Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 251957 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 357 PM EDT Tue Jul 25 2017 .SYNOPSIS...
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High pressure centered over the Great Lakes will track east and move overhead on Wednesday before drifting offshore on Thursday. Low pressure developing over the Great Lakes and Ohio Valley will push a warm front towards the region Thursday night. That low slowly moves across the Northeast through the end of the week. High pressure eventually builds in from the west for the weekend and moves offshore early next week.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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High pressure is centered over the Great lakes and will continue its journey to the Northeast tonight, moving over over our area by early Wednesday morning. Flow around the high will keep us in a more north and east flow regime. Winds will generally become light overnight and any directional component will favor the east. With easterly flow enmeshed over the area, we will see more clouds through the overnight period. Additionally, moisture trapped below the inversion will allow for some light showers/drizzle and some fog to develop. Overall, it will remain cool tonight with lows in the mid to upper 50s over the Poconos and northern New Jersey and low to mid 60s the Lehigh Valley, Delaware Valley, central/southern New Jersey and Delmarva.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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The high will slide across the region on Wednesday. As the high shifts overhead, winds should become more southerly through the day. Skies will remain pretty cloudy at the start of the day as the marine layer wins out. However, we should start to see some clearing out later in the morning and some sun will be present in the region. Temperatures will remain on the cool side and in the 70s. A few locations may break out, especially across the southern and western areas and some lower 80s will be possible there.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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Overview...Unsettled weather on tap for most of the late week period as intensifying low pressure slowly works across the region. High pressure returns for the new week. High pressure continues to drift offshore Wednesday night. Behind the departing high, a warm front lifts north through the region, and a warmer and more humid airmass will spread to the north and east. Highs on Thursday will climb into the low to mid 80s, and surface dewpoints begin to creep back up into the upper 60s and low 70s. Upper trough with several strong shortwaves then approaches ahead of a developing system upstream. That trough should be enough to trigger some late afternoon and evening showers and thunderstorms. From here, there is considerable uncertainty among the models. Low pressure develops over the Great Lakes and deepens over the Ohio Valley. This pushes another warm front into the region Thursday night, and then strong S-SW flow develops along much of the East Coast. Low level moisture increases throughout the region, and PWATs increase to over 2". The main problem is that the placement of the low varies among the models. The GFS keeps the low well south of the region Friday, and then that low passes through the Delmarva and moves offshore Friday night through Saturday morning. The NAM is a bit quicker and a bit farther north, bringing a slug of heavy precip across much of NJ Friday before a secondary low develops and slides across the Delmarva Friday night. It is key to note that the final frame of the 12Z NAM is 00Z Saturday, and the secondary low is right over DE Bay at that time. The 12Z CMC-GDPS is similar to the NAM in that it brings one low across the region during the day Friday with a slug of heavy rain, and then a secondary low sits over the region Friday night through Saturday, finally departing Saturday night. The ECMWF also features a round of precip Friday as one low passes through the region, followed by a secondary low with more precip Friday night and into Saturday. Given the uncertainty, will follow closely to the Superblend guidance, as well as WPC long range guidance. This means likely PoPs Thursday night and Friday, and then chance PoPs Friday night and Saturday. Will continue to mention a chance for thunderstorms, as well as heavy rain. There is the potential for some 2-4 inches of rain, which, in and of itself, could pose flooding problems. But given the mount of rain that fell over the last couple of days, it is important to follow this system, as additional flooding problems may occur during the late week period. Important to note the NAM showing a strong LLJ, generally on the order of 40-50 KT, that does not show up on the GFS. WPC Day 3 outlook (12Z Thursday to 12Z Friday) has most of the Delaware Valley and the Delmarva in a Slight Risk for Excessive Rainfall, and SPC Day 3 outlook (12Z Thursday to 12Z Friday) has the whole CWA under a Slight Risk for Severe Weather. Once this system departs, high pressure builds in from the west by Sunday, and gradually moves offshore through Tuesday. Temperatures will be near to just below normal with relatively comfortable humidity levels.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions this afternoon will start to deteriorate later this evening overnight as the onshore flow continues to stream low level moisture in the region. MVFR conditions overnight in a mix of low clouds and fog. Isolated showers are possible through this afternoon and tonight. Conditions will improve through Wednesday morning becoming VFR everywhere between 13Z-16Z. The winds will pick up out of the east before become more south to southeast through the day. OUTLOOK.... Wednesday night...Potential for MVFR or lower CIGs. Thursday...Potential for MVFR or IFR CIGs in SHRA/TSRA, mainly in the afternoon and evening. Thursday night through Friday night...IFR or lower conditions in moderate to heavy rain. Chance for TSRA as well. Saturday...Lingering sub-VFR conditions possible through the day in -SHRA. Saturday night...Improving conditions. Sunday...VFR.
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&& .MARINE...
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Relatively quiet conditions are expected to continue on the area waters through Wednesday. Seas are around 3 to 4 feet on the ocean with winds around 10 to 15 knots. Winds will lighten up overnight and again be around 10 to 15 knots on Wednesday. OUTLOOK... Wednesday night...Sub-SCA conditions. Thursday through Friday night...Generally sub-SCA conditions with gusts to 20 KT likely. Heavy rain with VSBY restrictions possible. Saturday...SCA conditions possible as low pressure works its way across the waters. Sunday...Improving conditions on the waters. RIP CURRENTS... A moderate risk for the formation of dangerous rip currents is expected for the New Jersey beaches in Atlantic, Ocean and Monmouth Counties owing to an underlying longer period swell and a relatively strong northeast wind (15-20 mph). Farther south along the coast, where the winds will be lighter and backed out of the north or northwest, the risk is lower.
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&& .TIDES/COASTAL FLOODING...
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Astronomical tides will continue to decrease as we get further from the new moon. However, onshore flow continues. Therefore, minor tidal flooding is expected along the New Jersey shore and Delaware Beaches and the nearby back bays. It is much less certain if minor flooding will occur along the shores of the upper Delaware Bay and the tidal Delaware River. For these locations the wind direction is not favorable for minor tidal flooding and unlike the last few days, there won`t be any heavy rain exacerbating already high tides. Therefore, will hold off on an advisory for the tidal Delware River, but will watch tidal levels along the lower Bay very closely this evening.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...Coastal Flood Advisory from 9 PM this evening to 2 AM EDT Wednesday for NJZ012>014-020-022>027. DE...Coastal Flood Advisory from 9 PM this evening to 2 AM EDT Wednesday for DEZ003-004. MD...None. MARINE...None.
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&& $$ Synopsis...MPS Near Term...Meola Short Term...Meola Long Term...MPS Aviation...Meola/MPS Marine...Meola/MPS Tides/Coastal Flooding...

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