Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 151936 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 336 PM EDT Tue Aug 15 2017 .SYNOPSIS...
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Weak low pressure moves off the Mid-Atlantic coast this evening, and then a cold front works its way through the East Coast late tonight. High pressure returns on Wednesday before moving offshore on Thursday. A warm front lifts north across the region Thursday night through Friday morning, followed by a cold front Friday night through Saturday morning. Weak low pressure may affect the area Sunday, then high pressure builds back for the new week.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Moisture-channel imagery nicely indicates the phasing of a fairly potent northern-stream shortwave trough with a weaker perturbation in the southern stream moving northeastward through the northern Mid-Atlantic this afternoon. In addition, an upstream perturbation is moving through the southern Mid- Atlantic this afternoon. The large-scale lift generated by these perturbations is producing several areas of convection this afternoon. Of particular concern is the Delmarva Peninsula and adjacent far southeast Pennsylvania and central/southern New Jersey, where fairly widespread quarter-to-half inch rains have fallen (save for areas near and in a corridor northeast of Philadelphia), with smaller swaths of 0.5-1.5+ inch totals where more intense convective cells have tracked. One area to watch the next few hours is central Delaware, where 1-1.5+ inches of rain fell through early afternoon, and convection is continuing to develop/organize near/northwest of there. Rain rates of 2-4 inches per hour are occurring with the strongest cells (which are not very fast moving), and this may cause some localized urban/poor drainage flooding through the early evening hours. Have indicated moderate to heavy rain in the grids through early evening generally south of the Mason-Dixon Line. Models are quick to rush the precipitation out of the area by 00Z, and I have generally followed suit. Skies will rapidly (but briefly) improve/clear as dry air advection upstream of the northern-stream trough rapidly occurs across the coastal Mid- Atlantic. Of note, however, is the latest HRRR simulation, which generates a band of convection in far northeast PA and adjacent eastern NY. This may clip far northern portions of the CWA early this evening before any ambient instability diminishes. The NAM Nest is also suggestive of this, but much further west (and generally out of our region), which seems to have more support from current observations at the moment. Did include some PoPs in the Poconos in the evening hours, but kept them fairly low to account for the uncertainty. Main question thereafter becomes fog formation. Ingredients are certainly in place, with residual low-level moisture from today`s precipitation and rapid drying aloft. A little concerned that weak surface west/northwest flow may be sufficient to mix the low levels (especially late tonight), preventing more widespread fog from forming (or maintaining itself). Given this uncertainty, will hold off on issuing a dense fog advisory for now, but would not be surprised if an advisory will be needed. At the very least, patchy dense fog is pretty likely, especially in susceptible valley/rural locations. Lows tonight will be near 60 in the Poconos, near 70 in the urban corridor, and in between elsewhere. Forecast was a combination of continuity, MAV/MET guidance, and projected dew points late tonight (using 2-m values from NAM/ECMWF as a starting point).
