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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 132334
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
734 PM EDT Sun Aug 13 2017

.SYNOPSIS...
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High pressure will influence our weather into Monday. A weak
disturbance in the upper atmosphere, approaching from the southwest,
is forecast to pass over our region on Monday evening. Tropical
Storm Gert is expected to pass well to our southeast and east on
Tuesday and Tuesday night. A frontal boundary is anticipated to
arrive from the north during the mid week period, then it should
meander near our region into the weekend.
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&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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High pressure over the Great Lakes region will into the area
through tonight. Meanwhile, the cold front that moved through
the forecast area last night will stall to our south across the
Carolinas and southeastern VA.

Overnight low temperatures in the 50s are attainable in the
typical sheltered valleys of northeastern PA and northwestern NJ
where the boundary layer is fully expected to decouple. Farther
south and east across the area, the floor for temperatures will
be in the low to mid 60s based on current and projected
dewpoints. However, a light northerly breeze may persist tonight
with the center of the high passing to our north while some mid and
high level clouds spread northward especially over the southern half
of the region. This would limit radiational cooling a bit and
potentially keep some spots in the urban I-95 corridor and coastal
plain slightly warmer.
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&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
High pressure will move off the New England coast and weaken on
Monday. Southerly return flow develops over the region around
the backside of the surface high and ahead of an upstream
shortwave trough over the Mid South/Ohio Valley. This will cause
the stalled frontal boundary to our south to return back
northward. However, with the steering flow rather weak, the
northward movement of the frontal boundary should be gradual.

Recent guidance has trended weaker/slower with the development
of a wave of low pressure along the front. Models consensus has
been to keep the warm front and more organized band of
convection generally south and west of the area tomorrow.
However, the 12Z NAM, 12Z ECMWF and about a quarter of both the
09Z SREF and 00Z NCAR CAM ensemble members still bring showers
into our far southern (mid Delmarva) and western (southeast PA)
late in the afternoon. The 12Z 3 km NAM Nest is even more
aggressive with the development of afternoon showers across much
of the CWA. This latter solution may be a bit overdone with the
bulk of the synoptic lift expected to remain upstream of the
region. Following model trends and consensus, PoPs were lowered
near the coast but raised for western (especially southwestern)
zones during the mid to late afternoon hours. PoPs were capped
in the 20-30 percent range with the expectation that coverage of
showers should be rather sparse. Forecast soundings from all
available guidance reveal a relatively stable environment that
is not supportive of thunderstorms (which makes sense with the
region located in the cool sector north of the front and with
increasing clouds helping to limit the degree of boundary layer
destabilization).

Trended cloudier than previous forecasts for Monday as model x-
sections show a deepening saturated layer (wrt/ ice) in the
700-400 mb layer. This pattern would indicate a rapid increase
in mid and hi clouds from SW to NE during the late morning and
afternoon. Owing to the cloud cover, weighted temperatures more
toward the cooler MET. Forecast highs are generally in the low
to mid 80s (slightly cooler in the Poconos and along the coast).

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
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A mid level disturbance is forecast to be overhead on Monday
evening before moving off the coast on Monday night. The models
are trending wetter for Monday night, so we will include at
least a slight chance for showers and thunderstorms at that
time. However, the NAM continues to appear quite overdone with
the rainfall amounts. Of course, you cannot go wrong forecasting
rain for central and southern Delaware and the adjacent
counties of eastern Maryland.

It appears as though Tropical Storm Gert over the western North
Atlantic will stay well enough to our southeast and east on Tuesday
and Tuesday night as not to impact our weather. However, a mid level
short wave trough is forecast to approach from the Great Lakes on
Tuesday before passing overhead on Tuesday night. We will include a
chance of showers and thunderstorms for Tuesday afternoon and
evening, mainly in eastern Pennsylvania and northern New Jersey.

The mid level short wave is anticipated to pull a frontal
boundary into our region from the north on Tuesday night. The
boundary is expected to remain over or near eastern
Pennsylvania, northeastern Maryland, Delaware and New Jersey for
the balance of the week into the weekend. The model guidance
differs in handling weak waves of low pressure travailing along
the front, so the timing of any precipitation in the forecast
remains difficult. Regardless, it seems as though the period
from Thursday into Saturday will be rather unsettled with a
chance of showers and thunderstorms each day.

High temperatures are forecast to favor the 80s during the
period from Tuesday through Sunday with lows generally in the
60s and lower 70s.
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&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR overall. There is the potential for MVFR visibilities
in the 07-12z timeframe at the more fog-prone terminals such as
KMIV, KACY, KABE and KRDG. For now, included a TEMPO for light fog
at KRDG and KMIV only. Northwesterly winds near 5 knots, becoming
light and variable at most terminals.

Monday...Other than some local MVFR visibilities (fog) possible
early, a developing VFR ceiling mostly in the 8000-15000 feet
range is expected. A few showers are possible mainly late in
the afternoon south and west of KPHL, however the chance is
rather low. Northeast to east winds less than 10 knots, becoming
southeast in the afternoon.

OUTLOOK...
Monday night and Tuesday...Mainly VFR. Late night and early
morning low clouds and visibility restrictions are possible. A
chance or slight chance of showers and thunderstorms.

Tuesday night through Wednesday night...Mainly VFR. Late night
and early morning low clouds and visibility restrictions are
possible.

Thursday through Friday...Mainly VFR with a chance of showers
and thunderstorms. Late night and early morning low clouds and
visibility restrictions are possible.
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&&

.MARINE...
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No marine hazards expected tonight and Monday.

Winds off the NJ coast become southerly this evening, then light and
variable winds overnight. A wind shift to E 5-10 kt during Monday
morning and eventually SE during the afternoon.

Seas in our coastal waters are generally around 3 ft and less than 2
ft in the Delaware Bay. Wave heights are not forecast to change much
through Monday.

RIP CURRENTS...

A medium-period (6-8 seconds) south-southeast swell and 3-4 feet
surf looks to continue into Monday. Wind speeds will be similar
Monday compared to today. However, the wind direction changes to
easterly during the morning and southeasterly by afternoon. Despite
the onshore flow, it is forecast to be light enough and therefore we
will go with a low risk for the development of dangerous rip
currents for Monday. This will be re-evaluated early Monday morning.

Tropical Storm Gert is forecast to pass well to our southeast and
east on Tuesday and Tuesday night. Swells from the system will
likely bring at least a moderate risk for the development of
dangerous rip currents for Monday night and especially Tuesday and
Tuesday night.
-- End Changed Discussion --

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Iovino
Near Term...Klein
Short Term...Klein
Long Term...Iovino
Aviation...Gorse/Iovino/Klein
Marine...Gorse/Iovino/Klein