Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 131431
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
931 AM EST Wed Dec 13 2017

.SYNOPSIS...
High pressure will slide across to the south of the region
today. A clipper system will cross the region late tonight into
Thursday. High pressure will return for Thursday night and
Friday. A low pressure system will move up the Mid Atlantic
coast on Friday, remaining well offshore. A brief return to high
pressure is expected for the weekend. Another system will
affect the region early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
With wind gusts across southern Delaware and parts of southeast
New Jersey falling below Wind Advisory criteria, will go ahead
and cancel the Wind Advisory. Will still have wind gusts 30-40
MPH through lunchtime, though.

Also, isolated snow showers will continue over the Poconos
through the late morning hours. Light accumulations are
possible, generally less than 1/2".

Otherwise, cold air advection continues with west-northwest
winds gusting 25-30 mph through this afternoon. Quite dry with
surface dewpoints generally in the single digits. Temperatures
in the 20s this morning will only top out around 30 this
afternoon, except for the Poconos, which is in the teens and may
only top out around 20. The resulting wind chill will stay in
the teens for most of the region, except for as low as 10 below
in the Poconos this morning, and then in the single digits this
afternoon.

Clouds may hang around the northern CWA for much of the day,
but the rest of the area should see some sunshine. Increasing
cloudiness near sunset will be possible as the next system makes
a rapid approach to the area.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
Clipper low should be entering the Mid-Atlantic region at the start
of the period, just downstream of a pivoting midlevel vort max in
the Great Lakes region. The surface low will weaken as it approaches
the spine of the Appalachians and redevelop offshore during the
overnight period, which suggests that the attendant precipitation
shield should be weakening as it approaches the CWA and
strengthening once it leaves the CWA (sorry, snow lovers).

Nevertheless, there will be snow overnight, but the combination of
waning dynamics, dry antecedent near-surface air, and orographic
shadowing should preclude snow amounts from exceeding a couple
inches to the northwest of the Fall Line and should generally be
under an inch to the southeast. Little if any snowfall is forecast
south of the Mason-Dixon Line. The snow should enter the region late
in the evening and be on the decrease by daybreak. Despite the light
amounts, the timing (and the subfreezing temperatures) suggests that
there will be impacts with the morning commute on Thursday. For
messaging purposes, it is important to stress that the lack of a
winter weather advisory (owing to light snow amounts) does not
equate to a lack of impacts. Some slick roads and treacherous travel
can be expected should the snow materialize as currently forecast.

Temperatures will not fall much from highs on Wednesday but should
drop a little bit during the evening hours, especially before the
denser cloud cover invades the region. Temperatures may reach their
minima prior to midnight before a slow rise in the warm-air
advection preceding the surface low`s trek/redevelopment offshore
through the night.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
An unsettled pattern continues through the extended period with
several waves of low pressure slated to move through the region.

Thursday...The clipper system quickly crosses the region early
Thursday, along with a strong shortwave. Snow will be falling early
and may make for a slick morning commute. The snow will start to
taper off through Thursday morning and we should see it end by early
afternoon across the forecast area. Winds may be gusty along the
coast, especially in the morning, but should subside through the day
as the low moves further offshore. Daytime highs will be fairly
cold across the area. Highs will range from the lower 20s across the
southern Poconos and northwest New Jersey to lower 40s across
southern Delaware and extreme southern New Jersey.

Thursday night through Friday night...High pressure will make its
way into the area overnight and cross the region on Friday. A
coastal low develops along the Carolina coast and makes its way up
the Mid Atlantic coast. Additionally, a couple of shortwaves will
rotate through the area on Friday. The models continue to show this
system remaining offshore and out of our area. However, so moisture
may reach into our area and we could see some snow fall towards the
afternoon and into the evening. Some mixing along the coast will be
possible but it looks cold enough that the majority of the area will
see only snow fall. Snow should end from west to east Friday night
and fairly quickly as the system moves further away from the area
and out to sea.

Saturday through Sunday morning...High pressure will slide across
the eastern United States on Saturday, bringing us a nice day of
quiet weather. The weather should remain quiet through at least
Sunday morning. Warmer air will start to make its way towards the
region this weekend but we won`t start to feel the difference until
Sunday.  Temperatures are expected to be in the 30s to lower 40s on
Saturday and in the upper 30s to upper 40s on Sunday.

