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-- Discussion containing changed information from previous version are highlighted. --
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.
-- End Changed Discussion --

&&

.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.
-- End Changed Discussion --

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.
-- End Changed Discussion --

&&

$$
Synopsis...Meola
Near Term...Klein
Short Term...Klein
Long Term...Meola
Aviation...Klein/Meola
Marine...Klein/Meola