Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 301929 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 329 PM EDT Tue May 30 2023 .SYNOPSIS...
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High pressure remains anchored over the Northeast through mid-week while weakening low pressure off the coast of the Carolinas will linger into Thursday. The low will continue to bring some scattered showers to the Delmarva Peninsula, but dry weather should prevail overall with increasing temperatures through the end of the week. A back-door cold front passes through on Saturday and introduces a cooler airmass for the weekend.
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&& .NEAR TERM /THROUGH WEDNESDAY/...
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High pressure south of the Canadian Maritimes will sag to the south and east tonight through Wednesday. Meanwhile, upper low over the Mid-Atlantic continues to dissipate. The other aspect to this forecast is smoke and haze from wildfires in Nova Scotia will spread into the region. More on that in a bit. In terms of sensible weather, onshore flow will continue tonight through Wednesday. Some light rain showers may continue to move into far southern portions of Delmarva with minimal, if any, QPF prior to this evening. Another round of light rain showers may impact these areas, again with minimal QPF, Wednesday afternoon. With onshore flow and increasing low level moisture spreading into the region, patchy fog and drizzle is likely for coastal areas and areas just inland from the coast late tonight through Wednesday morning. In addition, a stratus deck will spread farther inland. Clouds scatter out Wednesday morning, giving way to hazy sunshine in the afternoon. Lows tonight will be in the mid and upper 40s to low 50s. Highs on Wednesday will top off in the upper 70s to low 80s inland, and in the upper 60s to low 70s along the coast and just inland. Smoke from wildfires in Nova Scotia will then spread west into the region from the northeast. Latest HRRR and RAP vertically integrated smoke forecasts have one wave spreading into the region into this evening, and then another wave on Wednesday. Will go ahead and carry a wide swath of haze across much of New Jersey and southeast Pennsylvania this evening, and then a wider swath of haze across southeast Pennsylvania, most of New Jersey, and northern Delmarva. HRRR is also indicating the potential for smoke to mix down to the surface. Will go ahead and carry "patchy smoke" for parts of New Jersey and southeast Pennsylvania, including Philadelphia. Keep an eye out for any Air Quality Alerts that may be issued, and be prepared for the smell of smoke.
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&& .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
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A strong upper ridge will remain overhead for the short term period while upper troughing digs across the Southwest. The flow will be quite meridional aloft and the resultant blocking pattern will hold strong through the end of the week. At the surface, high pressure remains in control while a weak surface low spins off the coast of the Carolinas. Weak onshore flow Wednesday night will introduce a marine layer with low stratus/fog/light drizzle as moisture is trapped underneath a subsidence inversion around 900 mb. Further inland (Lehigh Valley to the Poconos) expect mostly clear skies and, with wind going light to calm, efficient radiational cooling, though warm advection will work to offset the cooling and lows will be a few degrees warmer than Wednesday night in the low 50s. In addition, expect that some haze/smoke will still be around into the early overnight, so maintained the patchy smoke mention. Temperatures will rebound quite quickly on Friday as the NBM is still holding firm with highs in the upper 80s and low 90s, while cooler along the coast. Latest guidance has continued the slight increase of 850 mb temperatures to around 17-19C which is very close to supporting highs in the 90s. NBM prob MaxTs are indicating a >60% chance of MaxTs >90F primarily along I-95 and west, though MOS guidance is still not budging from the upper 80s. Regardless, we`ll be seeing above average temperatures across the board. Forecast soundings show a very deep, dry boundary layer, so do expect that dew points will mix out in the afternoon and apparent temperatures should remain around actual air temperatures. A sea breeze should develop in the afternoon and will drop temperatures some as it progresses inland.