Sunday afternoon through Tuesday...Another system will approach the
region sometime around Sunday night or Monday. The models diverge
with how to handle another clipper system to our north Sunday and a
developing low to our south. For now, will continue to mention the
chance for precipitation starting Sunday afternoon through Monday.
Warm air will move into the region in the southwesterly flow and we
may see a period of rain or mixed precipitation through the region.
Temperatures look to remain cold over the northwestern zones and
likely to remain all snow there. Plenty of uncertainty exists
bringing lower confidence through this period.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR with west to northwest winds 15-20 kt with 25-35 kt
gusts. Winds may begin to diminish a bit after 18Z. High
confidence.

Tonight...Sub-VFR CIGs and especially VSBYs should be expected
with snow likely to most of the terminals through portions of
the night (general timing between 02Z and 10Z, with longer
durations likely north of PHL). Runway accumulations may occur,
but should be around an inch or less. Winds will begin from the
southwest around 10 kts but should become light and variable
from late evening onward. Medium confidence in CIGs/VSBYs (with
low confidence in occurrence/timing of snow); high confidence in
winds.

Outlook...

Thursday...MVFR or lower conditions possible in snow through
Thursday afternoon. Snow should clear out by 18Z and then mainly
VFR conditions expected. West to northwest winds around 10 to 15
knots. Confidence: Moderate

Friday...Mainly VFR conditions expected. MVFR or lower possible in
snow showers Friday afternoon and evening. Light winds.
Confidence: Low to Moderate

Saturday...Mainly VFR conditions expected. West to southwest winds
around 10 to 15 knots possible with higher gusts, mainly at KACY and
KMIV. Confidence: Low to Moderate

Sunday...Mainly VFR conditions expected. MVFR conditions in
snow/rain showers possible Sunday afternoon. Southwest winds around
5 to 10 knots. Confidence: Low

&&

.MARINE...
Gales continue across all marine zones at this time, with the
peak in the gusts likely through lunchtime. Latest model
forecasts a brief (around 6 hours) lull in winds (still
exceeding advisory thresholds) before increasing to near or just
above gale force off the Delaware coast again (albeit from
southwest winds rather than west/northwest), while areas farther
north likely stay at advisory levels.

Seas are running about a foot above model forecasts, so made
some adjustments to these through the late evening hours to
account for this.

At least small craft advisory conditions are expected tonight
as winds become more southwesterly in advance of the next
system. Gusts may reach gale-force again in the Delaware coastal
waters after midnight, but confidence is rather low at this
time. Some snow may develop, generally after midnight and
primarily for the New Jersey coastal waters, with visibility
restrictions likely should the snow materialize.

Outlook...

Thursday ...Small Craft Advisory conditions continue through
Thursday. Both winds and seas are expected to drop through the day,
falling below SCA criteria by Thursday evening.

Thursday night through Friday...Sub-advisory conditions are
expected.

Friday night through Saturday...Small Craft Advisory conditions
possible. West to northwest winds will gusts around 25 knots
possible. Seas will increase and near 5 feet.

Sunday...Sub-advisory conditions expected. Winds may near 25
knots and seas may near 5 feet late Sunday.

&&

.TIDES/COASTAL FLOODING...
The strong west to northwest winds occurring overnight have
lowered water levels on the tidal Delaware River. As these winds
are expected to continue through the day, water levels may be
low enough (i.e., around or below 2 feet below MLLW) to cause
some navigation issues during low tide this afternoon and this
evening. We have issued a special weather statement to address
this concern. Winds should diminish tonight, which will allow
water levels to return to more typical values.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Warning until midnight EST tonight for ANZ450>453.
     Gale Warning until 6 PM EST this evening for ANZ430-431-454-
     455.

&&

$$
Synopsis...Meola
Near Term...CMS/MPS
Short Term...CMS
Long Term...Meola
Aviation...CMS/Meola/MPS
Marine...CMS/Meola/MPS
Tides/Coastal Flooding...


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