Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 281740 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 140 PM EDT THU JUL 28 2016 .SYNOPSIS... High pressure will be anchored to the north through Friday. Meanwhile, a weak stationary front will be situated over the area with a wave of low pressure expected to track along it. A weak front will stall to our south and areas of low pressure will develop along this front through the weekend and bring periods of showers and thunderstorms. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 930 AM ESTF Update...The Flash Flood Watch (FFA) now goes into effect at 2 PM for E MD, DE, SE PA and S NJ. A rather diffuse WSW- ENE oriented frontal boundary bisected the CWA this morning. Dewpoints were in the 60s north of the front and 70s south of it. Following strong heating and boundary layer destabilization during peak heating, scattered showers and thunderstorms are expected to develop this afternoon where the axis of greatest moisture and instability will reside along and south of this old boundary. These storms will like contain torrential downpours as they develop in a moisture laden airmass where PWATS are forecast to be 2.0-2.25 inches. There is a signal in the hi-res CAM guidance (including the HRRR and NCAR ensembles) for locally heavy rainfall that would exceed 3-h FFG this afternoon and early evening in SE PA, S NJ and particularly across our Delmarva zones. The start time of the FFA for the tier of counties farther north into Bucks Co., PA and central NJ is still 6 PM. We will monitor the latest observations/mesoanalysis and model trends for consideration if an expansion of the FFA is needed farther north and west for tonight. The Storm Prediction Center has included our Delmarva zones in a slight risk and SE NJ, far SE PA in a marginal risk for severe thunderstorms due to the expected instability (MLCAPE 1500-2000 J/kg) along with a moderately strong westerly mid-level flow. Maximum temperatures should favor the lower 90s in our region. Readings should not get out of the 80s in the elevated terrain of the Poconos and northwestern New Jersey, and along the immediate coast. Heat index values are expected to remain several degrees below the excessive heat criterion for our northern counties and just below the criteria for the urban corridor and for our southern counties. The synoptic wind should be light allowing local influences to drive today`s wind direction. Speeds should be mostly less than 10 mph. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Precipitable water values of 2.2 to 2.4 inches will overspread our forecast area from the south for tonight. Conditions are expected to remain unstable, especially across our southern counties. Also, surface low pressure approaching from the west will result in the development of a favorable low level wind profile for organized convection tonight. The surface wind should begin to favor the east to south quadrant late tonight, especially on the coastal plain, while winds around 800 HPa will be from the southwest. The very high precipitable water values should result in heavy rainfall in any organized convection. As a result, the Flash Flood Watch remains in effect for southeastern Pennsylvania, central and southern New Jersey, Delaware and northeastern Maryland. The cloud cover and high humidity will keep temperatures from falling below the 70s for tonight except in the Poconos and far northern New Jersey. && .LONG TERM /FRIDAY THROUGH THURSDAY/... An area of low pres will ride alg the stalled front and bring rain to the area on Fri. There is still some disagreement in the mdls as to the extent of the rain and the locations of the heaviest, with the GFS indicating most rain n and very little s and the ECMWF having a more widespread event. For now, will just carry high chc everywhere. Mdls then clear things out Fri night into early Sat, so have lowered pops and removed them for the first part of Sat. Then there is more disagreement in the mdls into Sat as both mdls develop precip again but the ECMWF is more widespread into early Sun. However, by later Sun, both the GFS and ECMWF has another good shot of precip over the area as another wave moves by alg the front. After Sun, the ECMWF is much drier through midweek, keeping most of the period dry, while the GFS keeps Mon with a chc of precip and diurnal stuff on Tue. The drier ECMWF soln is supported by the CMC, so have leaned the fcst in that direction attm. Temps look to be a bit cooler than we have had recently, with temps not expected to hit 90 degrees in many areas thru the extendd pd. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Flying conditions will continue to deteriorate through tonight as thunderstorms with gusty winds and heavy rain will periodically impact all taf sites, but especially KPHL, KPNE, KTTN, KMIV, KACY through early Friday morning. Added vrb15g30kt winds to these TAF sites to account for potential for gusty winds in and near thunderstorms. Confidence is low for the timing of convection at any particular TAF site, but high that storms will be in the region through early Friday morning. Showers and storms will expand in coverage tonight and early Friday morning. This will result in ceilings dropping to MVFR at most sites around or after 04Z. Southeast winds will back to the east late tonight generally 10 knots or less, except in and around thunderstorms, but shift to the northwest and increase in speeds, gusting at 10-20 kts after 11Z Friday. Conditions gradually improve by late morning/early afternoon Friday with precipitation ending, and ceilings lifting Outlook... Sat through Sun...Shra/tsra will lower cigs/vsbys at times Otherwise VFR expected. Moderate Confidence. Mon...Mainly VFR. Moderate Confidence.
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&& .MARINE... The wind direction is forecast to favor the SE for much of the day, becoming more S late in the day and this evening. Winds speeds will be light, around 10 knots. The winds will be variable in some spots tonight but a general light SE wind is favored late tonight. Wave heights on our ocean waters will be 2 feet or less and waves on Delaware bay should be 1 foot or less. Scattered showers and thunderstorms are possible for this afternoon with numerous showers and thunderstorms anticipated for tonight. Outlook... Fri through Mon...There is a chance of marginal sca conds Fri night into erly Sat, but confidence is low. Otherwise, no marine headlines are anticipated thru the pd. RIP CURRENTS... There is a low risk for the development of dangerous rip currents for today due to the relatively light wind and tranquil seas. The probably risk for the development of dangerous rip currents on Friday is also low. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Flash Flood Watch from 6 PM EDT this evening through Friday evening for PAZ105-106. Flash Flood Watch through Friday evening for PAZ070-071- 101>104. NJ...Flash Flood Watch from 6 PM EDT this evening through Friday evening for NJZ009-010-012>015. Flash Flood Watch through Friday evening for NJZ016>027. DE...Flash Flood Watch through Friday evening for DEZ001>004. MD...Flash Flood Watch through Friday evening for MDZ008-012-015- 019-020. MARINE...None.
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