Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 282358 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 758 PM EDT Fri Apr 28 2017 .SYNOPSIS... A cold front over the upper Mississippi Valley will move east and through our region Saturday night. This front will become stationary near Delmarva on Sunday, then lift northward as a warm front on Monday, as low pressure moving through the Great Lakes drags a stronger cold front through our area Monday night. Another area of pressure organizing over the Arklatex on Wednesday will move northeastward toward the middle Atlantic coast by Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Cold front lies across central NY/PA, and will continue to track east this evening and will slowly work its way across the region late tonight through Saturday morning. Ahead of the front, surface dewpoints are mainly in the 40s to low 50s across northern/central NJ, the Lehigh Valley, and most of SE PA. Across much of southern NJ, DE, and NE MD, sea breezes have developed and pushed dewpoints into the 60s. A large MCS has developed over Indiana, and will, continue to slowly track to the east through the overnight. So far, few storm reports have been issued, but radar indicates widespread heavy rain, and numerous lightning strikes have been detected. Going through the overnight, this MCS and the aforementioned cold front will continue to approach. Models indicate the bulk of the instability across the Delmarva, correlating to the higher dewpoints. However, the forcing will mainly be aligned with the approach of the front itself, over northern zones. Think it will be hard for dewpoints north of I-195 to rise too much during the overnight, and this will inhibit convective potential. Will keep a swath of likely PoPs in the forecast for northern zones late tonight, but will keep thunder potential capped at chance. Slight chance/low chance PoPs should cover southern NJ, the Delmarva, and extreme SE PA. A warm night on tap with lows in the upper 50s and 60s. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/... Any lingering showers and thunderstorms will taper off Saturday morning. West winds develop behind the front, and will increase to 10-15 mph with afternoon gusts up to 20 mph. Skies become mostly sunny by late morning/early afternoon, and temperatures will quickly rise into the mid/upper 80s across most areas. Humidity levels will remain moderate as well, so there will be some discomfort during the mid-day heat. Scattered showers and tstms will again develop west of the area and move across the region later in the afternoon. We have continued with the chc pops for the area attm. && .LONG TERM /SATURDAY NIGHT 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 /00Z SATURDAY THROUGH WEDNESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...IFR VSBY in fog at KMIV/KACY late tonight. Otherwise, mainly VFR through around 08-10Z. SHRA and sct TSRA will move into the terminals during that time with localized MVFR conditions. Light winds become SW 5-10 KT late tonight. Saturday...Any lingering SHRA/TSRA tapers off in the morning. VFR for most of the day. West winds increase to 10-15 kt with gusts as high as 20 kt in the afternoon. Scattered late day/early evening SHRA/TSRA possible. OUTLOOK... MVFR possible in any lingering showers and thunderstorms Saturday night, especially ACY, MIV, and ILG. Predominantly VFR on Sunday, with east wind gusts up to around 20 MPH at all TAF sites. MVFR is likely at all TAF sites Sunday night into early Monday morning, mainly attributable to low clouds. A return to VFR is expected by late Monday morning, with southwest wind gusts up to around 25 kts during the afternoon. MVFR possible in SHRA/TSRA Monday night. Continued MVFR is possible in low clouds on Tuesday, with greatest confidence at ABE and RDG. Winds will shift to the northwest by Tuesday morning, with gusts up to around 25 kts possible during the afternoon. Wednesday is expected to feature predominantly VFR at this time. && .MARINE... Great conditions on the waters this evening and then the early overnight periods. Sct showers and perhaps a tstm may affect the NJ coastal waters and possibly into Del bay after midnight. Overall, winds will be mostly SW or W with speeds at or under 10 knots. Seas will remain mostly 3-4 FT on the ocean a with a med period swell. Across Del Bay, seas will remain 1- 2 ft at most. Fair weather Sat after a few morning showers and sct tstms. Increasing SW winds up to around 12-18 knots, looks like SCA flag will probably not be needed. 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 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 Record high temps listed tomorrow-Saturday where it is forecast to be within 2 degrees of record. PHL 90-1974 GED 88-1990 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Franck Near Term...MPS/O`Hara Short Term...O`Hara Long Term...Franck Aviation...Franck/MPS/O`Hara Marine...Franck/MPS/O`Hara Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.