Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 180727 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 327 AM EDT Tue Jul 18 2017 .SYNOPSIS...
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A weak front to our west will gradually dissipate through Wednesday as it slowly shifts eastward, meanwhile weak high pressure builds to our southwest during Wednesday. A cold front slowly moves across our area Thursday night and Friday before stalling near Delmarva into Saturday. This front however moves back northward as a warm front late Saturday into Sunday, then a cold front arrives on Monday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Patchy fog and stratus through just after sunrise this morning. Visibility may drop to less than a mile in some spots, but not expecting widespread dense fog that would require a Special Weather Statement or a Dense Fog Advisory. Skies clear out from mid-morning on. Upper trough lies across the East Coast, but H5 low pinches off and becomes a closed low over the Mid-Atlantic. Hot and humid conditions will be in place across the region as surface high pressure builds in from the west. Highs today will top off in the mid to upper 80s across most of the forecast area, except highs may come close to 90 in/around Philly, and will be in the low 80s along the coasts and in the mountains. Surface dewpoints will be in the upper 60s to low 70s, resulting in heat indices in the mid 90s on the I-95 corridor from around Trenton to Wilmington, and in the mid and upper 90s in the Delmarva away from the coast. This results in SB CAPE values from around 1000 J/kg to as high as 1500 J/kg. Some weak mid and upper level shortwave energy will dive into the base of the upper trough, and this may be enough to touch off some afternoon showers and thunderstorms for the forecast area mainly north and west of I-95. The Lifted Index will only be around - 1C, and there should not be much upper level flow with 0-6 km Bulk Shear around 10 KT. So any storms that develop may produce locally heavy rain, but will be slow to move, and may result in localized poor drainage flooding.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
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Any showers and thunderstorms will taper off in the evening with loss of diurnal heating. Skies clear out and winds become nearly calm. With abundant low level moisture in place, patchy fog and status likely to develop once again. Lows range from the upper 60s to low 70s, except in the mid 70s in Philly.
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&& .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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Summary...Hot and humid for much of the region through the weekend, although some local cooling takes place at times due to either some convection and/or the placement of a southward moving front Friday and Saturday. Synoptic Overview...As a ridge across the southern states shifts around through this entire period, a strong closed low centered near Hudson Bay Canada gradually shifts east-southeastward. This will push the northeastern part of the ridge southward later this week and into the weekend, with some help also from a short wave trough in the Midwest. This latter feature is forecast to amplify across the Great Lakes over the weekend and into the Northeast Monday. This will drive surface low pressure eastward with a cold front. This setup favors a hot and humid pattern across our area. The forecast challenge is centered mostly on the convective potential and timing. We used a model/continuity blend for Wednesday through Thursday night, then blended in the 00z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Wednesday and Thursday...Our region generally is within an area of weaker flow Wednesday as an upper-level trough is over the Mid Atlantic and a ridge to its west. There is a weakening front or surface trough that slowly shifts eastward Wednesday, and probably reaches the coast at night before potentially dissipating. An axis of some colder air aloft within the lingering upper-level trough in combination with the surface front/trough should serve enough as a focus for some mostly afternoon and evening convection. We cannot rule out a few pulse storms that are much stronger, however overall the weak flow should limit the threat to locally heavy rain. The coverage looks isolated to scattered therefore kept PoPs no higher than the lower chance range for now. For Thursday, the convective potential looks less certain as not much may happen until energy approaches from the Great Lakes late in the day along with a cold front. Much of the area may have some capping. Some guidance suggests that an MCS may develop in the Great Lakes region Thursday which then tracks east-southeast and across our northern areas at night. This scenario looks to depend on a short wave that ejects across the Northern Plains along the northern periphery of a ridge. For now, kept any PoPs to the afternoon and evening hours. It will be hot both days with daytime temperatures topping out in the low to mid 90s for much of the area, and any relief comes from cooling thunderstorms (and right at the coast). It will be humid as well although the dew points may lower some in the afternoon, especially Thursday, with plenty of boundary layer heating and the flow turns west to northwest for a time. This of course will have an impact on the peak heat indices, which as of now are forecast to remain just below advisory criteria. The heat index though will approach 100 degrees for a time each afternoon in the more sensitive I-95 urban corridor from Wilmington to Philadelphia to Trenton. For Friday and Saturday...Despite a cold front settling southeastward across the area Friday and stalling nearby, there is not much cooling behind it. It looks to be mostly a dew point front with less humid air lurking to the north and trying to settle southward at least into the northern half of the area. Friday may end up being dry, although this may be dependent on a possible MCS in the morning. As a short wave trough digs southeastward from the Midwest Saturday, downstream warm air advection and backing flow will allow the front to lift northward as a warm front. This could result in some convective development on Saturday. For Sunday and Monday...As an upper-level trough amplifies some across the Great Lakes into the Northeast, surface low pressure tracks to our north. We are anticipating that our region is within the warm sector Sunday as the warm front shifts to our north early on. The associated cold front should arrive Sunday night but not settle across our area until during Monday. Given increasing ascent from the incoming upper-level trough and the cold front moving through, some convection could be locally strong. The details of the convective evolution this far out is less certain as the guidance shows some differences with the amplitude of the upper-level trough. We kept in chance PoPs for most areas during this time frame. Peak heat indices will approach heat advisory criteria Sunday afternoon mainly from the Philadelphia metro area on south and east.
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&& .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. IFR and possibly lower conditions likely through after daybreak in fog/stratus, then VFR conditions. Scattered afternoon showers and thunderstorms may impact terminals north and west of I-95 with brief MVFR or lower conditions. KABE/KRDG most likely to be impacted, but chances of a storm over any given terminal are too low to warrant including mention of SHRA/TSRA in the TAF. Any SHRA/TSRA taper off in the evening. Patchy fog/stratus possible once again after 04Z. OUTLOOK... Wednesday and Thursday...Local MVFR/IFR possible each early morning due to fog/low clouds, otherwise mostly VFR. Some mainly afternoon and evening showers and thunderstorms are possible. South to southwest winds 10 knots or less, becoming west-northwest Thursday. Friday...Mainly VFR. West-northwest winds 10 knots or less. Saturday...Mainly VFR. Some afternoon showers and thunderstorms possible resulting in local restrictions at times. Light winds becoming south or southeast less than 10 knots.
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&& .MARINE...
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Surface high pressure builds in from the west as an upper trough remains along the East Coast. This results in a S-SE flow today through tonight with winds 10-15 KT across northern NJ ocean waters this afternoon, and 5-10 KT otherwise. Seas remain 2-3 feet. Isolated late afternoon/early evening showers and thunderstorms possible. Patchy fog this morning, and then again after midnight tonight, may result in localized VSBY down to 1-3 NM. OUTLOOK... Wednesday through Saturday...The conditions are anticipated to be below Small Craft Advisory criteria. Some fog may occur some mornings within a warm and humid airmass. Some showers and thunderstorms will be possible at times, especially later in the afternoon and evening time frame. Some thunderstorms may produce locally gusty winds, however coverage and timing is less certain. RIP CURRENTS... Thanks to a slight onshore flow, and an underlying long period swell, there is a moderate risk for the formation of dangerous rip currents along the New Jersey shore today. For the Delaware Beaches, winds are expected to be slightly off shore, leading to a continued low risk for the formation of dangerous rip currents.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse Near Term...MPS Short Term...MPS Long Term...Gorse Aviation...Gorse/MPS Marine...Gorse/MPS

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