Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 272230 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 630 PM EDT Thu Jul 27 2017 .SYNOPSIS...
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A warm front will move through our area later tonight. Low pressure will develop along the frontal boundary across the Ohio Valley, and it will move towards the area early Friday through Saturday. High pressure will return over our area for the late weekend and will persist into the week.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Weak cold front moving through the eastern Great Lakes will continue to track east this evening. Some light showers may continue to pop up across the area. Meanwhile, low pressure continues to organize and develop over the Midwest. A warm front extending out ahead of that low will lift towards the Mid- Atlantic this evening. With abundant low level moisture in place and increasing across the region, can expect patchy fog to develop late tonight. Weak cold front sags across northern zones, generally north of I-78, and skies may somewhat clear out. However, with that abundant moisture, do not think any clearing will last long. Warm and muggy tonight with lows in the 60s and low 70s.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... The start of a prolonged period of unsettled weather begins on Friday. Warm front ahead of the developing low seems to get hung up across the Mid-Atlantic. Several waves of low pressure will develop and intensify over the OH/TN Valleys along the boundary as closed H5 low digs into the OH Valley from the Great Lakes. The first surface low will lift into the Mid-Atlantic and Delmarva Friday afternoon, and showers and scattered thunderstorms will move into the Delmarva, DE Valley, and southern NJ during this time. Onshore flow ahead of the low will usher abundant moisture into the region. Surface dewpoints will rise into the upper 60s/low 70s throughout, and PWATs will range from 2-2.25". This first wave of showers and thunderstorms will produce heavy rain, mainly to the south of I-78. Hardest hit areas will be across northeast MD, DE, and southern NJ, where between 3/4-1" rain is possible during the day. Locally higher amounts are possible in training of thunderstorms. These areas are the areas already hit by heavy rain early this week. With 3 hour and 6 hour FFG values on the order of 2-4 inches, do not think this will be hard to reach, especially given the potential for training of thunderstorms. Therefore, have issued a Flash Flood Watch for much of southern NJ, parts of southeast PA, northeast MD, and DE starting Friday afternoon, though the bulk of the rain comes Friday night and Saturday. SPC has most of southern NJ and northern parts of the Delmarva in a Marginal risk for severe weather on Friday, and southern portions of the Delmarva in a Slight Risk for severe weather. Thunderstorms with damaging winds are possible, but big concern will be for the flooding. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Summer nor`easter this weekend with the potential for flash flooding increasing across portions of the region... A shortwave over the upper midwest as of this afternoon will carve out an an anomalous cutoff low across the mid Atlantic on Saturday which will persist into Tuesday. This will be the primary weather maker during this time frame with a summer nor`easter leading to heavy rainfall. The potential for flash flooding has increased and a flash flood watch is now in effect generally south of Interstate 78. Heavy rainfall...Shortwave energy rounding the base of the mid Atlantic trough will lead to cyclogenisis along a stalled frontal boundary near Delmarva from late Friday into early Saturday. As the mid-level trough closes off, the system will become vertically stacked on Saturday, with the surface low stalling off the Virginia Capes, before moving gradually northeastward on Monday. While there is model consensus through Saturday with the track of the low, the Canadian and NAM take the low further south and east on Sunday, while the GFS and ECMWF keep it locked in closer to the coast. We favored the GFS and ECMWF over the Canadian and NAM, as they have a better handle on the shortwave impulse associated with this system. That said, there is still considerable spread in the GEFS and EPS 500 hPa heights with the placement of the cutoff low, and this is leads to uncertainty in the track of the surface low. Both the GEFS and EPS means low positions are close to the op runs at 00Z Sunday. This is further evident in the ensemble spread in mean precipitation amounts, which is most acute in the I-195 to PA turnpike corridor. We have higher confidence in the potential for flash flooding southeast of this line, and lower to the northwest. Quite an impressive setup develops across the flash flood watch area on Saturday with an easterly 850 hpa jet of 50 kts developing (+3 to +4 STD DEV above normal), transporting deep later moisture into the region with Precipitable Water values in excess of two inches (+2 to +3 STD above normal). There are two potential bullseyes of heavy rainfall, one near the NJ coast, and another extending from southeast PA into Delmarva. With a frontal boundary stalled across Delmarva, rainfall across this area into southern NJ will also be convective in nature, as a broad area of instability sets up, as evidenced by MU CAPE, along with negative lifted and Showalter Indices. Additional rainfall is possible on Sunday, mainly across southern NJ and Delmarva. A return to fair weather is expected in the Mon - Wed time frame, with the next chance of showers on Thursday with an approaching front. Temperatures will be below normal from Saturday thru Monday, especially closer to the coast with an onshore flow. A return to near normal is expected thereafter. