Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 181610 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1210 PM EDT Sat Mar 18 2017 .SYNOPSIS... Low pressure across the Great Lakes region will redevelop just off the Mid Atlantic coast later today, then strengthen as it moves slowly out to sea south of Cape Cod Sunday morning. High pressure builds in briefly Monday, then a cold front moves through Monday night with a secondary cold front arriving Tuesday night. Strong high pressure is forecast to build in later Wednesday and Thursday, before shifting offshore Thursday night. A warm front will approach our area from the southwest later Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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1130 AM update...The Winter Weather Advisory for portions of NE PA and NW NJ has been cancelled with the threat of heavier/steadier snow bands on the northern side of a closed upper low shifted to the southwest the advisory area by almost 200 miles compared to yesterday`s 12Z model guidance. Minor snowfall accumulations (generally under an inch) is possible along the ridges with occasional light snow this afternoon and evening. Spotty freezing drizzle or light freezing rain may develop this afternoon and tonight but icing will mainly be confined to the highest ridges above 1000 ft in elevation as temperatures are above freezing in the valleys and precip shield is very organized. With the 700 mb closed low over Lake Erie now expected to dig southeastward toward the northern Piedmont region of VA late tonight, the potential for a convective band of moderate showers on the northern/western side of the circulation has shifted into south-central PA and the Chesapeake Bay region tonight. This band could shift into far southeastern PA (southwest of Phila) and Delmarva tonight. Forecast soundings indicate the effects of dynamical cooling allowing for wet snow to mix in all the way down into northeastern MD and possibly changeover to all snow along and north of the Mason-Dixon line. There is always a concern that the models are underdoing the strength of the dynamical cooling underneath the upper low. Accordingly, cannot rule out the potential for a Winter Weather Advisory (2+ inches of snow) for Chester-Cecil Counties and nearby locations but there is no guidance supporting that scenario at this time. Previous Discussion... A mid level low located near Lake Michigan early this morning will progress eastward reaching northwestern Pennsylvania and far western New York late in the day. A surface warm front is expected to remain across Virginia and the Delmarva today with an area of low pressure developing over Virginia during the afternoon. We are anticipating a cloudy day with an east to southeast wind around 5 to 10 MPH. The wind may begin to back toward the northeast late in the day. The onshore flow is forecast to bring low clouds into our region this morning. We continue to expect light precipitation for today, mainly in eastern Pennsylvania and New Jersey. Precipitation rates should increase a bit during the afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... The mid level low is forecast to drop over central Maryland and eastern Virginia late tonight. Deepening surface low pressure is expected to pass off the Virginia coast this evening before moving out to sea. The surface wind in our region is anticipated to become northeasterly and it should increase to 10 to 15 MPH, especially on the coastal plain. Cold air will begin to be drawn into our region and rain should begin to change to snow from north to south. An additional inch or two is possible in our northern counties with a light dusting possible as far south as the Philadelphia metropolitan area and central New Jersey. Low temperatures are expected to favor the 20s up north and the 30s elsewhere in our forecast area. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Synoptic Setup...A closed low is forecast to be in the East Sunday, however this shifts offshore later in the day and at night. Some ridging arrives during Monday, then another trough amplifies across eastern Canada Tuesday with this also extending across the Northeast through Wednesday. A shift in the pattern may then allow stronger ridging in the East Friday as a trough amplifies across the Plains and Midwest. The pattern will offer some warming, but also some shots of colder air once again. We used a model/continuity blend for Sunday through Monday night, then blended in the 00z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Sunday...A potent closed low is forecast to be sliding across the Mid Atlantic region in the morning, with a deepening surface low well offshore. While there are some differences in the details among the model guidance, the presence of the closed low overhead may assist in enough lingering lift in