Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 171231 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 831 AM EDT Thu May 17 2018 .SYNOPSIS... A nearly stationary front will extend from the central Appalachians through the central Mid-Atlantic through Friday night, with a series of surface lows moving slowly along the front. A stronger low will move from the Tennessee Valley tonight into the Northeast by Sunday, allowing the front to lift north before slowly dissipating this weekend. A cold front will approach the Atlantic coast early next week before stalling to the south by midweek. High pressure will attempt to nose its way into the northern Mid-Atlantic by this point. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The overall synoptic view: A stationary boundary lies just south of the Delmarva Peninsula, and extends back through the Ohio Valley. Weak low pressure lies on this boundary over West Virginia. Broad high pressure is centered over central Canada. Weak sub-tropical low pressure lies over the Florida Panhandle. Stationary boundary remains in place along the Mid-Atlantic today. Low pressure moves along the stationary boundary this morning, passes over the Delmarva and southern New Jersey, and moves offshore by early afternoon. Meanwhile, high pressure over central Canada builds to the south and east, and will be over the western portion of the Province of Quebec late this afternoon. Moderate to heavy rain is lifting through northern NJ and out of the region. Another area of moderate to heavy showers is moving over extreme SE NJ, and moving out to sea. Light to locally moderate showers and fog in place across the region. As the aforementioned weak sub-tropical low lifts to the north into the Southeast U.S. states and approaches the Tennessee Valley late this afternoon, some weak mid-level vortices associated with this low will lift to the north and east, and this keeps showers in at least the southern half of the forecast area throughout the day. Into this afternoon, some drier air sags into the northern half of the forecast area as that high builds to the south and east. Conditions remain cloudy, but showers will be confined to the Delmarva and southern NJ for the afternoon. With ML MUCAPE increasing to 200-300 J/kg, cannot rule out a few elevated thunderstorms, so will carry slight chance for thunderstorms in extreme southern portions of the forecast area. Highs will generally be in the 60s, but if some sun can break out in the southern Poconos, Lehigh Valley, and northern NJ, highs could get into the low 70s. Likewise for southern portions of the Delmarva. If the stationary boundary can lift a bit to the north, some warmer air may push highs into the low 70s for southern portions of eastern MD and for southern DE. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... High pressure over eastern Canada continues to build to the south and east and moves into northern New England tonight. Meanwhile, weak low pressure lifts to the north and east tonight, and begins to push the stationary boundary north as a warm front. It will be hard for showers to make it into the northern portions of the forecast area, so will keep Pops confined to chance for the southern Poconos and northern NJ. Otherwise, with increasing low level moisture, and PWATs building to around 2", widespread moderate to heavy rain showers will lift into the Delmarva, southeast PA, and southern NJ through the night. For areas north of I-195, generally 1/10-1/4" QPF expected. For areas where moderate to heavy rain will fall, generally 1/2" to 3/4", with locally up to 1", QPF is expected. The heaviest rain will move into extreme southern portions of the forecast area prior to daybreak Friday. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... The main focus in the long-term period remains the prolonged heavy rain event expected through early this weekend. The 00Z operational model suite is in fairly good agreement bringing widespread 2-4 inch rainfall to areas south of I-78 through Saturday, as a slow-moving upper low lifts from the Mid-South at 12Z Friday to the Mid-Atlantic by 12Z Sunday, becoming absorbed and sheared out by stronger northern-stream flow. Of particular importance are three meteorological phenomena present during this evolution: (1) a deep/southerly low-level fetch with backward trajectories into the eastern tropical Pacific and the Caribbean, which will provide highly anomalous (and deep-layer) moisture to the region Friday and Saturday, (2) a quasi-zonal/quasi-stationary front across the Mid-Atlantic, providing a low-level, subsynoptic-scale source of lift, and (3) favorable positioning of a slow-moving upper- level jet streak (extending