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FXUS61 KPHI 190042
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
842 PM EDT Thu Oct 18 2018

.SYNOPSIS...
High pressure centered just to our southwest tonight will move off
the mid-Atlantic coast on Friday. Deepening low pressure over
southeast Canada will pull a cold front through the area on
Saturday. High pressure will then build in from the west on
Sunday and Monday. Another cold front is forecast to cross the
area on Tuesday, followed again by high pressure on Wednesday
and Thursday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
No significant changes to the forecast. Temperatures are
dropping off fairly quickly in outlying areas, but should level
out to previously forecasted lows.

A cold night on tap with Freeze Warnings and Frost Advisories in
effect for the entire region, with the exception of the
immediate coastal zones.

As an upper-level trough continues to lift out of the Northeast
tonight, some weak ridging is expected to arrive from the west
overnight. This will allow surface high pressure to build much
closer to our area, although its center looks to be over
Virginia toward daybreak. This allows a bit of a pressure
gradient to remain over our area, however the flow is weaker and
with the cold air advection waning and becoming warm air
advection aloft through the night, the winds are expected to
decouple across much of the area. The airmass continues to be
rather dry and a mainly clear sky is expected. A lack of clouds,
a dry airmass and light to calm winds will set the stage for
efficient radiational cooling through the night despite some
warming aloft. We went with some of the colder guidance,
especially for the New Jersey Pine Barrens for example. It is
here where some temperatures will probably get to or slightly
below freezing, however the areal coverage of this is less
certain therefore held onto the Frost Advisory.

As for the rest of the area, opted to expand the Freeze Warning
to now include Upper Montgomery County and Berks County. The
Freeze Warning elsewhere is unchanged. The Frost Advisory was
expanded elsewhere including Delaware and Philadelphia counties,
as outside of the urban centers these two counties do have
several spots that radiate well. We did leave out the immediate
coastal zones though given the influence of the adjacent milder
waters. The typical areas that tend to decouple faster in the
evening will experience a quicker temperature drop, then all
areas will turn much colder overnight and especially closer to
daybreak.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
As a weak ridge slips offshore through midday, surface high
pressure will also shift to our southeast. This will result in a
return flow and therefore southwesterly flow will kick in. A
cold start to the day across the region, however given low-level
warm air advection through the day and sunshine will boost
temperatures well into the 50s and even lower 60s for much of
the area. As the boundary layer warms, a southwest breeze will
increase with a little bit of afternoon gustiness possible.

The next system will be amplifying across the Great Lakes
region through the day Friday, and with warm air advection ahead
of it some high level clouds should start to increase mainly
later in the day. Overall though, plenty of sunshine should be
the rule for the day on Friday. High temperatures are mainly a
blend of continuity and multi- model guidance.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Friday night through Sunday...A deep progressive upper trough will
be pushing eastward across the Great Lakes by the beginning of the
period. Associated low pressure will move slowly eastward across
southeastern Canada send a cold front through our area by late
Saturday. Isolated to scattered showers along with increasing cloud
cover and slightly above average temps will be the story late Friday
through Saturday night before the front passes. There may be a few
lingering showers overnight Saturday as well. The colder air in the
wake of this front will make for a chilly day Sunday with highs
about 15 degrees below normal along with breezy but mostly clear
conditions.

Sunday night though Thursday...Mostly benign weather is expected
during this time with westerly flow prevailing aloft and continental
airmass in place across most of the Mid-Atlantic region. A trough
will be pushing offshore by Monday as high pressure slides eastward
across the Mid-Atlantic region from the Ohio Valley offshore to the
western Atlantic. Another trough appears to move in by Tuesday, but
guidance differs in the timing, strength, and placement of this
feature. Nevertheless, low pressure will likely result across the
Great Lakes/New England region and push a weak cold front through
our area on Tuesday. Without sufficient return/moisture flow, only
expecting a few isolated showers possible around the Poconos during
this time with partly to mostly cloudy skies elsewhere. As this
system pushes east, more cool and dry continental air is ushered in
behind the front as high pressure builds from the northwest
Wednesday into Thursday. Temps will remain 5-10 degrees below normal
during this period, with the warmest day being Tuesday ahead of the
cold front.

&&

.AVIATION /01Z FRIDAY THROUGH TUESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR. LGT/VRB to nearly calm winds.

Friday...VFR with some afternoon high clouds. Light and
variable to calm winds, becoming southwest 8-12 knots from about
late morning on.

OUTLOOK...
Friday night and Saturday...Mainly VFR conditions, but
occasional sub-VFR possible with scattered showers. West winds a
bit gusty on Saturday.

Sat night and Sunday...VFR. Gusty northwest winds on Sunday.

Monday and Tuesday...VFR conditions expected.

&&

.MARINE...
NW winds 15-20 kt or so on the ocean waters will diminish
tonight to 10-15 kt. W winds become SW and increase to 15-20 kt
late Friday afternoon.

A Gale Watch is now in effect for the ocean waters Friday night,
but do not think widespread gales will be in effect for the
entire night, most likely for a 4-6 hour period starting around
midnight. A Small Craft Advisory is now in effect for DE Bay for
Friday night.

OUTLOOK...
Saturday...At least SCA conditions are expected. A few gale
force gusts are possible. Showers.

Sat night thru Sun night...SCA conditions expected. Gale
conditions possible Sat night into Sunday. Fair.

Monday/Monday night...Sub-SCA. Fair.

Tuesday...SCA conditions possible. Fair.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Freeze Warning from 1 AM to 9 AM EDT Friday for PAZ054-055-
060>062-103-105.
Frost Advisory from 1 AM to 9 AM EDT Friday for PAZ070-071-101-
102-104-106.
NJ...Freeze Warning from 1 AM to 9 AM EDT Friday for NJZ001-007>010.
Frost Advisory from 1 AM to 9 AM EDT Friday for NJZ012-013-
015>023-027.
DE...Frost Advisory from 1 AM to 9 AM EDT Friday for DEZ001>003.
MD...Frost Advisory from 1 AM to 9 AM EDT Friday for MDZ008-012-015-
019-020.
MARINE...Gale Watch from Friday evening through late Friday night for
ANZ450>455.
Small Craft Advisory from 6 PM Friday to 6 AM EDT Saturday for
ANZ430-431.

&&

$$
Synopsis...AMC
Near Term...Gorse/MPS
Short Term...Gorse
Long Term...AMC/O'Hara/Staarmann
Aviation...AMC/Gorse/MPS
Marine...AMC/Gorse/MPS
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