Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211433 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 933 AM EST Sun Jan 21 2018 .SYNOPSIS... High pressure moves offshore of the southeast coast through tonight. A warm front will be gradually moving north across our area later tonight and Monday as low pressure moves through the Great Lakes. As the low moves into southeastern Canada, a cold front is pulled across our area early Tuesday. High pressure will then build across the region for the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Some cooling aloft is occurring this morning, however with earlier radiational cooling a couple of low-level inversions are present. This is shown nicely on the 12z Sterling, VA and Upton, NY raobs. An area of expanding low-level clouds (2,500-4,000 feet) is present within the inversion heights. We probably will not be able to mix this moisture layer out during the day, although perhaps the southern extent of it can to some extent, therefore increased the cloud cover quite a bit from west to east. This may have an impact on afternoon high temperatures, but as of now no changes were made. Did adjust the hourly temperatures some more given varying values due to incoming cloudiness compared to remaining clear areas with no wind, however this should tend to even out more through the remainder of the morning and into the afternoon. Otherwise, surface 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 mid level 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. && .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 /15Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...VFR to areas of MVFR ceilings, with cloud bases mostly in the 2,500-4,000 feet range. Low confidence on the extent of the MVFR ceilings. Light and variable winds, becoming southwest less than 10 knots by late afternoon. Tonight...MVFR ceilings (lowering to 1,000-1,500 feet), then lowering to IFR late for most terminals. Some light rain/drizzle possible, especially towards daybreak Monday. Southerly winds less than 5 knots. 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... Rest of today...Sub-Small Craft Advisory conditions expected. High pressure over the Southeast U.S. continues to drift offshore. 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...Gorse/Robertson Near Term...Gorse/MPS Short Term...MPS Long Term...Robertson Aviation...Gorse/Robertson/MPS Marine...Robertson/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.