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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West-northwesterly midlevel flow will become established Wednesday with weak subsidence in the wake of the passing shortwave trough today. Northwest surface winds will become established, which (in combination with morning insolation) should aid in the rapid dissipation of fog. Wednesday looks to be a mostly dry day for the region with the lack of any synoptic-scale lift to speak of. However, the 12Z NAM Nest does develop an isolated storm or two in the northern CWA during peak heating - so a dry forecast is not exactly a slam dunk. On the other hand, coverage would be sparse enough, and confidence is certainly low enough, to preclude mention of any PoPs at the moment. With some subtle downsloping and ample surface heating expected, temperatures look to warm to just above seasonal averages. Forecast highs are near 80 in the Poconos, near 90 in the urban corridor, in the lower 80s at the beaches, and mid to upper 80s elsewhere. Used a combination of MAV (warm) and MET (cool) guidance, with some subtle extra weighting for the MAV given the cool bias of MOS this summer in such patterns.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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High pressure over the area Wednesday night will gradually slide offshore, and will then anchor itself over the western Atlantic on Thursday. Meanwhile, H5 trough digs into the Northern Plains and western Great Lakes, and surface low pressure organizes and develops. That low will slowly track east on Thursday, and a warm front extending out ahead of that low will lift north into the Mid-Atlantic and Northeast late Thursday and Thursday night. Several shortwaves will slide across the region as well, and this will touch off scattered showers and thunderstorms across the region Thursday afternoon and Thursday night. With the warm front north of the region on Friday, strong southerly flow will usher a very warm and humid airmass into the region. Although highs on Friday will generally top off in the mid to upper 80s, with the warmest temps (near 90) along and south of I-95 from Trenton to Philadelphia to Wilmington, surface dewpoints will be well into the lower 70s. This results in a heat index around 95 from Philadelphia to Wilmington, and possibly near 100 for northeast Maryland and Delaware. Cold front then approaches Friday afternoon and evening as low pressure over the Great Lakes lifts northeast into eastern Canada. Showers and thunderstorms are likely Friday with the passage of this front, but conditions only marginally cool off and dewpoints only marginally fall back into the upper 60s/low 70s on Saturday. A secondary cold front will pass through the region on Saturday afternoon, and then dewpoints drop back into the low/mid 60s on Sunday. Weak low pressure may have an impact on the region Sunday, hence slight chance PoPs, but confidence is low. High pressure then returns for the start of the new week.
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&& .AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Sub-VFR conditions will continue through late afternoon at the terminals with showers continuing at MIV/ACY through at least 21Z. Should see briefly improving conditions during the evening hours, though exact timing is not particularly high confidence given some disparities among the short-range models at this time. Fog is expected to develop and may become dense at most of the terminals tonight, with MVFR/IFR (potentially LIFR) conditions likely to develop by 06Z. Rapid improvement should occur after 12Z. Light/variable winds are forecast to become northwesterly but remain below 10 kts late tonight and Wednesday. OUTLOOK: Wednesday night...Sub-VFR conditions possible in fog/stratus. Thursday through Friday...Sub-VFR conditions in mainly afternoon and evening showers and thunderstorms. Late night and early morning fog/stratus possible as well. Saturday...Scattered showers and thunderstorms possible. Sunday...VFR.
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&& .MARINE...
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Seas have risen to 3-4 feet at the buoys off the NJ/DE coast as Hurricane Gert churns well offshore. Longer-period swell is beginning to reach the buoys (generally 9-12 seconds) and will continue to increase to around 11-13 seconds tonight and Wednesday. Seas are expected to build to 4-7 feet tonight and Wednesday, and have continued the small craft advisory to account for this. Meanwhile, storms will move through the region late this afternoon and early this evening before departing the entire area around midnight. Gusty/erratic winds/waves are likely near storms. Otherwise, southeast winds around 10 kts will veer to the northwest late tonight before veering to a southeast direction again by Wednesday afternoon. Some fog may develop near the coast and near Delaware Bay late tonight, with some visibility restrictions possible. However, this fog should dissipate rapidly during the morning, with a nice/dry day expected, save for the aforementioned higher seas during the morning hours, at least. OUTLOOK... Wednesday thru Sunday...Sub-SCA conditions expected through this week and into the early part of the weekend. Scattered thunderstorms may bring locally higher winds and seas. Southerly wind gusts 20-25 KT possible Friday afternoon as a cold front works its way east. RIP CURRENTS... The rip current risk remains moderate through this evening as onshore winds around 10 kts combine with gradually increasing swell during the period thanks to Hurricane Gert. The moderate risk is expected to persist Wednesday, with high seas and longer-period swell continuing. Swim only in the presence of lifeguards where safety prevails. Otherwise, you`re on your own with any rescue potentially critically delayed. There have been numerous drowning fatalities this year along the NJ coast.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory for hazardous seas until 2 PM EDT Wednesday for ANZ451>455. Small Craft Advisory for hazardous seas from 10 PM this evening to 2 PM EDT Wednesday for ANZ450.
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&& $$ Synopsis...MPS Near Term...CMS Short Term...CMS Long Term...MPS Aviation...CMS/MPS Marine...CMS/MPS

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