Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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811
FXUS61 KPHI 071951
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
251 PM EST Wed Dec 7 2016

.SYNOPSIS...
Low pressure off the mid Atlantic coast will move out to sea today
while another area of low pressure drifts through southeast Canada.
This low will continue to slowly drift eastward through the end of
the week and move offshore of the Canadian maritimes by Friday and
Saturday. With high pressure to our west, a strong west to
northwest flow will remain through the end of the week. High
pressure briefly affects the area Saturday night. An area of low
pressure is forecast to move north of the area Sunday into Sunday
night, bringing a cold front across the area.

&&

.NEAR TERM /THROUGH TONIGHT/...
Remainder of this afternoon...skies should generally brighten with
the lower clouds thinning out and clearing near sunset. Isolated
nw flow gusts 19 kt as observed.

Tonight...we will presume the subsidence inversion dissipates and
that so too, the low clouds continue their mid afternoon clearing
trend. However, a deck of mid or high clouds should arrive after
midnight from the west. light west to northwest wind. Fcst elements
are a 50 50 blend of the 12z/7 GFS/NAM MOS. Its possible that there
could be isolated frosty fog patches in the countryside in the light
boundary layer flow radiational cooling sitn (low confidence of
occurrence). Low temps above normal.

&&

.SHORT TERM /THURSDAY/...
Considerable mid and high cloudiness during the morning becoming mostly
sunny and breezy during the afternoon with westerly wind gusts expected
to increase to 25 mph. This cloud wind sequence in assn with the arrival
in the aftn of a ne-sw positive tilt 500 mb trough axis with 12 hr 100-140m
hfc overhead. max temps near normal.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
On Thursday, an area of low pressure will slowly be moving across
eastern Canada, before reaching the Canadian maritimes Friday into
Saturday, while high pressure remains across the central part of
the country. This will keep a steady west to northwest flow across
the area. A broad trough aloft will remain across the east coast,
with an elongated vorticity impulse moving across the area
Thursday and a weaker impulse moving across the area Friday. There
is not a lot of moisture across the area Thursday as it`s forecast
to remain northwest of the area, but the strength of the vort max
and the surface winds may help carry some flurries/sprinkles
across the area during the day. On Friday, the short wave is
weaker, but there is more enhanced low level moisture and steep
low-mid level lapse rates during the day, so there could be even
more widespread showers or flurries/sprinkles across the area.
Especially across our northwestern areas. By Saturday, another
short wave/vorticity impulse is forecast to move across the area
as the northwest flow begins to weaken as the low lifts farther
away and high pressure begins to approach from the west. There
will continue to be steep low-mid level lapse rates during the
daytime combined with some enhanced moisture, especially across
the northern areas. Therefore there will continue to be a chance
for some isolated showers/flurries across the northern half of the
area. High pressure briefly affects the area Saturday night,
providing dry conditions. Thursday through Saturday will be a cold
and windy period, with Friday being the windiest, and Saturday
being the coolest of the 3 day period.

On Sunday night, an area of low pressure is forecast to move
north of the area, pulling a cold front across the region. Another
low pressure is forecast to develop along the boundary, although
the GFS and ECMWF differ on where they develop the low. The GFS
develops this secondary low north of the area, while the ECMWF
develops it to our south. Sunday night looks like the most likely
period for precipitation, but it could linger into Monday if the
low does develop around or south of the area as the ECMWF
indicates, so we`ll keep a chance of precipitation into Monday.
Temperatures will be cold enough for portions of the area to
experience wintry precipitation. The exact timing and placement of
wintry precipitation is still to be determined, but as usual, the
most likely areas for snow would be the northern half of the area.

Dry conditions are currently expected for Monday night into
Tuesday as high pressure is forecast to briefly build to our south
and nose its way into our area. The GFS and ECMWF are showing the
potential for a coastal low to develop on the old frontal boundary
to our south by next Wednesday, which could bring some
precipitation to the southern half of the area. With cold
temperatures, it`s possible that a period of snow could develop
for the southern portions of the area.

&&

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Remainder of this afternoon...mvfr cigs in the process of advecting
eastward to leave behind sct clouds aoa 3000 ft. Wind generally
northwest with gusts under 20 kt.

Tonight...VFR.  W-NW winds will be light (under 10 kt).

Thursday...VFR multi layered cigs aoa 10000 ft during the morning-
midday with chance of an hour of cigs down to near 6000 ft during
midday with isolated sprinkles or flurries mainly higher terrain
of e PA. Then becoming VFR clear late in the day. Westerly wind
increasing, with gusts 22-28 kt during the afternoon as the caa
process develops more earnestly.

OUTLOOK...

Thursday night-Saturday...MVFR CIGS may be possible each morning
before lifting to VFR during the day. Scattered showers or
flurries/sprinkles are possible during the daytime of each day
which may temporarily lower conditions. Gusty west to northwest
winds 15- 25 knots, strongest winds Thursday and Friday.

Sunday...VFR early, lowering to MVFR then IFR by the afternoon and
overnight. Rain or snow possible late in the day and overnight.

&&

.MARINE...
SCA seas has been extended since easterly swells in the wake of
last nights short gale are slow to decrease. Guidance continues to
lower these swells/seas too fast. Pls see MWW or the CWF.

Eventually the seas should subside below the 5 ft sca criteria
overnight.

Thursday...anticipating the need for a westerly flow cold air advection
small craft advisory for many of the waters Thursday afternoon.
Have begun the process with a DE Bay SCA.

OUTLOOK...

Thursday night-Saturday...Small Craft Advisory levels winds expected
through the periods with west to northwest winds gusting 25-30 knots.

Saturday night-Sunday...Winds expected to drop below Small Craft
Advisory levels, but pick up again late Sunday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory for hazardous seas until 10 PM EST this
     evening for ANZ452>455.
     Small Craft Advisory for hazardous seas until 6 PM EST this
     evening for ANZ450-451.

&&

$$
Synopsis...Robertson
Near Term...Drag 251
Short Term...Drag 251
Long Term...Robertson
Aviation...Drag/Robertson 251
Marine...Drag/Robertson 251



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