005 FXUS61 KPHI 010939 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 439 AM EST Tue Dec 1 2020 .SYNOPSIS... A surface low will linger near the Saint Lawrence Valley through tonight before lifting northward into eastern Canada Thursday and Friday. High pressure will spread northeastward from the Southeast through Thursday before shifting offshore Friday. An upper-level system will likely affect the region late this week, with potential for a surface low to affect the area this weekend or early next week. && .NEAR TERM /THROUGH TONIGHT/... A now closed upper low will drift slowly northward into southeastern Canada today. The associated surface low will also follow suit as it continues to occlude from the warm sector which is now well offshore. Seasonably cold high pressure will remain in place across the Southeast through tonight which will yield a pressure gradient atypical of climatology in the post-system regime (southwesterly winds instead of northwesterly). Nevertheless, low-level cold air will advect in from the west through tonight. As a result, temperatures will hold nearly stead throughout the day in the 40s and dewpoints will fall into the 20s. The cold advection will result in some good boundary layer mixing this afternoon and early evening. This combined with 925 mb winds in the 25-30 kts range and a tightening surface pressure gradient will result in a breezy day across the forecast area. Expect southwesterly winds in the 15 mph neighborhood with gusts of 25-35 mph. As the low pressure system lifts north, mid-level winds switch from southerly to southwesterly pushing the dry slot offshore and ushering in mid-level moisture. The result will be increasing and lowering clouds during the day as the aforementioned cold advection ramps up. Guidance indicates sufficient forcing for light showers across the Poconos and adjacent areas of the Lehigh Valley and NW NJ during the day. Precip type will likely begin as rain then mix with and change over to snow in the higher elevations. As temperatures continue falling below freezing into the late afternoon and evening, the snow will begin to stick to at least some elevated surfaces and grassy areas likely resulting in a dusting of snow across the Poconos. Expecting generally less than a half inch of snow given very light QPF values, but can't rule out some isolated areas closer to an inch or more. Flurries may continue in areas of the Lehigh Valley and NW NJ into the overnight period as well. BUFKIT profiles suggest boundary layer mixing will continue after sunset given the continuing cold advection, but I'm skeptical on this since cold advection is not particularly strong and we won't have the sun on our side to support continued deep mixing. Temperatures will fall well into the 30s tonight with a continuing southwesterly wind around 10-15 mph. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... During the Wednesday through Thursday periods, the low pressure system will continue lifting away from the region lessening its impact to our sensible weather as high pressure begins moving offshore to our south. Flow aloft will become more zonal. Wednesday will be quite chilly with highs in the 40s area wide. Continued pressure gradient will result in another breezy day, but not as breezy as Tuesday with winds becoming a bit more westerly. Snow flurries and snow showers will likely continue across the southern Poconos throughout the day with little to no additonal accumulation. Temperatures will fall into the upper 20s to around 30 degrees Wednesday night. The southern stream will lift north through Thursday resulting in some increasing clouds later in the day, but overall another largely fair weather day is expected. Temperatures will be a few degrees warmer than Wednesday and winds notably light from the southwest. Temperatures fall into the 30s Thursday night. && .LONG TERM /FRIDAY THROUGH MONDAY/... Run-to-run model continuity remains nearly nonexistent today, with individual deterministic runs occasionally showing tantalizing similarities, only for large differences to show up in subsequent runs. The ECMWF continues to be an outlier with the evolution of the southern-stream system late this week into the weekend (much farther south), but there is at least some trend northwest toward the somewhat more agreeable 00z GFS/CMC, mainly with the northern-stream vort max affecting the region on Friday. Thus, confidence is a little higher the region will see some precipitation on Friday. The precipitation may start out as a wintry mix in the Poconos and Lehigh Valley, but this is conditional on the presence of precipitation (which may hold off until later in the day, and may be more southeast of these areas anyway). Did increase PoPs to primarily the chance category by afternoon, as most of the deterministic guidance (with ensemble support) generate some precipitation in advance of the northern-stream vort max (and some semblance of an ejected