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&& .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
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The forecast still remains more uncertain going into the weekend. The deterministic suite of guidance indicates a potent shortwave diving south out of New England and a back door surface front coming onshore on Saturday. Meanwhile, an area of surface low pressure will be sliding up the Atlantic coast, and both the GFS and ECMWF show the shortwave aiding in the low`s development before quickly evolving into a closed low itself. Both deterministic and ensemble guidance remains spread on the timing of the backdoor front as well as the strength and track of the surface low. The GFS and Canadian favor a very early Saturday morning passage with the ECMWF later. Ultimately, the setup is favoring a post-frontal cooldown on Saturday after a quite warm Friday with thickness values falling and cold advection with northeasterly onshore flow. Held with NBM which introduces some PoPs (20-40%) with frontal passage on Saturday, though moisture may be overall lacking with the front. Cool high pressure is expected to build in on Sunday with mostly dry conditions and high temperatures in the low to mid 70s. The beginning of next week looks to stay dry overall for now, but there is quite a bit of model spread regarding where this closed coastal low will end up. Current ensembles keep it a good distance offshore and mostly unimpactful to our area, though a surface boundary may sag south across the area sometime Monday. Run-to- run fluctuations continue essentially after the frontal passage Saturday, so stuck close to the consensus/NBM forecast here until we can narrow down possibilities over the next several days.
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&& .AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of this afternoon (through 00Z)...VFR. Hazy skies with no restrictions possible for KTTN/KPNE/KPHL. E winds 10 to 15 kt with gusts up to 20 kt, diminishing to 5 to 10 kt by 00Z. Moderate confidence. Tonight (00Z through 12Z)...VFR initially, then MVFR CIGs likely to develop at KTTN/KPNE/KPHL/KILG and IFR CIGs with MVFR VSBYs in fog, stratus, and drizzle will develop at KMIV/KACY after 09Z. E winds around 5 kt. Moderate confidence. Wednesday (12Z through 00Z)...Any sub-VFR conditions will give way to VFR. HZ develops in the afternoon. Cannot rule out VSBY restrictions in smoke, but probabilities are low. E-NE winds 5 to 10 kt, increasing to around 10 kt in the afternoon. Moderate confidence. Outlook... Wednesday night...Sub-VFR conditions expected with low stratus and fog, especially at I-95 terminals and KACY/KMIV. Some haze and patchy smoke at mid-levels through the early overnight. Variable wind less than 5 knots. Moderate confidence. Thursday...Some early morning fog/low stratus may continue sub-VFR conditions early, otherwise expect VFR for the remainder of the day. Northeasterly wind 5-10 knots becoming southeasterly in the afternoon. Moderate confidence. Friday...VFR. Northeasterly wind 5-10 knots. Moderate confidence. Saturday...Sub-VFR conditions possible with passage of a cold front. Northeasterly wind increasing to 10-15 knots with gusts to 25 knots behind the front. Low confidence. Sunday...VFR. Northeasterly wind 5-15 knots. Low confidence.
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&& .MARINE...
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NE winds 10 to 20 kt with gusts up to 25 kt will diminish through this evening and will be 10 to 15 kt after midnight. On Wednesday, winds will be NE 10 to 15 kt with gusts up to 20 kt. Seas of 5 to 7 feet will subside to 3 to 5 feet tonight through Wednesday. Small Craft Advisory for Lower DE Bay will end at 7 pm, and sub-SCA conditions then expected for DE Bay through Wednesday. Seas remain elevated on northern NJ ocean waters through tonight, then seas will be below 5 feet through Wednesday. For southern NJ and DE ocean waters, seas remain around 5 feet through Wednesday, and SCA remains in effect through Wednesday. Outlook... Thursday...No marine headlines expected. Northeasterly wind 5-10 knots. Seas 3-4 feet. Friday...No marine headlines expected. Southeasterly wind 5-10 knots. Seas 2-3 feet. Saturday...Small Craft Advisory conditions possible. Northeasterly wind 5-10 knots in the morning increasing to 15-20 knots with gusts to 25-30 knots in the afternoon. Seas 2-3 feet building to 5-7 feet from north to south. Sunday...Small Craft Advisory conditions possible. Northeasterly wind 15-20 knots with gusts to 25 knots and seas 5-7 feet in the morning. Conditions may improve later in the day.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...High Rip Current Risk until 9 PM EDT this evening for NJZ014- 024>026. DE...High Rip Current Risk until 9 PM EDT this evening for DEZ004. MD...None. MARINE...Small Craft Advisory until 7 PM EDT this evening for ANZ431. Small Craft Advisory until 6 AM EDT Wednesday for ANZ450-451. Small Craft Advisory until 8 PM EDT Wednesday for ANZ452>455. && $$ SYNOPSIS...Brudy NEAR TERM...MPS SHORT TERM...Brudy LONG TERM...Brudy AVIATION...Brudy/MPS MARINE...Brudy/MPS

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