Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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001 FXUS61 KPHI 220759 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 359 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/... The large-scale environment features a potent vorticity maximum in southeast Canada and adjacent New England with the vorticity lobe elongated southwestward to a second potent maximum in the central plains of the U.S. This second maximum is in the (gradual) process of being cut off from the polar jet as the northern and downstream vort max pivots northeastward into the Canadian Maritimes. Attendant with the New England vort max is an upper-level jet streak, with the Mid-Atlantic in the right rear quadrant. Combined with differential cyclonic vorticity advection from the central plains vort max and low-level isentropic ascent occurring along and north of an east-west baroclinic zone stretched across the southern Mid-Atlantic, large-scale ascent will be present for much of the day across the area, allowing for multiple rounds of scattered showers and isolated storms, particularly in the southern CWA (which is in closer proximity to the baroclinic zone). These rounds will be timed based on small-scale perturbations ejecting from the main vort max in the central U.S. Scattered showers and isolated storms are developing in the Delmarva Peninsula and far southern New Jersey early this morning in an environment of marginal/thin elevated CAPE with parcels generally rooted in the 925-850 mb layer. Though instability is weak, deep- layer shear is sufficient for updrafts to generate small hail (in addition to decent rain rates). The threat for severe storms is low, but the threat of lightning is high enough to include in the forecast this morning generally along and south of the Mason-Dixon Line. High-resolution simulations continue to generate scattered showers/isolated storms through the morning southeast of I-295 with at least light rain/showers northwest into eastern Pennsylvania and northern New Jersey. Though the general idea is present in most high- res guidance, there are major discrepancies remaining, including timing of the steadier precipitation across the area, locations with the maximum QPF, and the strength of the convective showers on the south side of the precipitation shield. Based on very-near-term trends, there is reason to place somewhat heavier PoPs in the grids early-mid morning with gradually lowering values by late morning through mid afternoon. Precipitation amounts look to be light, but locally heavier amounts are likely in stronger convective showers. 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. Temperatures will hold fairly steady through the day given the overcast conditions and scattered showers. This is a very low predictability pattern, as the process of upper lows cutting off is sensitive to several phenomena poorly resolved/predicted by numerical models (and not well understood physically). In addition, the aforementioned small-scale perturbations ejecting from the main vort max are poorly resolved by operational models and can be modulated by proximity convection (or other processes). As such, the forecast for today will be highly reliant on extrapolation of current observations/trends and will be subject to several potentially significant modifications over the course of the day. && .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 /08Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Low-confidence forecast today. Generally IFR CIGs/VSBYs at all TAF sites through around daybreak before CIGs rise to MVFR or low-end VFR during the morning. Light rain or showers are expected to move through the terminals this morning, with potential for occasional MVFR/IFR CIGs/VSBYs, especially if heavier rain falls. Improvement to VFR is expected by afternoon, but another round of rain may move in by late afternoon or early evening, especially south of KPHL. Conditions should improve by late evening at the terminals. Winds should generally be north or northwest with speeds around or below 10 kts 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. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Klein Near Term...CMS Short Term...CMS Long Term...Klein Aviation...CMS/Klein Marine...CMS/Klein

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