Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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399 FXUS61 KPHI 170802 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 402 AM EDT Thu Aug 17 2017 .SYNOPSIS...
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High pressure will move offshore today. A warm front will lift northward towards the area this afternoon and then moves through the area late tonight. A cold front will approach from the west on Friday, pushing through the region Friday night into Saturday. High pressure returns for Sunday through Tuesday. Another cold front will arrive late Tuesday or Wednesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A 1019 mb surface high was centered over upstate NY early this morning. Meanwhile, a rather diffuse stationary front was positioned south of the high across the coastal plain of Delmarva and southern NJ. The frontal boundary becomes better defined much farther to our west across the Midwest states as it interacts with a 998 mb surface low that was over southern MN. Early morning GOES-16 fog channel showing an area of fog over the NJ Pine Barrens. Mid-level clouds over northeast PA and northwest NJ as well as over the mid Delmarva have prevented radiational fog from developing thus far across the northern and far southern zones of the CWA. The fog across southern NJ could expand into southeastern PA through daybreak before fog quickly dissipates by mid morning. While an isolated pop-up shower cannot be ruled out near the aforementioned stationary front (mainly Delmarva), the morning will predominately be a dry one. Showers and storms could develop this afternoon mainly south of Philadelphia across the coastal plain of NJ and Delmarva, where higher dewpoints (low 70s) will yield a more favorable convective environment. Nonetheless, forecast soundings for these southern zones reveal only modest instability (MLCAPE values below 1000 J/kg). Models also show weak mid-level lapse rates that could potentially limit the strength of convective updrafts. Accordingly, there may not be many thunderstorms around this afternoon (capped coverage to isolated). Owing to a lack of deeper synoptic forcing available today, capped PoPs at 20-30 percent with the focus for convective development primarily being sea-/bay-breeze fronts. Without much change in the airmass, high temperatures today will be similar to yesterday- upper 70s/lower 80s at higher elevations in NE PA and NW NJ and along the coast; mid to upper 80s elsewhere.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...
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An area of low pressure will track east across the Upper Midwest tonight. A warm front associated with the system is expected to approach from the southwest this evening and then lift northeastward through the forecast area overnight. Models generally advertise a southerly low-level jet that strengthens downstream of the Midwest low tonight. Both operational global models and hi-res CAMs indicate increasing coverage of showers and storms overnight into early Friday morning as the interaction between this low-level jet and approaching warm front would yield forcing for ascent. PoPs increase to likely across NE PA and chance elsewhere by daybreak Friday morning. However, capped thunderstorms at a slight chance with only modest instability analyzed on model forecast soundings and given the unfavorable time of day (diurnal min). PWATs are forecast to increase to above 2 inches overnight, so any storms that do develop could contain heavy downpours. Increasing cloud cover tonight should keep temps elevated overnight and limit the extent of fog development. Forecast lows range from the mid 60s in the southern Poconos to the mid 70s in the urban I-95 corridor and coastal locales.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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Friday looks like it could be a very active day across the forecast area. The warm front should be north of the area by early Friday with strong southerly flow behind it. This will bring an influx of moisture to our area and we will see dewpoints rise back into the low to mid 70s across the region. Instability will increase across the area and the chances for thunderstorms will continue through out the day. CAPE values range widely depending on the model but there will be more than enough lift in place to sustain updrafts and the development of thunderstorms. SPC has placed our region in a marginal risk for severe storms with the main threat being damaging winds. What appears right not to be the greater threat for our area is potentially heavy rainfall. While we have had a few days to dry out across the majority of our area, there are still several spots that have 3hr FFG values between 2-3 inches. PWATs will be in the 2-2.5 inch range and heavy rainfall will be possible in any storm that develops. WPC has included us in their Day 2 excessive rainfall outlook for Friday as the combination of high PWATs, diurnal heating, and the cold frontal passage has us primed for heavy rainfall. Will not be issuing a watch at this time as the location of heavy rainfall is difficult to pinpoint but the potential is there for localized flash flooding. We have added the mention of heavy rain into the grids. The cold front will approach the region from the west on Friday, crossing the region later Friday into Saturday. Ongoing showers and thunderstorms with the potential for heavy rain will continue as the front crosses the area. Storms will end from the northwest to southeast early Saturday morning. The front will slowly move offshore through the day Saturday and really gets a good shove offshore as a shortwave/upper trough crosses through the region. With the front not clearing the region early, it will prevent the low level moisture from clearing out quickly so Saturday will remain a little muggy even though the sun should break through and be overall a pretty nice day. As the upper trough pushes through later Saturday, we should have some cooler and drier air make it into the region. High pressure will build in for Sunday and a period of relatively nice weather should continue for the start of the week with the high shifting offshore on Monday. Another cold front will approach the region from the west on Tuesday, crossing the area late Tuesday into Wednesday. Additionally, a surface trough may develop in advance off this next front and could serve as the focus for some convection to develop.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Fog development overnight has been limited to the NJ Pine Barrens (ACY and MIV terminals). Visibility at ACY has been as low as 1/4SM but is highly variable. Visibility at MIV has been steady in the MVFR range though it could temporarily become LIFR before 11Z or 12Z. Mainly VFR expected for today and this evening with the better chance for showers and storms this afternoon shifted to the south of the TAF sites. Increasing chances for showers and perhaps isolated embedded thunderstorms overnight tonight. Local/temporary restrictions to IFR possible if heavier showers or storms directly impact a terminal. Light and variable winds this morning will become SEly and then Sly this afternoon and tonight between 4 and 9 kt. OUTLOOK... Friday...Fog possible early morning. Sub-VFR conditions in showers and thunderstorms. Saturday...MVFR or lower conditions early in showers and thunderstorms. Conditions are expected to improve to VFR by Saturday the afternoon, may take longer at KMIV and KACY as they remain closer to the boundary. Sunday and Monday...Mainly VFR conditions expected.
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&& .MARINE...
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Winds and seas below SCA criteria today and tonight. Seas should hover around 3 feet through the period (lower in Delaware Bay). OUTLOOK... Friday...Mainly sub-small craft advisory conditions expected. Guidance shows that the gradient will tighten up and southerly winds will gust between 20-25 knots on Friday, mainly during the afternoon and evening. Additionally, seas will rise but are expected to remain below 5 feet. Showers and thunderstorms possible Friday through early Saturday, with locally higher seas and winds. Saturday thru Monday...Sub-Small Craft Advisory conditions expected on the area waters through Monday. RIP CURRENTS... Buoy observations indicate that the longer-period swells generated by Hurricane Gert have diminished (or at least becoming less frequent). With improving conditions today compared to yesterday, the risk for dangerous rip currents will be lowered to moderate.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...None.
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&& $$ Synopsis...Meola Near Term...Klein Short Term...Klein Long Term...Meola Aviation...Klein/Meola Marine...Klein/Meola

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