Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 100553 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1253 AM EST Sun Dec 10 2017 .SYNOPSIS... Low pressure will deepen as it continues to move away from our area overnight and into the Canadian Maritime region on Sunday. Weak high pressure over the area on Monday will be followed by low pressure on Tuesday, with an associated arctic front ushering in the coldest air of the season so far by Wednesday. Another fast moving low pressure system may affect the area Thursday night into Friday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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A very interesting first winter storm of the season. With the aid of very strong and deep large-scale ascent from the right entrance region of an intense upper-level jet streak and broad frontogenetical forcing in the 850-700 mb layer, bands of snow readily developed during the day across the area and persisted with little movement for quite some time. Last night`s NAM Nest identified one of these regions quite well (i.e., the Lehigh Valley into northwest New Jersey), though it was overdone with totals to the south and east. Unfortunately, there was little evidence of this in much of the other guidance until shortly before the event, with the coarser GFS (among others) much too dry in this area with the 00Z/9 simulation. The trends of consensus were generally in the right direction (higher totals to the north; lower to the south) last night, but still way too dry/low north/west of Philly and a little on the wet/high side to the south and east. One thing that most of the hi-res models appeared to pick up on reasonably well was the character of the snow through the event, with a period of partial melting of the snow during the afternoon (denoted by lower snow ratios and a lack of dendritic snow, with even a little sleet observed in some locations) straddled by two more dendritic periods along and east of the urban corridor, the latter of which certainly aiding snow totals along and east of the Delaware River. Farther to the west, the character was mostly dendritic for a longer portion of time, which (along with the favorable mesoscale banding), allowed for healthy snow ratios and resultant snow accumulations. Aside from a fast-moving band of snow in far southern New Jersey, the accumulating snowfall has ended across the area. Have cancelled all winter headlines with the 12:30 am update. The forecast is far from easy for the rest of the night. Skies are rapidly improving in the western CWA, and this is permitting temperatures to fall rapidly as drier air slowly seeps into the area. Patchy fog is developing west of the area, and visibilities are starting to come down in Berks County. If current trends continue, will need to add some fog mention in the grids for the rest of the night. Moreover, if fog becomes dense enough (as it has to the west in patchy locations), freezing fog will be possible. Will monitor trends closely for the next few hours to see if this becomes a more substantial threat. Otherwise, made some edits to temperatures and sky cover grids to match latest trends. Generally, this involved more rapid improvement to sky cover overnight, with a downward adjustment to temperatures as well (albeit minor).
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/...
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Potent vort max responsible for the winter storm Saturday will be moving northeastward away from the area today with quasi- zonal midlevel flow in its wake. Models indicate little in the way of smaller-scale perturbations moving across the region today. As a result, opted to reduce PoPs across the northwest today, as there is fairly reasonable agreement favorable ascent will be too far to the north and west. Skies should be variable today, with most model soundings suggesting at least partial (if not substantial) sunshine for most of the area, save for the Poconos and vicinity. There may be the development of scattered to broken low to midlevel clouds (which models have tended to underforecast this season so far) with the low-level cold air advection allowing for decent boundary-layer mixing today. This should also keeps west winds fairly elevated today, with gusts 20 to 30 mph possible across the area during the late morning and afternoon hours. Temperatures will struggle to warm with fresh snow cover and cold air advection (at least to begin the day). If clouds are more prevalent, this would slow warming even more. For now, kept temperatures below a consensus statistical blend by about 1 to 2 degrees across the area. Am a little worried this is still too optimistic in the northern CWA, where cloud cover may be more persistent and snow cover is generally higher than models are accounting for.
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&& .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
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The pattern for the extended forecast period features a longwave trof over the eastern US with several fast-moving shortwave trofs rotating through it. Partly due to their rapid movement these systems will not be able to pick up much moisture, but temperatures will be generally cold enough for any precip to fall as snow or at least a mix. An arctic surge around mid- week will bring the coldest airmass of the season so far into our area. Some moderation is possible by next weekend. Weak high pressure over the area on Monday should provide fair wx with near normal temps. Then Monday night into Tuesday low pressure is forecast to develop over the lower Grtlks and intensify as it moves east into New England. The associated strong cold front is forecast to cross the mid-Atlantic region Tuesday morning. Some WAA is forecast ahead of this front along with possible rain or snow showers, although mainly snow snow is expected N/W of PHL. Behind the front strong CAA and gusty NW winds will persist through Tuesday night and Wednesday. A weak ridge of high pressure with fair wx is forecast for THursday but temps should remain well below normal. Another fast moving system may bring snow showers to the area Thursday night, but forecast PoPs are only in the 20-30 percent range.
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&& .AVIATION /05Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Improvement to VFR expected; however, periods of sub-VFR VSBYs may occur with patchy fog as sky conditions improve. Any lingering snow at KMIV and KACY will end by 08Z. Winds generally west at 5 to 10 kts. Confidence high. Sunday...Generally VFR, though there may be a SCT-BKN cloud deck around 3000-5000 feet, especially northwest of Philly. West winds will increase to 10 to 15 kts with gusts 20-25 kts. Confidence medium. OUTLOOK... Sunday night and Monday...VFR. West wind. High confidence. Monday night...Restrictions psbl in areas of rain or snow. Average confidence. Tuesday...Mainly VFR with a possible rain or snow shower north. Afternoon Winds becoming west and gusting 25-30 kt. High confidence. Tuesday night and Wednesday...VFR WIND with flurries or snow showers possible e PA and nw NJ. West-northwest gusts 35 kt with isolated 40 kt possible, especially Wednesday. High confidence. Thursday...VFR conditions expected. Average confidence.
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&& .MARINE... SCA is in effect through Sunday for the ocean zones and Delaware Bay. Seas will continue to build some tonight. The pace indicated is based on obs which have been faster than the wavewatch guidance this afternoon. While a lull in the winds may occur tonight, westerly wind gusts of 25-30 knots are likely at several intervals through Sunday. OUTLOOK... Monday...Sub-SCA with fair weather. West wind. Mon night..Rain or snow likely and sub-SCA. Tuesday...SCA probable with increasing west winds and building seas. Tuesday night and Wednesday...Gale northwest wind 35-40 kt gusts as the coldest air of the season arrives. Wednesday night...Gales diminishing to SCA conditions. Thursday...Sub-SCA expected. && .CLIMATE...
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This section will be updated by 4 am. Record daily snowfalls for the date are listed below for Saturday, December 9th. Snowfall through 7 PM ACY 3.5-1933 1.2 PHL 2.9-1942 3.3 ILG 5.0-1928 3.3 ABE 9.1-2005 4.1
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 5 PM EST this afternoon for ANZ430- 431-450>455.
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&& $$ Synopsis...AMC Near Term...CMS Short Term...CMS Long Term...AMC Aviation...AMC/CMS Marine...AMC/Gaines Climate...Staff

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