from New England into the Canadian Maritimes). Until the upper-trough passes the Mid-Atlantic Saturday night and Sunday, the region will be in prime territory for periods of heavy rainfall, aided by warm-rain convective processes owing to the deep, nearly-saturated troposphere. Model soundings exhibit relatively meager instability (generally elevated, given the front`s position to the south of the area) through Friday night. Given this lack of CAPE below freezing, relatively little lightning is expected with the convective showers that develop. However, the warm-rain processes at work will generate highly efficient rain rates, and there is strong evidence that training of convection is possible at times through the event. The 00Z NAM and ECMWF output show widespread 3-6 inch rains for areas near and southeast of I-95, with the CMC and GFS a little bit on the drier side. These differences may mean little, though, as antecedent rainfall has made for saturated soils that may be able to handle little additional rainfall, particularly if occurring with heavier showers. Did not make changes to the flood watch with this forecast, as there remains a little uncertainty with the heavy rainfall potential on Saturday. Nevertheless, the output from operational models suggests another round of heavy rainfall is likely before the upper low progresses through the area. Additionally, the upper levels will begin to cool/dry during this phase of the event, which will likely mean greater coverage of lightning. There appear to be two phases to the heavy precipitation Friday and Saturday. The first occurs Friday and Friday evening as a weak perturbation impinges on the quasi-stationary boundary. Increasing low-level flow (850-mb winds 20-35 kts) will aid in pronounced isentropic lift near and poleward of the boundary, placing the southern CWA (generally I-76 southward) in a very favorable location for heavy rainfall. NAM and ECMWF are suggesting 6-hour totals of 1- 2 inches will be common, with locally higher amounts possible where the strongest convection develops. As the perturbation continues northward, the strongest lift moves off with it. However, the front will advance northward as well, and the region will become warmer/moister by Saturday. Another perturbation will lift northeastward during the day, and more convection should readily develop in the Mid-Atlantic just downstream. Convection may train in this environment as storm motions will be north-northeast with low- level flow from the south-southwest. This provides a favorable environment for convective regeneration nearly opposite to individual cell motion. Combined with the anomalous moisture in place, more heavy rainfall is expected Saturday, though the threat may become more localized in this regime. The upper low passes to the northeast Saturday night and Sunday, but there are indications a surface trough will remain in place on Sunday. With the origins of the system in the southern stream, it will remain warm/humid on Sunday (likely warmer, actually), with an environment still favorable for isolated/scattered convection. Kept slight-chance to chance PoPs on Sunday to cover this threat, which the 00Z ECMWF, in particular, is honing in on. PoPs may need to be raised if a more pronounced source of larger-scale lift is depicted in future simulations. Notably, a northern-stream trough passage (well to the north...in Canada) may provide such a source should its southern extension become stronger than currently advertised. A cold front will approach the area early next week as a stronger midlevel perturbation reaches the Northeast. An attendant surface low will likely develop along the front. Combined with differential cyclonic vorticity advection and associated midlevel cooling, more convection is expected to develop along the front Monday and Monday night. Increased PoPs slightly during this period, especially Monday evening, given the somewhat slower timing depicted by some of the guidance. This front will likely hang up again as it meets the sturdy subtropical western Atlantic ridge. With more upstream perturbations incoming, chances for convection continue into Tuesday and possibly beyond if the front does not progress too far south of the area. Models are depicting a drier period as a surface high noses in from the northwest around midweek, but this is not certain given the strong ridging to the southeast. Currently, there is no choice but to leave PoPs in for much of the rest of the long term at this time. I did trend temperatures to seasonal averages by midweek after a rather warm/humid start. && .AVIATION /13Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...IFR/LIFR CIGs this morning, lifting to