Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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018 FXUS61 KPHI 222216 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 616 PM EDT Thu Jun 22 2017 .SYNOPSIS... Low pressure will track well north of our area Friday and Saturday. An associated cold front crosses our region during Saturday as some tropical moisture moves ahead of and along it. As high pressure builds well to our southwest Sunday into Monday, a weak cold front moves through our area later Monday. High pressure then builds across the Mid Atlantic Tuesday and Wednesday, before shifting offshore Thursday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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The 6:30 pm ESTF update will include a few tweaks with temps and dewpoints. Sky, weather, and POPs were also adjusted across our far western zones as the showers and thunderstorms are dissipating a bit more slowly. The HRRR brings some precip to I95 corridor between 03-04z, but we feel this over done. Previous discussion... The first challenge of the near term forecast will be if and how quickly any convection associated with a low level trough to our west reaches our region by early this evening. Showers and thunderstorms have already developed across western PA, but it is unclear if they will reach our region before sunset, when we should see activity diminish (this convection appears to be very much diurnally driven as there is limited forcing in the mid and upper levels and model soundings show a low level inversion developing quickly this evening. Through the overnight hours, the closed low over south central Canada is expected to continue gradual progress east. As it does so, we may begin to see the next round of showers and thunderstorms move into our region in the pre dawn hours, though latest model timing indicates it is unlikely we will see this before daybreak.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... As the low and it`s associated cold front make slow progress east, it will interact with remnant moisture of TD Cindy. Through much of the day tomorrow, it looks like the axis of the deepest moisture and best lift generally stay to our north and west. Having said that though, it is likely that some convection will move into our region through the day especially west of the fall line. It is also likely that we will keep mid level clouds over our region for much of the day as the mid level cloud shield associated with this is already moving into SW VA. Cloud cover will be crucial in determining our severe potential tomorrow afternoon. Model soundings indicate that if we can see some clearing by mid day, we could have CAPE values in our region over 500 J/kg. Bulk shear remains limited (generally less than 30 kt) until very late in the day. Thus, any severe threat (hail and strong winds) will be conditional on clearing early enough in the afternoon. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Summary...Tropical moisture moves through Friday night and the first half of Saturday with a cold front, then lowering humidity levels during the second part of the weekend and next week. Some cooling next week as a trough settles into the East for a time. Synoptic Overview...An upper-level trough amplifies eastward from the Midwest and Great Lakes to the Northeast through Sunday. This will send a cold front our way, however the remnants of tropical system Cindy interacts with it. This feature, while possibly less defined at the surface, should have its mid level energy enhance our chances for locally heavy rain later Friday night and Saturday morning. The trough is then forecast to sharpen in the East through the first half of next week, before lifting out later Wednesday or Thursday. The presence of this trough will result in cooler temperatures for a few days, with perhaps some instability showers/thunder especially early in the week. We used a model/continuity blend for Friday night through Saturday night, then blended in the 12z WPC Guidance. Some adjustments were then made following additional collaboration with our neighboring offices. For Friday night and Saturday...As a trough amplifies eastward from the Midwest and Great Lakes region, surface low pressure tracks well to our north. An associated cold front however will be moving through our area Saturday. While this occurs, the remnant energy from tropical system Cindy gets caught up in the faster flow aloft as it moves northward and then accelerates east-northeastward. While the surface feature should be less defined, the model guidance shows the mid level energy holding together as it crosses especially the southern half of our area Saturday morning. There is an axis of very high moisture content air moving east with PW values of 2.0-2.5 inches. This moisture in combination with lift from the front and remnant tropical system Cindy will result in locally heavy rain. While there is some potential for localized flooding, the system should be moving through fairly quickly. As a result, we held off on a Flash Flood Watch at this time. This may have to be revisited though especially for the urban I-95 corridor. The main area of concern looks to be from the Philadelphia metro area on south and eastward. The forecast soundings generally show a tall and thin CAPE profile, which within a very moist air mass tends to lead to areas of locally very heavy rain. Backing up a bit, there should be ongoing convection to our west that spreads eastward Friday night with the front and then the tropical system remnants. We have an area of likely PoPs across