Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 201323 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 923 AM EDT Sun Aug 20 2017 .SYNOPSIS... High pressure will build in from the Ohio Valley today and then move offshore on Monday. A cold front will approach from the Midwest on Tuesday while high pressure remains anchored off the eastern seaboard. The front is expected to move through the area on Wednesday. Canadian high pressure builds into the region late in the week and into next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Only minor updates needed to reflect current temperature and dew point trends, but otherwise the forecast remains on track. The axis of the upper trough will move to the east of the area this morning with flow aloft becomes more zonal behind the passing trough. Meanwhile, surface high pressure will start to build in from the west. Some drier air will be across the region today and with little to no moisture around, we should see plenty of sunshine and no rain. The dry air will be slightly cooler than what was in place the past few days but maximum temperatures will only be about 2-3 degrees cooler than they were on Saturday. Expect highs to rise into the mid to upper 80s across the forecast area with some upper 70s to lower 80s across the southern Poconos and northwest New Jersey. With drier air in place, it will feel much more comfortable outside.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... The center of the high pressure will cross overhead tonight and then push offshore early Monday. Flow across the region will be light and skies should be relatively clear overnight. Cloud cover will start to increase toward daybreak, mainly across Delmarva, as return flow around the high starts to bring some moisture back into the area. Temperatures will respond to the relatively clear skies and light winds overnight and we should radiate pretty well. Lows will drop down into the low to mid 60s across much of the region with some upper 50s across the higher elevations. Temperatures in Philadelphia, nearby suburbs, and along the coast will be slightly warmer and in the mid to upper 60s. Even with moisture starting to increase, do not see much in the way of fog development overnight although a few patchy spots may develop, mainly toward daybreak. && .LONG TERM /MONDAY THROUGH SATURDAY/... High pressure moves offshore on Monday. Southerly return flow around the high develops across the Mid-Atlantic region. Meanwhile, a weak shortwave disturbance embedded in an belt of stronger westerlies residing to the north of the upper ridge is expected to approach from the Ohio Valley during the afternoon and then pass through the area Monday night. A few showers and storms could develop on Monday, especially west of the Delaware Valley (Delmarva-Reading-Lehigh Valley) late in the afternoon and evening when forecast soundings show instability increasing in concert with the arrival of deeper moisture from the southwest. Kept coverage isolated for the afternoon hours with the greater lift from the shortwave trough still to our west. Slightly higher coverage of showers and storms may arrive in our far western zones during the evening. Solar Eclipse Viewing Monday Afternoon... It is important to note the wide range of model solutions for cloud cover across the most of the area, basically ranging from clear skies (with a minimal cumulus deck and thin cirrus clouds) to cloudy skies. In general, RH fields in the 850 mb layer (closely represents the ceiling height of the cumulus field at the top of the mixed layer) for Monday afternoon have trended higher with the most recent 00Z models compared to 24 hours before. This trend seems loosely tied to models showing an earlier exit of the high and a more pronounced warm-air/moisture advection pattern on the western side of the high. Unfortunately, this means we increased cloud cover with the latest forecast update- to partly sunny roughly south/west of Philadelphia and to mostly sunny farther north/east. While confidence in the official (deterministic) sky cover percentages (seen in the NDFD graphics on our website) is are not not particularly high, (in a probabilistic framework) the likelihood of a having a decent view of the eclipse increases the farther north and east you go. Low pressure is expected to track from the Great Lakes into southeastern Canada on Tuesday. A cold front extending southward from the low will move through the Midwest states during this time. The focus for showers and storms on Tuesday will be along a pre- frontal trough to the lee of the Appalachians. Accordingly, the highest PoPs reside across our far western zones in eastern PA and far northwestern NJ. Tuesday is still expected to be the hottest day of the period with highs generally in the lower 90s. Showers and storms across northwestern zones may continue into the overnight and progress farther south/east across the forecast area later Tuesday night as the cold front approaches. The highest PoPs Tuesday night once again reside across the northwestern zones. While showers and storms are forecast with the front moving through on Wednesday, a morning-early afternoon timing of the fropa may prevent us from realizing our full convective potential (in terms of thunderstorm coverage and severity) than if it coincided more favorably with peak heating later in the day. Showers and storms will end from northwest to southeast late in the day Wednesday into Wednesday night as the front moves through the area. Canadian high pressure gradually builds into the area late in the week. Dry conditions and below normal temperatures can be expected Thursday through Friday with high pressure firmly in control of our weather pattern. Highs will be in 70s and lower 80s each day with lows in the 50s and 60s. && .AVIATION /13Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions are expected at the terminals through the TAF period. Relatively clear skies will be across the region today, with just some cirrus around as high pressure moves into the area. Mainly clear skies will continue through tonight. Fog may develop in a few locations overnight tonight but confidence is low and it has been left out of the TAFS at this time. Light winds this morning will become west to northwest by mid morning at around 10 knots or less through this afternoon. A few gusts around 15 knots are possible this afternoon. Winds will subside again and become slightly more southwest this evening before becoming light and variable overnight. OUTLOOK... Monday...Predominately VFR. Isolated showers and storms could develop late in the day and at night, mainly south and west of Phila terminals. Tuesday...Mainly VFR. Isolated to perhaps scattered showers and storms in the afternoon, especially across western terminals (RDG and ABE). Showers and storms could move into I-95 terminals Tuesday night with a cold front approaching from the NW. Wednesday...Showers likely with a chance of storms. Localized sub-VFR restrictions a possibility with this activity. Improving to VFR with showers ending from NW to SE late in the afternoon and evening. Expect a wind shift from SW to NW to occur in wake of fropa. Thursday...VFR and light winds.
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&& .MARINE... Tranquil conditions will continue across the area waters through tonight. Seas will be around 1 to 2 feet. West to northwest winds around 10 to 15 knots will start to turn more to the south/southwest this afternoon and evening. A few gusts around 20 knots are possible through tonight. OUTLOOK... Monday...Winds and seas below SCA thresholds. Tuesday...SCA possible late afternoon and night with S winds increasing to near 25 kt ahead of cold front. Building seas to 4-6 ft are forecast for the Atlantic coastal waters. Wednesday...Winds should decrease to below 25 kt but SCA conditions may continue into the morning as it may take longer for seas in the coastal waters to drop below 5 ft. Thursday...No marine hazards expected. .RIP CURRENTS... Winds will be northwest through this morning before turning to the southwest this afternoon. The overall risk for today looks to be low at both the Delaware beaches and the New Jersey shore but with a more southwesterly flow developing a few of the more prone locations, like LBI, may see a slightly increased risk through this evening. && .TIDES/COASTAL FLOODING... We will be watching the tidal departures closely for the Monday evening high tide, as winds will once again shift to more southerly, and astronomical tides should be relatively high thanks to the new moon on Monday. At this point minor flooding is possible, though some guidance continues to show water levels falling short of minor flooding thresholds. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...Meola/Di Spigna Short Term...Meola Long Term...Klein Aviation...Klein/Meola Marine...Klein/Meola Tides/Coastal Flooding...

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