Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 260409 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1209 AM EDT Wed Apr 26 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/...
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Onshore winds will advect ocean fog inland into coastal communities overnight. The fog may become locally dense in spots during the overnight with visibility reduced to as low as one-quarter mile at times. At least patchy dense fog could persist into the morning and affect the morning commute. A Special Weather Statement was issued and patchy dense fog wording was added to the forecast grids to highlight the threat to travelers. Previous Discussion... The water vapor imagery as of 01z shows the center of a well defined closed low just exiting the northeast coast of North Carolina. An area of heavier rain is moving across central to northern New Jersey with precipitation more showery to the south and west. Some lightning continues near and east of the closed low center, and a mesoscale analysis shows the axis of instability offshore although there is some MUCAPE across the southern half of our area. The lightning has been maintained offshore in the axis of higher instability, which is also more surface-based there. Since this should mainly stay offshore, removed thunder for our area. Made adjustments to the hourly temperature, dew point and wind grids to keep them current. Adjusted the PoPs based on radar trends and some assistance from the HRRR guidance, with highest values across the northern areas awhile longer. Transitioned the precipitation over to showers as this looks to be the mode over much of the area. However given the low celings and wealth of low-level moisture, some drizzle can occur outside of the showers. With abundant low-level moisture in place, stratus will be maintained and some forecast soundings show the inversion lowering some overnight. In addition, some fog is anticipated however how widespread it becomes or dense is a forecast challenge. The stratus may dominate, however we will continue with a fog mention although not anticipating the need for a Dense Fog Advisory at this time.
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&& .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 /04Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Overnight...IFR, locally LIFR, ceilings. Some fog/drizzle especially late will result in times of IFR, perhaps LIFR, visibilities although the extent and timing is less certain. Showers will continue for awhile, but taper off from southwest to northeast overnight. Northeast winds 10-15 knots with local gusts up to 20 knots, diminishing to 10 knots or less. Wednesday...IFR/LIFR in the morning due to low clouds and some fog, then conditions slowly improve to IFR/MVFR in generally the 15-18Z timeframe. Isolated to 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...
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A Dense Fog Advisory was issued for the coastal waters and lower Delaware Bay for tonight and the morning. Area beach webcams show dense fog while nearby AWOS (WWD, BLM) are reporting visibility under 1 NM. Just about all available model guidance has been indicating dense fog developing over the marine areas. There is low confidence in the dissipation time of the dense fog. Most of the guidance indicates improvement in visibility before noon local time, but with minimal mixing, an absence of dry air advection and moisture trapped underneath a very strong thermal inversion, there is a potential that the fog could continue into the afternoon in spots. Dense fog could even persist into or redevelop Wednesday night. Previous Discussion... The Small Craft Advisory for Delaware Bay is cancelled as winds are diminishing. The Small Craft Advisory for the ocean zones continues through Wednesday night, as winds diminish overnight from south to north however it should take awhile for the seas to subside. OUTLOOK... SCA-level seas likely persist through Friday with a return to sub-SCA conditions thereafter.
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&& .TIDES/COASTAL FLOODING...
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The Coastal Flood Advisory was expired for coastal areas of New Jersey and Delaware, including Delaware Bay as high tide has passed. Most locations received minor tidal flooding at high tide which was earlier this evening. The advisory was also cancelled for the zone that covered the tidal Delaware River downstream from the Commodore Barry Bridge and the extreme upper part of the Delaware Bay (including New Castle, DE and Salem, NJ counties). High tide at Reedy Point has just passed within the hour with the tidal gauge cresting just below minor coastal flood threshold. 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.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Dense Fog Advisory until noon EDT Wednesday for ANZ431-450>455. Small Craft Advisory until 6 AM EDT Thursday for ANZ450>455.
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&& $$ Synopsis...Franck Near Term...Gorse/Klein Short Term...MPS Long Term...Franck/MPS Aviation...Franck/Gorse Marine...Franck/Gorse/Klein/MPS Tides/Coastal Flooding...Klein

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