Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 290753 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 353 AM EDT Sat Apr 29 2017 .SYNOPSIS... A cold front is expected to slide south into our region tonight before stalling over Delmarva and south Jersey. The front is expected to return north as a warm front late tomorrow. Another cold front is expected on Monday night as an area of low pressure moves into the Great Lakes region. This low will drift northeast through eastern Canada through mid week. Another low pressure system will lift out of the lower Mississippi Valley towards the Mid Atlantic by late in the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... An area of showers continues to develop along an old weakening frontal boundary that remains stalled out across the area early this morning as a strong short wave/vorticity impulse moves across the area. There is enough lift to interact with the elevated instability to create some scattered thunderstorms as well. While we do not expect any severe weather to occur, any thunderstorms that do develop will produce a period of heavy rainfall, frequent lightning, and locally gusty winds. Some small hail may also be possible. This first round of showers and thunderstorms will move to the east and offshore around or shortly after sunrise as the first short wave passes to our east. Then we should have a break in shower/thunderstorm activity, before another round of showers and thunderstorms move into the area later this morning into the the noon hour as a second short wave/vorticity impulse moves across the area. This area should weaken move to our east as the second short wave passes to the east. Then yet another round of showers and thunderstorms is possible by late this afternoon into this evening as a backdoor cold front moves into the area and another short wave slides across the area as well. This batch of showers and thunderstorms will have more surface based instability to tap into and less of a cap. So it could be possible that an isolated severe thunderstorm could develop late today. Temperatures today will warm quite significantly and come withing a couple of degrees for some areas. GED is the one site that is actually forecast to reach a record. && .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/... The polar jet remains displaced well to our north in Canada, with an active southern stream across the conus thru the period. A southern stream closed low ejects northeast out of the four corners region on Sunday and through the Great Lakes on Tuesday. A broader long wave trough then sets up on Wednesday, extending from Atlantic Canada to the Colorado Rockies. There is general model agreement that this feature slowly progresses eastward, as a cutoff low develops across the southeast states. Overall, this will maintain a southwest flow regime with temperatures at or above normal. The two primary focuses for precipitation will be with a cold frontal passage Monday night, then within the overrunning area ahead of the southern stream cutoff low from Thursday into Friday. Some residual convection is possible through at least midnight on Saturday night across Delmarva and southern NJ, as a cold front moves slowly through this area, and stalls over central Delmarva through Sunday. The GFS appears to be overdone with QPF north of this boundary through Sunday, and have discounted it, given the drier surface air to the north and slight ridging aloft as well. The NAM and ECMWF are much drier, and the forecast reflects this, with Delmarva the exception. As the boundary stalls, we expect renewed showers and thunderstorms on Sunday. The high temperature forecast will be challenging on Sunday, with a maritime air mass within the easterly flow north of the front, while portions of Delmarva remains south of the front within the warm sector. We expect low-70s to low- 80s across Delmarva, with low-60s to low-70s across the remainder of the region. The exception will be some of the NJ barrier islands, where highs will not make it out of the 50s! The boundary is then expected to lift northward as a warm front on Sunday night. There is not much support for precip, so PoPs were kept in the slight chance category. Expect temperatures to remain steady or rise overnight. With the cold front still over the Ohio Valley on Monday, 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-mid 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. The post-frontal air mass will remain moist on Tuesday, with a surface trough traversing the region under a cyclonic flow aloft. Expect considerable cloudiness and a continued chance of showers, especially during the afternoon, and northwest of the NJ Turnpike/ I95. It should be too stable for any thunder but winds may gust up to around 30 MPH. Much cooler compared to Monday, but high temps still at or slightly above normal. For Wednesday, slight ridging builds in aloft and the column is dry, so fair weather is expected. 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 /08Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Showers and thunderstorms are moving across the area early this morning and will affect all TAF sites. With the showers, a period of heavy rain will cause a reduction in VSBYS and possibly CIGS. These showers and thunderstorms will move east of the area after sunrise, then another round of showers, possible thunderstorms, could affect the TAF sites later this morning. Another round of showers and thunderstorms is more likely later this afternoon which has a better chance of being stronger storms with reductions to conditions. Outside of the showers and thunderstorms, VFR conditions will prevail through the daytime hours. However, as a backdoor cold front sinks through the area later this evening and overnight, we expect lower clouds and light fog to develop. At this time, we only expect conditions to lower to MVFR, but there is the possibility that conditions could lower to IFR overnight. Winds are variable at many locations, but are generally out of the southeast to south for most places where there is any direction. Winds will increase out of the southwest this morning, before becoming west then northwest through the day and into the afternoon and evening. Winds could gust 15-20 knots for a period this afternoon as well. Behind the frontal passage, winds will become northeast this evening and 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. && .MARINE... Conditions on the coastal waters are expected to remain below Small Craft Advisory levels through today and tonight. Even though there are some strong winds above the surface, there is a strong inversion which will keep those winds fro mixing to the surface. However, winds could gust around 20 knots and seas could reach 4 feet at times through the day. There is a chance of showers and thunderstorms across the waters. Any thunderstorms will contain frequent lightning and may cause strong gusty winds. OUTLOOK... SCA likely Monday through Tuesday. Southerly winds increase Monday, with gusts up to 30 knots continuing into Monday night. Winds will become northwest by Tuesday morning, with renewed gusts around 25 knots possible. Seas building to the 5-7 Ft range on Monday, and then 7-9 FT Monday night, decreasing to around 5 FT late Tuesday. Seas may remain elevated into Tuesday night on the ocean, which would necessitate the extension of any Small craft Advisories. && .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...Johnson Near Term...Robertson Short Term...Robertson Long Term...Franck Aviation...Johnson/Robertson Marine...Franck/Robertson Climate...

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