the western/northern areas Friday evening, then these spread south and eastward overnight and increase to low end categorical toward daybreak for some areas. For Saturday, we confined an axis of likely to low-end categorical PoPs to mostly the morning hours. At least some clearing should then occur from west to east Saturday afternoon and especially at night with dew points dropping in the wake of the cold front. It is possible that a few showers or thunder develops in the afternoon especially across the eastern and southern zones where lingering instability is present before the front clears the coast. For Sunday and Monday...An upper-level trough is forecast to amplify across the East during this time frame. This should be accompanied by a secondary cold front at the surface later Monday. While Sunday overall looks dry with lowering dew points, some instability showers or low-topped thunderstorms will be possible Monday especially in the afternoon to early evening. The presence of the upper-level trough and accompanying cooling aloft will result in cooler surface temperatures especially more noticeable on Monday. For Tuesday through Thursday...The upper-level trough in place Tuesday into Wednesday may begin to lift out later Wednesday or Thursday. This will allow surface high pressure to build in from the west-southwest before shifting offshore Thursday. As the main part of the trough is overhead Tuesday, there will be the chance for some instability showers during the daytime. The dew points are anticipated to be in the comfortable zone due to the presence of the trough and associated cooler air mass. Some moderating of the temperatures is expected Wednesday and especially Thursday as high pressure moves offshore and a warm air advection pattern should begin. && .AVIATION /22Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tempo -SHRA was added to both KRDG and KABE between 23z and 02z to account for the showers and thunderstorms moving in from the west. Previous discussion... VFR conditions are expected to continue through at least 00Z, and likely through 06Z. After 00Z lower clouds and showers and thunderstorms are expected to approach the region from the southwest. Though the chance for showers and thunderstorms begins after 00Z, it looks more likely that precipitation will hold off in our region until closer to 12Z. With any showers and thunderstorms, MVFR conditions are likely, and IFR conditions are possible, especially with heavy downpours which are possible tomorrow morning. Conditions may improve to VFR for several hours after 18Z. OUTLOOK... Friday night and Saturday...Times of MVFR/IFR conditions with showers and some thunderstorms. Some heavier rain can significantly reduce the local visibility at times. The greatest coverage of showers should be late Friday night and Saturday morning, with improving conditions Saturday afternoon and night. Sunday...VFR overall. Monday and Tuesday...VFR overall, however a few mainly afternoon and early evening showers or a thunderstorm are possible each day.
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&& .MARINE... Winds and seas should stay below SCA criteria through Friday morning. By mid day seas and southwesterly winds will increase on the coastal waters and SCA conditions are likely to develop by early afternoon. Strong winds are possible in the vicinity of any thunderstorms that may develop especially Friday afternoon. OUTLOOK... Friday night and Saturday...A cold front will approach Friday night with some increase in a southwesterly flow. The forecast soundings indicate that the mixing is not all that efficient due to a very warm and humid air mass, however times of 25-knot gusts can occur and this should build the seas on the ocean to around 5 feet. As a result, a Small Craft Advisory was issued for only the ocean zones through 10z/6 AM Saturday. This advisory may need to be extended through much of Saturday prior to the frontal passage. Sunday through Tuesday...The conditions are anticipated to be below Small Craft Advisory criteria during this time frame, with subsiding seas during Sunday. RIP CURRENTS... We are forecasting that the low risk for the formation of dangerous rip currents will continue into tomorrow. However, even with a low risk...the bigger diurnal difference in the tide cycle as we approach the date of this months new moon could mean some rapidly changing conditions. && .TIDES/COASTAL FLOODING... High astronomical tide levels are expected in association with the new moon on Friday. That, combined with increasing southerly flow, is expected to result in minor coastal flooding with the afternoon and evening high tide for the Atlantic coastal areas and the shores of the lower Delaware Bay. There may be minor coastal flooding further up the bay, but it is unlikely, so will not expand the coastal flood advisory at this time. Minor coastal flooding may occur again with the high tide cycle Friday afternoon and evening, though the latest guidance has backed off slightly from previous runs. We will have a better idea of the potential tomorrow once we see the tidal departures this evening. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory until 9 PM EDT this evening for NJZ012>014-020-022>027. DE...Coastal Flood Advisory until 9 PM EDT this evening for DEZ003- 004. MD...None. MARINE...Small Craft Advisory from noon Friday to 6 AM EDT Saturday for ANZ450>455. && $$ Synopsis...Gorse Near Term...Johnson/Kruzdlo Short Term...Johnson Long Term...Gorse Aviation...Gorse/Johnson/Kruzdlo Marine...Gorse/Johnson Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.