Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182234 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 634 PM EDT THU AUG 18 2016 .SYNOPSIS...
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A weakening front will stall to our south into the weekend as high pressure tries to build in from the Canadian Maritimes on Saturday. Low pressure will deepen as it moves northeastward through the Great Lakes and eastern Canada Saturday night into Sunday. The attendant cold front will progress through our region Sunday and Sunday night. High pressure will then build in from the west during the first half of next week.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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An upper air analysis indicated a weak short wave centered near southern New England early this evening. This extended back into north-central Pennsylvania. The combination of this weak short wave with west-northwest flow aloft and instability to around 1000 J/KG is supporting mainly isolated showers from portions of northeastern Pennsylvania to north of Baltimore, MD. There has been little to no lightning and given the time of day and weak forcing, these small convective cores should start to diminish as they slide southeastward. We therefore kept some slight chance pops in for awhile longer for isolated activity mainly north and west of I-95, however removed the thunder mention. Outside of the showers, the cumulus has been dissipating from east to west and the sky cover grids were therefore adjusted downward. The combination of light winds overnight and not a lot of clouds may result in light patchy fog in the outlying areas. The hourly temperature, dew point and wind grids were adjusted with the latest observations, then the LAMP/LAV guidance was blended into continuity through the evening.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... Friday is expected to be dry across the area, even though there will be a weak surface trough across the area and a short wave/vorticity impulse will be sliding across the southern areas. This is mainly expected to bring an increase in cloud cover through the day, although it would not be surprising to see an isolated shower pop up during the day, especially across the south closer the the short wave, but chances are low enough to keep out of the forecast. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Friday night through Saturday night...The forecast area will be situated between a stationary front to our south across VA into the central Appalachians, high pressure to our northeast across the Canadian Maritimes into coastal New England and an area of low pressure to our northwest. The boundary-layer flow is expected to be predominately onshore (easterly, becoming southeasterly late Saturday) during this time while the ridge associated with the surface high noses southwestward toward the area and the front stalls to our south. Since the onshore flow will be light and warm SSTs in the nearby Atlantic, temperatures will still be near to slight above normal- lows in the low to mid 70s Friday and Saturday nights; highs in the 80s. The warmest spots in the cities could reach 90F on Saturday if marine stratocu isn`t stubborn to mix out. There may be a few shortwave troughs embedded in the stronger westerlies within the mid- and upper-levels that pass through the region during this time. Forcing from these disturbances could result in convective development, especially if the passage coincides favorably with peak diurnal heating each afternoon and evening. It should be noted that the subtleties of each one of the small-scale shortwave disturbances could easily get lost in the models; thus the predictability is limited. Kept PoPs low (mainly isolated coverage showers and storms) due to the inherent forecast uncertainty and lack of a deep/organized lifting mechanism. Sunday has the potential to be an active weather day across the mid- Atlantic region as a potent shortwave digs around the backside of a northern stream trough over the Midwest and Great Lakes. The upper trough is forecast to undergo amplification and acquire a neutral or even negative tilt just upstream of us. At the surface, a strong low pressure system for this time of year is forecast to cut well to our northwest through the Great Lakes and into eastern Canada. The system`s cold front will approach from the west Sunday afternoon. In response to the strengthening/backing southerly flow aloft, the stationary front to our south may begin to lift back northward as a warm front. Strong height falls, well-defined upper diffluence seen ahead of the trough are supportive of deep organized synoptic lift. There also looks to be a well-defined atmospheric river with a deep tropical moisture connection to the Pacific Ocean and Gulf of Mexico (and even Atlantic Ocean near the surface) ahead of the trough. With good model agreement, PoPs were increased to 60-70 percent for Sunday-Sunday night. The above setup favors a potential risk for heavy rainfall and severe thunderstorms on Sunday, especially with model consensus showing a favorable timing of day for convection to peak ahead of the cold front with a late afternoon or evening arrival of the front. However, there is still enough spread in the models and uncertainty in how the mesoscale details play out this far out for things to change. Additionally, there is uncertainty regarding how much northward progress the previously stalled front over VA makes during the day Sunday or if it remains stationary to our south before the main cold front catches up with this downstream boundary. This would have important implications on our weather as backed flow near the triple point could enhance the directional shear and provide a more favorable setup for tornadoes. Will highlight these threats in the HWO with emphasis on uncertainty. Monday through Thursday...High pressure builds in behind the departing cold front and any lingering showers will dissipate by late Monday morning. The high continues to build over the area through the week. Cooler air moves into the region and maximum temperatures will range from the mid 70s across the northern zones to mid/upper 80s across the southern zones. Dew points will drop considerably and we will see some much more comfortable and dry conditions across the region. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Isolated showers through early this evening mainly north and west of PHL, otherwise VFR. Some local MVFR visibility due to fog however may develop overnight, mainly at MIV, RDG, and TTN. Northwest winds up to 10 knots, becoming light and variable through the overnight at most terminals. Friday...VFR overall. Northwest winds 5-10 knots. OUTLOOK... Friday night...VFR. Saturday and Saturday night...Generally VFR. Slight chance of showers and thunderstorms, mainly during the afternoon. Sunday and Sunday night...Showers and thunderstorms likely with a cold front moving thru. Sub-VFR likely in heavier showers and storms. Monday...Mainly VFR conditions expected. Showers possible during the morning but improving through the day. Northwest winds with gusts of 15 to 20 knots. Tuesday...VFR conditions expected. Light north to northwest winds less than 10 knots. Sea breeze possible at KACY.
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&& .MARINE...
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Sub-Small Craft Advisory level winds will continue across the waters tonight through Friday. OUTLOOK... Friday night through Saturday night...Winds and seas below SCA threshold. Onshore flow 10-15 kt expected for most of the period. Sunday and Sunday night...Winds back out of the S or SE and strengthen ahead of a cold front Sunday. Winds shift out of the W behind the front late Sunday night. Winds and seas just below SCA levels. Monday and Tuesday...Mainly sub advisory conditions expected across the area waters through Tuesday. Northwest winds with gusts up to 20 knots. Seas around 2 to 4 feet. Seas may briefly near 5 feet on the eastern portions of the coastal zones. RIP CURRENTS... The risk for rip currents is low through this evening. It is expected to remain low through Friday.
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&& .EQUIPMENT...
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Due to equipment failure (A/C unit and computer), the KDIX WSR-88D radar remains out of service. Return to service is unknown at this time. KDOX and TPHL radars will help fill in the radar coverage gaps across southeast PA, southern NJ and Delmarva.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse/Klein Near Term...Gorse Short Term...Robertson Long Term...Klein/Meola Aviation...Gorse/Klein/Meola Marine...Klein/Robertson/Meola Equipment...

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