the morning especially closer to the coast. As a result, we kept some chance PoPs through mid morning. The system is forecast to move away during the course of the day with a tight pressure gradient in its wake. This should result in breezy conditions especially closer to the coast. As the entire system shifts east, clouds are anticipated to give way to increasing sunshine through the afternoon. For Monday and Tuesday...High pressure builds in briefly Monday before quickly shifting offshore. A cold front is forecast to arrive Monday night, however it looks like moisture is becoming more limited with this. There could be a better chance of some precipitation Tuesday night as a secondary cold front moves through with possible low pressure developing along it. This however may be shoved to our south as strong high pressure builds across the Great Lakes. We are expecting a milder airmass to be in place during this time frame, with the main cooling arriving Tuesday night in the wake of the secondary cold front. For Wednesday and Thursday...A potential start of a pattern change, at least for a time, looks to offer a trough developing in the Plains with downstream ridging. This ridging allows for strong high pressure near the Upper Great Lakes to build down into our area during the course of Wednesday, then it shifts offshore later Thursday. This setup allows for colder air to be in place with an initial northerly flow. However, we eventually get into a developing return flow as low pressure organizes across the Plains later Thursday with high pressure slipping offshore. We maintained a dry forecast and cooler temperatures. For Friday....A pattern shift looks to result in a strong trough across the plains which sends organizing low pressure toward the Great Lakes region. This, in combination with high pressure off the East Coast, will send warm air advection with a developing warm front approaching our area. This will result in the start of a warm up. While the main initial push of warm air advection and overrunning showers may be focused to our west, some showers may still arrive at some point Friday. Given timing and details uncertainty, any PoPs were kept on the lower side. && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions early this morning are forecast to lower into the MVFR, then into the IFR range between about 1000Z and 1500Z in response to a developing wind off the ocean. IFR conditions are then anticipated through the remainder of the TAF period. Light rain is expected for KTTN, KPNE, KPHL, KILG, KMIV and KACY. A mix of snow and rain is forecast to KRDG and KABE. A southeast to east wind around 5 to 10 knots is expected to develop for today. The wind should become northeasterly for tonight at speeds of 10 to 15 knots with gusts around 20 knots from KTTN/KPNE/KPHL/KILG southeastward. OUTLOOK... Sunday...MVFR ceilings possible to start, then VFR with thinning cloud cover through the day. Some light snow is possible in the morning south and east of PHL. Northeast winds around 15 knots with gusts up to 25 knots becoming north-northwest, then diminishing to 10 knots or less at night. Monday...Sub-VFR conditions possible at times mainly at night with a chance of showers as a weak cold front moves through. Tuesday and Wednesday...VFR overall. Northwest wind gusts up to 25 knots. && .MARINE... An east to northeast wind is forecast to be on the increase from north to south on our coastal waters from this afternoon into tonight. As a result, we will start a Small Craft Advisory at 2:00 PM for our waters from Sandy Hook down to Little Egg Inlet. We will begin a Small Craft Advisory at 11:00 PM for our waters from Little Egg Inlet south to Fenwick Island. We will start the Small Craft Advisory for Delaware Bay at 2:00 AM Sunday. OUTLOOK... Sunday...Strengthening low pressure offshore will result in a tight pressure gradient across our area. This combined with pressure rises and efficient mixing is expected to result in gale force gusts on the ocean (northeast to north flow). As a result, a Gale Warning has been issued through 18z. The winds should be a bit lower on Delaware Bay and therefore we opted to go with a Small Craft Advisory. The conditions however are expected to subside at night. Monday...The conditions should be below Small Craft Advisory criteria as high pressure briefly builds across the area. Tuesday and Wednesday...Small Craft Advisory criteria possible later Tuesday night and Wednesday, especially wind gusts, in the wake of a cold front. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 11 PM this evening to 6 AM EDT Sunday for ANZ452>455. Gale Warning from 6 AM to 2 PM EDT Sunday for ANZ450>455. Small Craft Advisory until 6 AM EDT Sunday for ANZ450-451. Small Craft Advisory from 2 AM to 2 PM EDT Sunday for ANZ430- 431.
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