Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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360 FXUS61 KPHI 271459 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1059 AM EDT Thu Jul 27 2017 .SYNOPSIS... A weak front will move across the area later today and tonight. Low pressure will develop along the front across the Ohio Valley and then move towards our area Friday through Saturday. The low will strengthen as it slowly moves away later Saturday and Sunday. High pressure will begin to build over the area next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Some light showers popping up in northern PA ahead of a weak trough moving into western NY. These showers should pass through the Lehigh Valley, Pocono Mountains, and into NW NJ from late morning through early afternoon. Still monitoring potential for convective development this afternoon. The first source of lift will be ahead of an upper shortwave trough that will be digging in from the Upper Midwest this afternoon. The bulk of the lift from this disturbance will pass through our northern zones (north of I-78) during the early to mid afternoon. Expect this first round of showers to be light across the north since morning cloud cover will limit destabilization in the boundary layer beforehand. Farther south, scattered showers and storms may develop late in the day (after 4 PM) owing to isentropic lift near the low-level warm front. Consensus among hi-res CAMs points to convective initiation occuring in E PA and S/C NJ between I-78 and the Mason-Dixon line). The official forecast reflects this scenario though we extended the axis of higher PoPs southward toward the axis of higher instability near the Chesapeake Bay. D1 Convective Outlook from SPC includes the entire region in a marginal risk for severe storms. The environment could become supportive of marginally severe wet-microbursts, especially across the far SE PA/S NJ/E MD/DE, where 70-degree dewpoints return, low-level lapse rates steepen and MLCAPE values are forecast to reach 1500 J/kg (possibly higher near the Chesapeake and Delaware Bays) late this afternoon. High temperatures range from the mid to upper 70s north of I-80 to the upper 80s in eastern MD.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Showers and storms will likely continue into tonight and possibly expand in coverage this evening as another shortwave disturbance/ remnant MCS (currently over the Midwest early this morning) arrives. Even though the majority of hi-res guidance (NAM Nest, HRRR, NCEP WRF ARW/NMM, and NCAR ensembles) focus the bulk of the convection over E PA and S/C NJ (between the I-78 corridor and the PA Turnpike), there is still some skepticism that it may be too far north. Accordingly, kept chance PoPs in the forecast for Delmarva as well. A somewhat slow W-NW storm motion (15-20 kt) that is oriented parallel to the H8 warm front could potentially provide a training setup of storms. Accordingly, there is a concern for locally heavy rainfall and even localized flash flooding tonight. After coordination with surrounding offices, held off on a Flash Flood Watch since heavy rainfall will likely be very isolated. However, we will mention this conditional threat in the HWO. Drier air behind the cold front should reach NE PA and NW NJ late tonight. Forecast low temperatures range from the low 60s in the Poconos to the mid 70s in the downtown Philadelphia, at the shore and in Delmarva. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... A prolonged period of wet weather will continue Friday and into the weekend as a front and low pressure moving along it will be across the Middle Atlantic region. The models seem to be slower in moving the low away Saturday, so pops have been increased for the weekend. Overall, total rainfall when all is done will be around 1-2 inches over nrn NJ and the Lehigh Valley, 2-3 inches over the Delaware Valley and srn NJ and 3 - 3.5 inches over Delmarva. Thunderstorms may bring locally higher totals to some areas. Eventually, flood watches may be issued for parts of the region, so pay attention to future forecasts. Temperatures will be mostly below normal Friday and into the weekend. Improved conditions are expected Monday which should last into the middle of next week. It`s possible that future fcsts may begin to show some possibility for some showers Monday, with the trend of the low pulling away slower beginning to show up. Confid in this is still limited, so we will keep the dry fcst for now. Temperatures will be near normal Mon/Tue and then rise to a little above normal for Wed. && .AVIATION /15Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. MVFR CIGs have been persistent over RDG and ABE for most of the night and intermittent at ILG. Low clouds (stratocu) will likely expand into the I-95 terminals as mixing deepens later this morning. CIGs should eventually increase to above 3 kft AGL between 18Z and 21Z this afternoon. A PROB30 group for TSRA was added for most TAF sites after 21Z today. The threat of storms will continue through the evening and possibly overnight. Uncertainty in location and coverage of storms precludes any mention of showers or storms in a prevailing group. VFR CIGs are favored for tonight (MVFR in storms) though there is some potential for low clouds to redevelop overnight. Fog could develop if there are breaks in the cloud cover. A S-SW wind under 10 kt can be expected today and tonight. OUTLOOK... Friday thru Saturday...Widespread lower CIGS/VSBYS with frequent showers and tstms. Sat night thru Monday...Mostly VFR. Scattered showers Sat night and near the shore Sunday with lower conditions possible. && .MARINE...
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A SW wind 5-10 kt will become S 10-20 kt this afternoon and tonight. Wind gusts could briefly approach 25 kt this evening but most areas should remain below SCA owing to weak mixing profiles. Seas on the ocean will be in the 2-3 ft range. OUTLOOK... Fri...Mostly sub-SCA conditions. Showers and tstms. Fri night...Winds and seas building to near SCA. Showers and tstms. Sat thru Monday...SCA conditions expected. Showers Saturday. RIP CURRENTS... After reviewing the latest trends in observations and guidance, it was decided to raise the risk for the development of dangerous rip currents to moderate for today. The beach day should start off as a low risk through the morning, but the combination of an underlying long (10 second) period southeasterly swell, increasing southerly winds to around 15 mph and building seas to around 3 ft this afternoon should enhance the risk to a moderate level.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...O`Hara Near Term...Klein/MPS Short Term...Klein Long Term...O`Hara Aviation...Klein/MPS/O`Hara Marine...Klein/Meola/MPS/O`Hara

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