843 FXUS61 KPHI 301946 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 346 PM EDT Thu Aug 30 2018 .SYNOPSIS... A cold front will continue to weaken as it moves to our south through the overnight, eventually clearing the area by Friday morning. High pressure will slowly build west from the Atlantic into our region through next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... The upper-level flow has become more zonal with the 250 mb jet farther north and a stronger segment from New England eastward. At the surface, weak low pressure near the Canadian Maritimes will shift out to sea while a weakening cold front is draped southwestward into the Mid-Atlantic. There is a lack of forcing with this lingering boundary, with perhaps terrain influences or other local affects to possibly force an isolated shower or thunderstorm. The instability decreases northward across the area due to drying that has taken place. While it remains humid across the area, the dew points are significantly lower compared to the last two days. This is resulting in noticeably lower heat indices. There is an outside chance that an isolated shower occurs into this evening, however this potential is rather low and therefore opted to remove the mention of precipitation for nearly the entire area. As we go through tonight, a subtle short wave trough embedded in the overall zonal flow is forecast to arrive in our area. Much of the guidance wants to strengthen the short wave energy with this as it arrives. Due to this, large scale ascent will increase late tonight. As this occurs, surface high pressure building across Ontario/Quebec Canada results in a developing northeasterly low-level flow into our area. Due to this, the flow veers with height late tonight and there are hints in some of the model guidance that a southeasterly low- level jet migrates northward toward morning. This all combined with the short wave energy should result in the development of some showers and possible thunderstorms late tonight from the southwest. The forecast challenge is the timing of this. Some guidance suggest that this is mainly a Friday thing. We therefore gradually increase the PoPs from the southwest late tonight and especially close to daybreak. In addition, low-level moisture and convergence with the onshore flow may result in some low clouds developing toward morning. Low temperatures are a blend of mostly MOS and continuity, and the hourly temperature, dew point and wind grids incorporated some hi- res guidance. It will be a muggy night for most (lower dew points across the far north as some additional drying is forecast to advect it), however not as warm as previous nights. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... Our region will reside in a zone north of a stalled but weak frontal boundary, with surface high pressure centered near northern New England. This will result in an onshore flow with cloudier and cooler conditions. It will still be on the humid side but not to oppressive levels. A subtle short wave trough is forecast to be generally shifting to our northeast during the morning, and there should be some showers with it. Given the onshore flow with some lingering short wave energy along with a northward shifting weak low- level jet (veering low-level flow), some showers and perhaps a few thunderstorms are expected. Some high-res guidance hinting at locally heavy rain as convection may be slow moving, however the overall coverage of heavier rain is less certain. The greatest instability should be found farther south across the area, with the influence from the surface high farther north and somewhat drier air resulting in less instability across our northern areas. We therefore trimmed back the thunder mention some northward and carried some lower PoPs across the far north. Due to the expectation of lots of clouds, some showers around and an onshore flow, high temperatures will be noticeably cooler. A multi- model blend with continuity was used overall for the high temperatures. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... The pattern will remain somewhat unsettled through the long term, especially through the weekend. Chances for rain showers and occasional thunderstorms will continue through the weekend. The chance for showers and thunderstorms will persist for each afternoon through the early part of next week. A relatively strong ridge will remain parked over the eastern U.S., preventing any major changes to the synoptic pattern, though it won't be strong enough to prevent the development of afternoon airmass thunderstorms. Dew points will remain fairly high, leading to more humid conditions through next week. A subtle short wave trough will continue to move across New England and southern Canada through Friday night. With most of the Mid- Atlantic sitting under the right entrance region of weak jet streak, and moisture advection off the Atlantic, chances for storms will continue through the weekend, though the greatest chances looks to be Friday night into Saturday morning with the strongest onshore flow. Highs will remain cooler Saturday with low 80s south and low to mid 70s north. Highs Sunday will return back to normal with mid 80s south and upper 70s north. High pressure will remain dominant through most of next week, with chances for rain and thunderstorms each afternoon. Steering flow and shear will remain weak, so convection will tend to be less organized. A short wave trough will ride around the building ridge of high pressure on Monday, which may increase the chance for rain and storms for the Labor Day afternoon. The ridge will continue to strengthen through the week, with potential for above normal temperatures through next week. Continued high dew points will result in heat indices in the mid to upper 90s. Guidance suggests the ridge will begin to erode by Friday as another cold front begins to move toward the area. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This afternoon...VFR with some cloud bases mainly between 5000-10000 feet. An isolated shower or thunderstorm is possible in the region mainly late this afternoon. North-northwest winds 8-12 knots, becoming west-northwest and diminishing some. A sea breeze should develop late this afternoon and possibly allow for a wind shift to southerly at ACY. Tonight...VFR with a developing ceiling of around 5000 feet through the evening, then an MVFR ceiling could develop toward daybreak at some terminals. Temporary visibility restrictions and lower ceilings are possible with any showers and thunderstorms, but confidence is low regarding coverage and timing. Winds becoming variable after 00Z, eventually settling out of the northeast less than 10 knots after 06Z. Friday...Sct showers and tstms. Some lower CIGS/VSBYS with NE flow off the ocean and a maritime airmass in place. MVFR psbl. Outlook... Friday night and Saturday...Sct showers and tstms. Some lower CIGS/VSBYS with NE flow off the ocean and a maritime airmass in place. MVFR psbl. Sat night thru Tuesday...Mostly VFR, but lower CIGS near the coast with sct showers/tstms mostly during the afternoon and evening. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of This afternoon...VFR with some cloud bases around 5000 feet. An isolated shower is possible. North-northwest winds 8-12 knots, diminishing some toward evening. A sea breeze right along the coast could reach ACY with a wind shift to more southerly. Tonight...VFR with clouds increasing a ceiling lowering. Some MVFR ceilings may develop close to daybreak from near the PHL area north and east. Some showers will be possible late with some visibility restrictions possible at times. Winds becoming light and variable this evening, then northeast 4-8 knots late. Low confidence regarding the MVFR ceilings and timing of showers. Friday...MVFR to times of VFR ceilings. Some showers and a few thunderstorms are expected with visibility restrictions at times, with potentially the greatest coverage from the PHL area south and west. Northeasterly winds increasing to around 10 knots. Low confidence regarding the ceiling heights. Outlook... Friday night and Saturday...Sct showers and tstms. Some lower CIGS/VSBYS with NE flow off the ocean and a maritime airmass in place. MVFR psbl. Sat night thru Tuesday...Mostly VFR, but lower CIGS near the coast with sct showers/tstms mostly during the afternoon and evening. && .MARINE... A Small Craft Advisory has been issued for Friday for the two northern New Jersey Atlantic coastal waters zones. Northwesterly winds, some nearshore east to southeast winds into this evening, will become northeast overnight and Friday from north to south. This flow will increase with the strongest winds (gusts to 25 knots) expected across the northern waters. Seas will also build into the 3- 5 feet range from north to south Friday (lower on Delaware Bay). Outlook... Low end SCA psbl into Fri night for the northern NJ waters, elsewhere sub-SCA. Sct shower and tstms. For the weekend and into early next week. Sub-SCA is expected with sct afternoon and evening tstms. Rip Currents... The risk for the formation of dangerous rip currents is expected to be low through this evening. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM Friday to midnight EDT Friday night for ANZ450-451. && $$ Synopsis...Davis Near Term...Gorse Short Term...Gorse Long Term...Davis/Miketta Aviation...Gorse/Miketta Marine...Gorse/Miketta