Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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223 FXUS61 KPHI 170154 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 954 PM EDT Fri Mar 16 2018 .SYNOPSIS... Low pressure remains over the Canadian Maritimes through Saturday, then weakens as it moves eastward. Meanwhile, high pressure slowly builds to our south and west tonight and Saturday. A weak area of low pressure will quickly slide to our south Saturday, then high pressure arrives for Sunday. Low pressure in the Central Plains late Sunday and Monday tracks eastward, resulting in a coastal storm for Tuesday and Wednesday which then moves northeastward to off the New England coast during Thursday. High pressure should start building in during Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... High pressure will extend from the Great Lakes to the Middle Atlantic coast overnight. As a result, we are expecting a mostly clear sky with perhaps some stratocumulus lingering over parts of the Pocono Region and far northern New Jersey. The west northwest wind is forecast to diminish. Minimum temperatures are anticipated to favor the 20s in our region. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... High pressure over the Mid-Atlantic moves out to sea Saturday morning, and another area of high pressure builds into the Great Lakes. Meanwhile, a weak wave of low pressure will approach the Mason-Dixon line and then pass off the Mid-Atlantic coast during the day. Most models have some kind of precip developing over southern portions of the Delmarva Saturday afternoon. GFS is the most robust in terms of QPF with up to 0.20" inches, and most other models are generally at or under 0.10" across southern portions of eastern MD and southern DE. With the cold airmass moving offshore, temperatures warm a bit compared with today, and will generally top off in the mid to upper 40s, and in the upper 30s to low 40s in the Poconos and northwest NJ. && .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... Summary...Less wind on Sunday however colder conditions compare to average look to continue. Coastal storm development looks likely next week, however the details/impacts are still uncertain. Synoptic Overview...An expansive upper-level trough in eastern Canada and the eastern U.S. is forecast to shift a little more northeastward through Sunday. A potent short wave/closed low is forecast to slide east from the Central Plains Monday, then as stronger energy drops southeastward from southern Canada a larger trough is forecast to develop into the East. Surface low pressure is forecast to be associated with the first impulse, which then redevelops off the Mid-Atlantic coast. The model guidance is in good agreement of a storm off the East Coast, however the track and therefore impacts are less certain. The placement will depend on the track of the short waves and their phase/amplification. The evolution aloft and the surface low track will be crucial in determining precipitation types/amounts and rates and therefore any impacts. Despite the uncertainty with the details, there continues to be a strong signal of a storm next week although the departure of the storm has slowed. For Saturday night and Sunday...A weakening short wave moving through a confluence area in the Mid-Atlantic to start Saturday night quickly shifts to the east. As a result, any light rain across portions of southern Delmarva ends very early and clouds decrease. Otherwise, the persistent trough aloft in the East is forecast to shift a bit farther northeastward during Sunday. This will allow weak high pressure at the surface to build into our area, resulting in less wind and more sunshine Sunday. For Monday...High pressure centered near south-central Canada is forecast to extend southeastward over our area to start Monday, which should setup a cold air damming scenario and potentially deliver drier low-level air. Significant energy ejecting eastward from the Central Plains will drive surface low pressure into the lower Ohio Valley by late Monday. Clouds are expected to increase mainly at night and some large scale forcing may start to arrive toward daybreak Tuesday in our southern areas. As a result, we maintained some slight increase in PoPs later Monday night mainly across our southern areas. For Tuesday through Thursday...As significant energy rolls eastward, the upper-level trough is forecast to amplify quite a bit into the East Tuesday and Wednesday as the northern and southern streams phase. The complexity of this involves interaction between the southern and northern streams as well as relative small scale short waves in both streams. All of this energy results in low pressure redevelopment off the Mid- Atlantic coast. While the details and therefore impacts are still uncertain, there continues to be a strong storm signal for this time frame. The ensemble guidance still shows the variability regarding the overall evolution/track of surface low pressure. We continued to follow closer to WPC which indicates a secondary low development in the Carolinas early Tuesday morning. The slowing of the storms departure looks to be due to some more blocking in place along with a more sharpening of the trough aloft. At least some guidance has a more strung out surface low and this may be due to all of the energy involved, therefore having trouble focusing on where the main surface low consolidates. The entire evolution, strength and track of the features at the surface and aloft will impact the thermal fields and thus precipitation types, amounts and rates (embedded stronger forcing). Given some timing uncertainty with the details, we did not go higher than likely PoPs at this point and did continue with higher chance PoPs through Wednesday night, then PoPs lower for Thursday. The hazards that need to be ironed out are, heavy precipitation (rain and/or snow) and flooding, strong winds and coastal flooding (due to potentially long duration east to northeast winds). We used a model blend of partial thicknesses to derive the precipitation types, however this is highly subject to change. The system should be moving to a position off the New England coast to start Thursday, with improving conditions across our area. For Friday...As a strong upper-level trough shifts eastward, short wave ridging is forecast to approach from the west. This will drive surface high pressure into our area during Friday with no precipitation currently anticipated along with a continuation of chilly conditions. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Overnight...VFR with a mostly clear sky. West to northwest wind around 5 to 10 knots. Saturday...VFR with increasing clouds. Cloud bases are expected to be mainly above 10,000 feet. West to northwest wind around 8 to 12 knots with gusts of 15 to 20 knots. Outlook... Saturday night and Sunday...VFR. North-northwest winds less than 10 knots, becoming west-northwest Sunday. Monday...Clouds increase mainly at night, with possible MVFR/IFR conditions developing toward daybreak Tuesday if precipitation arrives. Low confidence regarding any sub-VFR conditions. Tuesday and Wednesday...Widespread MVFR/IFR conditions with rain/snow and potentially gusty east-northeast winds. Low confidence continues with the details. && .MARINE... Small Craft Advisories remain in effect for tonight for the ocean waters for winds continuing to gust to 25-30 kt. Seas remain elevated at 4-6 feet as well. Wind gusts on upper DE Bay will diminish to sub-SCA criteria around midnight, and conditions subside to sub-SCA criteria on DE ocean waters and lower DE Bay by daybreak Saturday. Wind gusts may briefly subside to sub-SCA conditions on NJ ocean waters Saturday morning, but pick up in the afternoon back to 25 kt or so. Will run SCA through the day Saturday, despite a potential lull in SCA conditions in the morning for the NJ ocean waters. Outlook... Saturday night and Sunday...The conditions should be mostly below Small Craft Advisory criteria, however a northerly wind surge with gusts 20-25 knots may briefly occur Saturday night. Monday...The conditions are anticipated to be below Small Craft Advisory criteria, although winds should start to increase from an easterly direction later Monday night. Tuesday and Wednesday...Winds and seas increase as a coastal storm tracks off the coast. The magnitude of the winds and seas will depend on the track and strength of the coastal storm, however there is an increased chance for a period of gale force wind gusts. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ450>453. Small Craft Advisory until 6 AM EDT Saturday for ANZ431-454- 455. Small Craft Advisory until 1 AM EDT Saturday for ANZ430. && $$ Synopsis...Gorse Near Term...Iovino Short Term...MPS Long Term...Gorse Aviation...Gorse/Iovino Marine...Gorse/Iovino/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.