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Office: PHI

FXUS61 KPHI 141256

Area Forecast Discussion
National Weather Service Mount Holly NJ
756 AM EST Thu Dec 14 2017

Low pressure will move quickly to the north and east of the
area this morning. High pressure will build in from the west for
the rest of today through Friday. A low pressure system will
move up the Mid Atlantic coast on Friday, remaining well
offshore. High pressure then returns for the weekend. A weak low
pressure system will move through Sunday night into Monday with
another system expected around midweek.


745 AM update: The remainder of the Winter Weather Advisory has
been cancelled with snow moving out of northeastern PA and
northern NJ. There are additional snow showers farther south
across DE and southern NJ but with precip moving quickly to the
east and with temperatures above freezing, do not expect much
in the way of snowfall accumulations and impacts on travel. The
forecast (PoPs and Wx) has been adjusted accordingly.

Previous discussion...

A decently strong clipper low is moving through the northern Mid-
Atlantic this morning, with most of the area receiving a dusting to
around an inch and a half of snow with warm-air advection generated
precipitation that moved through the area earlier. Lingering light
snow persists north of I-78 as the surface low remains to the west.
However, it will quickly redevelop offshore as synoptic-scale lift
quickly increases in favorable left-front quadrant jet dynamics
combined with differential cyclonic vorticity advection from the
attendant midlevel vort max. This means that the zonally-oriented
band of snow across most of Pennsylvania should generally diminish
with time while rain/snow develop offshore during the next few
hours. However, as the surface low to the west propagates closer to
the western CWA, there should be a final push of snow across
northern portions of the CWA, where up to another inch or so is
possible through the morning rush hour.

As such, the winter weather advisory was not changed with this
update, though snow totals themselves will likely be below criteria
(at least in an areal average sense) with this event. I may chip
away at the advisory from south to north during the next few hours
should the snow be as far north as the latest HRRR simulations
suggest (versus the NAM). For context, the precipitation as
simulated yesterday was underdone and too far north in the WAA
regime (by about 100 miles), so I do not have much confidence in the
placement of the final push of snow this morning. With subfreezing
conditions continuing, expect slick roads for the morning commute,
especially those that remain untreated.

To the south, the dry slot has effectively ended the precipitation
(mainly along/south of I-76). Models are much too cold in this area,
and hourly temperatures have been a real struggle as a result.
Statistical guidance has trended warmer today, but I suspect this is
too aggressive where accumulating snow has fallen. Nevertheless,
given the cold bias in the southern areas this morning, I inched up
temperatures today in general.

Once the low passes this morning, west winds should pick up and
become gusty again, especially this afternoon. Gusts should be much
lower today versus yesterday, and with warmer temperatures, it will
simply be uncomfortable versus bone-chilling.


High pressure moves into the Mid-Atlantic tonight, and this will
cause the winds to subside after dark. Mostly clear skies to start
the night will set the stage for a sharp drop in temperatures during
the evening, likely exacerbated in areas where remaining snow exists
(i.e., especially in the north). Generally went below guidance for
lows, though with increasing cloudiness likely to occur late, there
is some uncertainty here. The increasing cloud cover will be aided
by a 250-mb jet streak encompassing much of the eastern U.S.
downstream of another midlevel vort max entering the Great Lakes
region. The main question is timing, with faster approach of the
clouds likely resulting in warmer lows. Current forecasts are for an
increase rapidly after midnight, which should be ample time for
temperatures to plummet in the evening.

Speaking of lows, they are forecast to range from single digits in
the Poconos to around 20 in the Philly metro area. Bundle up.


Friday 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. The models continue to show this
system remaining offshore and out of our area. However, some
moisture may reach into our area and we could see some snow/rain
fall towards the afternoon and into the evening. Additionally,
a couple of shortwaves will rotate through the area on Friday
with a another clipper system staying to the north of the area.
Guidance shows the moisture from the clipper drying out before
it reaches the region but with a strong northwest flow, we could
see some streamers make it from the lakes to our area.

Saturday through Sunday morning...High pressure slides across
the eastern United States Saturday, bringing some beautiful late
fall weather. The weather should remain quiet through at least
early Sunday before the next system arrives. Saturday may be a
bit windy, especially along the coast. Warmer air filters in
from the southwest but we won't really 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 Monday...A weak low pressure system
will approach the region sometime around Sunday night into
Monday. While the models are not too robust with moisture, it
seems more likely underdone and we expect a period of rain or
mixed precipitation through the region. However, with
differences in timing and strength of the low, confidence
remains low.

Tuesday through Wednesday...The models diverge with how to
handle a strong low pressure system to our north and the
attendant cold front. Indications are we will see some
precipitation occur sometime in the Tuesday/Wednesday timeframe
but confidence is low with respect to coverage and timing.


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

At 1245Z, conditions are improving to VFR with the SN already
exiting the area. However, RDG and ABE are still reported MVFR
CIGs in SN (though it is more likely BLSN). W-NW winds will
increase later this morning, which could potentially lead to
BLSN and localized reduced visibility to MVFR in spots where
fresh snow fell early this morning.

Gusty west or northwest winds will occur from 15Z to 22Z for
most of the area (speeds 10 to 15 kts with gusts to 20 kts or
so). High confidence.

Thursday night...VFR with northwest winds around or below 10 kts.
Increasing mid and high clouds late. High confidence.


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

Saturday...Mainly VFR conditions expected. West winds around 10
to 15 knots possible. Gusts up to 20 knots possible, mainly at
KACY and KMIV. Confidence: Moderate

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


630 am update: Gale warning continues for the Delaware coastal
waters, with models indicating peak winds between now and 10 am.
Latest model simulations show continued advisory-level gusts for
the New Jersey coastal waters until around 10 pm tonight.
Extended the advisory here, and will replace the gale warning
with a small craft advisory for the Delaware waters once the
gales come down.

Previous discussion...

Gales have developed at buoy 44009 this morning, so I have
issued a short-fused gale warning for the Delaware coastal
waters through noon today. Elsewhere, strong small-craft
advisory conditions will exist this morning before a slow
diminishing trend occurs this afternoon. Although winds may
linger near/slightly above advisory criteria early this evening,
not confident enough to extend the advisory at this time.

Seas should remain elevated through the day, but as northwest
flow develops this afternoon, there should be a diminishing
trend this evening.

Light snow should continue across the New Jersey coastal waters
this morning, with visibility restrictions likely. Fair weather
should set in this afternoon and tonight.


Friday...Sub-advisory conditions are expected.

Friday night through Saturday night...Small Craft Advisory
conditions possible. West to northwest winds with gusts around
25 to 30 knots. Gale force gusts possible but confidence is low
at this time. Seas will increase and near 5 feet on Saturday.

Sunday through Monday...Sub-advisory conditions expected.


MARINE...Small Craft Advisory until 10 PM EST this evening for
     Gale Warning until noon EST today for ANZ454-455.
     Small Craft Advisory until 6 PM EST this evening for ANZ430-


Near Term...CMS/Klein
Short Term...CMS
Long Term...Meola