Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 270344 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1144 PM EDT Sat May 26 2018 .SYNOPSIS... High pressure remains anchored well off the Southeast U.S. coast through the weekend. A back door cold front approaches tonight and then moves through the region Sunday, and will become nearly stationary over the Mid-Atlantic and stretching back to the south and west for the start of the new week. Several waves of low pressure will impact the region through Monday. Another cold front moves through the region on Tuesday. Weak high pressure briefly builds in from the north on Wednesday, and then the stationary boundary lifts north as a warm front late in the week. Moisture from the remnants of Subtropical Storm Alberto may affect the area late in the new week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 1140 PM Update...Based on latest analysis and high res model guidance, we have issued a Flash Flood Watch for most of our eastern PA zones (except Carbon, Monroe Counties) through most of central NJ north through the I-78 corridor. Expect that as backdoor cold front moves south overnight into the region this will act in tandem with upper support arriving from the west and PWATs near 2 inches to trigger heavy showers and storms that may tend to train repeatedly over the same areas. Biggest threat area overnight will be across the north near I-78 with the shower/storms and the associated threat shifting south by morning. Previous Discussion...The stronger thunderstorms from this afternoon and earlier this evening are beginning to weaken as daytime heating is waning. However, a surface trough remains across the area and has actually pushed back northward this evening. Meanwhile, a back door cold front continues to sink southward toward the area later this evening and overnight. Even through the stronger thunderstorms have ended, there will remain a chance of showers and thunderstorms through the overnight hours, especially as we go past midnight and through daybreak. There will likely be a lull in activity for most areas for the next few hours, then as the backdoor cold front approaches, and additional short wave/vorticity impulses, showers may become more concentrated across the northern half of the area. With some instability across the area, a few thunderstorms will remain possible. PW values remain 1.5-2.0 inches overnight, so some locally moderate to heavy rainfall is possible. Given the tropical air mass and heavy precipitation, patchy fog will also be possible, but widespread dense fog is not expected. It will be warm and humid, with low temperatures ranging from the low-60s to low-70s, around 10 to 15 degrees above normal. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... Quite a change on tap for Sunday as a backdoor cold front moves from northeast to southwest across the area during the day, with cold air advection in its wake. Meanwhile, above this very shallow low-level feature, the mean tropospheric remains out of the west-southwest, which will continue a transport of warm and anomalously moist air atop this boundary. Precipitable Water values will remain near 2.0 inches south of a PA Turnpike to I-195 line, as a maritime air mass remains in place north of the front. Meanwhile, several short wave perturbations will traverse an increasingly west-east boundary, which will become parallel to the mid-level flow. Closer to the front, there will be sufficient instability for convection, with MU CAPE up to 1,000 J/Kg indicating thunder potential just to the north of this feature as well. This all points to a continued heavy rain potential though Sunday, with Delmarva again being the focus, given the saturated ground and resultant lower threshold for flash flooding discussed previously. The greatest potential for thunder will be across Delmarva, where surface based CAPE may approach 2,500 J/Kg during the afternoon and evening just southwest of the front. While Bulk Shear remains weak, generally AOB 30 kts, steeper 0-3 Km lapse rates may favor gusty winds across this area with any convection that develops. There will be quite a contrast in high temperatures across the region. The post frontal, maritime air mass will feature cloudy skies and temperatures actually falling through the day (non- diurnal trend), with highs most likely being reached during the morning across northern NJ and northeast PA. Northward of the PA Turnpike to Atlantic City Expressway, temperatures will not rise much above overnight lows. To the southwest, high temperatures will range from the mid-70s to mid-80s. Of course, this is all dependent on the frontal passage timing, with the higher resolution given greater weight given the shallow nature of back door fronts. To the north of the front, cloudy skies, northeast wind gusts around 20 mph (closer to 30 mph along the northern NJ coast), and the moist air mass will make it feel somewhat raw outside, especially compared to the recent string of warm days! && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Showers and thunderstorms taper off Sunday evening as low pressure moves offshore. Another low develops to the west Sunday night and passes through the Delmarva along a stationary boundary south of the area late Sunday night and into Monday morning. This may keep lingering showers in the forecast through Monday morning, and then conditions should dry out in the afternoon as a back door cold front sags southward through the Northeast. Skies remain cloudy for much of the day as an onshore flow ushers a cool and damp airmass into the region. This will also keep the temperatures down, as highs will generally top off in the mid and upper 60s at the beaches and coasts, and generally in the low to mid 70s elsewhere. High pressure passes to the north on Tuesday, and then a back-door cold front moves through the region late in the day. Ahead of that boundary, though, a warm and humid airmass will be in place. Highs top off in the mid and upper 80s with surface dewpoints well in the 60s. Scattered convection looks to develop over central PA and may move into southeast PA, western NJ, and the Delmarva. Will follow guidance and cap PoPs at slight chance. The moisture and the instability will be there for convection over the area, it just remains to