Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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931 FXUS61 KPHI 221348 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 948 AM EDT Sat Apr 22 2017 .SYNOPSIS... A cold front will remain south of the region this weekend while high pressure builds in from the northwest. An area of low pressure developing along the boundary to our south will move off the Southeast coast Sunday night. The offshore low will then turn northward Monday and track just off the eastern seaboard through the middle of next week. A cold front is forecast to pass through the area Thursday or Thursday night but may quickly return back northward as a warm front on Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A frontal boundary will remain stalled out south of the area today with our area on the northern side of the boundary with northeasterly flow this morning, eventually becoming northwesterly later today. Meanwhile, an elongated trough aloft extends from southeastern Canada southwestward into the central plains with our area under the influence of southwest flow aloft. This southwest flow aloft will help keep enhanced low-mid level moisture in place through the day today. With several vorticity impulses expected through the day, there will be enhanced lift which will lead to scattered showers through the day. Showers will be mostly light with some pockets of moderate rainfall possible at times. Models are depicting a second round of somewhat steadier precipitation moving in late this afternoon generally south of I-78. Hi-res guidance appears to be converging on the timing (generally 3 pm to midnight), with the best chances likely south of the Mason- Dixon Line. Therefore, kept PoPs elevated late this afternoon in the southern CWA with this second steadier round. Notably, the NAM/NAM Nest timing looks a little faster than other guidance (HRRR, RAP, WRF-ARW, WRF-NMM), and the coarser operational guidance (GFS, ECMWF) appears to agree, which would mean most of the precipitation would be done by early evening. There is some question regarding storm potential this afternoon, with the NAM and WRF simulations depicting an environment of marginal instability remaining present near Cape May and Georgetown. Other models indicate negligible CAPE present. Felt the need to include a slight chance of storms in these areas this afternoon given the models so far underplaying the convective potential north of the baroclinic zone in general, but not very confident of this. With the northeasterly-northwesterly flow north of the frontal boundary, and amount of cloud cover and showers expected through today, temperatures are not expected to warm very much through the day with highs generally in the upper 50s to low 60s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... The cutoff process of the central U.S. vort max continues tonight, with the upper low expected to be in the vicinity of the Mid-South by 12Z Sunday. Operational models generally agree that a perturbation ejecting from this low will be exiting the Mid-Atlantic tonight, followed by a brief period of subsidence as downstream ridging commences along the coast of the Southeast. The current forecast depicts a drying trend across the CWA during this period, with PoPs generally confined southeast of I-78 Saturday evening to far southern Delmarva/southern NJ after midnight. There may even be some partial clearing in the southern Poconos and vicinity by late in the night. Overnight lows are expected to be on the chilly side, with temperatures in the 30s to around 40 northwest of the Fall Line to the mid to upper 40s in the urban corridor and areas southeast. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Surface high pressure/low-level ridge axis extends anticyclonically from the Southern Plains to the northern Mid Atlantic/Northeast states. Subsidence underneath the ridge should provide an opportunity for most of the CWA to dry out on Sunday. PoPs were decreased considerably but there still remains a low chance for showers from the Delaware Bay southward. Did not want to entirely close the door on the possibility of lingering precip across these far southern zones with some of the models still supporting the stationary front to get hung up nearby to our south. A cutoff low developing over the Mid South late in the weekend is forecast to gradually progress eastward over the Southeast states Sunday night-Monday then offshore Monday night. The closed low then looks to transition to an open wave and turn northward Tuesday in response to strong downstream ridging over the western Atlantic. Models generally show this system tracking parallel to the eastern seaboard but remaining offshore during this time. The period beginning Sunday night for locales south of the Mason-Dixon Line (Monday farther north into eastern PA and NJ) through Tuesday or Tuesday night is setting up to be wet though it`s a bit premature to label this a total washout. Rainfall does not look to be particularly heavy since the region is located on the stable, cool side of the frontal boundary. Persistent onshore flow and clouds will result in below normal temperatures (particularly for daytime highs) Sunday through Tuesday. However, highs on Sunday may wind up being near normal across NE PA and NW NJ if clouds are able to clear out early enough to allow for stronger heating to take place in the afternoon. Conditions should dry out Wednesday as the offshore low passes north of our latitude and a shortwave ridge builds