Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 140240 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 940 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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Mesoscale update #2: Increased 700 mb FGEN forcing has been sufficient to develop an area of snow across Delmarva this evening. Radar is filling nicely with snow quickly reaching the ground across most of that area. Moderate snow is now occuring in Millville, NJ as well. Precipitation should continue to advance northeastward over the next couple of hours. Little change from below. Mesoscale update #1: Modeling has really keyed on a band of snow to form in association with WAA coming northward. Snow could be moderate at times with this band given the lift in place. Soundings also a quick saturation and not that deep of a dry layer, so virga concerns are more limited. Also the snow growth region this evening is favorable for higher ratios in the 15:1 range as this band moves northeastward. Timing looks to be around 10 pm in the Delmarva and Eastern PA and closer to Midnight as you get further northeast into New Jersey. Given slightly increased QPF based on the mesoscale modeling snowfall, amounts are now forecasted to be a touch higher in most locations. A quick inch or even slightly more could fall in a two to three hour period. Roads will likely get slick and hazardous quick this evening and we have expanded the advisories eastward some. Overnight, modeling shows a dry slot working northward through Delmarva and into SE PA/NJ before additional snow comes in across the advisory area by sunrise. Near term summary: 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 the Lehigh Valley into northern NJ 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 and in New Jersey near I-295, including Chester, Delaware, Philadelphia, Lower Montgomery and Lower Bucks counties, along with Camden/NW Burlington NJ areas 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 /03Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Snow develops this evening with MVFR ceilings and IFR/LIFR visibilities. The LIFR visibilities will be confined to a period of generally moderate snow moving northeast across the region from 2-5Z. Snow will accumulate to runways and average an inch or so for most TAF sites with amounts around two inches for KABE and KRDG. 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. Moderate confidence with snow higher confidence with winds. 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 15-25 KT gusts, perhaps a few KT higher at KACY. High confidence. 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...
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SCA conditions expected for the rest of tonight as winds have decreased. There may be a brief lull in SCA conditions on DE Bay. SCA conditions will prevail through Thursday. Gusts may increase close to gale force again as we head into 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.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Winter Weather Advisory until 10 AM EST Thursday for PAZ054- 055-060>062-070-071-101>106. NJ...Winter Weather Advisory until 10 AM EST Thursday for NJZ001- 007>010-012-015-017>019. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Thursday for ANZ430-431- 450>455.
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&& $$ Synopsis...O`Hara Near Term...Gaines/MPS Short Term...MPS Long Term...O`Hara Aviation...Gaines/MPS/O`Hara Marine...Gaines/Miketta/MPS/O`Hara

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