Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 211924 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 324 PM EDT Mon Aug 21 2017 .SYNOPSIS...
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High pressure will be centered in the western Atlantic through Tuesday. A warm front resides in eastern Pennsylvania. A cold front will approach from the west late Tuesday and slowly exit off the coast on Wednesday. Canadian high pressure builds into the region on Thursday and should to be the primary influence on our weather into next Monday.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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Solar eclipse has actually diminished some of the cumulus across the area this afternoon, though it may briefly increase again as the sun has a few more hours to isolate fully before dusk. Main feature of concern is a subtle short wave passing eastward through PA/MD/VA, where a few cells have initiated in response to increase forcing for ascent along higher terrain to our west. Modest flow and veering wind profiles aloft may support brief strong to severe storms as they move eastward. Additionally, moist advection in increased southerly flow will make it possible for any of these cells to produce heavy rainfall. After sunset and with the loss of diurnal heating, any ongoing thunderstorms will gradually begin to decrease in intensity, with only isolated showers and thunderstorms continuing overnight. Subsidence in the wake of the wave combined with increasing moisture in low levels will support the development of areas of fog and stratus overnight, particularly outside of the Philadelphia metro. Cloud cover and increased moisture will keep low temperatures several degrees above climatological normals.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
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Initially cloudy conditions in the morning will gradually scatter out as the low levels begin to mix. Southerly flow will strengthen ahead of an approaching upper trough and its attendant cold front, leading to warm/moist advection and heat indices around 100 degrees for the Philadelphia to NYC metro corridor. A heat advisory has been issued and is valid from 1 pm through 8 pm Tuesday. Apart from heat, there will be increasing chances of thunderstorms by evening as the upper trough approaches the area, though the primary forcing for ascent and best chance for any development will largely be to the north and west. At the moment, some of the deterministic models seem a bit overblown with overall convective coverage. Have sided more with higher resolution data showing the best chance of thunderstorms over higher terrain at least through the evening. Modest flow with veering profiles will once again support the potential for strong to severe storms capable of damaging winds and heavy rainfall.
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&& .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... On Tuesday, a longwave trough associated with an upper low near James Bay is forecast to amplify over the Great Lakes region as shortwave energy digs around the backside of the trough. The surface low will deepen in response to increasing height falls as it tracks northeastward across Ontario and Quebec. The attendant cold front will progress eastward through the Midwest. Looking farther downstream and closer to our area, models show development of a pre- frontal/lee trough over central PA and Potomac Highlands during the afternoon hours. Tuesday will clearly be the hottest of the next 7 days with forecast highs in lower 90s (except cooler 80s near the coast and at higher elevations in NE PA/NW NJ). The humidity will also be a factor with dewpoints reaching the mid 70s across Delmarva and southern NJ and lower 70s just about everywhere else. Peak heat indices in the upper 90s to near 100F are likely for the urban I-95 corridor and points south/east. With the expectation that focused lift will remain to our west near the pre-frontal trough on Tuesday, convection over the forecast area will likely be disorganized and generally isolated in coverage (perhaps widely scattered across NE PA where the terrain could locally enhance lift) during the afternoon. Slightly better chances for showers and storms arrive Tuesday evening as convection along this trough propagates eastward into our area. PoPs are highest in our far western zones (west of the Delaware Valley and near the eastern shore of MD). Showers and storm are then expected to move eastward into the Delaware Valley and coastal plain at night as the cold front catches up with the pre-frontal trough and moves into the area. Even though the environment should be supportive of showers and storms continuing into the night owing to the arrival of deep synoptic lift, PoPs decrease slightly with eastward extent due to some uncertainty concerning the intensity and coverage of convection later with the nocturnal boundary layer tending to stabilize further into the night. The approach of a low- and mid-level jet ahead of the cold front will yield increasing shear profiles (0-6 km bulk shear 35-45 kt) by evening. Additionally, weak to moderate amount of instability (MLCAPE 1500 to 2000+ J/KG) is forecast to be available to fuel convective updrafts. Some storms could be strong to severe, primarily during the