Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 100249 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 949 PM EST Sat Dec 9 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Untreated surfaces will be slick tonight as temperatures are below freezing. Be especially cautious driving on bridges and overpasses. Low pressure will continue to move farther away from our area tonight as it strengthens. An upper air analysis shows a potent 250 mb jet across the Mid-Atlantic and Northeast, with our region within the right entrance region. In addition, strong positive vorticity advection is occurring downstream of a potent short wave. There are also areas of frontogenetic forcing within the 700 mb to 850 mb layer. This forcing continues to result in several bands oriented south-southwest to north-northeast, which is producing more focused heavier snowfall. These bands continue to shift eastward and will weaken with time as the aforementioned 250 mb jet shifts east and our region moves out of the favored right entrance region. This process is ongoing at this time as the snow is ending from the west. Therefore the snow will continue to end from west to east this evening into the overnight hours. Since the snow is ending across the western areas, the Winter Weather Advisory is cancelled for several counties. However, untreated surfaces will continue to be slippery. The rest of the Winter Weather Advisory continues for now, and the Winter Storm Warning will be kept since the snow there will be ending over the next few hours. Otherwise, some adjustments to the PoPs, temperature, dew point and wind grids to keep them current. The PoPs may need to be lowered eastward a bit faster with later updates. No major changes to the low temperatures at this time. The main upper-level trough arrives over our area overnight, however snow showers with it are expected to remain well to our west.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... A general trough of low pressure will still be over the region on Sunday leading to a mix of sun and clouds across our area. Some moisture off of the Great Lakes has an opportunity to interact with the higher terrain of the southern Poconos and NW NJ, leading to a few snow showers and minor accumulations. However, the wind direction looks too westerly for a more widespread event. Winds will actually be the main story with bufkit analysis continuing to show the potential for 20-25 mph gusts in the afternoon. Temperatures will warm into the 30`s, likely a few degrees below a MET/MAV blend based on the snow cover. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... 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. && .AVIATION /03Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Improving conditions for KRDG and KABE this evening, then this spreads eastward into the overnight hours. The MVFR/IFR ceilings and/or visibilities will therefore improve to VFR as snow ends from west to east, mostly everywhere by 06z. Northwesterly winds 10 knots or less, becoming westerly. Sunday...VFR. Westerly winds 10-15 knots mainly from late morning through mid afternoon, with gusts up to 25 knots at times. 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...
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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.
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&& .CLIMATE...
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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...Winter Weather Advisory until 1 AM EST Sunday for PAZ070-071- 102-104-106. NJ...Winter Weather Advisory until 1 AM EST Sunday for NJZ008>010- 012>022-025>027. Winter Storm Warning until 1 AM EST Sunday for NJZ023-024. DE...Winter Weather Advisory until 1 AM EST Sunday for DEZ001-002- 004. Winter Storm Warning until 1 AM EST Sunday for DEZ003. MD...None. MARINE...Small Craft Advisory until 5 PM EST Sunday for ANZ430-431- 450>455.
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&& $$ Synopsis...AMC Near Term...Gorse Short Term...Gaines Long Term...AMC Aviation...AMC/Gaines/Gorse Marine...AMC/Gaines Climate...

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