Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 171009 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 609 AM EDT Sat Mar 17 2018 .SYNOPSIS... Low pressure will remain cutoff over the Canadian Maritimes this weekend. Meanwhile, an area of low pressure will pass to our south today, then high pressure builds in from the north and west tonight and Sunday. A weak cold front progresses southward through the region Sunday night. An area of low pressure tracking across the Mid South on Monday will move eastward along the Virginia-North Carolina border Monday night. This low is expected to deepen as it moves northeastward off the Mid-Atlantic coast on Tuesday. Another wave of low pressure may develop along the slow-moving frontal boundary, passing south and east of the region on Wednesday. High pressure should start to build in Thursday and Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A low pressure system and an associated upper level trough continues to linger over Atlantic Canada keeping the area in a broad cyclonic flow and this will continue to be the trend through today. Most of the region will see a dry day however there will be a wave sliding by just to our south and this still looks to potentially clip southern parts of the Delmarva with a period of light rain this afternoon...mainly affecting areas south of Dover, DE. Farther north through SE PA and NJ, morning sunshine will give to filtered sun by the afternoon as high clouds increase due to the aformentioned system. Winds will be a little breezy but not as windy as Friday. Afternoon highs will be mainly in the 40s except some upper 30s across the southern Poconos. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Weak wave to our south quickly moves offshore through the evening with an otherwise mainly clear and chilly night on tap as a NW flow continues to dominate. Lows will be mainly in the 20s except around 30 over much of the Delmarva as well as the Philadelphia metro area and upper teens over the southern Poconos. && .LONG TERM /SUNDAY THROUGH FRIDAY/... The focus in the long-term period continues to be on the midweek storm. A positive note is there has been a clear trend in the guidance over the past couple of cycles, including last night`s 00Z run, toward a more progressive solution that would favor a suppressed storm track for the first wave of low pressure Monday night-Tuesday and even more notably for the second wave Wednesday- Wednesday night. If these trends were to hold up, the extent, duration and severity of impacts would be much lower than what models were indicating 1-2 days ago. Given this storm is still 3-5 days out and the predictability skill for such a complex setup (with a parade of disturbances in play along with the potential for phasing to occur) is limited, it`s a bit premature to rule out a shift back in the other direction that would put us back in play for a long duration, higher-impact event from late Monday night through Wednesday night. The bottom line is the poor run-to- run continuity and lingering spread among the operational models and the individual ensemble forecast systems hinders our ability to provide specific details with high certainty, on things such as rain and snow accumulations, where the rain/snow line sets up and how it evolves over time, and the magnitude of winds/beach erosion/coastal flooding. Keeping the abovementioned caveats in mind, a huge motivating factor for cautiously maintaining our message for a potentially high-impact event is the vulnerability of our infrastructure following the recent Nor`easters this month: Trees have been weakened/damaged, restoration efforts are still ongoing, and soils in E PA and NJ are still very saturated. This all makes our region more susceptible than usual to another round of power outages and flooding IF a worst-case scenario would come to we don`t want to let our guard down too soon. Concerning the rain vs. snow aspect of this storm... The track of the primary surface low will likely be south of our region (along the VA-NC border) while a secondary coastal low develops near Norfolk and tracks northeastward over or nearby the Gulf stream. Synoptically, this would be a favorable pattern for snow across the region. However, thermal profiles look to be marginally supportive of accumulating snow (more likely a rain-snow-sleet mix) at least initially, especially in the I-95 corridor and coastal plain, given the lack of cold air from a storm system of Pac NW origin and the influences of a high sun angle in late March. A transition to wet snow would be possible at some point as (1) Canadian high pressure building eastward allows cold air to drain southward into the region and (2) the coastal low deepens off the Mid-Atlantic coast while the upper low reaches our longitude. The latter would favor strong dynamical cooling although it is highly conditional on deep ascent and heavier precip rates setting up over our region- which is becoming increasingly doubtful given the latest trends. High pressure builds in behind the storm later Thursday into Friday. Cold (5-10 degrees below normal) and dry conditions would be expected in this pattern. && .AVIATION /10Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...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. Tonight...VFR. NW winds less than 10 knots. Outlook... Sunday and Monday...VFR. NW winds of 10 kt or less on Sunday may briefly back out of the W-SW ahead of a surface trough Sunday evening, then become N-NE late Sunday night-Monday morning. High forecast confidence. Monday night and Tuesday...Onset of precip and MVFR/IFR conditions likely to occur late sometime Monday night or Tuesday morning. At this point, a wintry mix (rain, snow, sleet) would be favored for I- 95 terminals, rain farther S/E (ACY, MIV) and snow or dry conditions farther N/W (ABE/RDG). Confidence on ptype and amounts of each ptype are still low at this point. NE winds increase and become very gusty with the strongest winds expected near the coast (e.g., ACY) on Tuesday. Tuesday night through Wednesday night...Following the first part of the event Monday night-Tuesday, a second wave of low pressure could prolong the precip, IFR conditions and gusty NE winds. However, there is less support for this scenario from the latest model guidance with the potential for drying conditions, especially along and north of PHL. Forecast confidence is low. && .MARINE... Winds should diminish below SCA levels by morning over the southern Delaware Bay as well as DE coastal waters so SCA expires at 6 AM for these areas. Otherwise, expect marginal SCA conditions to continue through the day for the NJ coastal waters. The winds here could briefly subside below SCA levels this morning before increasing again this afternoon. Heading into tonight, winds should diminish below SCA levels for all areas by early this evening so SCA over the NJ waters ends at 6 PM. Outlook... Sunday and Monday...Winds and seas below SCA criteria. A W-NW wind direction on Sunday becomes N-NE Sunday night and eventually E on Monday. Monday night through Wednesday...NEly winds should quickly strengthen late Monday night and Tuesday to gale force while seas build as a low pressure organizes to our south. NEly gales and high seas will likely continue through Wednesday. A brief period of storm force winds would be possible if the low deepens fast enough but that is still highly uncertain at the point given the southward trend with the storm track. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ450>453. && $$ Synopsis...Klein Near Term...Fitzsimmons Short Term...Fitzsimmons Long Term...Klein Aviation...Fitzsimmons/Klein Marine...Fitzsimmons/Klein is the U.S. government's official web portal to all federal, state and local government web resources and services.