Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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616 FXUS61 KPHI 282055 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 455 PM EDT Wed Jun 28 2017 .SYNOPSIS... A broad area of high pressure will slowly drift eastward into the western Atlantic through the end of the week. A cold front will gradually sag southeastward from the Great Lakes and southeast Canada this weekend to the Ohio Valley and southern New England early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Expect the mid level cumulus field to dissipate shortly after sunset. An elongated cirrus shield associated with the mid and upper level low over the Boundary Waters region could move over our region late tonight. Even still, it looks like radiational cooling will be rather efficient over night. Therefore, stayed close to the previous forecast which is slightly below the NAM and GFS MOS guidance especially north and west of the fall line and over the Pine Barrens. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... With the surface high to our east, and increasing pressure gradient in response to the surface low over the Great Lakes region moving further east, expect low level southwesterly flow to develop and increase. In response, we should see a considerable warming trend tomorrow, with highs about 5 to 10 degrees higher than today. Also, late in the day as the low continues to slowly propagate eastward, we could see isolated showers and thunderstorms generally from the Lehigh Valley to NW NJ and further northwest. I have kept it at a slight chance though as the low should stay well to our west through this time and the upper level jet will be too far north to provide much in the way of lift across our region. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
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A long duration of hot and humid weather is in store for the region, with chances of precipitation increasing this weekend with occasional chances continuing next week. Operational models are in reasonably good agreement with the synoptic-scale details through the long-term period. Smaller- scale vorticity maxima within the southwest upper-level flow developing this weekend and large-scale trough passage next week have typical strength/timing differences among the model suite, with no obvious model biases noted. As such, pretty much loaded the grids with a model blend through the long term with minor modifications for better office collaboration, somewhat lower weighting for the 12Z CMC late in the period owing to its increased discrepancies from consensus, and dampened PoPs Monday onward owing to larger uncertainties associated with vort max timing. At 00Z Friday, a potent vort max will be moving through southern Ontario and the adjacent Great Lakes with downstream ridge amplification fully commenced in the Northeast. After a rapid warmup on Thursday, this will be followed by a rapid increase in surface dew points on Friday and Saturday. With highs near or above 90 in the urban corridor, this leads to a rather uncomfortable period for the area. I was a little hesitant to increase dew points to the values of MAV/MEX MOS, which have been a little on the high side in similar patterns so far this spring/early summer. As such, I dropped heat indices somewhat both Friday and Saturday, especially since model soundings show well-mixed soundings during the afternoons. With a rather deep mixed layer, I strongly suspect MOS is overdoing surface moisture, especially on Friday. Much stronger weighting was given to MET guidance Friday and manual adjustment Saturday given NAM BUFKIT soundings. Regarding the aforementioned vort max, as it slides eastward into far northern New England, an upstream vort max will move into the Upper Midwest by Friday afternoon. This will be the start of prolonged southwesterly midlevel flow in the eastern U.S. Several perturbations in the faster flow will progress northeastward from the central U.S. to the Northeast for the following several days. With a strong Bermuda high remaining influential into the southeastern U.S., this will force the associated vorticity maxima to elongate in a positively-tilted orientation as they progress northeastward. Upstream perturbations will then amplify the vorticity with decreasing latitude (farther southward) with time. The result of this above-surface pattern is a frontal boundary moving at a snail`s pace southeastward as midlevel flow becomes parallel to its orientation, broadly from New England to the Great Lakes and southwestward this weekend. Several embedded surface waves of lower pressure will progress northeastward along this boundary, likely enhancing convection in surrounding regions. Precipitation chances increase gradually Friday through Sunday in this regime from northwest to southeast. Models are in reasonably good consensus showing a particularly strong wave moving through the Mid-Atlantic region Saturday afternoon through Sunday morning. This brings about the most challenging aspects of the weekend forecast. The first is highs on Saturday, with models/statistical guidance trending noticeably downward with temperatures on Saturday as potential sky cover/precipitation influences may inhibit maximum warming. The second is figuring out timing/location of highest PoPs. Decided to keep a conservative approach for this forecast, both by lowering max temperatures slightly on Saturday and by capping PoPs to chance Saturday through Sunday. Sunday may end up being fairly dry if the vort max projected to move through Saturday night is followed by fairly strong transient ridging (as depicted to some degree by the GFS and ECMWF). The 12Z CMC, meanwhile, brings a quick-to-follow perturbation through the area on Sunday, generating more convection in much of the region. With so much uncertainty with these small-scale details, it is formidably challenging to get too specific with PoPs at this point. There are decent indications of a push of drier surface air moving into the area by Sunday night as the cold front makes its strongest progress into the region. For early next week, models are showing a somewhat drier day Monday upstream of the perturbations moving through during the weekend followed by a potent trough moving through the Northeast Tuesday and Wednesday. Kept slight chance to chance PoPs for much of the area (highest north) Tuesday and Wednesday, but kept Monday mostly dry given the reasonably strong consensus at this point. However, with the front failing to make progress southeastward through the region, temperatures will remain warm, and humidity will remain high.
