Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 122259
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
659 PM EDT Thu Oct 12 2017

.SYNOPSIS...
High pressure to our north will nose its way down into our area
tonight, then offshore Friday. A weak surface trough is
forecast to move onshore to our south Friday, then lift toward
the area Friday night through into Saturday night. A cold front
is expected to affect the area late Sunday into Sunday night.
High pressure is then expected to build into the area Tuesday
through Thursday. A dry front may clip our northern areas late
Tuesday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
A remnant low pressure system was analyzed near the OH-PA
border this afternoon. Meanwhile a strong (1032 mb) surface high
was centered over southern Quebec. The high is forecast to
strengthen a bit tonight as it shifts southeastward toward
northern New England. A distinct U-shape pattern can be seen (in
the observational analysis and in short-range model forecasts)
in the isobars to the south of the high over New England and mid
Atlantic, indicative of cold air damming (CAD) down the eastern
side of the Appalachians.

Looking aloft, GOES-16 low- level water vapor satellite imagery
(channel 10) captures a flat upper shortwave trough moving
eastward into southern New England.

Stratus at the top of the marine layer has resulted in a cloudy
day across most of the area. However, the combination of dry
air advection south of the Canadian high and subsidence behind
the aforementioned shortwave trough has lead to breaks in the
cloud cover across the I-80 corridor and in the coastal plain of
central NJ. Expect these breaks in the stratus to eventually
reach the Delaware Valley by this evening with more significant
clearing occuring across northeastern PA and northwestern NJ.
Meanwhile, areas to the south and west will remain socked in low
clouds.

Patchy rain/drizzle occurring from RDG-PHL-ACY southward during
the first half of the afternoon is expected to gradually shift
southward over time, eventually becoming confined to Delmarva
after sunset. There is indication from some of the guidance for
rain/drizzle to expand back northward into southern NJ and
southeastern PA late tonight in response to weak isentropic lift
once the low-level flow starts to veer (from easterly to
southerly) around the departing high. Overall, PoPs were lowered
for the period with drizzle favored over measurable
rain/showers.

The setup for radiational cooling tonight appears to be
greatest in northeastern PA and northwestern NJ where more
persistent breaks in the clouds are expected. Otherwise, the
clouds and E-NE winds will prevent temperatures from dropping as
much from afternoon readings. Forecast lows range from the
lower 40s north of I-80 to the lower 60s at the DE beaches.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
The center of high pressure will continue to progress eastward
across Nova Scotia on Friday and eventually offshore by the end
of the day. The high will continue to ridge down the eastern
seaboard with a residual CAD wedge over the area.

There is a bit of uncertainty regarding cloud cover for
tomorrow, especially as you travel northward across the forecast
area. At least the Delmarva region should be overcast for the
start of the period. Stratus should expand northward during the
day but when and how quickly this occurs remains to be seen.
Official forecast reflects a middle solution between the
aggressive/cloudier NAM-based guidance and the slower/less
cloudy GFS. These clouds will also have an impact on
temperatures. Forecast highs similar to today, ranging from the
upper 50s across the southern Poconos and near 70F in southern
Delaware.

A low chance for light rain/drizzle was confined to areas along
and south of the Mason-Dixon Line at the start of the period,
gradually expanding northward across eastern PA and southern NJ
throughout the day.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
By Friday night, high pressure to our north will be moving
farther offshore of the Canadian Maritimes, and a surface trough
is forecast to move onshore to our south. This trough is
forecast to lift toward our area Friday night through Saturday
and into Saturday night. With the high to our northeast, and the
trough lifting toward our area, we will be under the influence
of a moist east to southeast flow. This will keep low level
moisture across the area. Friday night through Saturday night
could be a fairly cloudy period, with the chance for areas of
light rain or drizzle and fog. The drizzle and fog have the best
chance Friday night/Saturday morning, then again Saturday
night/Sunday morning.

By Sunday, the trough weakens as it lifts to our north, hen a
southwest flow at the surface and aloft develops across the
area. With this return flow, Sunday will be the warmest day in
the extended period. A cold front begins to approach late in the
day Sunday, then passes through Sunday evening into Sunday
night. It would not be surprising for a few showers to develop
doing the day if enough instability develops. With the frontal
passage late Sunday, there will be an increase in shower chances
late Sunday into Sunday night. Depending on how fast the front
moves, showers could linger into early Monday morning.

Once the front moves through, dry weather returns through
Thursday as high pressure builds across the region Tuesday
through Thursday. A weak frontal boundary may clip our northern
areas Tuesday, but it is forecast to be a dry boundary.

&&

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

MVFR CIGs from stratus deck has been resistant to erosion so
far today. Expect gradual improvement to VFR from E to W through
23Z as these lower clouds scatter out. Patchy -RA/DZ near RDG,
PHL, MIV and ILG should progress south of the terminals by
sunset this evening.

Mainly VFR tonight before stratus redevelops toward morning. A
return to MVFR CIGs are forecast to occur from south to north
between 09Z-15Z Friday but confidence in the timing that is
advertised in the 18Z TAFs is lower than normal. MVFR CIGs are
favored to persist into the afternoon Friday although CIGs may
improve to above 3 kft AGL late in the day.

E-NE winds 10-20 kt with gusts 20-30 kt (highest in this range
will be near the coast) this afternoon will weaken toward
sunset. Winds will generally decrease to 10 kt or less tonight
into Friday.

OUTLOOK...
Friday night...MVFR CIGS, possibly lowering to IFR overnight.
Chance light rain or drizzle and fog.

Saturday...MVFR or IFR CIGS early, possibly improving to VFR
during the day. Chance of l

Saturday night...Fog and low clouds developing overnight
leading to MVFR or IFR conditions. Slight chance of light rain
or drizzle and fog.

Sunday-Sunday night...Improving conditions through the day.
Frontal boundary late Sunday into Sunday night with rain showers
possible leading to temporary lower conditions. Gusty southwest
winds 20-25 knots, becoming northwest behind the front Sunday
night.

Monday-Monday night...VFR. Northwest wind gusts 15-20 knots.

Tuesday...VFR.

&&

.MARINE...
Will cancel the Gale Warning for the ocean waters and will
convert to a Small Craft Advisory, as gusts have fallen below 34
kt.

A Small Craft Advisory remains in place for the Delaware Bay.

OUTLOOK...

Friday night-Saturday night...Wind gusts likely to lower below
advisory levels, but seas expected to remain elevated through
the night Friday into Saturday night.

Sunday-Sunday night...Winds and seas both forecast to be near
Small Craft Advisory levels.

Monday-Tuesday...Winds and seas expected to be below advisory
levels.

&&

.TIDES/COASTAL FLOODING...
The times of high tide have passed. Water levels will continue
to subside. The Coastal Flood Advisories have been cancelled.

This should be a one tide cycle event. The onshore flow will
begin to weaken tonight. Also, the astronomical tides associated
with the overnight high tide are lower than those during the
day.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ450>455.
     Small Craft Advisory until 6 AM EDT Friday for ANZ430.
     Small Craft Advisory until 11 AM EDT Friday for ANZ431.

&&

$$
Synopsis...Robertson
Near Term...Klein/MPS
Short Term...Klein
Long Term...Robertson
Aviation...Klein/Robertson/MPS
Marine...Klein/Robertson/MPS
Tides/Coastal Flooding...



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