Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 260850 AAA AFDPHI Area Forecast Discussion...UPDATED National Weather Service Mount Holly NJ 450 AM EDT Wed Apr 26 2017 .SYNOPSIS...
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The coastal low that has affected the East Coast the last 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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 07Z surface analysis shows a 1003 mb low pressure centered over the lower Delmarva peninsula. A clear counter-clockwise circulation is evident on a loop water vapor satellite imagery and even radar imagery with the center of the nearly vertically- stacked low about 80 miles to the E-SE of Wallops Island. Overall coverage of showers have decreased area wide last night but there is still are few areas of showers over S NJ, SE PA and northern Delmarva that seems to be associated with a mid- level shortwave trough/jet streak that is pivoting westward on the northern side of the cyclone. Enhanced convergence near the system`s bent back warm front and inflow of modestly unstable air over the Gulf stream could be responsible for focusing a line of heavier showers that is moving downstream toward the Phila. PoPs were updated for today to reflect a steady decrease in showers from SW to NE today with generally scattered coverage this morning, becoming isolated early this afternoon. It should be noted that guidance, even hi-res CAM models, will likely have a hard time capturing these small-scale features, so monitoring real-time observations had a huge influence in the forecast but continual refinements to the PoPs/weather grids will likely be needed today beyond a few hours out in time. 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). && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/... 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. && .LONG TERM /THURSDAY THROUGH TUESDAY/... On Thursday, our area will be under the influence of return flow ahead of an approaching cold front. 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. Bring 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 /09Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. IFR/LIFR CIGs with stratus will persist this morning. Some gradual improvement in the CIGs to MVFR are expected, possibly as early as as 13-16Z at RDG/ABE and 16-19Z for Phila terminals. ACY/MIV/TTN are the most susceptible out of all the TAF sites to remain IFR for most of the day. A return to IFR is expected with cloud bases lowering below 1 kft AGL sometime this evening. Current LAMP and MET guidance shows further lowering to LIFR between 06-12Z. NE winds 10-15 kt early this morning will gradually weaken over time to 5-10 kt by afternoon. Light and variable winds are expected tonight. OUTLOOK... Thursday...MVFR CIGS could continue into Thursday morning, before improving to VFR during the day. 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. && .MARINE...
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The Small Craft Advisory has been converted to a hazardous seas with winds falling below 25 kt last night. Waves are currently around 7-10 ft and will gradually subside to 4-7 ft late this afternoon and tonight. OUTLOOK... Thursday-Thursday night...Small Craft Advisory for hazardous seas extended 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...
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Tidal departures early this morning are 1 to 1.25 above astronomical prediction. These departures should decrease further today owing to much lighter N-NE winds today compared to yesterday. The upcoming high tide this morning, which is the lower astronomical tide will not be accompanied by another round of minor flooding. We will continue to monitor the subsequent 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.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Dense Fog Advisory until noon EDT today for ANZ431-450>455. Small Craft Advisory for hazardous seas until 6 AM EDT Friday for ANZ450>455. && $$ Synopsis...Robertson Near Term...Klein Short Term...Klein Long Term...Robertson Aviation...Klein/Robertson Marine...Klein/Robertson Tides/Coastal Flooding...Klein

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