Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 132045 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 345 PM EST Wed Dec 13 2017 .SYNOPSIS... A fast moving low pressure system will move across the area tonight and then offshore Thursday. A ridge of high pressure will build over the area later Thursday through Friday. A low pressure system will move up along the coast Friday night. More high pressure will affect the area much of this weekend. Another few disturbances will be across the area beginning later Sunday and lasting into early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Winds will diminish going into this evening to less than 10 mph shortly after sunset, and then will become light and variable as the evening progresses. Clipper low moving through the Great Lakes will be over the Ohio Valley after midnight, and then will pass through New Jersey and move offshore just after daybreak Thursday. The low looks to weaken as it approaches the spine of the Appalachians between midnight and daybreak Thursday, but seems to strengthen a bit once it is through the Appalachians in the pre-dawn hours. As a result, light snow develops this evening, first over the Lehigh Valley, and then spreads east through midnight. Around an inch or so of snow accumulation is expected over the Lehigh Valley and into the Poconos, but then the snow really gets going as the low intensifies. Best lift and dynamics will be to the north, where an additional 1- 2" of snow is expected across the Lehigh Valley, Pocono Mountains, and into northern New Jersey for a total of 2-3". Temperatures will be cold enough to support 15-20:1 snowfall ratios, so it will not take much liquid QPF to result in accumulating snow. Although borderline for a Winter Weather Advisory, feel it is prudent to issue one for Berks, Lehigh, Northampton, Upper Montgomery, Upper Bucks, Hunterdon, Warren, Sussex, Carbon, and Monroe counties, as the bulk of the 2-3" of snow will accumulate just prior to and during the Thursday morning commute. For the swath in southeast Pennsylvania, including Chester, Delaware, Philadelphia, Lower Montgomery and Lower Bucks counties, 1-2" of snow is expected, and this is close to Winter Weather Advisory thresholds of 2", so the Advisory has been issued based on the impacts of the snow falling during the Thursday morning commute. Parts of northern New Jersey could get close to 2", and for southern New Jersey and the Delmarva, accumulations should be less than an inch, and not nearly as widespread as the accumulations will be to the north. Lows tonight range from the teens in the Poconos to the 20s elsewhere.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/... Snow tapers off fairly quickly after sunrise Thursday morning, but not before dropping up to 1/2" of additional snowfall across northern zones. Behind strengthening and departing low, west to northwest winds will increase to 15-20 mph with gusts up to 25 mph starting in the late morning hours. The pressure gradient relaxes during the afternoon. Temperatures moderate a bit Thursday afternoon compared to highs today, but will still be below normal. && .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... Weak high pressure will be across the Middle Atlantic region Thursday night and Friday. It will bring mostly fair and continued cool weather to the area. Temperatures will remain some 5 to 10 degrees below normal. A quick moving low pressure system will move just off the NJ/DE coastal areas Friday night. This could bring some light precipitation (most likely snow) to parts of SE NJ Friday evening. Overall, confid in this is low however with the system looking disorganized and the trends are to minimize any qpf over the area. We`ll just keep the slgt chc pops for now, with this low and an upper trough crossing the area. Much of the upcoming weekend will be fair, with high pressure building over the area. Sunny weather is expected most of Saturday and much of Sunday before clouds increase later Sunday. Temperatures will still be a little below normal Saturday before moderating back to normal levels Sunday. A few showers are possible Sunday afternoon in a warm advection pattern, but again confid in this is rather low attm, so we`ll just have slgt chc or small chc pops only for this time. A better chc for rains (a some higher elevations snows) will develop Sunday night and last into Monday as a upper trough begin to affect the area. We`ll have chc pops for Sunday night and slgt chc pops for Monday. We`ll carry the low pops into Tuesday as well, with the different models having varying solutions for this period. Most of the precip will be rain with the temperatures Monday and Tuesday having moderated back to above normal by then. A few snow showers across the southern Poconos are possible however. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Snow develops this evening, resulting in MVFR/IFR CIGs and IFR VSBYs at KRDG-KABE-KTTN-KPNE-KPHL, though the lowest CIGs/VSBYs will be at KRDG/KABE, where the snow will be heaviest. At these terminals, around 2" of snow will accumulate. At KTTN-KPNE-KPHL, around an inch of snow is expected. For KILG-KMIV-KACY, snow will not be as widespread and long-lived, but brief IFR conditions are possible. NW winds 15-20 KT with gusts up to 30 KT will diminish fairly quickly to 5-10 KT early this evening, and then will become LGT/VRB tonight. Thursday...Snow tapers off fairly quickly from west to east just after 12Z Thursday, and conditions improve to VFR between 13-15Z. Winds shift to the NW and increase to 10-15 KT with 15-20 KT gusts, perhaps a few KT higher at KACY. Outlook... Thu night thru Fri...VFR. Fri evening thru Sat morning...Mostly VFR. Few showers psbl. Sat afternoon thru Sunday...Mostly VFR. Sunday night thru Monday...Mostly VFR. Slgt chc showers. && .MARINE... Gales will diminish fairly quickly going into this evening over DE Bay and the DE coastal ocean waters, and SCA conditions expected for the rest of tonight. There may be a brief lull in SCA conditions on DE Bay. For the NJ coastal ocean waters, gales continue through this evening, before diminishing to SCA levels. As low pressure approaches from the west, there looks to be another surge in Gale force winds late tonight and into Thursday morning. Otherwise SCA conditions will prevail through Thursday. Snow will develop late tonight and into Thursday morning, mainly on the NJ coastal ocean waters, and this will result in VSBY restrictions. Outlook... Thu night...SCA conditions early, diminishing. Fair, Fri thru Fri evening...Sub-SCA. Chc rain/snow. Fri night thru Sat...SCA with low end Gale psbl. Chc rain/snow. Sat night thru Sunday...Mostly sub-SCA. Mostly fair. && .TIDES/COASTAL FLOODING... The strong west to northwest winds have lowered water levels on the tidal Delaware River. Although winds will diminish this evening, water levels may be low enough (i.e., around or below 2 feet below MLLW) to cause some navigation issues during low tide into this evening. We have issued a special weather statement to address this concern. Water levels will return to more typical values later tonight. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory from 8 PM this evening to 10 AM EST Thursday for PAZ054-055-060>062-070-071-101>106. NJ...Winter Weather Advisory from 8 PM this evening to 10 AM EST Thursday for NJZ001-007-009. DE...None. MD...None. MARINE...Gale Warning until 10 PM EST this evening for ANZ450>453. Gale Warning until 6 PM EST this evening for ANZ430-431-454- 455. && $$ Synopsis...O`Hara Near Term...MPS Short Term...MPS Long Term...O`Hara Aviation...MPS/O`Hara Marine...Miketta/MPS/O`Hara Tides/Coastal Flooding...

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