Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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446 FXUS61 KPHI 272009 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 409 PM EDT Sat May 27 2017 .SYNOPSIS... A weak surface low will move off the coast of Virginia tonight. Weak high pressure will build into the Northeast on Sunday. A stronger low pressure system will move into southeast Canada on Monday, which will bring a cold front through the area. A couple of reinforcing cold fronts will move through the region during midweek before a surface high approaches the East Coast by the end of the week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Weak high pressure across the area tonight with an upper disturbance crossing through this evening. You can expect a continuation of mostly cloudy skies with some showers across Delmarva and southern New Jersey. A few lighter sprinkles may also occur across ern Pennsylvania and New Jersey this evening. Clouds will tend to thin out later tonight once the short wave passes the area. Tonight`s low temperatures will drop into the low/mid 50s up north and upper 50s across metro Philadelphia and Delmarva areas. Winds will be light tonight. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... The area of weak high pressure will be offshore Sunday morning. Low pressure will be advancing towards the area from the Ohio Valley. Overall much of the daytime hours will be dry, but clouds will be increasing over the area during the day. A couple showers are possible across the southern Poconos or Berks County by late afternoon. High temperatures tomorrow will be much like today`s, with low/mid 70s in most areas. It will be cooler across the shore areas and over the higher elevations up north. Winds will be mostly E or SE at 5 to 10 mph. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Primary forecast concern is chances for storms Sunday night into Memorial Day. Operational models have come in a little faster with the system moving through the region to close out the holiday weekend. Currently, the relevant vorticity maximum is in the Rockies and adjacent Great Plains. The trough axis is strongly positively tilted at this time, but with the aid of a kicker vort max in central Canada, the western side of the central U.S. vort max will pivot rapidly eastward on Sunday, with the trough axis becoming more neutrally tilted Sunday night as it approaches the Great Lakes and Ohio Valley. Attendant surface low will intensify as it tracks northeastward from the southern plains to the vicinity of Lake Erie by 00Z Monday, with a broad region of warm air advection downstream into the Mid-Atlantic. Isentropic ascent poleward of the associated baroclinic zone will permit the development of widespread precipitation in the central Appalachians, the Mid-Atlantic region, and portions of the Northeast. A predecessor vorticity maximum Sunday afternoon/evening, in conjunction with increasingly favorable large-scale dynamics (including favorable positioning of an upper-level jet streak and downstream difluent flow), will allow for deep lift and substantial cooling of the mid and upper levels of the atmosphere. As a result, elevated instability will increase to the north of a quasi-stationary boundary positioned in the southern Mid-Atlantic. Though the strongest ascent and most widespread precipitation should be to the north and west of the area Sunday night (in closer proximity to the surface low), models indicate 35 to 45 kts of 925-850 mb layer flow orthogonal to the baroclinic zone in the Mid-Atlantic. With such strong ascent in play, think chances are quite good for a round of storms Sunday night into at least early Monday. Raised PoPs quite a bit during this time frame, given the reasonable agreement among the recent simulations of the NAM/GFS/CMC/ECMWF. Less obvious is the residual environment within the warm sector on Monday in advance of a cold front moving through the region. Latest models swing the front through the region rather early in the day, indicating that the time window for recovery of instability will be limited at best. As such, think chances of convection beyond Monday morning are somewhat more limited, though given the model variability run-to-run regarding frontal passage timing...not particularly confident of that assessment. Nevertheless, felt it wise to reduce PoPs from southwest to northeast late Monday morning onward, particularly given the dry looking simulations of most of the operational guidance. Tuesday looks generally like an "in-between-systems" day as the main vort max swings northeastward into New England and southeast Canada. The next perturbation will dig southeastward into the Midwest on Tuesday and then eastward through the Northeast on Wednesday as it rounds the semi-permanent longwave trough in eastern Canada. Associated cold front should generate another round of showers as it moves through, which looks to be Wednesday (with timing uncertainty becoming even more pronounced). Instability looks more limited with this system, so kept mention of thunder out of the grids for now, though models may be underplaying this a little bit (i.e., not sure if I`m buying the pronounced inverted-V boundary layer the GFS is depicting). The 12Z CMC, e.g., provides some evidence of more vigorous convective development on Wednesday. Thereafter, forecast looks a little drier as surface high pressure becomes established. The 12Z GFS provides a prolonged break in the active pattern, keeping somewhat drier northwesterly flow in the region through the following weekend, while past runs and the most recent 12Z CMC offer a strong low developing in the Ohio Valley into the Mid-Atlantic by next weekend. Latest ECMWF has a pretty dissimilar solution to either camp. Given the large uncertainty during this period, stuck quite close to the inherited forecast Thursday onward. