Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 282337 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 737 PM EDT Tue Mar 28 2017 .SYNOPSIS... Surface low pressure will move offshore of the Mid-Atlantic tonight, then drift into the open Atlantic tomorrow. Canadian high pressure builds into the Northeast late Wednesday and Thursday. Low pressure develops in the central U.S. on Thursday, moves east into the Mid- Atlantic on Friday, and progresses offshore Saturday. High pressure returns to the area by late in the weekend. Another surface low may affect the region by the middle of next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Weak low pressure is located along a stationary front near the mouth of the Delaware Bay this evening. Meanwhile, a cold front is pushing through central NY/PA. Low pressure will lift to the east-northeast this evening, followed by the cold front later tonight. Areas of showers and drizzle continue across most of the area, especially focused across eastern Pennsylvania and central/northern New Jersey. Along with the rain and drizzle, there is also some patchy fog across the area. These showers will continue to drift eastward through this evening and begin to dissipate later this evening as the surface low and mid-level trough and vorticity impulse slide east of the area as well. However, until the cold front moves through overnight and toward daybreak, there will continue to be cloudy skies and some light fog. It remains possible for some isolated thunderstorms, mainly across southern New Jersey and the central/southern Delmarva where any instability could linger south of the front/low pressure. Lows tonight will range from the mid and upper 30s across the Poconos and northern NJ to the lower 40s for the rest of NJ and SE PA, and in the mid to upper 40s in MD/DE. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... High pressure builds in from the north, bringing plenty of sunshine. A slight downsloping flow will help moderate the temperatures, and highs will top off in the mid 40s in the Poconos, upper 50s across northern NJ and the Lehigh Valley, low 60s across central and southern NJ and SE PA, and in the mid to upper 60s in the Delmarva. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... The long term period starts off quiet as a surface ridge axis passes overhead Thursday morning. Surface flow becomes northeasterly with time, and the cool/maritime origins of this flow suggest Thursday may be a bit cooler than Wednesday, especially if increasing cloudiness occurs early in the day. This increased cloudiness would commence downstream of another potent southern-stream vorticity maximum progressing eastward from the central plains. Broad southerly low-level flow amplifies downstream ridging in the eastern U.S., with substantial warm advection in much of the Midwest. However, with the retreating surface high in eastern Canada, this is a favorable setup for cold air damming east of the Appalachians, particularly with the more southerly track of the aforementioned vort max. As with the systems affecting the area early this week, I think the models are overly aggressive sweeping the baroclinic zone/associated warm front north through the region Thursday night and Friday. Though the 12Z GFS is a bit of an outlier with the southern/slower track, the disagreement in the midlevels between the 12Z CMC/ECMWF do not provide confidence enough to discard the GFS solution, which would favor a somewhat colder scenario for our area (though the GFS is itself switching winds to south way too fast in such a setup). For temperatures, generally went somewhat below guidance Thursday night and Friday, and my suspicion is that I am still too warm during both periods. The implications of this are most important for the southern Poconos and Sussex County, NJ, where precipitation will likely begin Thursday night and temperatures will once again flirt with the freezing mark. Model guidance is at least somewhat suggestive of a wintry mix of precipitation in this region, and thermal profiles suggest the presence of a warm nose, which may mean that freezing rain and/or sleet would be possible. For now, kept things simple with a mix of rain and snow in this area (particularly with surface temperatures forecast slightly above freezing), but later shifts may need to include mention of sleet or freezing rain in this region. By some time on Friday, temperatures are expected to warm here enough for precipitation to be all rain, but given lackluster performance with models this winter scouring out this cold air in an accurate manner, current forecast may be overly optimistic (at least Friday morning). A sustained southerly low-level fetch downstream of the surface low moving through the region on Friday brings PWATs > 1.0 inch (approaching 1.25 inches) to central/southern portions of the CWA. With aid of upper-level