Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182234 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 634 PM EDT Fri Aug 18 2017 .SYNOPSIS... As low pressure tracks well to our north, a cold front moves through later tonight into early Saturday. High pressure then builds in late Saturday and remains in place into Monday before shifting offshore. The next cold front is scheduled to arrive during Wednesday, then Canadian high pressure builds in for Thursday and Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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630 PM update...Cancelled the Severe Thunderstorm Watch for portions of the western zones. Will maintain the watch elsewhere at this time, and no changes to the Flash Flood Watch although the western part of this watch has now a much reduced threat. Otherwise, very warm and humid airmass in place across the region with surface dewpoints well into the 70s, and even approaching 80 in coastal portions of southern NJ. This results in an unstable environment with SB CAPE values ranging from up to 1500 J/kg across northern NJ to 2500-3000 J/kg across southern NJ and the Delmarva. Given the amount of instability across the region, and strong lift associated with an approaching mid-level shortwave, active weather on tap for this evening. In addition, with PWATs close to 2.5 inches across the region, continuing to expect heavy rain and training of thunderstorms. The Flash Flood Watch remains in effect until midnight, but went ahead and added all of DE and all of NJ to the Flash Flood Watch, as model guidance continues to show a precip max across the southern most portions of the forecast area, mainly associated with weak low pressure developing along the front. Low pressure currently north of the Great Lakes will lift to the north and east tonight. This will drag a cold front through the East Coast during the overnight hours. Pre-frontal trough has developed across central NY and PA out ahead of the front, and that pre- frontal trough will trigger showers and strong to severe thunderstorms across much of the forecast area this evening. The first storms that have developed are currently moving into the Lehigh Valley, but that activity will spread east through the early evening hours. Cold front swings through the region after midnight and during the pre-dawn hours. After sunset, with loss of diurnal heating, should lose some of the instability that would lead to severe storms. However, flooding threat remains in place until the storms end, as the abundant low- level moisture remains. The last of the storms look to move off the NJ coast around midnight. However, abundant low level moisture remains in place as the drier airmass will not get here until Saturday morning. As a result, can expect widespread fog to develop when rain ends and skies start to clear out. Cannot rule out patchy dense fog. Lows tonight remain in the upper 60s to low 70s, except for the Delmarva where lows will be in the mid 70s and the Poconos, where lows will be in the low 60s.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... Cold front should be offshore early Saturday morning, and once west to northwest winds increase a bit, the drier airmass spreads east, and any lingering fog will burn off. Skies clear out in the afternoon. Dewpoints drop about 10-15 degrees, generally into the mid 60s, but temps will be fairly warm, topping off in the upper 80s to near 90 across the urban corridor of I-95 from Trenton to Wilmington, and in the Delmarva. Upper trough approaches from the west, and this may bring isolated late day showers and thunderstorms to far western portions of the forecast area. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Summary...Lowering of the dew points through Sunday although remaining warm to hot, then moistures increases again with some convection especially Wednesday with the next cold front. A noticeably cooler and drier airmass arrives for Thursday and Friday in the wake of the cold front. Synoptic Overview...An upper-level trough is forecast to exit the Northeast Saturday night and Sunday, then the flow aloft turns more zonal through early next week. Another trough is forecast to amplify across eastern Canada and the Great Lakes to the Northeast Tuesday into Thursday, which then remains in the Northeast on Friday. We used a blend of continuity/guidance for Saturday night through Sunday night, then blended in the 12z WPC Guidance. For Saturday night and Sunday...The axis of an upper-level trough is forecast to slide across the region Saturday night, then quickly move across and exit New England Sunday. A short wave embedded within this trough is forecast to move