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Weak high will be across the area tonight and Sunday. An upper disturbance will be over the region through early tonight. A generally VFR fcst is expected over the area. Clouds will prevail in many areas the first half of tonight and then they will diminish overnight. Cloud CIGS will be mostly 040-050. A few light showers are crossing the southern parts of our area attm and they will continue into the early evening. It`s possible that a TSTM may develop across Delmarva later this afternoon. The pops looked low enough to exclude from KILG`s TAF, however. VFR should continue into Sunday with clouds increasing again later in the afternoon. Winds will be mostly 5 to 10 knots thru the period. Wind directions will be W or SW this afternoon (but S at ACY), then winds will switch to onshore (E/SE) tonight and remain that way into Sunday. OUTLOOK... Sunday night and Monday: MVFR/IFR CIGs/VSBYs are reasonably likely, especially Sunday night. Scattered showers and storms expected, especially Sunday night and Monday morning, with conditions potentially improving during the afternoon. East to southeast winds around 10 kts Sunday night transitioning to west by Monday afternoon after cold frontal passage. Confidence below average. Monday night and Tuesday: Slight chance of showers/storms north/west of KPHL. Some chance of low stratus or fog on Monday night. Primarily VFR on Tuesday. Winds under 10 kts, likely transitioning from west to north to east during the period. Confidence below average. Tuesday night and Wednesday: Generally VFR. Slight chances of showers, primarily during the day Wednesday. Winds generally west or southwest 5 to 15 kts, with perhaps some gusts to 20 kts or so during the afternoon. Confidence below average. Wednesday night and Thursday: VFR expected. Winds generally west 5 to 15 kts. Confidence average. && .MARINE... Weak high pressure will remain across the waters tonight and Sunday. An upper air disturbance will bring some showers across Delaware Bay and the adjacent DE/srn NJ ocean waters through this evening. Conditions will remain sub-SCA through the period with winds mostly under 10 knots. Seas will be 2-3 ft on the ocean and 1-2 ft across Del bay. RIP CURRENTS... A low risk for the development of dangerous rip currents exists for the rest of today. For Sunday, an onshore flow is forecast to strengthen to around 10 to 15 kts, and this may result in the rip current risk approaching moderate especially for the coast of New Jersey. OUTLOOK... Sunday night through Thursday: Generally sub-advisory winds and seas expected. However, winds/seas will be close to advisory thresholds Sunday night and Monday as a storm system moves through the area. In addition, there is some risk for visibility restrictions Sunday night and Monday before a cold front moves through the area later in the day. A couple of chances for showers/storms exist through the period, especially Sunday night and Monday and again on Wednesday. && .TIDES/COASTAL FLOODING... A Coastal Flood Advisory remains in effect for tonight`s high tide. It will be in effect for the coastal counties of New Jersey and Delaware, and for the counties along Delaware Bay and the lower Delaware River up to near the Commodore Barry Bridge. Model guidance continues to suggest fairly solid minor coastal flooding will occur tonight, generally somewhat worse than last night and somewhat better than the night before. With the increasing onshore flow through the evening hours, these projections still appear reasonable. Nevertheless, advisory thresholds may be somewhat harder to reach on the Delaware coast and the coast of Delaware Bay, where the past 24 hours of offshore winds appear to have had a somewhat greater impact in lowering water levels. Minor coastal flooding may also occur during Sunday night`s high tide, as astronomical tides remain high and onshore winds should continue. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT Sunday for NJZ012>014-020>027. Coastal Flood Advisory from 11 PM this evening to 3 AM EDT Sunday for NJZ016. DE...Coastal Flood Advisory from 8 PM this evening to 2 AM EDT Sunday for DEZ002>004. Coastal Flood Advisory from 11 PM this evening to 3 AM EDT Sunday for DEZ001. MD...None. MARINE...None. && $$ Synopsis...CMS Near Term...O`Hara Short Term...O`Hara Long Term...CMS Aviation...CMS/O`Hara Marine...CMS/O`Hara Tides/Coastal Flooding...

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