Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 192320 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 720 PM EDT Sun Mar 19 2017 .SYNOPSIS... Low pressure southeast of Cape Cod will move farther out to sea tonight and Monday while high pressure builds over the eastern seaboard. A weak disturbance will pass through the region Monday night. A cold front from this system will stall to our south on Tuesday. A wave of low pressure will track along the boundary. High pressure builds in from the northwest Wednesday before the center of the high retreats off the Mid-Atlantic coast by Friday. A warm front is expected to move into our region Friday into Saturday. The boundary may start to return back southward as a cold front late in the weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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730 pm update: Needed to make more adjustments to temperatures as they have been a little bit slower to fall than expected, though there has been a sharper decrease noted in the past 15 minutes. Also adjusted the winds downward this evening, as they were running a bit too strong, especially in eastern Pennsylvania. 630 pm update: Hourly temperatures were nudged upward early this evening, as they were running 3-7 degrees too cool in several locations. Also adjusted temperature trends tonight based on latest high-resolution guidance and on where remaining snowpack exists, given the expected favorable nocturnal radiational cooling environment that will occur after sunset. Minimum temperature adjustments were generally minor, but did edge temperatures downward a degree or two northwest of the I-95 corridor in general. Previous discussion... A ridge of high pressure over the Ohio Valley will push east towards the area tonight. Skies are expected to remain clear and winds will become light. Temperatures will drop back to below normal readings with low 20s north, mid 20s in the Lehigh Valley and low 30s across metro Philadelphia and Delmarva.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... Spring begins Monday morning at 6:29 AM. Monday will be a pleasant day across the Delaware Valley with high pressure in control. Sunny skies early will slowly cloud over late as the next disturbance approaches late. Overall, a nice day however. It will start the day chilly, but readings will warm to close to normal with low 50s south and mid/upper 40s north. Winds will generally be from the NW or W at 8 to 12 mph during the day. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... A rather flat large-scale pattern over the CONUS to start the period Monday evening will quickly amplify Midweek with ridge building over the Rockies and Great Plains as a downstream trough over eastern Canada digs southward into Great Lakes and Northeast states. This pattern change will be preceded by a weak disturbance (currently located over the Intermountain West) passing through the northern Mid-Atlantic region Monday night. The synoptic pattern then looks to transition to ridging over the eastern CONUS late in the week once the upper trough progresses off the east coast. A storm system over the Southwest U.S. is then forecast to track eastward and eventually help break down the East Coast ridge by next weekend. Monday night...The aforementioned shortwave disturbance that passes through our region is expected to be accompanied by some precipitation. Yesterday`s model runs were in agreement that the showers to our west would dissipate as they moved eastward into the forecast area Monday night. While timing of these showers haven`t changed much over the past 24 hours, today`s 12Z model runs maintain these showers a bit longer into the night as they pass through the region. Accordingly, PoPs have trended upward a bit. QPF ranges from less than a tenth inch across NE PA/NW NJ to around a quarter inch in Delmarva. Thermal profiles indicate rain for most of the area except there is a brief potential for sleet to mix in at the onset north of I-80. Tuesday...Subsidence in wake of the Monday night system will provide us with the opportunity for some sunshine and if that happens, it could be the first day with above normal highs since March 9th. Our forecast highs reflect this happening. However, NAM forecast soundings introduce some skepticism with the potential for stubborn stratocu with moisture trapped underneath the subsidence inversion. Tuesday night and Wednesday morning...A strong 1040 mb surface high over southern Manitoba builds southeastward toward our region. NW flow around the high signifies a return to winter cold as we technically transition to astronomical spring. A wave of low pressure is expected to develop along a west-to-east oriented cold front to our south. Some of the details still need to be resolved, including