Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 181044 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 644 AM 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 /THROUGH TONIGHT/...
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Minor changes to the forecast, mainly for hourly PoPs and weather through this evening, with the 630 AM ESTF update. Impressive rainfall totals of 1.5-2.5 inches in an hour were reported when the line of storms went through the Lehigh Valley very early this morning. The line has since picked up forward speed, lessening the likelihood that downstream areas in northern NJ get rainfall amounts this high. Additional scattered showers and storms have developed farther south across the Delaware Valley- Delaware Bay area just after sunrise. The HRRR continues to have a decent handle on this area of convection and were blended with earlier forecast grids for the latest update. Previous Discussion... An active weather pattern returns to the area, bringing the threat of severe thunderstorms and heavy rainfall/flash flooding especially this afternoon and evening. A sub-1000 mb surface low that was located over the northern Great Lakes early this morning will track northeastward across southeastern Ontario and western Quebec through tonight. The attendant cold front with this system will track eastward from the Midwest today. The front will move through the forecast area tonight. An area of showers and embedded isolated thunderstorms was moving into our far western zones in eastern PA just before 4 AM. Radar trends indicate this activity has become better organized overnight, which is likely in response to a strengthening low-level jet and isentropic lift across the baroclinic zone that is positioned to the southwest of the surface warm front. Hi-res CAMs show this trend continuing with convection expanding southward as it progresses eastward into the Delaware Valley early this morning and eventually the coastal plain later this morning. PoPs were increased accordingly with the 330 AM ESTF update to account for this first round of precip. Rainfall amounts around 2" are possible in isolated locations. Expect a break in the showers and storms behind the initial line later this morning and early afternoon. Meanwhile, breaks in the clouds are expected to develop from west to east during this time, which will help destabilize the atmosphere this afternoon. Owing to strong heating and anomalously high moisture content in the boundary layer (surface dewpoints well into the 70s), 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 was issued 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. SPC has included most of the forecast area in a slight risk for severe thunderstorms for this afternoon and evening. 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.
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&& .SHORT TERM /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 NIGHT 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 /12Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Low clouds are moving in from the west early this morning. CIGs are already in MVFR range at ABE and RDG. Expect MVFR CIGs to spread into the I-95 terminals toward daybreak. A line of showers and storms currently near ABE and RDG at 0830Z will progress eastward this morning. This activity could also expand southward, impacting Phila airports between about 11 and 14Z. Brief IFR visibilities are possible with these storms. Expect improvement to VFR behind this initial line of showers and storms from west to east later this morning and into the early 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 from 2 PM EDT this afternoon through this evening for PAZ060>062-070-071-101>106. NJ...Flash Flood Watch from 2 PM EDT this afternoon through this evening for NJZ001-007>010-012-013-015>019. DE...Flash Flood Watch from 2 PM EDT this afternoon through this evening for DEZ001-002. MD...Flash Flood Watch from 2 PM EDT this afternoon through this evening for MDZ008-012-015-019-020. MARINE...Small Craft Advisory from 11 AM this morning to midnight EDT tonight for ANZ450>455. Small Craft Advisory from 2 PM this afternoon to 10 PM EDT this evening for ANZ430-431. && $$ Synopsis...Meola Near Term...Klein Short Term...Meola Long Term...Meola Aviation...Klein/Meola Marine...Klein/Meola is the U.S. government's official web portal to all federal, state and local government web resources and services.