Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 060852 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 352 AM EST Tue Dec 6 2016 .SYNOPSIS... High pressure over the northeast will give way to developing low pressure off the Carolinas into tonight. Meanwhile, low pressure nearing James Bay sends a cold front thru our area on Wednesday, sweeping the Carolinas low well offshore. The low pressure over James Bay will move to Atlantic Canada into Friday, while high pressure over the central United States moves into the middle Atlantic this weekend. A warm front may approach our region by Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... High pressure was centered over the forecast area early this morning while low pressure was located in Mid South. The high will retreat into New England today as the low lifts northeastward toward the Ohio Valley. This primary low will weaken this afternoon as the system transfers its energy to a secondary low organizes along the warm front near the NC coast. National radar mosaic loop early this morning shows a broad shield of precip with this storm over the Southeast states that had advanced northward into southern VA. Deep convection and organized thunderstorms was ongoing across the northern Gulf Coast states. The leading edge of the rain will reach our southern most zones (southern DE-eastern MD) by midday. The rain is then expected to expand northward into the Philadelphia metro by mid afternoon and then the I-78 corridor closer to sunset. Locations near I-80 may see wet snow or sleet mix in with the rain initially. Moderate rain will likely be falling over the southern half of the forecast area during rush hour. A Winter Weather Advisory was issued for the Carbon and Monroe Counties in PA and Sussex County, NJ starting at 4 PM (see short term section for a discussion on accumulations and impacts). There is still some uncertainty in the onset timing of precipitation in our northern most locations (north of I-80) but it could arrive as early as the evening rush (around 4-6 PM). It will take some time for the column to saturate and precipitation to reach the ground with a residual dry airmass in place, so a best case scenario would be for precipitation to hold off until after the commute home since these northern-most zones will likely see snow at the onset. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... The coastal low moves off the coast (somewhere between KORF and KHSE) early this evening. The surface low will deepen to around 1000 mb tonight as it heads out to sea. Steady precipitation will continue into the evening across the area. Ptype will be rain across the majority of the forecast area except the far north. Numerous hi-res models show a band of heavier precipitation arriving during the evening across NE PA and NW NJ. Wet snow may mix in with the rain at times farther south into the Lehigh Valley and I-78 corridor but little or no snowfall accumulation is expected. Forecast soundings indicate sufficient dynamical cooling during this time to change precip to all snow in the southern Poconos and in NW NJ (roughly NW of I-287). The latest thinking is these areas may see several hours of moderate to locally heavy snowfall this evening. We are forecasting snowfall accumulations of 2-4 inches mainly along the ridges above 1000 ft in elevation from roughly I-80 northward. An elevated warm layer should surge northward in concert with the arrival of SW low-level jet, turning precip into all liquid overnight across the far north. Some of the colder model solutions, including the NAM, RGEM, and NCEP WRF NMM show temperatures remaining steady at or just below freezing at higher elevations in the southern Poconos during the overnight. Leaned more toward these colder solutions as some of global models tend to erode the residual colder layer near the surface too quickly. The FRAM based on input from our forecast database gives us a few spots with several hundredths of an inch of ice across northern Carbon and Monroe Counties. The rain will gradually taper off from SW to NE late tonight but spotty drizzle may persist thru daybreak. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... A long wave trough over the central US during the mid-week will gradually move east, with a ridge building over the eastern US this weekend, followed by a shortwave traversing the upper MS Valley and Great Lakes early next week. The primary uncertainty is with regard to shortwave energy moving through the region on Thursday, and the amplitude/timing of the next shortwave early next week. For Thursday, model disagreement centers on the amount of moisture associated with the shortwave. The NAM/GFS appear to be too robust, with a questionable amount of moisture given the synoptic pattern. We leaned more toward the ECMWF, and concentrated PoPs across the northwest, where the strongest lift and deepest moisture coincide. Overall, expect a chance of rain/snow showers on Thursday, with a dusting of snow possible across the higher terrain. A cold front moves through the region Thursday night, with gusty northwest winds to follow through Friday. Model soundings show the potential for winds to gust up to around 30 mph Friday afternoon. During this time frame, some snow showers may also work their way into the area, especially northwest of I-95-276-76, where the BTV Snow Squall Parameter values are the highest. Moving into the weekend, snow showers may continue across northwest NJ and the Poconos into Saturday. The next storm system approaches on Sunday, with the aforementioned model uncertainty regarding the attendant shortwave amplitude/timing. We leaned more toward a GFS/ UKMET compromise, in between the slower ECMWF and faster Canadian. This would bring an increase in cloudiness on Sunday, with precip overspreading the region Sunday night. The GFS thermal profiles support wintry precip initially with a changeover to rain Monday, while the slower ECMWF is much warmer, suggesting a primarily rain event. Given the antecedent cold air mass in place, wintry precip is plausible at the onset, with a changeover to rain. && .AVIATION /09Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR this morning. Rain will move into the area from S to N this afternoon. Expect CIGs and VSBYs to drop to MVFR levels shortly after the rain starts. A band of moderate rain is expected to move through the area between approximately 21Z this afternoon and 03Z this evening, resulting in IFR conditions. RA may could briefly mix with SN at RDG and ABE, which could result in temporary restrictions to LIFR. Rain tapers off from SW to NE late tonight, but low clouds will remain over the area. Consequentially, IFR conditions will linger into the overnight. N-NW winds around 5 kt will veer from the NE later this morning and then increase out of the E this afternoon. Winds then shift out of the NE tonight. Winds may become gusty from the I-95 terminals eastward late this afternoon and tonight. OUTLOOK... Wednesday thru Saturday... Predominantly VFR at all TAF sites through the period. Northwest winds may gust up to around 25 knots Thu night into Friday. && .MARINE... A Gale Warning was issued for tonight for the coastal waters of DE and S NJ as well as the lower DE Bay. Easterly winds will strengthen to the north of a coastal low. BUFKIT soundings from several models (non-GFS) indicate the potential for 35-40 kt gusts over these southern zones tonight. A SCA was issued for the northern coastal waters and for the upper DE Bay with gusts 25-30 kt possible tonight. Seas will build to 4-7 ft in response to the strengthening wind field. OUTLOOK... Wednesday...Although northwest wind gusts are expected to remain below 25 knots, seas are forecast to remain elevated, at or above 5 FT. Thus, a Small Craft Advisory for hazardous seas may be needed for the ocean waters. Wednesday night and Thursday...Sub-SCA winds and seas anticipated. Thursday night thru Friday night...Northwest winds are forecast to increase Thursday night, with frequent gusts meeting SCA criteria, along with isolated gale force gusts possible late Friday. Saturday...Sub-SCA conditions are anticipated at this time. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Winter Weather Advisory from 4 PM this afternoon to 6 AM EST Wednesday for PAZ054-055. NJ...Winter Weather Advisory from 4 PM this afternoon to 6 AM EST Wednesday for NJZ001. DE...None. MD...None. MARINE...Gale Warning from 6 PM this evening to 6 AM EST Wednesday for ANZ452>455. Small Craft Advisory from 6 PM this evening to 6 AM EST Wednesday for ANZ450-451. Small Craft Advisory from 6 PM this evening to 4 AM EST Wednesday for ANZ430. Gale Warning from 6 PM this evening to 4 AM EST Wednesday for ANZ431. && $$ Synopsis...Franck Near Term...Klein Short Term...Klein Long Term...Franck Aviation...Klein/Franck Marine...Klein/Franck

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