Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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960 FXUS61 KPHI 281619 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1219 PM EDT Fri Jul 28 2017 .SYNOPSIS... A weak front will remain across the area today. Low pressure will develop along the front near the Tennessee Valley today and move along the front south of our region tonight and Saturday. The low will then strengthen as it moves offshore Saturday night and Sunday. High pressure will build in for Sunday and Monday and remain into the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... Showers and thunderstorms over the DC/BWI area tracking to the east and moving into northeast MD. With SB CAPE values 2000-3000 J/kg across the Delmarva, there is plenty of instability and with surface dewpoints in the low 70s, there is plenty of moisture with which to work. These storms are slowly moving, so once they do make it into the region, the threat for flooding and flash flooding will begin. This afternoon and tonight... The stage is set for the second high impact heavy rainfall event in less than a week across the southern tier of the forecast area this afternoon and tonight. The greatest concern over the next 24 hours will be flash flooding. Here are a few points on this topic: 1) Forecast confidence for the occurrence of heavy rainfall is high. 2) There could be numerous occurrences of flash flooding late this afternoon and tonight (i.e., not just a highly isolated/ localized risk as we commonly see in summer-time flash flood events). 3) A southward trend in the axis of heaviest QPF has been noted the model guidance last night. This makes meteorological sense as the sustained deeper convection needed to produce heavy rainfall rates (greater than 2" per hour) will be favored in the unstable warm sector along and south of the stationary front. We have seen several events recently where this has happened and the heaviest rainfall shifted farther south than earlier indications. We opted to trim the northern tier of counties along I-195 in central New Jersey and north of the PA Turnpike in eastern Pennsylvania from the Flash Flood Watch. Otherwise the Watch remains unchanged. 4) The axis of heaviest rain is looking like it could potentially fall over the same areas south of the Mason-Dixon line that were drenched with 3-6" (locally higher amounts) of rainfall since last weekend. Since area basins and soils have not had time to recover, it will not take nearly as much rain to flood. 5) The flash flood risk ramps up in Delmarva late this afternoon in concert with diurnal convection that develops along a stationary front nearby. Complex of showers and embedded thunderstorms containing torrential downpours is then expected to become better organized as it expands northward into SE PA/S NJ this evening. Heavy rainfall is expected to continue into the overnight. There will also be a threat for a few strong to severe thunderstorms late this afternoon and into this evening, mainly across Delmarva. SPC has the southern two-thirds of the CWA (south of I-78) in a D1 marginal risk for severe storms, which represents a downgrade from the slight risk across Delmarva with yesterday`s D2 outlook. The main convective threat would be locally damaging winds from wet microbursts. Cloud cover, especially over the southern half of area, should limit how much the atmosphere can destabilize during the day, thus placing some uncertainty on the severe setup. && .SHORT TERM /SATURDAY/... The models are now trending back to a progressive solution that pulls the coastal storm away from the area faster than yesterday`s runs were showing. There will still be rains and gusty winds over the area Saturday morning, but the rains will end from N/W to S/E during the day and into the evening. Kept the 2 PM Saturday ending time for the Flash Flood Watch, but it may be able to be cancelled earlier if this latest trend holds true. Winds will become gusty as the low deepens and pulls away. Highs will only be in the 60s over the Lehigh Valley and points N/W while highs in the low 70s will be over the Delaware Valley and central NJ. Highs over srn Delmarva and the srn tip of NJ could reach the upper 70s to low 80s on Sat. