991 FXUS61 KPHI 160555 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 155 AM EDT Fri Oct 16 2020 .SYNOPSIS... High pressure over the western Atlantic continues to drift out to sea tonight. A cold front slowly approaches from the west on Friday, and low pressure develops on that front. That low pressure lifts through the region Friday, and then the cold front swings through Friday night. High pressure builds in from the west on Saturday, then moves out to sea Sunday. A frontal boundary approaches from the west next week, but remains west of the area into the middle of next week. Several waves of low pressure will develop on that boundary. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... A longwave upper trough is slowly digging southward and propagating eastward across the CONUS today. This trend will continue through the overnight as as cold front approaches the forecast area. At the surface, high pressure will remain centered south of the Canadian Maritimes and the parent low pressure will occlude and race north into eastern Canada. The latest guidance continues the trend of slowing the front down through the area. It still hasn't reached our far NW areas yet and may not for a few more hours. It could take until dawn to reach the Delaware Valley and then off into southern NJ and Delmarva after that. Pops/wx/winds have been adjusted accordingly. The winds will shift with the front, and then the rains will arrive one to two hours after that. As for the rain tonight, still expecting some scattered showers to move into the area along and in the wake of the slow moving front. However, the primary forcing for ascent doesn't arrive until Friday, so rainfall amounts will remain light (less than a tenth of an inch) where rain does fall. Clouds will move in quickly along and just ahead of the front. Winds will remain southerly overnight until the front passes around 5-10 mph. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/... The longwave trough will continue to amplify throughout the day Friday as it continues eastward. A shortwave located across the Ohio Valley will aide to sharpen the trough and tilt it negative Friday night as it approaches the Mid-Atlantic. Significant height falls will result and surface-based low pressure will develop along the cold front offshore and race northward toward New England. Winds will become northwesterly behind the front around 5-10 mph. Showers will be ongoing and increasing in coverage and intensity by daybreak Friday. The best synoptic forcing in relation to the jet to our north appears to arrive mid-day and into the evening, so rain will be widespread and moderate to locally heavy throughout the latter half of the day and into the evening. As the forcing departs into the overnight, showers will become more scattered and lighter and temperatures will begin dropping into the 40s. Clouds will quickly clear out of the area from west to east by daybreak Saturday as the rain ends. Storm total QPF looks to be generally 0.75" to 1.25" with the highest amounts from around the Philly metro and northeastward toward the NYC metro. The lightest amounts will be across the western portions of the forecast area with storm total values of 0.50" or less for Berks and Carbon Counties. Given the dry antecedent conditions and longer duration of this event, flash flooding is not expected. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Cold front will be east of the area by Saturday morning, and high pressure builds in from the west. Temperatures will be below normal with highs mainly in the 50s, but near 60 in some spots. High pressure will be over the area Saturday night. Strong radiational cooling conditions will set up with the high nearly overhead with clear skies and nearly calm winds. A Frost Advisory may be needed for a good portion of southeast Pennsylvania, most of New Jersey, and portions of Delmarva, as lows will drop into the low to mid 30s. For Philly and the immediate coasts of New Jersey and Delaware, lows will be in the 40s. Cannot rule out some spots with lows at or below freezing, but at this time, that will be isolated. Will not mention frost in Carbon, Monroe, and Sussex (NJ) counties, as the growing season ended there in September. High pressure moves offshore on Sunday. Return flow sets up, and temperatures warm into the 60s. Going into the new week, high pressure remains over the western Atlantic, and a cold front approaches from the west. There is considerable uncertainty as to how far east that front will be able to get, but it should mainly remain west of the local forecast area. Several weak waves of low pressure will develop on that boundary and will track to the north and west. As a result, will carry slight chance to low end chance PoPs for far northern and western portions of the forecast area. If the 12Z GFS is right, could have more in the way of light precip making it into more of the region, and if the ECMWF is right, it should mainly be dry until late next week. Temperatures will be slightly above normal levels. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today... A surface cold front is just west of KRDG and KABE this morning with steady rain falling behind the front. The real feature of interest is a shortwave that is currently located in NC and can easily be seen on water vapor. This wave will ripple northeast through the morning hours and will cause widespread rainfall over the terminals. The cold front itself will cross the zones through the next couple of hours with it mainly being a wind shift and a slow reductions in cigs. The wave over NC will then mover overhead around 15z. Due to this, have targeted the greatest restrictions in visibilities and ceilings around this time. The other tricky part here is how much ceilings will lower. GFS and NAM forecast soundings have been very consistent in showing a brief lowering to MVFR across the western TAF sites (KRDG and KABE) with a then slow recovery to VFR by this afternoon. Towards KPHL mostly MVFR cigs are forecast. The highest confidence for IFR cigs looks to be towards KMIV and KACY, or towards the greater area of lift. Moderate confidence. Tonight...Skies slowly clearing from west to east with cigs rising to VFR. Rain will also be coming to an end as the upper level wave ejects northeast. Northwest winds around 10 to 15 kts. High confidence. Outlook... Saturday through Saturday night...VFR. NW winds 15-20 kt, diminishing to 10-15 kt in the afternoon, then LGT/VRB at night. High confidence. Sunday...VFR. S winds 5-10 kt. High confidence. Monday through Tuesday...VFR. S winds less than 10 kt. High confidence. && .MARINE... The SCA for Delaware Bay and the DE coastal waters was cancelled earlier since winds have subsided across these areas. On the NJ waters, the Small Craft Advisory conditions will continue through the evening with winds gusting 25-30 kts and seas near 5 feet. Winds and seas will diminish overnight, but will increase slowly again throughout the day Friday as a cold front moves offshore. Small Craft Advisory conditions appear probable again by late tomorrow afternoon and into the overnight period Friday night with winds gusting around 25 kts and seas 3-5 feet. Outlook... Saturday...SCA conditions with NW winds 15-20 kt with 25-30 kt gusts. Conditions subside to sub-SCA levels by Saturday night. Sunday through Tuesday...Sub-SCA conditions expected. && .EQUIPMENT... The Dover, DE WSR-88D radar (KDOX) will be out of service for approximately the next 7 days. This is for the replacement of the generator, fuel tanks and accompanying components, which is all part of the NEXRAD Service Life Extension Program. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...MPS Near Term...Staarmann Short Term...Staarmann Long Term...MPS Aviation...Haines/MPS Marine...MPS/Staarmann Equipment...WFO PHI