Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 260155 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 955 PM EDT THU AUG 25 2016 .SYNOPSIS... A weak cold front moves across the region on Friday, and then remains nearly stationary just south of the region through the weekend. High pressure builds to the north this weekend and then moves off the New England Coast on Monday. Another high settles over the area into the middle of the new week, but a series of weak disturbances will pass through the region during this time. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... A cold front was located over the Great Lakes and Midwest regions this evening. A line of showers and storms along the front was moving into western PA/NY. The front and associated convective activity will continue its progression downstream and should be on our western doorstep shortly after midnight. This activity is expected to weaken as it travels into an increasingly stable environment farther east. Convection should be disorganized and weak by the time it reaches our far western zones in eastern PA (e.g., Berks, Carbon, Monroe Counties) by around 2 AM but there is a brief window for an isolated thunderstorm. Cannot rule out a few leftover showers or sprinkles to be maintained farther downstream into the Delaware Valley and NJ coast by early morning. Hires CAM guidance generally supportive of this scenario. Boundary layer decoupling has already taken place west of the I-95 as noted by calm winds. These areas (specifically the river valleys) would be favored for the development of radiational fog tonight. However, debris clouds from the upstream convection is expected to move in overnight, limiting our fog threat in these inland areas. Farther east toward I-95 and the coastal plain, a S-SW winds around 5-10 mph will likely continue overnight in many places. This will also hinder the setup for fog overnight. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... Latest models trended faster with the weak cold front. GFS is the most progressive, showing the cold front arriving at the I95 corridor by 15Z. Given the upper level ridge and strength of the low level southwesterly flow ahead of the front, this appears to be too fast. Still, did speed up the timing slightly (closer to the NAM), with the front now arriving at the I-95 corridor near 18Z. Given the faster arrival of the front, expect heat index values to fall short of heat advisory conditions (though we may not see significant cold air advection just behind the front, expect there will be some dry air advection through the day). The heat index is forecast to be between 100-105F range in southern DE and adjacent eastern MD, which is awfully close to heat advisory criteria. We do not expect much in the way of precipitation with an absence of organized lift and with a convective lid being provided by a strong mid-level subsidence inversion on the northern periphery of the upper ridge. An isolated shower is possible near the coast during peak heating but deeper convection needed for thunderstorm development is not favored in this environment. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Cold front continues to push south of the region Friday night. Behind that front, high pressure builds into the Great Lakes region and will slowly track east through the weekend. Northerly flow ahead of the high will usher a much dryer airmass into the region on Saturday with surface dewpoints falling from the upper 60s to near 70 Friday evening to the upper 50s to around 60 by Saturday afternoon. High temps will only be a few degrees cooler compared to Friday, but conditions will feel more comfortable due to the lower humidity levels. The center of the high will pass over central NY/PA Saturday night, and then moves off the New England Coast on Sunday. Return flow sets up behind the departing high, and returns flow sets up with surface dewpoints creeping back up into the mid and upper 60s, making it a bit more humid compared to Saturday. Cold front then approaches on Monday and slowly passes through the region on Tuesday. Isolated showers and thunderstorms are possible Monday afternoon and evening, and then there is a better chance for showers and thunderstorms on Tuesday with the passage of the front. There currently is not much upper level support, so will keep POPs capped at low end chance for now. Thereafter, surface high pressure settles over the Great lakes and Ohio Valley into the middle of the week. However, a series of weak upper level shortwaves and vort maxes will pass through the region, touching off isolated to scattered showers and thunderstorms during that time. Will go no higher than low chance for POPs. Temperatures will continue to run several degrees above normal through the middle of next week, but should as hot as Friday. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mainly VFR conditions tonight. There may be an isolated shower in our western terminals between 06-08Z. Patchy fog is possible but the setup does not look as favorable for as it did before with convective debris clouds moving in late tonight. Winds shift from SW to W and then NW during the morning behind a cold front. There is a small chance (too small to mention in the TAFS at this time) that MVFR ceilings could develop in wake of the fropa Friday morning. OUTLOOK... Friday night through Sunday night...VFR. Light N winds will become NE on Saturday afternoon, and then will become SE on Sunday. Monday through Tuesday...Mainly VFR. Isolated to scattered showers and thunderstorms possible. && .MARINE... Gusts at or above 20 kt will continue through the overnight hours, but are expected to stay below SCA criteria at this time. By tomorrow, winds should begin to decrease. OUTLOOK... Friday night through Tuesday...High pressure passes well north of the waters this weekend. A cold front approaches on Monday and slowly passes across the waters through Tuesday. Sub-Small Craft Advisory conditions expected to continue across the waters. For today...The risk for the formation of dangerous rip currents is low for the New Jersey shore and the Delaware beaches tomorrow. However, if winds remain backed more out of the south than what we are currently forecasting, then a moderate risk could develop at our NJ beaches. The probable risk for the formation of dangerous rip currents on Saturday appears to be low at this time. Sunday through Wednesday...model ensemble guidance on every successive cycle continues to send 2-4 ft long period (15 to 17 second) east southeast swell into our waters from the central Atlantic - Gaston. It could be even higher but we`re taking the conservative approach at this time from this distant offshore hurricane. This is all dependent on size, strength and track but there is little doubt that swells will build here...just how much is the question. Swimming and wading dangers will probably increase. For now we`re anticipating a moderate or greater risk for Monday through Wednesday and possibly starting Sunday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Johnson/MPS Near Term...Klein Short Term...Johnson/Klein Long Term...MPS Aviation...Johnson/Klein/MPS Marine...Johnson/Klein/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.