Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211121 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 621 AM EST Sun Jan 21 2018 .SYNOPSIS... High pressure remains offshore of the southeast coast through tonight. A back door cold front will approach the area from the north through Monday, but likely not make into our area. Meanwhile, a warm front will be moving into the area later tonight and linger near the area through Monday night as an area of low pressure moves through the Great Lakes. This low will pull a cold front across the area early Tuesday as it moves into southeastern Canada. High pressure will build across the area for the end of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Fluctuating temperatures through sunrise with some areas skies clearing out and radiating, and clouds spreading into other areas and temperatures have held steady and risen. This trend continues through the morning. Clouds will thicken and lower as they spread into the region from the west. Otherwise, high pressure over the Southeast U.S. will drift offshore as low pressure over the northern Great Lakes will track to the east and pass well north of the region today. Meanwhile, closed H5 low over the Southwest U.S. will move into the Central and Southern Plains, and surface low pressure intensifies over the Oklahoma Panhandle and begins to lift to the north and east later today. Weak surface trough passes through the region this morning, and weak CAA will allow for a cooler airmass to overspread the region. However, late in the day, return flow sets up behind the departing high, and WAA begins ahead of the developing low, as heights and thicknesses rise. High temperatures today will be cooler than Saturday, generally topping off in the low 40s in the Poconos and northern NJ, and otherwise in the mid to upper 40s. Southeast NJ and the Delmarva will get into the low to mid 50s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... Low pressure continues to develop and strengthen over the Central Plains as it lifts to the north. Abundant moisture in the 1000-700mb layer overspreads the region, and thicknesses continue to slowly rise. Latest BUFKIT soundings keep surface temperatures above freezing throughout, even in the Poconos and northern NJ, so no major ptype issues, as precip will remain all rain. With limited surface moisture and not much in the way of upper level support, PoPs will be low, and not expecting much beyond light rain and drizzle with the increasing moisture. Lows tonight will range from the low to mid 30s north and west of the Fall Line, and otherwise in the mid to upper 30s. && .LONG TERM /MONDAY THROUGH SATURDAY/... On Monday, low pressure will be moving through the mid- Mississippi Valley. As this happens, the warm front across the area will try to lift northward, but will likely not make much progress as it remains dammed up across the area. Along and north of where the frontal boundary stalls out, some light rain/drizzle and patchy fog could remain through the day Monday. It looks like temperatures will remain above freezing, so the threat for freezing rain/drizzle has decreased. South of the front, dry conditions will likely move back in and some clearing may take place. Overnight Monday into Tuesday morning, as low pressure moves through the Great Lakes, the warm front will continue to slowly move northward, before a cold front is expected to sweep across the area. Again, north of the warm front, some light rain/drizzle and patchy fog could remain into the evening. Then as the cold front approaches and moves through the area, we expect a period of moderate to heavy rainfall. Guidance is showing the potential for some weak instability within the warm sector across southern New Jersey and Delaware and Maryland. So we have introduced isolated thunderstorms across these areas during the morning hours Tuesday as the rainfall associated with the frontal passage move through. Dry air will move in behind the cold front during the afternoon. Winds will become gusty 20 to 25 mph behind the front as well. For Tuesday night into Wednesday, the low pressure system moves across eastern Canada, while a secondary cold front moves across the area. We expect dry conditions across the area as any precipitation is forecast to remain to our north. Gusty conditions are expected on Wednesday as winds will be gusting at least 25 to 30 mph. High pressure returns for Thursday and Friday, but builds offshore Saturday, leading to dry conditions through Saturday. Winds will be less gusty than Wednesday, around 15 to 20 mph. && .AVIATION /11Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR. Cirrus clouds this morning will lower and thicken throughout the day. Skies become OVC050 in the afternoon. Low confidence that MVFR CIGs will develop in the afternoon, but SCT025 possible. VRB winds 5 KT or less become SW less than 10 KT. High confidence. Tonight...MVFR CIGs develop in the evening, becoming BKN010-015, and then lowering to IFR late. Light rain/drizzle possible, especially towards daybreak Monday, with sub-VFR conditions. S winds less than 5 KT. Moderate confidence. OUTLOOK... Monday...Sub-VFR conditions expected during the first half of the day. Conditions likely improve during the day, especially across southeast PA, southern NJ, and Delmarva. West winds may gust 15-20 knots, especially PHL southward. Moderate confidence. Monday night-Tuesday..Conditions lower to MVFR then IFR everywhere with periods of rain associated with a frontal passage. Conditions improve back to VFR behind the frontal passage. Southeast winds become westerly behind the front and begin gusting 25-30 knots. Moderate confidence. Low-level wind shear is possible to develop Monday night into Tuesday morning. Tuesday night-Thursday...VFR expected. Northwest winds with occasional gusts Tuesday night, becoming more gusty 25-30 knots Wednesday-Wednesday night. Less gusty around 20 knots Thursday. High confidence. && .MARINE... Today...Sub-Small Craft Advisory conditions on tap for today. High pressure over the Southeast U.S. continues to drift offshore today. Meanwhile, a weak trough will pass through the waters this morning. NW winds 10 KT or less will back to the SW this afternoon. Tonight...Conditions remain below SCA levels. SW winds less than 10 KT, becoming 10-15 KT with gusts to 20 KT over DE ocean waters by daybreak Monday. OUTLOOK... Monday...Conditions expected to remain below advisory levels, but winds may periodically gust around 20 knots. Monday night...Conditions expected to return to Small Craft Advisory levels. Tuesday-Wednesday...Small Craft Advisory level wind continue, but may reach gale force. Wednesday night-Thursday...Conditions likely to remain at Small Craft Advisory levels. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Robertson Near Term...MPS Short Term...MPS Long Term...Robertson Aviation...Robertson/MPS Marine...Robertson/MPS

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