Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 250117 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 917 PM EDT Sat Sep 24 2016 .SYNOPSIS... High pressure north of the Great Lakes will build to the south and east towards the Northeast this weekend, and will be centered over New England by Monday morning before moving offshore by Monday night. A cold front approaches Monday night and passes through the region on Tuesday. Low pressure then dives into the Northeast for the midweek period, eventually departing as high pressure builds in from the west for the end of the new week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Cloud cover had continued to erode from north to south this evening. The only remaining clouds are across srn Delaware and they will be gone from there shortly. We expect mostly clear skies overnight, and winds will become light and variable most places as high pressure begins to nose its way down from the north. This will allow for good radiational cooling to take place, and a seasonably cool night is expected across the area with near normal temperatures.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... High pressure will continue to affect the area from the north through Sunday. Mostly clear skies, except some cirrus through the day, are expected. Temperatures will remain pretty close to normal. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Quiet conditions for the start of the long term period, and then a period of unsettled weather for the midweek period before high pressure returns for next weekend. High pressure centered just to the north of the local forecast area Sunday night will drop into New England by Monday morning. With clear skies, a dry airmass, and a light pressure gradient over the area, can expect strong radiational cooling for areas away from the urban centers. For most areas, the coldest temps of the season, and in quite some time, will occur. Temps across the Pocono Mountains and into northern NJ will be the coldest, dropping into the upper 30s to near 40. Some patchy frost is possible prior to daybreak. For the rest of the region, lows will generally be in the 40s, but the ocean will keep coastal areas several degrees warmer, generally near 50. MD/DE zones, as well as the Philly metro area, will also have the warmest temps. High pressure over New England on Monday continues to drift to the south and east during the day. Meanwhile, a cold front will push through the OH/TN Valleys, and will end up west of the Appalachians by Monday evening. Mid and high clouds will build in from the west ahead of that front, but based on latest latest models, precip should hold off until Monday night. Another mild day on tap for Monday with highs in the mid 60s in the mountains to the north and in the low to mid 70s elsewhere. Warm front lifts north through the region Monday evening ahead of that cold front, and increasing low level moisture will spread into the region. Surface dewpoints will climb into the upper 50s/low 60s, and resulting PWATs will range from 1.75-2". As a result, can expect occasionally moderate to possibly locally heavy rain at times as the front passes through the region Monday night into Tuesday morning. Front may get hung up just offshore, and this may keep showers in the forecast for eastern zones through the afternoon, while conditions dry out to the west. Thereafter, there is considerable uncertainty in the forecast for the midweek period. Surface low pressure along with an upper trough will track across Canada, ending up over the Great Lakes on Tuesday. The uncertainty lies in where that trough will end up, but models suggest that at least the base of the trough, if it is not a closed low, will end up either right over the Mid-Atlantic area or just to the south. Several waves of surface low pres will develop around this upper trough and pass just south and east of the region. For now, will keep slight chance PoPs in the forecast from Wednesday through Friday. Latest ECMWF keeps the closed upper low over the area into the weekend while the GFS has it offshore and has high pressure building into the area. Temps will be near or slightly below normal through the period. && .AVIATION /01Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR expected overnight with clear skies and light north winds. Winds will likely switch back to a north-northeast direction around daybreak for the morning hour Sunday, before switching back to a northwesterly direction for the afternoon. OUTLOOK... Sunday night...VFR. Light winds. Monday...VFR. South winds possibly gusting to 20 KT late in the day. Monday night through Tuesday...MVFR with the potential for IFR or lower in SHRA as a cold front works across the region. Improvement possible Tuesday afternoon over western areas, but conditions will be slow to improve over eastern areas. Wednesday through Thursday...Predominantly VFR, but occasional showers may briefly reduce CIGs/VSBYs.
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&& .MARINE... A Small Craft Advisory will remain in place as seas are expected to remain around 5-6 feet. Winds have dropped below 25 knots, and should remain below advisory levels through Sunday. However, wind may gust around 20 knots at times. OUTLOOK... Sunday night...Long period swells associated with Tropical Storm Karl will persist, but ocean seas will average 3-4 feet. Monday through Thursday...Cold front approaches on Monday and moves across the waters Monday night through Tuesday morning. Strong southerly flow may result in 25-30 KT wind gusts during that time, in addition to 5-6 foot seas. Sub-SCA conditions then expected Wednesday and Thursday. RIP CURRENTS... Both long period swells and building seas suggest conditions may be more representative for a moderate risk of rip currents. A moderate risk for the formation of dangerous rip currents will likely continue into Sunday with long-period southeasterly swells from Tropical Storm Karl expected to impact the eastern seaboard. && .CLIMATE... The high temperature yesterday, September 23rd, at Allentown, PA (KABE) was 90 degrees. While this did not set a daily high temperature record, it marked the second latest 90-degree day in the calendar year at KABE since 1970. During this span, the only other date that reached 90 degrees this late in the year was October 8, 2007. Note: A high temperature of 90F on October 8, 2007 was also the latest occurrence of a 90-degree day in a calendar year on record for KABE. Allentown historical climate records go back to 1922. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until noon EDT Sunday for ANZ450>455. && $$ Synopsis...MPS Near Term...Robertson/Po Short Term...Robertson Long Term...MPS Aviation...Robertson/MPS/Po Marine...Robertson/MPS Climate...

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