Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 250159 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 959 PM EDT Fri Mar 24 2017 .SYNOPSIS... A warm front over the southern tier of New York will become nearly stationary near our region thru Tuesday, as several waves of low pressure move along it. A cold frontal passage is expected Tuesday night, followed by high pressure from Canada, which will build into our area Wednesday through Thursday. A warm front may approach our region by Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... High pressure was anchored off the Southeast coast this evening. Strong WAA occurred earlier today, resulting a late day temperature spike. SWly return flow around the offshore high will result in a much milder night than in recent nights. Nonetheless, there will still be a wide range in temperatures across the forecast area as sheltered valleys across the far northwestern NJ and northeastern PA have started to decouple tonight. Temperatures in these colder spots have already dropped into the upper 30s and lower 40s. Meanwhile, a 10 mph S-SW wind at the surface has helped keep the boundary layer mixed across southeastern PA-central NJ southward where temperatures are still in the low to mid 50s. Based on current temperature/dewpoint observations this evening, fog is favored north of I-78, especially in the river valleys. Fog development is favored late tonight and into the early morning hours when a backdoor cold front progresses southward into our far northern zones. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... The warm front that lifted through our region earlier today, is expected to stall and then possibly retreat slightly south, though still staying just north of our region. As it does so however, a weak low will propagate along the front, bringing a chance for precipitation on both sides of the front. As with today, the GFS shows the most extensive QPF as compared to the other models. Given how weak the low is that is expected tomorrow, think this is overdone. Temperature wise, with our region firmly in the warm sector we should be well above normal with highs ranging from the mid 50s to lower 70s despite persistent cloud cover. One caveat though is that if the northeasterly flow develops earlier than forecast (at this point we don`t expect to see that wind shift until later in the day), it could temper the warming trend. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... An unsettled period of weather is in store Sunday thru Tuesday. At the mid-levels across the conus, a generally zonal northern stream flow will be in place, while several shortwave disturbances traverse the southern stream. Meanwhile at the surface, Canadian and sub-tropical high pressure systems will maintain a frontal boundary in the vicinity of our region until a more substantial frontal passage in the Tuesday night time frame. Temperature-wise, at or above normal through the period The primary uncertainty will be the location of the aforementioned boundary, along with the timing of several areas of low pressure that ride along it, and accompanying areas of precipitation. Most notably, the NAM and GFS have trended stronger and further south with the backdoor front, which brings colder temperatures and a more robust easterly flow to our region for the second half of the weekend. For the most part, the models remain consistently too warm for freezing rain Sunday night into Monday morning, but the cold air is close enough that it cannot be totally discounted. Given the trends with respect to the placement of the front within an overrunning regime, there is now an increasing chance of freezing rain or drizzle Saturday night into Sunday morning. In terms hydrology, given the above normal temperatures through the long term, the snow pack north of I-78 will continue to melt. The model consensus brings additional rainfall amounts of one to two inches across this area through Wednesday, most of this spread out between more substantial precip Sunday night- Monday and Tuesday night. Unless there are significant changes, we expect rises on area streams and rivers, but no flooding concerns at this time. There is also the potential for spotty minor coastal flooding along the ocean front for the late Sunday and early Monday high tides. As the frontal boundary moves southward in response to Canadian high pressure building to the north, scattered showers are possible across our entire area Saturday night into Sunday. Model soundings depict rich low-level moisture within the post-frontal maritime air mass, which will be conducive to low clouds, drizzle, and perhaps patchy fog. Two meter temperatures may be close to freezing Saturday night into Sunday morning, mainly near and to the north of I-80, per the higher resolution NAM, WRF-ARW, and NMM. While antecedent conditions are not that cold, there is some potential for showers and drizzle to result in a light ice glaze on elevated surfaces. There is uncertainty with regard to ice glaze Saturday night into Sunday morning, but the probability has increased, and we have included a mention in the HWO (per our directives). This will need to reassessed with future forecast packages, as we`re looking for continued run to run model consistency. If confidence increases, an ice map will be posted to our website, and further advisory headlines will be considered. As low pressure moves through the Ohio Valley into the Great Lakes from Sunday night into Monday, more substantial overrunning precip is likely during this time frame. As mentioned previously, the models have exhibited run-to-run consistency with regard to the thermal profile Sunday night into Monday morning, which does not favor freezing rain. In addition, boundary layer winds are sufficiently strong enough to advect warm air northward through the low-level column, despite somewhat of an in-situ cold air damming signature. In addition, the lack of low-level dry air will limit any evaporational cooling. Again, given the proximity of the cold air, we cannot rule out freezing rain, but the probability of it occurring is low. Expect showers to continue Monday afternoon into Wednesday, with another round of more substantial precip likely centered on Tuesday night, associated with a cold frontal passage. We do not anticipate any p-type issues during this period. There is uncertainty with the timing of this front, particularly given the time range, with the GFS being the most progressive. At this time, we expect a gradual return to fair weather on Wednesday, continuing into Thursday. By Friday, a warm front may approach the region, along with an increase in cloudiness and a chance of showers. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions will continue through at least 06Z, and will likely continue through 18Z. A few models depict low stratus or fog developing in the southern Poconos and getting close to KRDG and KABE after 09Z tonight. Confidence in fog has increased slightly but enough to add a TEMPO IFR VSBY group in the 00Z TAF for RDG and ABE. Forecast soundings show marginal LLWS tonight with a sharpening nocturnal inversion and wind speeds increasing to 35-45 kt at or above 1500 ft AGL. Between 18Z and 00Z Saturday afternoon/evening, ceilings and visibility will start to lower across the region from N to S as rain showers move in. Southwesterly winds 5-10 kt tonight. After 12Z, winds will gradually shift from southwesterly to westerly through midday, before shifting to northeasterly late in the day behind a backdoor cold front. OUTLOOK... Saturday night-Tuesday...MVFR or IFR conditions with a chance of rain possible through the period. && .MARINE... Extended the SCA for the NJ coastal waters thru 8 AM Saturday. This extension was mainly for seas but there is still a diminishing chance for 25 kt gusts thru about midnight. Guidance has been underdone with wave heights, which are currently 5-6 ft at our offshore buoys. These seas should gradually subside overnight in response to the weakening winds. Once winds and seas drop below SCA criteria, sub-SCA conditions are expected through the day time hours on Saturday. OUTLOOK... Saturday-Saturday night...Conditions expected to fall below advisory levels. Sunday-Sunday night...Small Craft Advisory levels possible. Monday-Tuesday...Winds may be below advisory levels, but seas could remain elevated leading to Small Craft Advisory levels. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ450>453. && $$ Synopsis...Franck Near Term...Johnson/Klein Short Term...Johnson/Klein Long Term...Franck Aviation...Johnson/Klein/Robertson Marine...Johnson/Klein/Robertson is the U.S. government's official web portal to all federal, state and local government web resources and services.