Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 182029 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 329 PM EST Wed Jan 18 2017 .SYNOPSIS...
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Low pressure over the western Atlantic waters will continue to drift out to sea tonight. Meanwhile, high pressure over the Midwest will build east and then move offshore at the end of the work week. Weak low pressure will impact the region this weekend, and then a stronger and more complex area of low pressure will impact the Mid- Atlantic and Northeast early in the new week.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Shortwave axis has exited the coast as of late afternoon, with any lingering sprinkles and rain showers diminishing, as subsidence becomes dominant through the column. Yet a weak low-level inversion remains across the region, with ample moisture below it, which is maintaining considerable cloud cover. This cloud cover will persist, with breaks developing after midnight, and partial clearing from south to north beginning toward dawn. With the inversion in place, northwest wind gusts will be limited, in addition to the decoupling of the boundary layer this evening. Wind gusts will range from 15 to 25 mph, on the lower end north of I- 78, and on the higher end over Delmarva. Winds will diminish rapidly north of I-78 this evening, and more gradually toward midnight over Delmarva. In terms of low temperatures, sky cover will be the determining factor, as winds become light and variable. With greater cloud cover north of I-78, we adjusted temperatures slightly upward. Elsewhere, with less cloud cover toward dawn, we went slightly colder. With abundant low-level moisture in place, light and variable winds, and clearing skies over portions of the area, there is a window of opportunity for the development of patchy fog. Both model soundings and indices also support this. Low temperatures are forecast at or below freezing northwest of the NJ Turnpike/ I-95, as well as the Pinelands of NJ. Thus, where fog develops across these areas, it may freeze on contact. This could lead to slippery conditions, especially on untreated surfaces.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
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Patchy fog that does develop could linger into mid-morning, given maintenance of the low-level inversion with a building ridge aloft, and subsequent high pressure at the surface and light winds. The deeper moisture will continue moving north of the area, with some lingering clouds north of I-78 in the morning. Otherwise, a mostly sunny day develops, with some scattered clouds below the low-level inversion. Dry weather is expected, with no support for precipitation. As for high temperatures, generally 10 to 15 degrees above normal, from the low 40s in the Poconos to mid 50s in Delmarva. Northwest winds 5 to 10 mph.
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&& .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
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A prolonged period of warm and wet weather is on tap for the Long Term period. Broad area of high pressure along the Eastern Seaboard will move offshore Thursday night as weakening low pressure approaches from the west. Weak secondary low will form over the Mid-Atlantic, and the primary low will dissipate as is tracks from the Great Lakes into eastern Canada. This will send a quick shot of precip into the Mid-Atlantic and Northeast, but not expecting much more than 0.25" QPF or so. Best chances will be from midday Friday through Friday evening. Conditions dry out after midnight Friday night. One concern is timing of onset of precip for the Poconos and areas to the north of I-80. If precip develops before noon on Friday, surface temps may not yet have risen above freezing, and there could be a brief initial period of rain/freezing rain. Otherwise, with onshore flow, temperatures will be warm enough for precip to be all rain. Weak high pressure then builds across the region Friday night and Saturday. Patchy fog may develop Friday night and Saturday morning, as with weak flow, there will not be much of a chance for the moisture to scour out. Conditions will otherwise be dry and mild on Saturday with highs in the upper 40s to mid 50s. High pressure moves offshore Saturday night. Meanwhile, an upper trough will move into the West Coast sometime early in the weekend and will become a closed upper low over the Southern Plains and Gulf Coast states during the day Sunday. As this system organizes and develops, it will send waves of precip out ahead of it and into much of the East Coast. The first wave will develop over the TN Valley and lift towards the area on Sunday. Both the GFS and the ECMWF bring measurable precip into the region by Sunday afternoon, and then the bulk of the rain approaches Sunday night and Monday as the main system lifts into the Mid-Atlantic region. An area of concern: high pressure will nose its way into eastern Canada and northern New England. Depending on the placement of the high, winds could take on more of a NE/E flow as opposed to an E/SE flow. This could then usher a shallow area of cold air into mainly northern zones, and would result in a period of wintry weather sometime Sunday night and Monday morning. Latest track, based on both the GFS/ECMWF, keeps temperatures above freezing, so will keep precip as all rain throughout the CWA. There may be a brief break in the precip late Monday morning, but the main low will lift through the region Monday afternoon and evening, bringing another round of potentially moderate to locally heavy rainfall. With deep onshore flow, PWATs will build to around 1.5". Pressure gradient tightens up, and with strong low level jet passing through, can expect E winds to increase to 15-25 MPH with 25- 35 MPH gusts, mainly on Monday. Rainfall ends fairly quickly from SW to NE Monday night through Tuesday morning. Behind that departing system, a deep area of low pressure will be entrenched over the northern Great Lakes region, and several shortwaves will rotate around the low and into the local area. This may touch off some showers during the mid-week period. During this time, Arctic air remains bottled up to the north. Temperatures will average above normal, with highs nearly 10 degrees above normal.
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&& .AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...MVFR conditions across the region as of late afternoon. Expect continued, gradual improvement into this evening, with all TAF sites becoming VFR by 00Z. Northwest winds gusting up to 20 knots will subside this evening, becoming light and variable at times overnight. Thursday...Predominantly VFR conditions. There may be some SCT030 ceilings developing around noon. Northwest winds 5 to 10 knots. OUTLOOK... Thursday night...VFR. LGT/VRB winds less than 5 KT. High confidence. Friday through Friday night...Sub-VFR conditions possible in -RA. SE winds 5-10 KT on Friday become SW less than 5 KT Friday night. Moderate confidence. Fog possible Friday night. Saturday...Improving conditions Saturday morning. VFR. West winds 5 KT or less. Saturday night...LGT/VRB winds. Mainly VFR, but cannot rule out passing shower. Sunday...E winds 10-15 KT. IFR conditions possible in RA. Sunday night through Monday...IFR and lower conditions possible in RA. E winds 15-20 KT with 20-30 KT gusts. LLWS possible with 60-65 KT LLJ Monday afternoon.
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&& .MARINE...
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With northwest flow in place across the waters, areas of fog had dissipated by the early afternoon. Small Craft Advisory continues for the southern NJ ocean and lower Delaware Bay waters, mainly for seas reaching 5 FT on the ocean, and wind gusts up to 25 knots on the ocean and lower bay. We canceled the Small Craft Advisory for the upper Delaware Bay waters. The latest observation trends and model guidance favor wind gusts remaining below 25 knots. For the remainder of tonight into Thursday, high pressure moves over the waters, with winds and seas below Small Craft Advisory. OUTLOOK... Thursday night through Sunday morning...A weak pressure gradient will result in sub-SCA conditions during this time. VSBY restrictions possible in rain on Friday, and in fog Friday night and Saturday morning. Sunday afternoon through Monday night...Easterly flow increases to 15-20 KT with 25-30 KT gusts starting Sunday afternoon, and then gale force gusts of 35-40 KT likely Monday and Monday night. Moderate to heavy rain with low VSBY expected. Conditions improve late Monday night, and winds decrease to sub-SCA levels. However, seas on the ocean should remain above SCA criteria. Tuesday...Ocean seas remain above SCA criteria.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 9 PM EST this evening for ANZ431- 452>455.
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&& $$ Synopsis...MPS Near Term...Franck Short Term...Franck Long Term...MPS Aviation...Franck/MPS Marine...Franck/MPS

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