Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 282001 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 401 PM EDT Tue Mar 28 2017 .SYNOPSIS... Surface low pressure will move east from the Mid-Atlantic region into the open Atlantic tomorrow. Canadian high pressure builds into the Northeast 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 lies along a stationary front just over the Delmarva. Meanwhile, a cold front is pushing through central NY/PA. Low pressure will lift to the northeast this evening, followed by the cold front later tonight. The first area of rain is pushing northeast through northern NJ and the Poconos, and patchy fog is developing in its wake. There will be some locally dense fog, but it is not widespread enough to warrant a Dense Fog Advisory. Latest HRRR indicates a secondary area of showers and possible thunderstorms near the Delmarva. This is also shown in the 12Z NAM and 12Z GFS. This area then lifts to the north and east through this evening as low pressure moves off the Atlantic coast. E-NE flow on the northern side of the stationary front is keeping NJ/PA stable, and just south of the front is a warmer and more unstable airmass, with 250-500 J/kg SB CAPE across DE/MD. Much higher SB CAPE values can be found across VA (1000-2000 J/kg) where storms are beginning to develop. Would expect storms to move into the Delmarva, but do not think they will last long as they track to the north and east. Will keep chances for thunderstorms capped at isolated. Wave of low pressure departs early this evening, but it will take some time for the cold front behind it to pass through the region. It will then take even more time for northerly winds to increase enough for drier air to filter into the region. Patchy fog will develop this evening, and will persist until ending prior to daybreak Wednesday. 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 /20Z TUESDAY 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. LGT/VRB winds this evening, becoming N 5-10 late tonight. 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. && .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 Short Term...MPS Long Term...CMS Aviation...CMS/MPS Marine...CMS/MPS Equipment...Drag

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