Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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969 FXUS61 KPHI 270505 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 105 AM EDT Tue Sep 27 2016 .SYNOPSIS... A cold front will move across the area later tonight, then stall off the Mid Atlantic coast on Tuesday. An upper low will meander over the eastern United States for the rest of the week, lifting to the northeast and departing sometime this weekend. Meanwhile, several waves of low pressure will develop along that stalled boundary and move up the East Coast. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1230 AM ESTF update...Updated the hourly PoP/Weather grids thru this afternoon based on recent radar trends and hi-res CAM models. Expect precip to expand in coverage/become better organized overnight as it moves eastward thru the area. Current thinking is the heavier rainfall amounts will reside south and east of I-95 (mid Delmarva and far southern NJ). A weak shortwave impulse was detected on water vapor satellite imagery across southeastern VA. This disturbance will lift northeastward ahead of the main upper trough toward the Delmarva coast and enhance rainfall thru daybreak across these southern areas. These same southern areas are positioned near the pre-frontal theta-e ridge axis, where locally higher (elevated) instability will provide a more favorable environment for deeper convection that is needed to produce heavier rainfall rates and even isolated thunderstorms. Rainfall amounts of one-half to one inch are expected in these southern zones, while areas along and northwest of I-95 should see less (some spots may only receive less than one-quarter inch). Previous Discussion... An upper air analysis early this evening shows a strong closed low centered around the upper Great Lakes region. A narrow ridge axis is located across New England. Some warm air advection is noted around 850 MB across much of our region north of a warm front. A cold front will be arriving overnight and in advance of this the large scale forcing is forecast to increase, especially after Midnight. Some weaker forcing is ongoing just to our west which is well ahead of the cold front but north of a warm front. An analysis showed a weak short wave tracking across the northern Mid Atlantic. This along with some warm air advection, which is helping to strengthen an inversion, should produce enough lift within a moistening airmass to get some showers going especially across the southern areas initially. The more organized showers are expected overnight and toward daybreak from west to east. A review of many forecast soundings from various model guidance indicates that while some instability develops through the night, the profile becomes saturated which tends to really limit the instability. In this case, thunder may not occur. Given some uptick in the instability though along with some lightning to our west and southwest this evening, left in an isolated mention across the southern areas. As the moisture increases, some local downpours should occur as well. Some adjustments were made on the hourly grids based on the obs, then the LAMP/LAV guidance was blended in for the temperature and dew point grids. The POP increase was slowed a bit eastward as we await for the more organized forcing. Temperatures will be milder than last night given the cloud cover in place.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... The cold front continues its eastward movement but it looks like it gets hung up as it moves through the I-95 corridor and starts to slow down considerably. This will keep the precipitation hanging around longer and areas to the south and east of I-95 may see rain through much of the day. Although, precipitation should remain lighter as the best forcing moves away from the region. Areas to the west of I-95 will start to clear out by mid to late morning with the sun peeking through and starting to help dry out the area. Areas that break out on Tuesday will be warmer than those that remain stuck under relatively thick cloud cover. Highs into the low to mid 70s across eastern Pennsylvania and central to northern New Jersey with southern New Jersey and much of Delaware remaining in the upper 60s to near 70. If the front moves further east faster, then temperatures may rise a couple degrees above forecast due to less cloudiness. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... An extended period of unsettled weather is on tap for the rest of the week, and possibly into the upcoming weekend. Nearly vertically stacked closed low, along with several strong shortwaves, will dive into the Great Lakes and Ohio Valley Tuesday night. Meanwhile, frontal boundary remains stalled along the Mid- Atlantic. With these shortwaves rotating around the low and interacting with the stalled front, several waves of low pressure will develop over the Southeast U.S., and will lift to the north along the East Coast. Where the uncertainty lies is where each low will actually track, and where the axis of rainfall will develop. First swath of rain looks to come up the coast sometime Wednesday afternoon, with the bulk of the rain falling Wednesday night and Thursday. Latest GFS has a swath of rain across central NJ and into E PA, whereas the ECMWF has that swath over southern PA, MD, and VA. These details have yet to be worked out, but with onshore flow ahead of the low, abundant low level moisture will stream into the region, allowing for at least some periods of moderate to locally heavy rain. Another wave of low pressure looks to come up the coast on Friday, but there is less certainty with that track. The closed upper low, meanwhile, does not look to even begin to depart until sometime Saturday night into Sunday morning, and even then, models are not in good agreement as to the timing and placement of it. Therefore, will carry chance PoPs into the weekend. Regardless of where the rain does end up falling, given the onshore flow that will be in place, can at the very least expect cloudy and cool weather through the week with temps running a few degrees below the seasonal averages, with highs ranging from the low to mid 60s across northern zones to the low to mid 70s across southern zones. Latest WPC QPF forecast suggests about 2-5 inches of rain across the region, but that is over the entire period ending on Monday. Given how dry the area has been, this should not result in widespread flooding, and this rain is much needed. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR ceilings will lower to MVFR overnight as as a cold front arrives. Showers will develop and increase in coverage overnight, and an initial round may be more scattered. The showers will result in lower visibility at times. A period of IFR ceilings are possible especially during showers. A few isolated thunderstorms are possible overnight and early Tuesday morning, however coverage is too low to include in the TAFS at this time. Southerly winds mainly less than 10 knots, becoming west-northwest toward morning. Tuesday...Conditions are expected to improve to VFR quickly north and west of PHL early, with lower ceilings hanging on the longest at MIV and ACY as the cold front slows. It is here where showers are expected to last the longest along with MVFR ceilings. West to northwest winds up to 10 knots, then diminishing some in the afternoon and becoming southwesterly to perhaps locally light and variable. OUTLOOK... Tuesday night through Saturday...Sub-VFR conditions likely in showers and fog. Best chances for more widespread rain appear to be on Thursday and Friday. Forecast confidence: low. && .MARINE... A Small Craft Advisory is in effect for all waters from tonight through Tuesday morning. Southerly winds around 10 to 15 knots will increase as the gradient tightens ahead of an approaching cold front. In addition, seas will start to build and will near 5 feet by late tonight. The cold front will slow down as it nears the coastline and is expected to move offshore Tuesday morning. Winds just ahead of the front will be out of the south to southwest and will quickly turn to the west-northwest behind the front. highest gusts are expected to occur during the morning just ahead of and behind the front, with a quick drop off as the front moves slowly to the east. Seas will remain elevated through the mid morning, and will then subside, falling below 5 feet by late morning. OUTLOOK... Tuesday night through Wednesday...Winds and seas below SCA threshold. Wednesday night through Saturday...E winds 15-20 KT with 25-30 KT gusts possible across northern ocean waters. A period of gale force gusts is possible Wednesday night. Seas may build to 6-8 FT. A SCA will likely be needed for the ocean waters for at least Wednesday and Thursday. Low confidence in forecast thereafter, but SCA may continue to be needed into the start of the weekend. RIP CURRENTS... A moderate risk for the formation of dangerous rip currents continues through this evening. A moderate risk for Tuesday especially in the morning as winds and seas subside behind a cold front. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ430- 431-450>455.
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