/Robertson/MPS Marine...Klein/Robertson/MPS Tides/Coastal Flooding...Staff

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