Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 222343 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 743 PM EDT Sat Apr 22 2017 .SYNOPSIS... Several waves of low pressure will move west to east along a stalled frontal boundary to our south from the Tennessee Valley into the Carolinas through Sunday. High pressure will also be anchored to our north. By Monday, another low pressure system will organize over the Carolinas and move up the east coast through the middle of next week. Another frontal boundary will attempt to approach from the north as high pressure becomes more dominant across the region later next week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... The front to our south will continue to push farther south overnight as high pressure begins to build in from the west. Meanwhile, the northern half of a trough aloft to our west begins to flatten out tonight as it approaches the area, while the southern half turns into a cutoff low. There will continue to be showers into this evening as several vorticity impulses and enhanced moisture continue to move across the area in the southwest flow aloft. But as the flattening short wave approaches, the vorticity weakens as will the lift across the area. Showers will come to an end from west to east through the late afternoon and evening, eventually clearing the coast shortly after midnight. A few showers could linger on the southern Delmarva through sunrise Sunday. Overall rainfall will be mostly light in nature, but a few pockets of moderate rainfall will be possible. There is no instability across the area with a stable airmass in place, so we`ve removed the chance of thunderstorms from the forecast. If there were to be any, they would be confined to southern portions of the Delmarva. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... Weak high pressure briefly builds across the area early Sunday, before moving offshore Sunday. The frontal boundary to our south remains well south through the day, but may begin advancing northward late. Sunday may actually be a pretty nice day for much of the area as dry air above the surface will be in place. There could be some cumulus build up during the day along with some cirrus overhead, but there should be a fair amount of sunshine across the area, unless the cirrus gets too dense. Any precipitation that would occur during the day would be confined once again to southern portions of the Delmarva. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Sunday night: The stalled frontal boundary should be far enough south to get a break in the light overrunning rainfall for most of the region with light rain beginning to move northward in Delmarva as moisture intersects the boundary and is condensed in the cooler airmass. Low temperatures will generally be in the 40`s and stayed close to the MET/MAV blend in this period for most weather elements. Monday through Tuesday night: A strong ensemble signal is present that low pressure will organize over the Carolina coastline. However, some spread is present on exactly where this will occur. Two different parts will be interacting to form this low. The first part being the persistent stalled frontal boundary and second is a low pressure system coming from the Caribbean, bringing tropical moisture northward. Model and ensemble guidance to some extent are struggling a bit on the interaction of these two features, fairly typical for a few days out. This will have a large impact on just how much rain falls as the low moves northward throughout this timeframe. For now, the forecast gives a lean toward the drier GFS and ECMWF. On the other hand, the Canadian and the UKMET are are further west and wetter. Incorporating the wetter CMC and UKMET has out a QPF forecast a bit higher than WPC along with the GEFS and some other ensembles. A concern is that the GFS and ECMWF are having issues resolving the convection as the low moves northward. In some aspects this system QPF wise is a reminder of the March storm with model tendencies. So would not be surprised to see this system trend further west based on potential convection issues. The good news in terms of excessive rainfall is that we will still be in a cooler airmass with lower PW values than ideal (max around an inch) for very heavy rainfall and flooding concerns. Still over the course of 36-48 hours, 1 to locally 2 inches of rainfall could fall with the highest amounts over Delmarva with totals under an inch north and west of Philadelphia. Low clouds, rain and an easterly onshore flow will keep temperature ranges small and rather chilly with highs in most spots only in the 50`s. This forecast trends several degrees cooler than MET/MAV. Winds should pick up as well from the east with some gusts around 20 mph at times, not a favorable environment for mixing but with the low pressure nearby a tighter pressure gradient will be present. Wednesday