Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 041129 AAA
AFDPHI

Area Forecast Discussion...UPDATED
National Weather Service Mount Holly NJ
629 AM EST Sun Dec 4 2016

.SYNOPSIS...
High pressure over the region today will progress offshore tonight.
A weak disturbance will pass  through the area tonight into early
Monday. High pressure builds back into the Mid-Atlantic states
Monday afternoon and night. An area of low pressure over the Gulf of
Mexico tracks northeastward  and toward the region Tuesday. This low
is forecast to pass to our south Tuesday afternoon and evening and
then move off the Mid-Atlantic coast late Tuesday night and
Wednesday. A strong cold front likely moves through our area
Thursday as another area of low pressure tracks across the
Northeast states. This low will strengthen over the Canadian
Maritimes Friday and Saturday while high pressure builds in from
the central U.S.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Low clouds are stubbornly hanging on this morning over the forecast
area in spite of model RH forecasts which indicates clouds should be
breaking up or dissipating. Anyway, the cloud cover is keeping temps
a few degrees above the forecast overnight mins in most areas. Winds
have diminished and become more northerly as the center of
surface high pressure approaches from the west.

For today, high pressure will continue to move across the area with
light NW to N winds continuing. The low clouds should tend to break
up during the morning but by afternoon high-level cirrus will be
spreading in from the SW ahead of the next approaching shortwave
trof. Afternoon max temps are forecast to be a few degrees cooler
than yesterday across the area.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
A fast moving shortwave trof and associated surface pres trof will
cross the area tonight. Winds become southerly with some decent low-
level warm advection and upward VV ahead of the trof. Some increased
moisture should be available as clouds are streaming NE from the NW
GulfMex already. Our forecast area should see a quick shot of fairly
light precip between midnight and dawn on Monday.

Based on the forecast temperature and moisture profiles, it looks
like p-type will be mostly snow in the Poconos and NW NJ, a mix of
rain and snow N/W of PHL and north-central NJ, and mostly rain
elsewhere. Some snow accumulation, up to around and inch, is
possible where the precip remains mostly snow.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
A northern-stream shortwave disturbance and associated surface
trough is forecast to quickly swing through the region early
Monday morning. Models have trended toward a more progressive
system, which would equate to a faster ending time of precipitation
Monday morning. This trend is good news for the Monday morning
commute in northeastern PA and northwestern NJ as any snowfall
accumulations after sunrise should be minor. We may still see up
to an additional half inch of snowfall in the higher elevations
along and north of I- 80 before it all ends later that morning.

High pressure briefly builds into the region Monday afternoon and
night. Expect skies to clear from southwest to northeast during the
afternoon. High temperatures Monday afternoon range from the upper
30s in the Poconos to the lower 50s from Philadelphia and points
south/east. A favorable radiational cooling pattern will initially
be in place Monday night with a break in clouds and calm winds
underneath the high. High clouds will start to overspread the area
from the southwest late tomorrow night as low pressure approaches
from the Gulf Coast states; which may curb cooling across the south.
Low temperatures near or slightly below freezing are forecast for
all of northeastern PA/northwestern NJ, and rural locations west of
the Phila metro in southeast PA and in the NJ Pine Barrens.

Models advertise a double-barrel low approaching the Mid-Atlantic
region from the south and west on Tuesday. The system is predicted
to feature a primary surface low that cuts into the Ohio Valley
and a secondary low forming on the lee side of the southern
Appalachians Tuesday morning. This secondary coastal low will
likely track up the Southeast coast before passing to our south
late in the day Tuesday and offshore sometime Tuesday night.
Precip from this system will spread in from the southwest,
arriving the Delmarva during the morning rush. The onset of precip
across our northern-most zones may be delayed until afternoon
where a residual dry layer near the surface will be more apparent.
Thermal profiles look warm enough for the ptype to be rain for
most of the region. However, precip at the onset may briefly start
out as wintry mix west of the Fall Line before transitioning to
liquid as warm as a warm nose aloft becomes increasingly established.
The cold outlier model solutions show surface temperatures
hovering in the mid 30s (too close to freezing for comfort) in
our coldest spots north of I-80 (southern Poconos in PA, Sussex
County in NJ). It`s too soon to rule out the outside possibility
of freezing rain if temperatures in these coldest spots wind up
verifying just a few degrees lower than currently forecast.
Evaporative cooling, particularly early on in the event, could be
a mechanism to keep temperatures near freezing in these colder
spots if models are underdoing the magnitude of the residual dry
layer in the boundary layer. Since freezing rain is not the most
likely outcome, kept snow/ice accumulations out of the forecast
for Tuesday and Tuesday night.