jet coupling, substantial differential cyclonic vorticity advection, and considerable low- level isentropic ascent, widespread rain should develop on Friday and continue through Friday night as the low approaches the Mid-Atlantic coast. QPF looks quite decent, with widespread 0.5-1.5 inch totals possible (and perhaps more, if some simulations verify). The potential for storms seems limited at this time, given the CWA`s position generally along/north of the surface low track. However, if the warm sector is able to move more poleward than currently forecast, convection may also be a consideration (particularly for Delmarva). The surface low is expected to move offshore by Saturday, but wraparound showers may still occur, particularly north of the Mason-Dixon Line. Only gradually lowered PoPs late Friday night and Saturday given this possibility. Winds will switch to the north, but temperatures may actually be a little warmer than Friday`s readings given greater potential for sunshine, subtle downsloping, and boundary-layer mixing. However, not sold on the very warm MEXMOS readings at this point given the origins of the upstream surface high. Sunday and Monday should generally be dry as upstream ridging moves through the region. Temperatures will be near to slightly above seasonal averages. Another in a series of southern-stream systems is projected to move into the Ohio/Tennessee Valleys by early next week. With increased large-scale ascent downstream of the attendant surface low, precipitation will probably break out Monday night and Tuesday across the area. This low`s track would be farther to the west, which suggests this system will be warmer, giving our region greater potential for convection. Did not add thunder to the grids at this point given track/timing uncertainties, but think this is a good bet given relatively decent agreement among the model suite. Models suggest a reinforcing system may approach the area midweek, keeping the QPF train chugging. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...MVFR/IFR conditions possible in showers and fog early. Isolated thunderstorms possible early, but confidence not high enough to warrant inclusion in the TAF. Sub-VFR CIGs/VSBYs will continue for much of tonight in fog after showers depart. Improvement to VFR not until prior to daybreak Wednesday. VFR conditions will prevail through the day Wednesday. North- northeast winds around 5 knots are expected to become northwest overnight, and increase through daybreak. Winds will become gusty during the day Wednesday. OUTLOOK... Wednesday night through Thursday night...VFR with generally light winds. Confidence well above average. Friday through Saturday...Sub-VFR conditions likely, with lowered CIGs and VSBYs and periods of rain. East or southeast winds Friday around 10 kts becoming northerly and potentially gusty on Saturday. Confidence average. Saturday night and Sunday...VFR with north or northwest winds around 10 kts with at least some potential for gusts to 20 kts. Confidence average. && .MARINE... VSBY reductions possible in showers and fog tonight, with improvements not until well after midnight. Unlimited VSBYs expected on Wednesday. Sub-SCA conditions tonight through Wednesday, but some gusts to 20 KT possible Wednesday morning/early afternoon, mainly on the ocean. OUTLOOK... Wednesday night...Marginal small craft advisory conditions possible, especially off the New Jersey coast. Thursday and Thursday night...Sub-advisory conditions expected. Friday through Saturday...Marginal advisory conditions possible. Rain likely. Saturday night and Sunday...Sub-advisory conditions expected. && .TIDES/COASTAL FLOODING... Though astronomical tides will be gradually diminishing through the week now that we are past the new moon, the threat of minor tidal flooding along the NJ and DE Atlantic coasts increases late in the week. This is a result of a low pressure system bringing a prolonged period of on shore flow. The tide of most concern at this point is the high tide on Friday evening/late Friday night. By this tide cycle, it will take a surge of 0.8 to 1.0 feet to reach minor flooding thresholds, which is possible but still uncertain (it will be dependent on how quickly the on shore flow develops and how strong it will be by then). At least one source of guidance shows water levels reaching minor tidal flooding thresholds with the Thursday evening/night high tide, but that seems unlikely as the latest forecast depicts onshore flow either developing right around or just after the time of that high tide. && .EQUIPMENT... KNEL appears to be reading 10F too warm the past several days. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...CMS Near Term...MPS/Robertson Short Term...MPS Long Term...CMS Aviation...CMS/MPS/Robertson Marine...CMS/MPS Tides/Coastal Flooding... Equipment...

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