Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 291341 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 941 AM EDT Sat Apr 29 2017 .SYNOPSIS...
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An initial cold front will continue to dissipate in our region today, meanwhile a stronger colder arrives early this evening and is expected to settle southward tonight before stalling over Delmarva and south Jersey. The front is expected to return north as a warm front Sunday night. Another cold front is expected Monday night as an area of low pressure moves into the Great Lakes region. This low will drift northeastward through eastern Canada through mid week. Another low pressure system will then track towards the Mid Atlantic by late in the week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A surface analysis shows an initial cold front weakening and tending to dissipate across parts of our region this morning. The upper- level flow is such to allow for several short waves to track eastward, however a stronger cold front will be approaching from the north-northwest late this afternoon. A weakening convective cluster is approaching southeastern Pennsylvania and northeastern Maryland, as of 13z. The lightning has weakened considerably with this with mainly just a batch of showers heading eastward. This should fall apart as it tracks farther to the east. Some new convection may try and develop on its southern flank where a bit more warmth and more humid air is located. There then should be a break until some additonal showers and thunderstorms may arrive late this afternoon as the stronger cold front arrives along with another short wave. This batch of showers and thunderstorms will have more surface-based instability to tap into and less of a cap. So it is possible that an isolated severe thunderstorm develops late today especially south and west of Philadelphia. Temperatures will warm significantly this afternoon, as long as the convective cloud debris thins out. The 12z Sterling, VA RAOB has a temperature of +22C in about 960-900 mb layer. This may put some spots within a couple of degrees of record highs (GED is forecast to reach a record). The hourly temperature, dew point and wind grids were adjusted based on the latest observations, then the LAMP/LAV guidance was blended in for the next few hours. The sky cover was adjusted upward for much of the area given quite a bit of convective cloud debris moving through. The temperature rise may therefore be slowed a bit for at least some areas for a time.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... The backdoor cold front will continue to push through the area this evening and overnight. The round of showers and thunderstorms mentioned for the late afternoon and evening will slide east and move offshore during the evening hours as the short wave/vorticity impulse associated with it moves to our east as well. As the front sinks to our south, a northeast-east flow will develop across the area. There could be some light fog develop overnight, but we do not expect much if any dense fog at this time as winds will remain around 5-10 mph through the night and dewpoints are expected to drop enough behind the front that RH values do not reach high enough for widespread fog for most places. With the front near the area, it is possible that some isolated/scattered showers may develop overnight through daybreak Sunday north of the frontal boundary. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Sunday...there is some question on how far south the front will reach before stalling. The general trend with the 00Z models was to bring the front further south (possibly entirely south of our forecast area). Where the front stalls will have implications for both high temperatures Sunday as well as precip chances. As mentioned by the previous shift, the GFS appears to be overestimating the precip north of the front given relatively dry air in the cool sector. Therefore, focused the PoPs through this period on Delmarva closer to the front. The front is expected to return north as a warm front either late in the day tomorrow or tomorrow night. Monday...With the cold front still over the Ohio Valley, expect a mostly sunny and dry day. A brisk south- west flow will develop, with wind gusts up to around 30 MPH in the afternoon. Temperatures will be well above normal, with highs in the low 80s across the urban corridor. With the cold frontal passage Monday night, showers and thunderstorms are likely. While the timing is not ideal for severe weather and instability is limited, the wind field is strong and Precipitable Water values approach 2.0 inches, so gusty winds and heavy downpours are likely with any thunderstorms, the primary focus being northwest of the NJ Turnpike/I-95. Tuesday and Wednesday...in the wake of the cold front, temperatures will be about 5 to 10 degrees lower, with highs generally in the 60s and 70s. Models have trended faster with the dry air advection behind the front, so I trimmed back on the PoPs on Tuesday. It is looking mostly dry both days. Unsettled weather during the Thursday and Friday time frame. A broad trough to our west will maintain a southwest flow aloft, along with moisture of Gulf of Mexico origins moving up the eastern seaboard, within an overrunning regime. Heavy rainfall is certainly possible, with WPC guidance indicating the potential for 1-2 inches across our region, but there is still inherent uncertainty given it`s Day 6-7. Stay tuned. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...Lingering localized light fog dissipates by 15z, otherwise mainly VFR with ceilings at and above 5000 feet. Some showers should affect areas mainly from near PHL to ILG and RDG through late morning, then some additional showers and thunderstorms should arrive later this afternoon. Any shower or thunderstorm will briefly result in conditions, especially visibility, lowering to MVFR/IFR. Confidence is on the lower side regarding coverage and timing of showers/thunderstorms. Southwest winds increasing to around 10 knots becoming west-southwest, with some gusts to 15-20 knots possible this afternoon. Tonight...A few showers or thunderstorms mainly in the evening with locally reduced ceilings/visibilities, otherwise VFR for a time. However, a cold front settles southward and the winds will turn to the northeast allowing for cooler air to move. This should result in MVFR to possibly IFR ceilings to develop toward morning. West- northwest winds mostly 10 knots or less this evening, becoming north and northeast overnight. OUTLOOK... Sunday...Mostly VFR conditions expected. Sunday night...MVFR or IFR conditions are possible with low clouds and fog. Monday...Mostly VFR conditions are expected although MVFR conditions are possible with any showers or thunderstorms. Monday night...Widespread MVFR or IFR conditions with thunderstorms likely. An abrupt wind shift to westerly is expected with a cold front Monday night. Tuesday and Wednesday...Mostly VFR conditions expected.
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&& .MARINE...
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Conditions on the coastal waters are expected to remain below Small Craft Advisory levels through tonight. Even though there are some strong winds above the surface, there is a strong inversion which will keep those winds from mixing to the surface. However, winds could gust around 20 knots and seas reach 4 feet at times through the day. There is a chance of showers and thunderstorms across the waters. Any thunderstorms mainly late today and this evening will contain frequent lightning and may produce strong gusty winds. OUTLOOK... Sunday...winds and seas are expected to be below SCA criteria. Monday and Monday night...building seas and increasing winds are expected by Monday morning. Southerly gusts near or above 30 KT are possible Monday evening. Tuesday and Wednesday...winds will shift to southwesterly and then westerly behind a cold front expected early Tuesday. Gusts on Tuesday may linger near 25 kt. Elevated seas may linger into early Wednesday.
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&& .CLIMATE... Record high temps listed today where it is forecast to be within 2 degrees of record. PHL 90-1974 GED 88-1990 TTN 88- 1974 & 1888 **Record or number 2 warmest April on record expected** Presuming our forecast temps these last 2 days of April are accurate, we are assured of a top 2 warmest April on record in much of our forecast area. Sunday is probably the critical day for determining record or not. Below: April projected within the top April average temps, the normal for April and the period of record (POR). This includes Friday`s (28th) high and low temperatures through 4 PM. Sundays max temps may occur at 1 am and the min temps Sunday evening. PHL normal 54.0 POR 1874 59.5 2017 Projected warmest ever: still could slip .2 degrees. 59.4 1994 58.5 1921 58.4 2010 57.9 1941 ABE normal 49.9 POR 1922 56.9 projecting record and almost cant miss a record. 56.4 1941 54.7 1994 ACY normal 51.7 POR 1874 57.6 projecting record and almost cant miss. 56.3 2010 56.1 2011 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Gorse/Johnson Near Term...Gorse Short Term...Robertson Long Term...Franck/Johnson Aviation...Gorse/Johnson/Robertson Marine...Johnson/Robertson Climate...

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