Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 092301 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 601 PM EST Sat Dec 9 2017 .SYNOPSIS...
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Low pressure off the Mid-Atlantic coast will deepen as it moves northeast tonight 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, as of 5 PM, was located east of the New Jersey coast. This low will strengthen as it tracks northeastward through tonight. 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 are slowly shifting eastward and should weaken later this evening as the aforementioned 250 mb jet shifts east and our region moves out of the favored right entrance region. We therefore continue with ending the snow from west to east later this evening into the overnight hours. However, did increase the snow amounts a little and included additional accumulations in the advisory/warning segments. The advisory and small area of a warning all goes through 06z/1AM to simplify it. The western edge of the advisory in Pennsylvania was to end at 00z/7PM, however given ongoing snow with some heavier bands this was extended. Otherwise, some adjustments to the PoPs, temperature, dew point and wind grids to keep them current. The HRRR was used to help assist with some of the PoP adjustments.
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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 /00Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...MVFR/IFR ceilings and visibilities with snow. LIFR visibilities will occur at times with moderate snow. Gradual improvement to VFR after the snow moves east of the area later this evening and into the overnight hours. The snow should end by 06z. North to northwest winds 10 knots or less, becoming westerly late. Sunday...VFR. Westerly winds 10-15 knots from late morning through the 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 now in effect through Sunday for the ocean and Delaware Bay. Seas will continue to build this afternoon and evening. 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 over 25 knots are likely at several intervals through Sunday. Visibilities may be reduced in rain (mixed precip close to the coast) through the evening. 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... Record daily snowfalls for the date are listed below for Saturday December 9. Note...compaction and melting on contact may limit amounts below our forecast in some of our forecast area. ACY 3.5-1933 PHL 2.9-1942 ILG 5.0-1928 ABE 9.1-2005 && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Winter Weather Advisory until 1 AM EST Sunday for PAZ060>062- 070-071-101>106. NJ...Winter Weather Advisory until 1 AM EST Sunday for NJZ001- 007>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...Winter Weather Advisory until 1 AM EST Sunday for MDZ008-012- 015-019-020. 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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