Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 172239 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 639 PM EDT Sat Mar 17 2018 .SYNOPSIS... Low pressure over the Tennessee Valley moves off the Mid- Atlantic coast tonight. High pressure currently over the Northern Plains states will gradually build to the south and east on Sunday, and will move off the Mid-Atlantic coast sometime Monday or Monday evening. Canadian high pressure then re-establishes itself north of the Great Lakes. Meanwhile, low pressure currently over the Southern Plains will build east and emerge over the Mid-Atlantic Monday night. It then intensifies as it moves out to sea on Tuesday. Another low may form over the Mid-Atlantic Wednesday or Wednesday night. High pressure returns from the north to close out the work week. Low pressure then approaches next weekend. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... After some light precipitation this afternoon/early evening, quiet conditions are expected overnight. Light rain associated with an approaching short wave/vorticity impulse and low pressure sliding eastward along a frontal boundary to our south will continue to move east late this afternoon into this evening. Once this short wave passes to our east, any precipitation will come to an end around sunset. A backdoor cold front is then expected to approach from the north this evening, and may stall across the area overnight, although it will remain dry. Only an increase in mid-high level clouds is expected overnight across the northern half of the area. Overnight lows are expected to remain few degrees below normal. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... Sunday is expected to be another quiet day. The backdoor cold front that pushes into the area overnight remains stalled out across the area, before an area of low pressure approaches later in the day, which will begin to push the front fully across the area. Sunday is expected to be dry, with very little in the way of cloud cover expected. Daytime highs are expected to remain a couple of degrees below normal. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Overview: Complex storm system still on track to impact the region Tuesday through Wednesday, but details are still uncertain. 12Z GFS and 12Z ECMWF keep the storm and most of the precip to the south, as well as the 12Z CMC-GDPS. The 12Z NAM, however, is somewhat of an outlier, bringing the heavier and more widespread precip farther north. High pressure builds over the over the region Sunday night builds to the south and east and moves off the Mid-Atlantic coast on Monday. Conditions will be dry through Monday. Meanwhile, low pressure over the Midwest on Monday will track to the east through the Tennessee Valley Monday night and will emerge between the North Carolina coast and the Delmarva Peninsula by Tuesday morning. At the same time, high pressure centered over central Canada will press to the south and east and will be centered just north of the Great Lakes Tuesday morning, and this high slowly builds east during the day. In terms of sensible weather and QPF there are still too many variables to determine what will happen with this storm. There are several things going against a significant snowfall: 1) A lack of Arctic air in place ahead of the storm. 2) Higher late March sun angle (Tuesday is the First Day of Astronomical spring). 3) The position of the high to the north is too far west to lock in cold enough air at the surface. That does not mean that we are out of the woods, however. The 12Z NAM, though an outlier, is a bit farther north with the position of the low and has the high a bit farther to the south and east. This brings more in the way of cold air into play, and brings the heavier precip up into central and NJ and southeast PA as opposed to southern NJ and Delmarva. This track would bring more snow to the region during that time. Opted to use a blend of Superblend and WPC guidance given the uncertainties that lie with this storm, and did not make many appreciable changes to the previous forecast. This results in snow north of I-78, and otherwise snow, rain, or a wintry mix elsewhere. Generally rain during the day and snow at night. Please continue to use our snowfall probability maps to get an idea for the worst case scenario regarding this storm, as although main forecast keeps snow amounts low, there is the chance for significant accumulations. It is a low chance, but still a chance. Primary storm moves out to sea Tuesday night, and then another storm forms off the Mid-Atlantic coast on Wednesday. This one seems too far south to impact the region, but wrap-around precip may spread west. High pressure closes out the work week and another low approaches for the weekend. Temperatures will be well below normal through the period. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions continue through tonight. West to northwest winds gust around 20-25 knots late this afternoon into this evening. Gusts drop off after sunset, but remain around 5-8 knots through the night. Sunday...VFR conditions continue through the day Sunday. Northwest winds 5-10 knots, gusts may reach 15-20 knots in the afternoon. OUTLOOK... Sunday night through Monday...VFR. Monday night and Tuesday...Onset of precip will be Monday evening over the Delmarva as rain. By late Monday night and Tuesday, widespread MVFR/IFR conditions. Snow for KRDG/KABE, and wintry mix for KTTN/KPNE/KPHL/KILG. Wintry mix and then plain rain for KACY/KMIV. Low confidence on precip type, though. NE winds 15-20 kt with gusts up to 30 kt. Tuesday night through Wednesday...Additional precip possible with continued MVFR/IFR conditions. Low confidence. Wednesday night and Thursday...VFR. && .MARINE...
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The Small Craft Advisory was dropped at 6 p.m. The trend towards lowering winds and seas continues. Sub-SCA conditions expected through Sunday. OUTLOOK... Sunday night and Monday...Sub-SCA conditions. Monday night and Tuesday...SCA conditions with gales likely Tuesday. VSBY restrictions in rain, but wintry mix possible. Tuesday night and Wednesday...Gales continue Tuesday night. SCA conditions Wednesday. Additional VSBY restrictions in rain/snow. Thursday...SCA conditions continue.
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&& .FIRE WEATHER... Relative humidity values on Sunday will drop into the 20s again, but winds are expected to be less than Saturday. Even though fuels have been drying, they are expected to remain above critical levels as well. No enhanced statements are expected at this time. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...None.
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