perturbation from the southern stream). For the weekend, the GFS and CMC both produce a surface low that lifts somewhere near the East Coast through the period, providing considerable precipitation to the area. The CMC has been the most consistent with this, and given the generally similar look of the GFS, felt that keeping PoPs through the weekend was required. Temperatures will be cold enough at night for some wintry mix in this scenario, primarily northwest of the Fall Line (but probably not exclusively). Meanwhile, the ECMWF remains bone-dry during this period, keeping the southern-stream storm detached from the northern stream and shunting it offshore. Instead, it develops the cyclone well off the coast before migrating it northwest toward New England and the Canadian Maritimes next week as a digging vort max moves into the Northeast. Such a vort max would likely produce some light precipitation immediately downstream, and the ECMWF timing for our area would favor Sunday night and Monday (again, temperatures cold enough for a wintry mix for areas northwest of the urban corridor). Meanwhile, the GFS/CMC generally keep the area dry to start the week with cold northwest winds in the wake of the simulated weekend system. Yikes. Needless to say, forecast confidence for this entire period is unusually low. Did not stray too far from the previous forecast, keeping a broad brush of PoPs all four days and temperatures near or slightly below average given the presence of troughing in the area through the period. Expect low predictability to continue for another few cycles, as the complex interactions with multiple systems in both the polar and subtropical streams are of reduced scientific understanding. && .AVIATION /09Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...A tricky aviation forecast for today. Ceilings will increase in coverage after 12Z and lower to around MVFR at most terminals. Confidence is higher for prevailing MVFR ceilings at RDG and ABE, but periods of MVFR ceilings are probable at all other terminals. have covered this potential with TEMPO MVFR ceilings during the impacted periods. Southwesterly winds will increase after 12Z to 10- 15 kts with gusts of around 25 kts expect after 15Z. Moderate confidence. Tonight...Ceilings are forecast to scatter out and lift between 00Z and 03Z. Southwesterly winds around 10 kts with some lingering gusts possible through 06Z. High confidence. Outlook... Wednesday...Mainly VFR. Westerly winds 10-15 kts with gusts of 20-25 kts. Winds will diminish after 21Z. High confidence. Thursday...VFR. Southwesterly winds 5-10 kts. High confidence. Thursday night and Friday...Restrictions possible with a chance of rain. There is some potential for a wintry mix northwest of PHL, but this appears more likely northwest of RDG/ABE. West to southwest winds 5 to 15 kt. Very low confidence. Friday night and Saturday...Unusually low confidence for this period, with a wide range of weather outcomes possible. Bottom line is that restrictions are possible, but details are quite unclear. && .MARINE... Southwesterly winds will increase through this morning into the afternoon to around 20-25 kts with gusts up to 30 kts. Seas will remain elevated, especially across the ocean waters with heights of 7-10 feet expected. Seas will begin to diminish overnight to around 5-7 feet and winds will also diminish somewhat. Expect Small Craft Advisory conditions to continue overnight for the ocean waters. Outlook... Wednesday...Seas will continue diminishing throughout the day and especially into the overnight to below 5 feet across the ocean waters. Southwesterly winds will remain elevated with gusts of 20-25 kts across all waters. Expect a Small Craft Advisory will be needed through at least the daytime hours and possibly into the overnight period. Thursday...Winds and seas should remain below SCA criteria. Seas 2-4 feet with a westerly wind around 10-15 kts. Friday and Friday night...Sub-advisory conditions expected. Saturday...Very low confidence, but cannot rule out marine headlines. && .TIDES/COASTAL FLOODING... A round of spotty minor tidal flooding is possible this morning on the Atlantic coast, especially for northern and central New Jersey. Currently think the threat is too low and isolated for advisory issuance. Freshwater runoff may contribute to water levels higher than guidance, especially for Raritan Bay. Additionally, minor flooding cannot be ruled out on the tidal Delaware River for the early afternoon high tide. Will monitor observations closely this morning to determine if models are once again much too low. For now, think conditions will remain below advisory thresholds. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Wednesday for ANZ450>455. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431. && $$ Synopsis...CMS Near Term...Staarmann Short Term...Staarmann Long Term...CMS Aviation...CMS/Staarmann Marine...CMS/Staarmann Tides/Coastal Flooding...CMS