Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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768 FXUS61 KPHI 181834 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 234 PM EDT Fri Aug 18 2017 .SYNOPSIS... A cold front will approach from the west and move through the region tonight into early Saturday. High pressure returns for Sunday through Tuesday. Another cold front will cross the region on Wednesday followed by high pressure for the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Severe Thunderstorm Watch #455 is in effect until 9:00 PM. Surface dewpoints well into the 70s across the region, making it feel quite oppressive. Some breaks in the clouds will help destabilize the atmosphere. Owing to strong heating and anomalously high moisture content in the boundary layer, forecast models indicate a moderately unstable environment (SBCAPE of 2000-3000 J/kg) to sustain strong convective updrafts this afternoon. Storms are expected to initiate just ahead of the cold front across the higher terrain in central and northeastern PA during the mid afternoon. These storms are then expected to move eastward into eastern PA late this afternoon and the I-95 corridor this evening. Convection may linger into the overnight near the coast. CAMs have come into much better agreement with the above- mentioned timing of storms. A Flash Flood Watch remains in effect for most of the CWA for this afternoon and evening. The Watch does not include the southern Poconos (higher FFG) and southeastern NJ into southern DE (higher FFG and greater uncertainty if storms maintain intensity to produce excessive rainfall rates by the time they move into our far southeastern zones late this evening or overnight). PWATs are forecast to increase to between 2.25-2.5, which is +2 to +3SD above climo for this time of year. While storm motion should be fast enough to preclude widespread heavy rainfall, upstream corfidi vectors show a potential for convective training that could lead to locally heavy rainfall totals. With NWP support from CAMs, there is a potential for isolated rainfall amounts of 3-6 inches. The flash flood threat may peak after sunset this evening as convection (and associated cold pools) starts to weaken with the loss of heating. Storm motion may subsequently decrease with high rainfall rates (2+ inches per hour) still occurring. Have added damaging winds to the weather grids with that being the primary severe thunderstorm threat. However, isolated large hail and even a tornado is possible this afternoon and evening. Today will feel very muggy with dewpoints rising into the mid 70s and even near 80F across Delmarva and far southern NJ. Heat indices are expected to peak into the mid 90s across the urban I-95 corridor and near 100F in Delmarva. Held off on a Heat Advisory but heat indices will only be a few degrees below issuance criteria. Humid conditions continue into tonight, especially along and east of I-95, where forecast lows are in the mid 70s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... The cold front should be offshore by Saturday morning with all precipitation having come to an end. As the upper trough pushes through later Saturday, we should really start to see the dry air filter into the area. Saturday will be a fairly nice day with the sun shining across the region. Models are showing a slug of moisture crossing the region as the upper trough moves through. However, it appears to weaken as it moves over the mountains to our west and peters out before reaching our area. We will continue to keep the forecast mostly dry at this time, although an isolated shower or thunderstorm can not be ruled out later in the day Saturday. Best chance for anything to reach the ground looks to be across the northwestern zones so have included a slight chance for showers later Saturday. && .LONG TERM /SATURDAY THROUGH THURSDAY/... High pressure will build in for Sunday and a period of relatively nice weather should continue for the start of the week with the high shifting offshore on Monday. Another cold front will approach the region from the west on Tuesday, crossing the area on Wednesday. Models continue to show a surface trough developing on Tuesday in advance of the cold front. This trough, combined with a shortwave moving through the mid levels, will spark off some convection on Tuesday. Additional showers and thunderstorms will be accompany the cold front on Wednesday, with the storms ending from west to east on Wednesday evening. A few showers may linger early Thursday, mainly along the coast. High pressure, accompanied by some cooler air, will build down from Canada and across our area for the end of the week. && .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. MVFR/patchy IFR conditions slow to scatter out, but should lift to VFR early in the afternoon. Showers and storms are expected to redevelop after 18Z this afternoon mainly in vicinity of western terminals (ABE-RDG), then move eastward through the I-95 terminals this evening (between 23-03Z) and eventually our eastern terminals (MIV-ACY) late in the evening. A period of IFR restrictions are likely when these storms move through. Winds will predominately be out of the south today and tonight. Speeds will generally be 10 kt or less. However, they could increase to above 10 kt for a period this afternoon with occasional gusts up to 20 kt possible. Much stronger winds are possible with storms. OUTLOOK... Saturday...Fog possible early. Otherwise, mainly VFR conditions expected. Northwest to west winds around 10 to 15 knots. Sunday...Mainly VFR conditions expected. Westerly winds around 10 knots or less. Monday and Tuesday...Mainly VFR conditions expected. Southerly winds around 10 knots or less. && .MARINE... A SCA was issued for the coastal Atlantic waters and Delaware Bay from late this morning through this evening. Despite less than ideal mixing profiles in southerly flow (warm air above the relatively cooler waters), 25 kt winds are only 200-300 ft off the deck. Deeper mixing on land could easily transport these higher winds down to the surface in our nearshore waters. Seas in the coastal waters are forecast to increase to around 5 ft late this afternoon and evening. Strong thunderstorms mainly this evening could contain locally gusty winds, requiring Special Marine Warnings. OUTLOOK... Saturday through Tuesday...Sub-Small Craft Advisory conditions expected on the area waters through Tuesday. RIP CURRENTS... A moderate risk for the development of dangerous rip currents is forecast for today for DE and NJ shore, given increasing southerly winds and seas. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch until midnight EDT tonight for PAZ060>062- 070-071-101>106. NJ...Flash Flood Watch until midnight EDT tonight for NJZ001- 007>010-012-013-015>019. DE...Flash Flood Watch until midnight EDT tonight for DEZ001-002. MD...Flash Flood Watch until midnight EDT tonight for MDZ008-012- 015-019-020. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ450>455. Small Craft Advisory until 10 PM EDT this evening for ANZ430- 431. && $$ Synopsis...Meola Near Term...Klein/MPS Short Term...Meola Long Term...Meola Aviation...Klein/Meola/MPS Marine...Klein/Meola/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.