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... A couple of showers could linger across southern Delmarva or SE NJ Sunday into early Monday, but by-in-large fair weather will return to most of the area. High pressure near Chicago Sunday will begin to usher in drier air from the N/W. Temperatures will be a few degrees below normal with highs in the upper 70s to low 80s over the srn areas and low/mid 70s across the north. Lows will be in the 50s N/W and low 60s S/E. The fair weather should extend deeper into next week as the high continues to move across the Ohio Valley and then the Middle Atlantic region. This and and rising H5 heights will contribute to temperatures climbing back to above normal readings by mid-week. Highs could reach the upper 80s with a few low 90s possible by Wed/Thu. An approaching front cloud bring a few tstms thu afternoon mostly to the N/W areas. && .AVIATION /16Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Still some lingering MVFR CIGs at KACY, otherwise VFR. Showers and thunderstorms developing south and west of the terminals this afternoon are expected to expand northeastward later in the day. This activity is expected to organize/become widespread after sunset this evening and continue into the overnight. MVFR conditions are anticipated but locally heavy rainfall will lead to poor visibility (LIFR) at times tonight. The heaviest rainfall and poorest visibilities is expected to setup just south of PHL. NE winds around 5 KT this morning will veer slightly out of the E and increase to 5-10 kt this afternoon, then back out the NE again and increase to 10-15 kt tonight as surface low pressure develops to our south. OUTLOOK... Sat/Sat night...Widespread MVFR/IFR slowly improving from N/W to S/E. Winds becoming gusty. Sun...Mostly VFR, but a few showers S/E. Sun night thru Tue...VFR expected. Patchy overnight fog psbl. && .MARINE... A NE wind around 5-10 kt will develop across the coastal NJ waters as the morning progresses, but may briefly shift out of the E this afternoon. A NE wind will strengthen tonight to around 10 kt in the DE Bay and adjacent coastal waters and to around 15 kt farther north in the NJ coastal waters. Winds should quickly ramp up across these northern areas to around 20 kt with gusts 25-30 kt early Saturday morning. OUTLOOK... High end SCA or low end Gale conditions for Sat into Sat night for both the ocean and the bay, Gale watch continues for now on the ocean with some uncertainty remaining wrt locations and intensity of Psbl Gale. The Gale Watch was expanded southward into the DE coastal waters and lower DE Bay. SCA expected Mon into Tue on the ocean. Sub-SCA later Tuesday. RIP CURRENTS... Based on the latest trends and guidance, we will carry a low risk for the development of dangerous rip currents today. While the flow will become onshore today, wind speeds will be light. Waves in the surf zone are expected to be in the 2-4 ft range. A high risk is likely Saturday through Sunday, and possibly into Monday. && .TIDES/COASTAL FLOODING... Minor coastal flooding is possible Saturday night and Sunday morning along the oceanfront. While astronomical tides lower, a surge potential of 1-2 feet may put some oceanfront sites into the minor category at the time of high tide. There is the potential for high surf with breakers of at least 8 feet along oceanfront beaches on Saturday and Sunday. A high surf advisory may be needed. Beach erosion is also possible. && .CLIMATE... Daily rainfall records for today and Saturday at our climate sites are listed below... 7/28 7/29 ACY 2.05 (2012) 2.21 (1884) PHL 8.02 (2013) 3.53 (1980) ILG 2.34 (1914) 1.85 (1913) ABE 3.00 (1969) 1.64 (1979) TTN 2.35 (2012) 2.84 (1961) GED 2.80 (2016) 1.07 (1969) RDG 3.57 (1969) 2.51 (1961) MPO 2.15 (1969) 4.59 (1969) July total ABE: 8.21" is the #8 wettest July on record. The wettest July on record is 10.42" set in 1969. RDG: 8.02" is the #8 wettest July on record. The wettest July on record is 13.85" set in 2004. July average temps: Projecting, for now, PHL 0.9 above normal, losing 1.3 positive departure from the values through the 26th. Ditto approximately the same loss at our other CLI sites will show MPO a little below normal, TTN and ACY near normal, and all other stations above normal. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch through Saturday afternoon for PAZ070-071- 101-102-104. NJ...Flash Flood Watch through Saturday afternoon for NJZ016>027. DE...Flash Flood Watch through Saturday afternoon for DEZ001>004. MD...Flash Flood Watch through Saturday afternoon for MDZ008-012- 015-019-020. MARINE...Gale Watch from late tonight through late Saturday night for ANZ431-450>455. && $$ Synopsis...O`Hara Near Term...Klein/MPS Short Term...O`Hara Long Term...O`Hara Aviation...Klein/O`Hara Marine...Klein/O`Hara Tides/Coastal Flooding... Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.