Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 150538 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1238 AM EST Fri Dec 15 2017 .SYNOPSIS... High pressure will build in across the area tonight through Friday and then weaken. A low pressure system will move up the Middle Atlantic coast Friday afternoon and evening. High pressure will return for most of the weekend. A weak low pressure system will move through Sunday night into Monday with another low expected for Tuesday. High pressure will build in for the middle part of next week. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Current forecast looks pretty much on track with winds becoming light NW and temps falling in the teens and low 20s most areas (except a bit warmer over northern Delmarva). Some patchy CI extends fairly far upstream and should continue to move across the area overnight, which may somewhat limit further radiational cooling. Will not make any changes to forecast mins attm. Previous discussion... High pressure builds into the region tonight. This will allow winds to become light, especially late this evening and overnight as the pressure gradient weakens. High clouds associated with an upper jet streak will spread into the area this evening and overnight before eroding from south to north toward early morning. We currently do not anticipate this cirrus deck to have a significant impact on radiational cooling. Accordingly, forecast low temperatures are on the cooler side of the guidance with lows in the teens area wide (slight cooler in the Poconos and slightly warmer in the cities. As far as how we get there: temperatures should drop quickly after dark despite the continued NW winds 5-15 mph given the tail end of the stronger post-frontal CAA surge is projected to occur 21Z-00Z this evening, then continue to fall but at a slower clip late this evening and overnight. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/... The next shortwave disturbance is forecast to dig around the backside of the large scale trough across the Midwest/Great Lakes region on Friday. There looks to be another weaker shortwave farther south that will be tracking eastward into the central Appalachians. The latest 12Z models trended toward slightly more pronounced with (at least some degree of) of phasing between these two systems as they approach the Mid- Atlantic region in the afternoon. This is evident in the upper- level jet streak that strengthens to 180-190 kt at 250 mb) just to our south by the afternoon as well as coastal cyclogensis that occurs near the NC/VA border midday. Large-scale lift in the left-exit region of the upper-level jet streak and in WAA/southwesterly flow ahead of the trough will likely lead to the development of light precipitation during the afternoon over coastal plain and possibly back into the I-95 corridor. Given the trends seen in today`s guidance, PoPs were increased to chance for the I-95 corridor and to likely for Delmarva and southern NJ by late afternoon. Although temperatures in the coastal plain may be in the mid to upper 30s early in the afternoon, evaporative cooling should bring them down to freezing as precip arrives. Accordingly, ptype looks to be primarily snow for this event with the possibility of a brief rain/snow mix at the onset near the coast. The progressive pattern makes this a short-duration event, limiting the risk for more substantial snowfall accumulations. Forecast amounts that went out with the afternoon forecast package are 1" or less for Delmarva and southern New Jersey. There is some support for mesoscale banding (through snow bands would be much more transitory than what we saw with the December 8-9 event) that would lead to locally higher amounts with frontogenesis potentially occurring on the northwestern side of the developing surface low. For the I-95 corridor, kept snowfall accumulations out of the forecast for now with them predicted to be on the north/western fringe of the organized lift and precipitation shield. We may need to expand light accumulations a bit farther back to the north/west if trends continue this way. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... The offshore low (from the short term) will continue to pull away Friday night and pops will decrease through the night. Any accumulations of snow across srn NJ or coastal Delmarva will be light after 00Z Saturday. High pressure will begin to build in by morning. Low temperatures Saturday morning will bottom out in the in the teens or low 20s far north and mid/upper 20s other areas. The weekend looks to be mostly fair at this point with high pressure affecting the area. Temperatures will be a little below normal for mid December, but not the the harsh chill like we had the past few days. Clouds will increase later Sunday as a weak system begins to arrive from the south/west. This system will be rather disorganized and is not even shown producing qpf on the 12Z GFS model. Nonetheless, since the other models are showing some precip, we have put some chc pops are in the fcst for these areas. Mostly rain S/E with some snow psbl N/W. Low confid overall. Monday through Tuesday night will have mostly fair weather for the area. Under upper ridging, temperatures will rise back to above normal values with highs Monday and Tuesday in the 50s over Delmarva and metro Philadelphia and mostly mid/upper 40s for the Lehigh Valley and central NJ areas. It`s possible that a few showers may occur, but confid is rather limited and thus any pops in the grids attm are in the slgt chc range. Chance pops will be found Tuesday across the N/W areas with a front affecting those areas. Wednesday and beyond will be mostly fair as another high pressure areas moves in. Most of the operational 12Z models have this period as dry. Temperatures should be back near normal values. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR. Winds will decrease to 5- 10 kt this evening and 5 kt or less by late tonight. High confidence. Friday...VFR and light/variable winds in the morning. By the afternoon, expecting widespread MVFR ceilings across the area. Confidence is increasing in light snow developing along and especially south/east of I-95 terminals. A light SW winds around 5 kt should develop. Moderate confidence in the flight category and timing of the snow. Outlook... Fri night...VFR conditions returning to S/E terminals with snow ending during the late evening. VFR elsewhere. Sat thru Sunday...Mostly VFR. Sunday night thru Tuesday...Mostly VFR. Few showers psbl. && .MARINE... Wind gusts near 25 kt will remain possible for the next few hours along the northern and central NJ shore. Otherwise, winds and seas should stay below small craft advisory criteria through the rest of today. Snow will likely develop over the DE Bay and the coastal waters of southern NJ and DE Friday afternoon. Outlook... Fri night/Saturday...SCA conditions developing on the ocean behind the departing low. Mostly sub-SCA for Delaware Bay. Sat night thru Monday...Sub-SCA expected. Showers psbl Sun night. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 5 AM EST early this morning for ANZ450-451. && $$ Synopsis...O`Hara Near Term...AMC/Klein Short Term...Klein Long Term...O`Hara Aviation...Johnson/Klein/O`Hara Marine...Johnson/Klein/O`Hara is the U.S. government's official web portal to all federal, state and local government web resources and services.