be seen if there is enough of a trigger for storms to develop. During this time, what is currently Subtropical Storm Alberto over the eastern Gulf of Mexico will lift north and affect the eastern Gulf Coast states. By Wednesday, the remnant low of this storm will lift into the Midwest. This also pushes that stationary front north as a warm front. High pressure over the Northeast moves offshore by Wednesday night, and then that warm front will move into the Delmarva and Delaware Valley. Unsettled weather then possible for the end of the week and start of the weekend as the remnant low of Alberto meanders over the Northeast and Mid-Atlantic. Best chances for precip will be Thursday through Friday. Given high levels of uncertainty, will generally run with chance PoPs for next weekend. However, given that this low will arrive as a warm and tropical airmass will be over the area with dewpoints well in the 60s, there is the threat for heavy rain and flooding during this time. && .AVIATION /04Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions will continue for the first half of the overnight period, unless a shower/thunderstorms affects a TAF site. MVFR will develop across much of the area mainly in low clouds as well as some patchy fog. Localized IFR restrictions are possible due to lower ceilings and heavy downpours in showers and thunderstorms, but were left out of the TAF at this time due to the uncertainty of occurrence. Winds out of the southwest 5 to 10 kts, becoming light and variable at times. Sunday...Conditions are expected to continue to lower to IFR during the morning Sunday. While ceilings and visibilities may temporarily improve later in the morning or early afternoon, they are expected to lower once again later in the afternoon and evening. Winds will shift from the southwest to the northeast during this time frame, and may gust up to around 20 mph, especially at KPHL. Outlook... Sunday night...IFR/LIFR CIGs as marine airmass overspreads the region. Showers and thunderstorms taper off in the evening, then there is a chance for more showers late at night. VSBY restrictions in fog/mist possible. NE winds less than 10 kt. Monday...Lingering IFR CIGs and showers in the morning. MVFR CIGs in the afternoon. NE winds less than 10 kt become SE late. Tuesday...Mostly VFR. Potential for afternoon SHRA/TSRA. W-NW winds less than 10 kt. Wednesday...Mostly VFR. Light East winds become SE 10-15 kt. Stronger winds possible at KACY. Potential for SHRA/TSRA late in the day and SHRA at night. Thursday...SHRA/TSRA possible throughout the day. S winds 10-15 kt. && .MARINE... Tonight...Sub-SCA conditions are expected at this time. Sunday...A backdoor front will progress southward across the coastal waters. Winds will shift from southwest to northeast along with the frontal passage. The strongest northeast winds are expected across the northern coastal waters of NJ (ANZ450-51), where gusts of 25-30 kt are most likely to occur. There could even be brief gale force gusts in the coastal waters off Monmouth County (ANZ450). Seas in the ANZ450-51 may build to 5 ft in response to the strengthening onshore flow. A SCA has been issued in accordance with the frontal passage timing, as northeast winds will rapidly increase behind the front, as follows: ANZ450 at 10Z and ANZ451 at 13Z. Outlook... Sunday night...Any lingering SCA wind gusts will diminish in the evening. NE winds 10-15 kt. Ocean seas 2-4 feet, and waves on DE Bay 2 feet or less. VSBY restrictions in fog possible. Monday through Thursday...Winds and seas below SCA levels. Rip Currents... Through this evening...Southwest winds 10 to 20 mph with waves of 3 to 4 feet in the surf zone. Thus, a low risk for the development of dangerous rip currents. A back door cold moving southward across the area on Sunday will cause winds to shift from the south-southwest to the northeast. While swell periods are expected to range from 5 to 7 seconds, an onshore flow will develop along the NJ oceanfront during the day, with wind gusts up to 20-30 mph. This will build waves in the surf zone up to 3-5 feet. In particular, the rip current risk will be greatest for the Monmouth and Ocean County beaches, where the winds will be strongest (25-30 mph), waves in the surf zone will be highest (4-5 ft), and where a northeast flow yields more of an onshore component due to the orientation of the coastline. In terms of the risk for the development of dangerous rip currents: a solid moderate risk of rip currents is expected for the Monmouth and Ocean County beaches, perhaps approaching high in some spots. Further south, a moderate risk for the Atlantic and Cape May County beaches, and a low risk for the Delaware beaches. Remember, a low risk of rip currents does not mean no risk! Life threatening rip currents often still occur near jetties, reefs, and piers. The vast majority of rip current victims swam at unguarded beaches. ALWAYS SWIM IN THE PRESENCE OF A LIFEGUARD. Sunday will certainly not be a `beach day` given cloudy skies and the chance of showers, with cooler than average temperatures in the mid-60s to mid-70s, along with northeast wind gusts 20-30 mph. && .TIDES/COASTAL FLOODING... Spotty minor tidal flooding is possible with the Sunday evening high tide along the NJ and DE oceanfront, Delaware Bay, and the late Sunday night high tide along the tidal portion of the lower Delaware River. In particular, the potential for minor flooding is greatest along northern NJ coastline, where onshore flow will be stronger and more persistent. && .EQUIPMENT... The Sandy Hook NOS tide gauge (SDHN4) remains out of service. CORMS is addressing the problem. There is currently no estimated time for a return to service. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch until 10 AM EDT Sunday for PAZ060>062-070- 071-101>106. NJ...Flash Flood Watch until 10 AM EDT Sunday for NJZ007>010- 012>015-017>020-022-025>027. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM Sunday to midnight EDT Sunday night for ANZ450. Small Craft Advisory from 9 AM Sunday to midnight EDT Sunday night for ANZ451. && $$ Synopsis...MPS Near Term...Fitzsimmons/LF/Robertson Short Term...LF Long Term...MPS Aviation...LF/Robertson/MPS Marine...LF/MPS Tides/Coastal Flooding... Equipment...

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