overhead. With surface winds quickly becoming southerly and subsidence underneath the ridge, expect a warmup into the 70s on Wednesday (except cooler along the coast and at higher elevations in NE PA/NW NJ). The warming trend continues into Thursday with the region situated deeper within the warm sector ahead of a cold front. Temperatures on Thursday will ultimately depend on the timing of this front, which is still uncertain this far out. We will continue to side close to WPC guidance, yielding highs in the 80s across most of the region (cooler again near the coast and in the mountains) with the assumption that the fropa occurs after peak heating. Added a chance for showers and thunderstorms during the afternoon and evening but PoPs were generally kept low due to the aforementioned timing uncertainty of the front. The next low pressure system is forecast to organize over the Southern Plains and then track northeastward through the Midwest and Great Lakes region late in the week. In this pattern, a warm front would eventually lift northeastward through the region but a large temperature gradient that may potentially exist across the front and high uncertainty in fropa timing greatly degrades forecast confidence for high temperatures on Friday. We would have the opportunity for 80s again on Friday if the warm front is quick to progress through, but if it is delayed, onshore flow on the cool side of the warm front will yield highs 10-20F lower (a scenario similar to what we saw yesterday). Forecast represents an in-between solution, which is lower than the latest WPC guidance. && .AVIATION /13Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Conditions vary across the TAF sites from IFR across the south, to VFR north, and MVFR/VFR in between. The overall trend is expected to be improving conditions through the day, although there will remain scattered showers across the entire area through the day. All sites should be VFR by this evening and overnight. The showers will begin to slide southeast this evening and should clear most areas except southern New Jersey and central/southern Delmarva where should could continue into the overnight hours. Winds should generally be north to northeast this morning, then become more north to northwest by this afternoon through tonight. Speeds are expected to be around 10 or kts or less through tonight. OUTLOOK... Sunday...Predominately VFR with showers staying well south of the terminals. There is some potential for MVFR CIGs during the late morning/afternoon near ACY/MIV if stratocu becomes BKN. NE winds 5- 10 kt in the morning veer out of the E-SE by late afternoon. Sunday night and Monday...Rain moves spreads back northward and CIGs lower to MVFR late Sunday night and Monday. Low confidence in IFR CIGs, but if it does happen, it would more likely occur at the southern most zones (ILG/MIV/ACY). Onshore winds continues. Monday night through Tuesday night...IFR restrictions likely with period of rain. Persistent NE winds will be strongest (10-20 kt) Tuesday afternoon and evening. Wednesday...Improving conditions. MVFR CIGS may linger into the first part of the morning before trending to VFR later in the day. && .MARINE... Scattered showers and isolated storms will continue today across the New Jersey/Delaware coastal waters and in Delaware Bay. Outside of gusty/erratic winds near storms, winds will stay below advisory criteria through tonight, with seas generally 2-4 feet. There may be some patchy fog this morning, but visibilities are expected to remain above 1 SM. Chances for showers decrease late tonight. OUTLOOK... Sunday...NEly winds in the morning, becoming Ely in the afternoon. Winds will increase from S to N (10-15 kt in DE Bay and NJ coastal waters, 15-20 kt in DE coastal waters). Although there is no SCA headline issued, there is still a chance that seas could reach 5 ft in our southern coastal waters where the gradient is strongest. Sunday night...E-NE winds increase to 15-25 kt late (strongest winds south). Seas will also increase, likely reaching or exceeding 5 ft in DE coastal waters and possibly farther north adjacent to the southern NJ coast. SCA likely needed for at least ANZ454-455. Monday through Tuesday night...SCA expected across the region with strengthening E-NE winds 20-30 kt. There is an outside chance for gales Tuesday when GEFS probability of greater than 30 kt sustained winds increase to 20-30 percent while seas greater than 9 ft increase to 50-70 percent in our outer coastal waters. Wednesday...Winds expected to drop below advisory levels, but seas may remain elevated. && .TIDES/COASTAL FLOODING... An extended period of onshore flow will occur Sunday through Tuesday night. The strongest winds are currently forecast for Tuesday, when low pressure tracks up the Southeast coast. Meanwhile, astronomical tides will be elevated with a new moon Wednesday. The above setup may lead to minor coastal flooding if the onshore flow sufficiently increases leading up to the higher of two high tide cycles. The most vulnerable high tide would be Tuesday evening when tidal departures of around 1 ft above astronomical would be enough to produce coastal flooding. Early Saturday morning ETSS guidance has Lewes, Atlantic City and Sandy Hook reaching minor coastal flood stage but just below the advisory criteria. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...CMS/Robertson Short Term...CMS Long Term...Klein Aviation...CMS/Klein/Robertson Marine...CMS/Klein Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.