IFR/MVFR this afternoon. Occasional rain showers through the day will result in MVFR VSBYs. Light east winds. Tonight...IFR CIGs/VSBYs in rain and fog. Light NE winds. Outlook... Friday through Saturday night...Periods of sub-VFR with occasional showers and possibly storms, especially on Saturday. Torrential rainfall is possible with the showers. Winds east- northeast 10 to 15 kts with stronger gusts, especially near the coast, Friday and Friday night. Winds become southeast/south Saturday and Saturday night, with potential for gusts continuing, especially near/southeast of PHL. Moderate confidence. Sunday through Monday...Periods of sub-VFR possible, though conditions may become predominantly VFR. Chances for showers/storms late afternoon through Sunday night and again Monday afternoon. Winds southwest or west 5 to 15 kts. Low confidence. && .MARINE... Winds and seas are expected to stay below SCA conditions tonight through Thursday. Some visibility restrictions are possible in rain showers and mist through this period. Outlook... Friday through Saturday...Small craft advisory conditions expected as east-northeast winds increase to 15 to 25 kts with gusts to 30 kts. Seas will also meet/exceed five feet through the period. Winds should diminish on Saturday, however (Friday night for Delaware Bay). Periods of rain, heavy at times, and local visibility restrictions likely. Small craft advisory was issued for Friday and Friday night for the Atlantic waters and for Friday and Friday evening for Delaware Bay. Saturday night through Monday...Sub-advisory conditions expected with occasional chances for storms. && .HYDROLOGY... Numerous rounds of showers and thunderstorms will continue across the region through the end of the week and into the weekend. Heavy rainfall is possible during this period, with the heaviest rain falling Thursday night through at least Friday night. The heaviest rain is forecast along and south of the I-78 corridor. The current forecast calls for 2-4 inches of rain between this afternoon and Saturday. This does not include the rain that fell the last few days. Up to three inches have already fallen across portions of southern New Jersey and the Delmarva. A variety of flooding types are all possible within the Flood Watch we issued. First, low-lying and poor drainage flooding is the most likely type of flooding to develop. Second, flash flooding will be possible under the stronger thunderstorms. Flash flooding occurs quicker than other types flooding due to the torrential rain thunderstorms can produce in a short amount of time. Flash flooding is most likely to occur across urban areas where rainfall runoff is maximized and where small creeks and streams respond quickly. The rain has to go somewhere, and it eventually leads to larger streams and rivers. This is the third type of flooding possible across the Flood Watch area. If you live across the Passaic, Raritan, and Rancocas basins in New Jersey or the Neshaminy, Schuylkill, and Brandywine basins across Pennsylvania and Delaware, keep an eye on later forecasts. These forecasts, as well as all of our hydrologic forecasts, can be accessed via our Advanced Hydrologic Prediction Service website. Tides are also expected to increase through the end of the week as well. This could exacerbate flooding across those areas where fresh water and tides meet. && .TIDES/COASTAL FLOODING...
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Updated 830 AM Thursday... The latest total water level forecasts continue to indicate that each high tide cycle through Friday morning should remain below advisory criteria...though a few places such as Cape May may get close with the high tide this evening. However, with onshore flow forecast to strengthen during the day Friday, there is a potential for a more widespread occurrence of minor coastal flooding with the Friday evening/night high tide cycle.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flood Watch from 6 PM EDT this evening through late Friday night for PAZ060>062-070-071-101>106. NJ...Flood Watch from 6 PM EDT this evening through late Friday night for NJZ007-009-010-012>027. DE...Flood Watch from 6 PM EDT this evening through late Friday night for DEZ001>004. MD...Flood Watch from 6 PM EDT this evening through late Friday night for MDZ008-012-015-019-020. MARINE...Small Craft Advisory from 8 AM Friday to 6 AM EDT Saturday for ANZ450>455. Small Craft Advisory from 8 AM Friday to midnight EDT Friday night for ANZ430-431. && $$ Synopsis...CMS Near Term...MPS Short Term...MPS Long Term...CMS Aviation...CMS/MPS Marine...CMS/MPS Hydrology...Kruzdlo Tides/Coastal Flooding...Klein/Fitzsimmons

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