Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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264 FXUS61 KPHI 252155 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 555 PM EDT Tue Apr 25 2017 .SYNOPSIS... Low pressure will track northeast just off the Delmarva and New Jersey coastline through Wednesday. Meanwhile, low pressure will move from the upper Mississippi Valley into James Bay, and send a weak cold front into our area Thursday night and Friday morning. A backdoor front is expected to move through the region Saturday evening. Another low pressure system moving from the lower Plains into Quebec and its associated fronts will affect the area Monday into Tuesday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... The water vapor imagery early this evening shows a well defined closed low centered near the coast of North Carolina. An area of significant dry mid level air is working northward on its eastern flank. An area of heavier rain is approaching our coast and this appears to be with a strong short wave rotating northward on the eastern side of the closed low. This batch of heavier rain will move onshore this evening, although it may weaken some as it gets farther inland. Some lightning has been occurring in this area, and a mesoscale analysis shows an axis of mainly elevated instability extending northwestward across our south/southeast zones. However, the more significant instability remains offshore. As a result, a few isolated thunderstorms cannot be ruled out for these areas this evening. Otherwise, only changes made were to adjust the hourly temperature, dew point and wind grids to keep them current. Some tweaks were made to the PoPs this evening based on radar trends and some assistance from the HRRR guidance. Otherwise, surface low pressure centered over eastern North Carolina will continue to slowly lift to the north and and east along the East Coast this evening, and should be just east of the Delmarva Peninsula by daybreak Wednesday. The rain tapers off between 00-03Z over the Delmarva, southern NJ, and SE PA, and then between 06-09Z over northern and central NJ and the Poconos. Some lingering showers will be possible throughout the region overnight, but additional rainfall will be light. Once the main rain ends, the pressure gradient weakens as the low lifts to the north. Winds take on more of a northerly direction, and some cooler air will filter into the region. With abundant low-level moisture in place, stratus will occur along with some fog. Not quite certain that dense fog will develop, but there may be some patchy areas where the visibility drops to one quarter of a mile. As of now, do not think a Dense Fog Advisory would be needed. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Low pressure off the NJ coast will continue to drift to the north on Wednesday. Upper level low will be just to the west of it, and with several strong shortwaves passing through the region, can expect some showers throughout the day. Otherwise, fog gradually dissipates through the morning, but clouds will remain across the region for most of the day. Winds back a bit to the N/NW by afternoon, but the gradient will be light, and winds will remain less than 10 MPH. Highs generally top off in the 60s, but a few spots may come close to 70. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... Overall - Warm pattern shaping up. The closed low that is currently over the southeast US finally exits the area early Wednesday night. With the polar jet displaced far to the north, several disturbances traversing the southern stream jet will aid in building a stout sub tropical ridge off the southeast US coast from Friday into the early part of next week. As another closed low ejects out of the midwest on Monday, a stronger and more active cold frontal passage occurs in the late Monday into early Tuesday period. Wednesday night into Thursday should feature dry weather under brief ridging aloft. With some partial clearing Wed night, light and variable winds within the col between weather systems, and abundant low-level moisture given recent rainfall, patchy fog is likely. The fog will burn off early Thursday, with partly sunny skies expected. A light southeast flow will maintain cooler temperatures at coastal locals. Highs will be around 10 degrees above normal. A weak cold frontal passage Thursday night into Friday will lead to considerable cloudiness, but moisture will be limited, with only a slight chance of showers across the region. In fact, with the strong subtropical high building over the southeast/orientation of the mid- level flow, this front is expected to washout/stall in the vicinity of the I-95 corridor. With deeper moisture and mid- level support exiting the region, expect a return to partly to mostly sunny skies by Friday afternoon. A weak synoptic flow should encourage a robust sea breeze. Highs will be several degrees warmer than Thursday, around 15 degrees above normal, but far from daily record highs. Clouds increase again Friday night into Saturday with another cold front moving toward the region. There is considerable uncertainty regarding the timing of this front, with model solutions varying from Saturday morning to Saturday night, with a backdoor