and Wednesday Night: Fairly good agreement is present to linger a few showers into early Wednesday, with the CMC and ECMWF on the slower end. Temperatures should start a rebounding trend with perhaps a few breaks in the persistent gloomy overcast late Wednesday. Stayed close to ensemble guidance on most elements. Thursday through Saturday: A busy late week and weekend in Philadelphia. unfortunately, the forecast is riddled with more uncertainty by late week with temperatures and further rain chances. A good piece of news is that high pressure will become more dominant resulting more sunshine and a break from the gloom most of next week will bring. Current thinking is that precipitation from any fronts will stay northwest of the region but concede rain chances may have to be added as more information comes in. The modeling is bringing another front a little further south with each run for the later half of the week with a cooler/wetter trend. This could still reverse given 5-7 days out. Temperatures will likely warm-up with a good shot for a more persistent southerly flow on Thursday and Friday. The movement of the approaching front will determine if the region sees a more persistent flow from the north and east with more clouds and highs in the 60`s or if the front stays north of the region allowing for highs to push well into the 80`s or warmer (00z EC temps at 18z +5 F even yield 90 in a few spots by Saturday. Will keep consistency from the previous forecast and not swing one or way or another given the 4 standard deviation spread with the 00z ensemble guidance by next weekend with temperatures. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. The back edge of the showers is sweeping across the NRN/WRN areas attm and will likely end across the Del Valley shortly after 00Z. The showers will clear the SE TAF sites 02Z-03Z. Conditions should remain VFR overnight with a decrease in low/mid clouds. A area of high clouds will remain over the area overnight. Conditions will probably no favor fog formation with the drier air arriving from the north and the high clouds remaining over the area. There is some small chc for some fog perhaps at MIV/ILG. We chose to keep it out of the TAFS attm. Winds, which have switched to N or NW in most areas, will remain there this evening and overnight. The winds speeds will be mostly 10 knots or less. Winds will then become northeasterly during the day Sunday, eventually becoming southeasterly late in the day. Wind speeds Sunday are generally expected to be 5 to 10 knots. OUTLOOK... Sunday night: VFR. Monday and Monday night: MVFR and IFR spreading from south to north for KMIV, KACY and KILG with rain. VFR KPHL and north. East northeast winds 10-15 knots. Tuesday through Wednesday: MVFR and IFR with several periods of rain. Perhaps some intervals of VFR for KABE and KRDG where more breaks in the rain are likely. Rain will end on Wednesday from west to east with a slow improvement to VFR. Easterly winds becoming northerly, gusts near 25 knots on Tuesday from the east decreasing to under 10 knots from the north on Wednesday. Wednesday night and Thursday: VFR. Winds under 10 knots. && .MARINE... Winds and waves are expected to be below Small Craft Advisory levels tonight through Sunday. Although seas will be around 4 feet and winds could gust around 20 knots at times. OUTLOOK... Sunday night: Seas building to around 5 feet late in response to low pressure formation south of the region. Easterly winds increasing from south to north as well but staying under 10 knots northern ocean waters and 20 knots southern ocean waters. Monday through Wednesday night: SCA seas and gusts throughout most of the period. Wind gusts peaking from the east around 35 knots Tuesday afternoon and evening on the ocean waters. High confidence for SCA conditions with a low to moderate confidence in Gale gusts. Both the GFS and ECMWF so a surge of 40 knot wind at 925 mb moving into the region at that time but conditions for mixing aren`t expected to ideal late Tuesday. Winds decreasing and turning northerly on Wednesday. Seas will also decrease starting on Wednesday. Thursday; Seas likely still over 5 feet with a slow decrease. && .TIDES/COASTAL FLOODING... A look at ensemble guidance shows the potential for minor coastal flooding centered around the high tides on Tuesday for the Delaware Bay and oceanfront. A persistent onshore flow will be in favor of the tidal flooding along with the proximity to a new moon. However, the low pressure system is not expected to strengthen much below 1000 mb at this time. Both of those factors support the ensemble guidance with keeping any tidal flooding in the minor category. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gaines Near Term...Robertson Short Term...Robertson Long Term...Gaines Aviation...Gaines/Robertson/PO Marine...Gaines/Robertson Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.