Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 261703 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 103 PM EDT Wed Apr 26 2017 .SYNOPSIS... The coastal low that has affected the East Coast the past several days will begin pulling away from the region today, but will be slow to exit. A cold front will move across the area Thursday night, with a second Saturday night. A warm front is expected to lift across the area Sunday night, followed by another cold front on Monday. && .NEAR TERM /THROUGH TONIGHT/...
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1230 PM ESTF: cloudy with areas of drizzle and isolated showers spinning west and southward through the forecast area into this afternoon due to the close proximity of the vertically stacked low pressure system off the mid Atlantic coast. VERY few changes to the 330 am fcst temps/dews. From the 330 AM fcst discussion for temps today. Model guidance continues to show considerable spread regarding temperatures today. MOS guidance from the MAV (highs in the 70s) is 5-10F warmer than the MET (highs in the 60s) due to differences in how much sunshine develops this afternoon. There does not appear to be a notable mechanism to mix out the marine stratus today with a strong low-level inversion remaining in place, light N-NEly winds and no notable source of dry/cold air advection. Accordingly, forecast weighted more toward the cooler and cloudier NAM-based guidance though not quite as extreme. If breaks in the clouds were to develop this afternoon, forecast temperatures would need to be adjusted upward by several degrees (which would be more likely to occur farther inland toward the eastern shore of MD and our far western zones in eastern PA). Tonight...The coastal low will continue to weaken tonight as it moves out to sea, passing south of Long Island/southern New England. Marine moisture will remain trapped underneath the subsidence inversion. Any stratus that mixes out this afternoon should redevelop quickly after dark tonight. At least patchy fog is anticipated late this evening and overnight, but the extent of the fog should be limited by the presence of stratus beforehand. It should be noted that some of the guidance shows dense fog forming late tonight but we have not quite bit on that due to the previously mentioned reasons. Cloud cover should curb how much cooling takes place tonight. Forecast lows are in the 50s. Nearly calm wind.
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&& .SHORT TERM /THURSDAY/...
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On Thursday, our area will be under the influence of return flow ahead of an approaching cold front. Brighter...especially south of I78 during the afternoon. Still a lot clouds. warmer. Wind becomes southeast to south.
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&& .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... It is possible that an isolated shower may develop across northeast Pennsylvania and northern New Jersey as a weak short wave/vorticity impulse approaches the area. However, the best chances for rain are expected to be during the overnight hours as the front moves across the area and a stronger short wave/vorticity impulse moves across the area. Although much of the guidance dissipates the more widespread showers as they approach, they do keep some scattered showers across our area. The front moves offshore Friday morning and any showers will dissipate and/or move offshore through the morning as well. The remainder of Friday will remain dry as weak high pressure briefly affects the area. By Friday night, another frontal boundary is forecast to begin approaching the area from the north as it crosses Pennsylvania and New York. The front will cross through the area during the day Saturday, then stall to our south Saturday night into Sunday. There is some slight timing differences in the model guidance on when the front moves through. But as a couple of short wave/vorticity impulses move across the area Friday night through Saturday night, there will be a chance for isolated/scattered showers. The front will stall to our south during the day Sunday, which should allow Sunday to be dry. Being the front is a back- door front coming from the north, it will bring cooler conditions on Sunday. But the front will not stay to our south very long as it is expected to lift back northward as a warm front Sunday night/Monday morning. While there is a chance of showers Sunday night into Monday morning, the more likely time period for any precipitation would be later Monday into Monday night as a cold front approaches from the west. Being we should enter into the warm sector during the day, we should warm quite nicely, and in turn some instability should build ahead of the front. Therefore there will be a chance of thunderstorms Monday afternoon and Monday night. The front will move offshore by Tuesday morning, but a secondary surface trough may move across the area during the day Tuesday. With a short wave/vorticity moving across the area during the day, there could be some isolated showers across northeastern Pennsylvania and northern New Jersey. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of this afternoon...Cigs 800 to 1800 feet continue to improve- rise with a few remaining patches of IFR drizzle, mainly NNJ. Probably goes MVFR to possibly VFR cigs by 22z most everywhere. Light northeast wind. Tonight...IFR conditions in st/fog should redevelop in nearly calm wind though it may take until near 06z for IFR to redevelop at the inland sites, dependent on how warm it gets there late this afternoon. Thursday...Ifr or MVFR conds in st/fog to start the day, probably improves to VFR sct-bkn clouds aoa 3500 feet during the afternoon. Wind becoming southeast to south. OUTLOOK... Thursday night...Generally VFR. A chance of showers overnight which could temporarily reduce conditions to MVFR. Friday...VFR conditions expected. Friday night...VFR early. Patchy fog and low CIGS may develop overnight. Saturday...Becoming VFR during the morning, then a chance of showers during the afternoon. Conditions may temporarily be reduced with any showers. Saturday night-Sunday...Low clouds and fog may develop overnight Saturday and continue into Sunday leading to reduced conditions. There will also be a chance of showers.
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&& .MARINE...
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The Small Craft Advisory hazardous seas continues through Thursday. Waves are currently around 7-9 ft and will gradually subside to 4-7 ft late this afternoon and tonight. OUTLOOK... Thursday night...Small Craft Advisory for hazardous seas through Thursday night. Friday...Winds expected to remain below advisory levels, but seas may remain elevated to advisory levels. Friday night-Sunday...Winds are expected to remain below advisory levels. Seas may approach 5 feet at times, but mostly remain around 4 feet.
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&& .TIDES/COASTAL FLOODING... We will continue to monitor the high tide this evening since it will be the higher one that occurs with a new moon. If the water does not drain away from the coast fast enough, we could see some localized minor flooding at that time. ETSS and a few of the higher NYHOPS ensemble members predict levels reaching minor threshold (but below local advisory criteria) at Lewes, Cape May and Reedy Point while the majority of the remaining guidance (including ESTOFS) keeps it short of minor. && .CLIMATE...
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Presuming our forecast temps these last 5 days of April are accurate, we are assured of a top 3 warmest April on record in much of our forecast area. Sunday is the critical day for determining record or not. Below: April projected within the top April avg temps, the normal for April and the period of record (POR). PHL normal 54.0 POR 1874 59.4 1994 59.2 2017? 58.5 1921 58.4 2010 57.9 1941 ABE normal 49.9 POR 1922 56.6 projecting record 56.4 1941 54.7 1994 ACY normal 51.7 POR 1874 57.2 projecting record 56.3 2010 56.1 2011
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory for hazardous seas until 6 AM EDT Friday for ANZ450>455.
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&& $$ Synopsis...Robertson Near Term...Drag/Klein 103 Short Term...Drag/Klein Long Term...Robertson/Miketta Aviation...Drag/Robertson 103 Marine...Drag/Klein/Robertson 103 Tides/Coastal Flooding... Climate...

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