Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 252209 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 609 PM EDT MON JUL 25 2016 .SYNOPSIS... A cold front will approach from the west and move through the region early Tuesday morning, before stalling near or just to the south of our area. This front will gradually lift north by the end of the week with areas of low pressure developing along this boundary. && .NEAR TERM /THROUGH TUESDAY/... Ongoing SVR weather. Excessive heat flags have been taken down. A new Severe Thunderstorm Watch (#416) was issued at 330 PM. This watch includes eastern MD, DE and southern NJ and is in effect until 10 PM. The remainder of the CWA is covered under Severe Thunderstorm Watch #415, which is in effect until 8 PM. The atmosphere continues to destabilize during peak heating this afternoon. 19Z mesoanalysis from SPC indicates strong instability (MLCAPE values ranging from 2000 J/kg across our northern most zones to 3000 J/kg across Delmarva) supportive of robust convective updrafts. The primary threat with these storms is damaging winds. Even though mid-level lapse rates are weak and there isn`t an abundance of dry air aloft, the environment is conducive for strong downbursts with a inverted V sounding below the LCL. The HRRR had a good handle on the location of convective initiation with the lee trough just to our west as well as the more organized convection that is tracking across southern NY. We expect the first round of convection to organize along the lee trough and then accelerate eastward toward the I-95 corridor between about 4-6 PM, reaching the coastal plain of NJ before sunset. The second line of convection with the upper shortwave trough is expected to move through the I-78/I-80 corridors between 6 and 9 PM. A more organized QLCS (and higher damaging wind potential) is likely with this second line owing to enhanced bulk shear and deeper lift ahead of the shortwave trough. You can see this starting to form upstream across north-central PA. There is still uncertainty regarding the strength of these storms in this line by the time it drops southward toward the Philly metro since it will likely be around or just after sunset. It will depend on how much the airmass is worked over from the first round of storms this afternoon. The threat of severe thunderstorms should be trending downward after sunset with the loss of heating and as the airmass stabilizes in the cold pool behind the storms. However, the severe threat may continue into the late evening and overnight across the Delaware Bay region (southern NJ and Delmarva) given this extremely unstable airmass will likely not have been overturned by prior convection. The other concern will be locally heavy rainfall that could result in flash flooding as storms develop in a moisture-rich environment (PWATS 2-2.25), especially if multiple rounds of storms affect the same area. We`ll also have to watch how the orientation of the convective line evolves over time. If the strongest part of the line bows out, the trailing southern end of it may become oriented more west to east and allow for storms to train along it. The threat would then transition from winds to flash flooding across these southern areas later this evening and maybe into the overnight. The heat headlines are set to expire at 6 PM. Heat indices this afternoon were in the 100-110F across much of eastern PA and NJ, and 105-115F in Delmarva. The heat will temporarily break with the thunderstorms. Patchy fog may develop overnight, especially in sheltered river valleys and rural areas, if clouds clear before the drier air moves in behind a cold front. && .SHORT TERM /TUESDAY NIGHT/... The cold front is expected to slow down as it moves southward through the Delmarva early Tuesday morning. There is still uncertainty regarding how quickly the front pushes southward. Fronts tend to get hung up in the Delmarva this time of year and in this pattern with zonal steering flow aloft. Added a chance for showers and storms during the afternoon hours across the southern half of DE and adjacent eastern MD with the front likely nearby. Unfortunately, we won`t get much of a break from the heat tomorrow as post-frontal CAA progged to be minimal. Forecast highs are in the low to mid 90s across most of the area (except cooler in the higher terrain of northeast PA/northwest NJ and along the coast). However, it will be noticeably less humid across eastern PA and central/northern NJ with dewpoints in the upper 50s and lower 60s. Farther south, where the front gets hung up, dewpoints will still be in the upper 60s and lower 70s. Afternoon heat indices should peak near 100F in the Delmarva. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... The medium range models are in good agreement keeping high pwat air just to our south as a sfc boundary stalls in the southern portion of the forecast area. Higher humidity values and a slight chance for storms will continue near the Delmarva. Heat index values in the mid 90s are still expected from Philadelphia southward. In response to an approaching shortwave, heights build on Thursday and 1000-850mb thicknesses increase resulting in temps climbing a couple of degrees warmer compared to Wednesday, with heat index values in the mid to upper 90s. As higher pwat air returns...and shortwave energy moves through the region...widely scattered to scattered storms also return Thursday. The ECMWF/GFS/Canadian models are coming into better agreement developing a broad upper trof over the eastern U.S. into the weekend...bringing temperatures down closer to normal values for late July/early August. With pwat values still 1.5-2.0" through Monday, and weak shortwaves moving through the upper trof axis, daily afternoon and evening showers and storms will be possible through the remainder of the long term period. && .AVIATION /22Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...VFR conditions continue this afternoon. South to southwest winds around 10 knots or less with a few gusts up around 20 knots through the rest of this afternoon. Scattered showers and thunderstorms are developing to the north and west of the terminals and will continue to push to the east through this evening. Expect showers and thunderstorms to impact the terminals, mainly between 20-02Z, with MVFR/IFR conditions possible. Tempo groups have been added to indicate the most likely timing of storms at the terminals. Overnight...Behind the storms, winds will shift more to the west, slightly northwest, but remain under 10 knots. Patchy fog will develop in the more prone areas as winds go light and skies largely clear. Tuesday...Patchy fog early, clearing by around 15Z. Then VFR conditions expected for the remainder of the day. Westerly winds around 10 to 12 knots through the afternoon. Outlook... Tuesday night through Friday...Mainly VFR conditions expected. Scattered showers and thunderstorms Thursday and Friday, with MVFR/IFR conditions possible at times. && .MARINE... Winds and seas will be just below small craft advisory criteria through tomorrow. Southern wind gusts to 20 knots will continue to occur through this evening. Seas will increase from 2 to 3 feet to 4 feet in our coastal waters. Stronger thunderstorms developing on land this afternoon could threaten the waters this evening. Locally strong winds and higher seas are possible with these storms. Outlook... Tuesday night through Friday...Sub-advisory conditions are expected to continue on the area waters through much of the week. Expect higher winds and waves in and near scattered showers and thunderstorms that become more likely at the end of the week. Rip Currents...A moderate risk for rip currents continues through this evening. Tuesday through the most of the week: with the Atlantic Basin continuing quiet tropically, unless we get a strong onshore flow...RC risk this coming week should be generally low. Tstms and pockets of chilly upwelling water may the greater concerns. There is some sign of colder upwelling along the coasts this evening. && .CLIMATE... As of 3 PM EDT, the temp at PHL was 96 which has tied the record high for this date. ABE was 95, also tying a record high, and ILG was 96. This also has tied the record for this date. The last time the temperature reached or exceeded 98F at PHL was three years ago on July 18, 2013. The last 100-degree day at PHL occurred four years ago on July 18, 2012. Record high temperatures for Today through Thursday are below. Site 25th 26th 27th 28th ---- ---- ---- ---- ---- PHL... 96-1899 101-1892 101-1940 100-1941 ABE... 95-1999 98-1940 98-1955 97-1949 ACY... 99-2010 96-2011 99-2005 98-1999 ILG... 96-1987 99-1894 100-1894 101-1894 TTN... 97-1999 99-1892 100-1894 101-1894 GED... 99-2010 97-2012 98-2005 99-1949 RDG... 96-1999 99-1940 98-1955 99-1941 MPO... 90-1999 89-1949 91-1955 93-1949 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...99 Near Term...Klein Short Term...Klein Long Term...99 Aviation...Meola Marine...Meola Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.