component. In addition, there is also uncertainty regarding the precip potential, with quite a bit of spread amongst the model solutions. For now, we maintained chance PoPs, including thunder on Saturday. Kept thunder out of the forecast for Sunday given more stable maritime airmass, except Delmarva, with closer proximity to the frontal boundary. Still some uncertainty with high temperatures on Saturday, which will be dependent on the location of the front, but expect them to be similar to Friday. There is model agreement that the front will be just to our south- west on Sunday, with the area under the influence of a northeast low- level flow. This would favor a stratocumulus deck and perhaps some sprinkles, mainly across the higher terrain. Kept thunder out of the forecast for Sunday given stable maritime airmass, except Delmarva, with closer proximity to the frontal boundary. High temps still be several degrees above normal. Moving into early next week, more showers and thunderstorms likely Monday and Monday night with the next cold frontal passage. Expect a return to drier weather on Tuesday. Continued above normal temps. && .AVIATION /22Z TUESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...IFR conditions this evening with periods of rain tapering off to some showers from south to north later this evening. Temporary LIFR conditions possible in heavier bands of rain during this time, and an isolated thunderstorm may near ACY/MIV through early this evening. Once the rain ends, stratus will continue along with some fog, and IFR/LIFR conditions are expected. East-northeast winds 15-20 knots with gusts up to 30 knots, diminishing to 10 knots or less later this evening and overnight. Wednesday...IFR/LIFR in the morning due to low clouds and some fog, then conditions will be slow to improve to IFR/MVFR after about 15Z. Scattered showers are possible especially through midday. Northeast to north winds mainly 5-10 knots. OUTLOOK... Potential for fog leading to visibility restrictions Wed night and MVFR ceilings persisting into Thursday. Predominantly VFR Thursday night into Saturday. MVFR possible in low clouds Sat night into Sunday. && .MARINE... Just a gale force gust or two across the northern waters but this should be short-lived and therefore the Gale Warning has been cancelled and changed to a Small Craft Advisory. On the ocean zones, the advisory goes through Wednesday night given the elevated seas that are forecast. On Delaware Bay, opted to go with a Small Craft Advisory through 02z tonight as some gusts have been close to 25 knots thus far. The winds are expected to diminish from south to north tonight. Areas of fog should develop tonight, and localized VSBY less than 1NM is possible. Fog gradually dissipates Wednesday morning. OUTLOOK... SCA-level seas likely persist through Friday with a return to sub-SCA conditions thereafter. && .TIDES/COASTAL FLOODING... The Coastal Flood Advisory remains in effect for coastal areas of New Jersey and Delaware, including Delaware Bay, and for the far lower part of the Delaware River. Low pressure located over eastern North Carolina this afternoon will drift northeastward reaching the waters off Delaware and New Jersey on Wednesday. The tight gradient and resulting brisk onshore flow will remain along the coasts of Delaware and New Jersey into this evening before wind speeds begin to diminish. The surge around this morning`s high tide ranged from +0.5 to +1.0 foot along the Delaware and New Jersey coasts. Based on the trends this afternoon and on the forecast guidance it appears as though the surge will be mostly in the +1.0 to +1.5 foot range for this evening`s high tide. The surge is expected to result in widespread minor flood in coastal areas of Delaware and New Jersey and along much of Delaware Bay. Localized minor flooding is anticipated on the far lower part of the Delaware River. It appears as though the tidal Delaware River above the Commodore Barry Bridge area may just reach the minor flooding threshold. However, the impacts should not be widespread enough there to warrant a Coastal Flood Advisory. We are not anticipating any coastal flooding along the upper eastern shore of Chesapeake Bay. The diminishing wind overnight along with the lower astronomical tides on Wednesday morning should preclude another round of minor flooding with Wednesday morning`s high tide. We will keep an eye on the higher astronomical tides on Wednesday evening. If the water does not drain away from the coast fast enough, we could see some localized minor flooding at that time. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory until midnight EDT tonight for NJZ012>014-020>027. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Wednesday for NJZ016. DE...Coastal Flood Advisory until midnight EDT tonight for DEZ002>004. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Wednesday for DEZ001. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ450>455. Small Craft Advisory until 10 PM EDT this evening for ANZ430- 431. && $$ Synopsis...Franck Near Term...Gorse/MPS Short Term...MPS Long Term...Franck/MPS Aviation...Franck/Gorse/MPS Marine...Franck/Gorse/MPS Tides/Coastal Flooding...Iovino

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