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&& .AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mostly VFR conditions are expected through the TAF period. After 18Z Thursday, there is a slight chance of showers and thunderstorms across portions of southeastern PA including KRDG and KABE. However, coverage is too limited to include in the TAFs at this time. We are starting to see a sea breeze develop as of 19Z. This could affect KACY by 21Z (shifting winds to southerly), but is not expected to move much further inland this evening. By 12Z tomorrow though, we should see winds across the region shift to south southwesterly and increase to 10 to 15 kt with gusts above 20kt. OUTLOOK... Thursday night and Friday...VFR conditions expected. Slight chance of storms northwest of KPHL, which may locally lower CIGs/VSBYs. Southwest winds 5 to 15 kts (stronger during the day), with gusts to 20 kts possible. Friday night through Sunday...Mostly VFR, but chances of storms through the period, especially Saturday afternoon through Sunday. Locally lower visibility and ceilings likely. West to southwest winds 5 to 15 kts, with gusts to 20 kts possible during the day (more so Saturday). Sunday night and Monday...A potential dry period, with mostly VFR conditions. Light winds, primarily westerly.
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&& .MARINE...
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Winds and seas will begin to build mid day Thursday. Small craft advisory conditions are expected to develop by mid afternoon along the Atlantic coastal waters. On the Delaware Bay, it is unlikely that winds will get above 25 kt through the day time hours. However, southerly wind gusts above 20kt are possible on the lower bay. OUTLOOK... Thursday night and Friday...Advisory conditions likely, with gusts near gale force possible Thursday evening. Friday night through Sunday...Advisory conditions may linger Friday night, with generally sub-advisory conditions thereafter. However, a period of stronger winds may occur Saturday night as well. Frequent chances of storms through the period. Sunday night and Monday...A drier period may occur. Sub-advisory conditions are expected. RIP CURRENTS... The outlook for tomorrow is that the low risk for the formation of dangerous rip currents will continue into at least tomorrow morning. Depending on how quickly the winds increase tomorrow afternoon, the risk may increase to moderate by late in the day, but that is uncertain at this time. Low risk does not mean NO risk and it is always advised for ultimate safety, swim in the presence of the lifeguards. They have the flotation devices that can more easily save a life. Where the surf zone waters are steeper, there may be fewer rip currents, but there, the danger of wave related injury increases. An ongoing DE 6+ year study shows the 10-20 year and 40-60 year age groups most vulnerable. The 10-20 year olds surf zone injuries are associated with body boarding and body surfing, while the 40-60 year old waders are knocked down by wave action, especially with back turned to the ocean. Males are statistically far more likely to be injured or lose their life in the surf zone but as June 15 reminds us...ANYONE is vulnerable.
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&& .CLIMATE... Monthly avg temp for June PHL projecting 74.5 or 1.2 degrees above the average of 73.3 ABE projecting 70.8 or 1.7 degrees above the average of 69.1 Seven of our eight long term climate sites will average above normal probably from about 8 tenths of a degree for TTN/ILG to as much as 2 degrees for GED. Mount Pocono is our only below normal average and projecting 1-1.5 below). These projections are based on our 330 am forecast. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 2 PM Thursday to 6 AM EDT Friday for ANZ450>455. Small Craft Advisory from 6 PM Thursday to midnight EDT Thursday night for ANZ431. && $$ Synopsis...CMS/Arcomano Near Term...Johnson Short Term...Johnson Long Term...CMS Aviation...CMS/Johnson Marine...CMS/Johnson/Drag Climate...Drag

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