through Saturday night, although it may shear out to the east-northeast. While some drying is expected to move in, enough lingering moisture and instability combined with lift may allow for some convection to arrive from the west before weakening. We therefore carried some slight chance PoPs in Saturday evening across parts of the western zones. The flow aloft then turns more zonal on Sunday with surface high pressure building in. This should be accompanied by more pronounced drying, and with plenty of heating during the day Sunday the dew points are anticipated to lower to more comfortable levels. A northwesterly breeze on Sunday may be light enough to allow for a sea breeze to develop in the afternoon. For Monday and Tuesday...The zonal flow aloft continues at least into Tuesday. This drives high pressure eventually offshore as it weakens some later Monday. The next upper-level trough amplifies eastward from the Midwest and Upper Great Lakes Tuesday. This will start to back the flow more southwesterly, and there may be a lee- side trough present on Tuesday. As the moisture begins to increase once again combined with the possibility for some convergence and enough instability, some convective development is possible Tuesday afternoon. We will carry slight chance PoPs as a result for Tuesday. Any isolated convection especially inland should tend to diminish during the night. It will turn more humid during this timeframe. For Wednesday through Friday...The prospects for convection will depend on the timing of an incoming cold front Wednesday. This will be due to an upper-level trough arriving from the west, however a pre-frontal trough may serve as the primary focus for showers and thunderstorms. We will continue with chance PoPs with the idea for at least scattered convection possible Wednesday. As of now, the cold front should be offshore to start Thursday and Canadian high pressure builds southeastward, which arrives over our area on Friday. A noticeably cooler and drier airmass overspreads our region for Thursday and Friday. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. IFR conditions, especially visibility, occurring west to east as a line of showers and thunderstorms move through. These have cleared KABE and KRDG, and as of 2230z are from KTTN to KPHL/KPNE to KILG. Strong wind gusts can occur as well. Once storms end, abundant low level moisture remains in place. IFR/LIFR CIGs/VSBYs should develop due to fog/stratus from late tonight through Saturday morning. VFR conditions develop Saturday morning. NW winds increase to 5-10 KT late Saturday morning, and back to the W-SW in the afternoon. Isolated late day SHRA/TSRA possible at KRDG/KABE. OUTLOOK... Saturday night and Sunday...VFR overall. Southwest winds 5 knots or less becoming northwest Saturday night, then northwest winds near 10 knots Sunday. Monday and Tuesday...Mainly VFR. Southerly winds around 10 knots. Wednesday...Sub-VFR at times with some showers and thunderstorms possible as a cold front arrives. Southwest winds 5-10 knots, becoming west.
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&& .MARINE... SCA remains in effect for the waters. Gusts to 25 KT will develop on DE Bay this evening, and gusts to 25 KT along with 5 foot seas will develop on the ocean waters this evening. Showers and thunderstorms will affect the waters as well this evening, some of which may require Special Marine Warnings. A Severe Thunderstorm Watch is in effect for DE Bay until 9 pm. Tranquil conditions develop after midnight tonight, and will remain in place through Saturday. Fog may develop late tonight and into Saturday morning, which could reduce VSBYs down to 1 NM. OUTLOOK... Saturday night through Wednesday...The conditions are anticipated to be below Small Craft Advisory criteria. RIP CURRENTS... A moderate risk for the development of dangerous rip currents remains in place for DE and NJ beaches through this evening. Thunderstorms will also impact the beaches through this evening. The rip current risk for Saturday is currently outlooked as low. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch until midnight EDT tonight for PAZ060>062- 070-071-101>106. NJ...Flash Flood Watch until midnight EDT tonight for NJZ001- 007>010-012>027. DE...Flash Flood Watch until midnight EDT tonight for DEZ001>004. MD...Flash Flood Watch until midnight EDT tonight for MDZ008-012- 015-019-020. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ450>455. Small Craft Advisory until 10 PM EDT this evening for ANZ430- 431. && $$ Synopsis...Gorse Near Term...Gorse/MPS Short Term...MPS Long Term...Gorse Aviation...Gorse/MPS Marine...Gorse/MPS

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