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Weak cold front approaches from the north this evening, and then a warm front approaches from the south and west late tonight. Low pressure approaches on Friday. MVFR CIGs look to continue into this evening. Weak cold front sags to the south, and then there may be some clearing in the evening, but isolated showers are possible as well. Conditions somewhat clear out in the evening, but with abundant low level moisture in place, expecting MVFR CIGs/VSBYs to develop late tonight and into Friday morning. IFR conditions are possible at southern terminals (KPHL/KMIV/KACY/KILG). Showers develop at those southern terminals Friday morning. For KPHL, showers and thunderstorms with heavy rain will move across the airfield after 18Z Friday. Light winds, generally 5 KT or less, on tap through Friday morning. East winds then increase to 5-7 KT. OUTLOOK... Saturday...Widespread IFR expected in low clouds and heavy downpours. Northeast winds may gust up to 25 knots and there is the potential for low-level wind shear across the terminals. Sunday...IFR may continue at ACY, MIV, and ILG. Improvement to MVFR elsewhere. Northeast winds may continue to gust up to around 20 knots. Monday and Tuesday...MVFR conditions may persist into Monday at ACY and MIV, otherwise VFR conditions. North winds generally around 10 knots both days.
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&& .MARINE... South to SW winds 10-20 KT this evening will become light by Friday morning, then will become E-SE 10-15 KT Friday afternoon. Occasional gusts to 25 KT possible at DE and southern NJ ocean waters this evening, but do not think they will be widespread or frequent enough to warrant a Small Craft Advisory. Seas will range from 2-4 feet. Widespread showers and thunderstorms will impact the waters Friday afternoon with reduced visibilities, gusty winds, and rough seas. OUTLOOK... We issued a Gale Watch for the New Jersey waters, including the waters east of the mouth of Delaware Bay, from Saturday morning into Saturday night. SCA conditions may persist thereafter from Sunday into Monday. RIP CURRENTS... A moderate risk for the development of dangerous rip currents continues through this evening due to the combination of onshore flow and an underlying long (10 second) period southeasterly swell. A moderate risk for the development of dangerous rip currents is outlooked to continue into Friday as well. A high risk is possible from Saturday through Monday. && .HYDROLOGY... A Flash Flood Watch has been issued for much of southern NJ, parts of southeast PA, MD, and DE. 3 hour and 6 hour FFG values range from 2-4", and with widespread showers and thunderstorms producing heavy rain, these values are likely to be met. Will forecast a Storm Total of 2-4" inches of rain from tonight through Saturday, however, with training of thunderstorms, significantly higher rain amounts are likely, especially in areas of DE and southern NJ that already received heavy rain and flooding early this week. Please be aware of road conditions, and be aware for Flash Flood and Flood Warnings that may be issued, especially Friday night and Saturday. && .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... Record daily rainfall 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 #8 wettest 10.42 is the record in 1969 RDG 8.02 13.85 is the record 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. && .EQUIPMENT... Dew point readings at KDOV continue to measure too high compared to surrounding locations and should be treated as unrepresentative of the area. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch from Friday afternoon through Saturday afternoon for PAZ060-070-071-101>106. NJ...Flash Flood Watch from Friday afternoon through Saturday afternoon for NJZ012>027. DE...Flash Flood Watch from Friday afternoon through Saturday afternoon for DEZ001>004. MD...Flash Flood Watch from Friday afternoon through Saturday afternoon for MDZ008-012-015-019-020. MARINE...Gale Watch from late Friday night through late Saturday night for ANZ450>454. && $$ Synopsis...99 Near Term...Iovino/MPS Short Term...MPS Long Term...Franck Aviation...Franck/MPS Marine...Franck/MPS Hydrology... Tides/Coastal Flooding... Climate... Equipment...

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