The secondary low is forecast to have moved off the Mid-Atlantic
coast by Wednesday morning. The back edge of the rain should taper
off from southwest to northeast during the day as the low tracks
farther offshore. A shortwave disturbance rounding the base of the
large-scale upper trough will progress through the region on
Thursday. Light precip may move back into the area as early as
Wednesday night and continue into Thursday. Partial thicknesses
from the GFS/ECMWF/CMC indicate a potential for snow or a
rain/snow mix in the Poconos and rain elsewhere.

A strong cold front will move through later on Thursday (probably
either afternoon or night). Northwest winds behind the front will
usher in the coldest air of the season thus far. With daytime
highs only in the 30s, Friday and Saturday could be the coldest
days since last February. A tight pressure gradient resulting
from an impressive 1040 mb high over the central CONUS and a
deepening low over the Canadian Maritimes will yield breezy
conditions on Friday. Wind chills in the teens are possible area
wide with single-digit wind chills in the Poconos Thursday night
and Friday night.

&&

.AVIATION /10Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

VFR conditions should continue at all the TAF site through today and
this evening. The low cloud cover with ceilings 4000 to 5000 ft has
been very persistent but some breaks should appear as the day
wears on. Winds today will be NW 10 kt or less as a ridge of high
pressure moves over the area.

Ceilings will begin to lower this evening ahead of low pressure
moving in from the west. MVFR conditions are expected to develop
overnight in light rain and fog. A mix of rain and snow is likely at
ABE and RDG so visibility may be further reduced there.

OUTLOOK...

Monday...Precip, mainly RA, ends by mid morning. RA may mix with
SN at northern terminals (RDG/ABE/TTN). MVFR or IFR around
daybreak will improve to VFR from SW to NE during the morning and
early afternoon.

Monday night...VFR.

Tuesday and Tuesday night...RA moves back into the area from
southwest to northeast on Tuesday. Conditions will likely lower to
MVFR shortly after precip onset and then IFR later Tuesday and
Tuesday night.

Wednesday and Wednesday night...Steady rain tapers off, but lower
clouds may linger.

Thursday...Generally VFR, but scattered showers are possible during
the day which may temporarily lower conditions. Gusty northwest
winds 25-30 knots develop behind a cold front during the
afternoon.

&&

.MARINE...
Sub SCA conditions are expected through today and tonight. NW
winds should remain a bit gusty for a while today but will
diminish by this evening as a ridge of high pressure moves
overhead. Seas will gradually subside along with the wind. Winds
overnight will become light and variable, however visibility may
be reduces in areas of rain and fog after midnight.

OUTLOOK...

Monday and Monday night...Winds and seas below SCA criteria.

Tuesday through Wednesday...Onshore flow strengthens on Tuesday.
SCA may be needed for winds by Tuesday afternoon. SCA for winds
and seas likely Tuesday night and Wednesday. Winds may reach gale
force for a period late in the day Tuesday, particularly across
the coastal waters of DE and southern NJ as well as the lower DE
Bay.

Wednesday night...Winds likely drop below SCA levels, but seas
could remain above 5 feet.

Thursday and Thursday night...Northwest winds increase behind a
cold front. A SCA will likely be needed. Gust near gale force are
possible beginning Thursday night.

&&

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

&&

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


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