afternoon and evening hours, with this setup. The latest D2 Convective Outlook from SPC includes the southern Poconos in a slight risk for severe storms with a marginal risk extending southeastward into the I-95 corridor. The latest 00Z models have trended faster with the cold front as it moves through eastern PA and NJ early Wednesday morning. The front will then likely slow down when it reaches Delmarva and perhaps eventually stall just south of the area across southern VA Wednesday afternoon. Accordingly, it is looking more probable that the majority of the forecast area dries out on Wednesday, especially north of the Mason-Dixon line. While PoPs were also lowered farther south, the close proximity to the cold front means that there is still a chance that showers and storms linger into the day. Additionally, some of the models (most prominently, the 00Z NAM) develop a wave of low pressure along the stationary front in southeastern VA. More organized convection could shift back northward toward Delmarva and possible far southern NJ by evening. High pressure over Canada and the Great Lakes region then starts to build southeastward toward the area on Thursday. This expansive high will remain in control through the weekend. The end result will be an extended stretch of pleasant weather Thursday through Sunday. Highs in the upper 70s/lower 80s and and lows ranging from the mid 50s to lower 60s are forecast each day and night, respectively. While the forecast remains dry for these four days, there looks to be increasing cloud cover heading into the weekend and potentially a risk for an isolated shower with 1) the approach of the upper trough and 2) light onshore flow helping to moisten low-levels over time. && .AVIATION /19Z MONDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Cumulus field with bases around 3500-6000 ft will gradually diminish after sunset, though stratus and fog will likely increase overnight, mainly for sites outside of KPHL. Best chance of any thunderstorms this afternoon and evening will be at KRDG/KABE, then chances will decrease overnight though an isolated shower or thunderstorm will be possible. Southerly winds decrease and become light and variable overnight before strengthening again tomorrow. There will be a better chance of thunderstorms tomorrow evening as a cold front slowly approaches from the west. Outlook... Tuesday...Early morning fog should dissipate by mid morning, leading to VFR for most of the day. Showers and storms develop during the afternoon but current thinking is coverage should initially be isolated and mainly confined to western terminals (ABE/RDG). Tuesday night...Potential for MVFR or IFR restrictions with slightly higher chances for showers and storms. This activity should progress to the east-southeast through the night. Wednesday...Scattered showers and isolated thunderstorms mainly for terminals S/E of PHL and earlier in the day. May start off MVFR with lower CIGs but improvement to VFR is likely from NW to SE during the day. A wind shift from SW to NW can be expected early in the day w/ fropa. Wednesday night through Friday...VFR and light winds.
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&& .MARINE...
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Winds and seas remain tranquil across the waters tonight in weak southerly flow. Patchy fog development will be possible overnight, but should dissipate shortly after sunrise. An isolated shower or thunderstorm is also possible overnight. Outlook... Tuesday...SCA was issued for the Atlantic coastal waters of NJ and DE for late Tuesday afternoon and night. S-SW winds are expected to strengthen late in the day to 15-25 kt. Isolated gusts to 30 kt are possible mainly during the evening when the winds will be strongest. Seas will quickly build in response to the wind field to 4-6 ft. Wednesday...A wind shift from SW to W-NW should occur on Wednesday behind a cold front. Winds will also decrease as well. There is still a possibility that the SCA may need to be extended into Wednesday morning if seas take a bit longer to subside below 5 ft. Thursday and Friday...Winds and seas below SCA criteria. RIP CURRENTS... A moderate risk for rip currents continues into this evening as the southerly swell period continues to gradually increase. A high risk of rip currents will likely be issued for tomorrow as southerly flow increases, allowing higher surf and longer period swells to develop.
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&& .TIDES/COASTAL FLOODING...
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Tides are expected to generally remain below coastal flooding thresholds, but there may be a few localized areas of brief minor flooding at times of high tide.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...Heat Advisory from 1 PM to 8 PM EDT Tuesday for PAZ070-071-102- 104-106. NJ...Heat Advisory from 1 PM to 8 PM EDT Tuesday for NJZ010-012-015- 017>019. DE...Heat Advisory from 1 PM to 8 PM EDT Tuesday for DEZ001. MD...None. MARINE...Small Craft Advisory from 3 PM Tuesday to 6 AM EDT Wednesday for ANZ450>455.
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&& $$ Synopsis...Drag Near Term...Di Spigna Short Term...Di Spigna Long Term...Drag Aviation...Di Spigna Marine...Drag/Di Spigna Tides/Coastal Flooding...

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