how far north does the precipitation shield expand and how quickly does the colder air arrive. Our deterministic forecast calls for rain mixing with or perhaps briefly changing to snow as colder air progress southward before it ends early Wednesday morning across our far southern zones (south of Philadelphia). However, it`s much more practical to think in a probabilistic framework this far out. Our experimental probabilistic snowfall guidance shows very low probabilities (about 10 percent) for over an inch of accumulation in our southern zones. Wednesday afternoon through Thursday night...Cold and dry conditions with high pressure building over the region. Temperatures will struggle to rise during the day on Wednesday as heating if being offset by CAA. Afternoon highs both days in the 30s N/W of the Fall Line and 40s S/E are 10-20 degrees below normal. Friday and Saturday...A warming trend will be in store as high pressure moves offshore and broad southerly return flow ensues up the eastern seaboard. The Mid-Atlantic region will be positioned between a building ridge over the Southeast states and a disturbance moving around the northern periphery of the ridge. The track of this shortwave trough will depend on how amplified the ridge becomes, but it could track far enough south to bring a period of shower to the region on Friday. Saturday is trending drier with models showing the next storm system still organizing over the central CONUS. Saturday also looks to be the warmest day of the period when the region will be situated well within the warm sector ahead of the next system. Highs in the 60s will make be reminiscent of February 2017! Saturday night and Sunday...Models try to develop a rex-type blocking pattern (to varying degrees of strength) over the Eastern U.S. as the ridge over eastern North America tries to rebuild while a closed low gets caught in the weak steering flow residing underneath. This could lead to an unsettled weather pattern heading into the second part of the weekend with the upper low nearby. There is also a potential for wintry precip north of I-78 as strong high pressure centered over eastern Canada is in a favorable position to draw colder air southward. Forecast confidence is very low with the details unclear this far out. && .AVIATION /23Z SUNDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 00Z TAFs...VFR through Monday afternoon. Mostly clear skies tonight followed by increasing upper level clouds tomorrow, with developing CIGs near 10 kft by late afternoon. NNW winds should decrease below 10 kts tonight slowly becoming more westerly during the day tomorrow. Winds should generally remain around 10 kts Monday. OUTLOOK... Monday night...A period of MVFR conditions possible with SHRA likely. Tuesday...Moisture trapped underneath subsidence inversion may result in stratocu and MVFR CIGs. NW winds increasing to 10-15 kt by the afternoon. Tuesday night and Wednesday...A disturbance passes to our south. Predominately VFR with the bulk of the precip likely staying south of the terminals. If forecasts happens to trend northward by about 50 miles, then a period of RA or RA/SN mix would be possible mainly from PHL southward. NW winds will become breezy on Wednesday with gusts near 30 kt possible. Wednesday night through Thursday night...VFR and light winds. Friday...Mainly VFR but MVFR could develop if showers reach the area. S-SW winds increase to 10-15 kt with gusts to 25 kt.
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&& .MARINE...
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Winds have really dropped off on the coastal waters (well below small craft advisory criteria), but the seas remain quite elevated (generally 6-9 feet at this time). Needed to adjust the wind forecast accordingly, but the advisory continues overnight owing to the high seas. OUTLOOK... Monday night through Tuesday night...No marine hazards anticipated. Monday and Tuesday...The conditions are expected to be dropping below Small Craft Advisory criteria Monday, and then remaining below for Tuesday. Wednesday...N-NW winds increase as high pressure builds southeastward. Widespread SCA likely though there is a brief potential for gale force gusts in concert with rapid pressure rises and deeper mixing.. Wednesday night through Thursday night....Winds and seas below SCA criteria. Friday...Sly winds strengthen to near 20 kt. Marginal mixing potential with warmer water above the cooler waters but cannot rule out the potential need for a SCA.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until noon EDT Monday for ANZ450>455. && $$ Synopsis...Klein Near Term...CMS/O`Hara Short Term...O`Hara Long Term...Klein Aviation...CMS/Klein/O`Hara Marine...CMS/Klein/O`Hara

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