Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 161637
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1137 AM EST Sat Dec 16 2017

.SYNOPSIS...
A cold frontal boundary across the Great Lakes will move
southeastward through the region today and tonight. This front will
then comes back northward through the region as a warm front on
Sunday. Another cold front will then move through the region during
the middle of next week. To end next week, an area of low pressure
will track northeast into the Great Lakes bringing a warm front
through our region.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1130 AM Update...
Snow showers have been slow to start with radar indicating some
virga aloft across the southern Poconos. The 12Z model suite has
also delayed the onset. The 12Z NAM and 15Z RAP have shifted the
QPF further north. As a result, have delayed the onset of light
snow showers into the 2-4 PM time frame generally in the
vicinityof I-80 in NJ and Carbon/Monroe counties of eastern
PA. Amounts across this area should range from a dusting to one
half of an inch. Cannot rule out isolated one inch amounts
across some of the higher terrain, with the higher snow ratios
in place. The snow showers should end early this evening.

The remainder of the forecast remains on track.

Previous Discussion...
Today, the attention turns to locations generally along and north of
Interstate 80 for snow chances. No major changes to the previous
forecast with today`s event. Another mid level shortwave trough will
approach our region through the day. In response, the cold front
currently over the Great Lakes will approach the region this
afternoon. Further, the right entrance region of an upper level jet
will be over the northern half of the region. As mentioned by the
previous shift, the best synoptic scale lift should  stay north of
our region. However, models continue to indicate a ribbon of low
level frontogenesis across southern NY. This, combined with the
nearly saturated boundary layer and equivalent potential vorticity
values near or below 0 just south of the frontogenesis, indicates
there should be enough instability for some snow showers to progress
as far south as the I80 corridor. Moisture is limited, as is the
window of opportunity for any snow showers, so any accumulations are
expected to be less than an inch.

Further south, the combination of a building surface ridge over the
Coastal Plains and be further removed from any forcing, should keep
the area dry.

Forecast temperatures are below most guidance. The reason for this
is that models tend to have a warm bias the day after a fresh snow
pack. Thus, expect highs to range from the upper 20s in the southern
Poconos and NW NJ, to the lower 40s across Delmarva.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
Overnight, the latest mid level short wave trough moves further away
from the region. The cold front should continue to slide south
through the region. However, by this point, models don`t depict any
frontogenesis, but there should be subsidence thanks to the building
east coast ridge. Thus, expect the overnight period to be dry across
the region.

Undercut guidance considerably for overnight lows as models have had
a strong warm bias the last few nights. Though the snow pack may not
be much of an issue (much, if not all of it could melt during the
day today), clearing skies and light winds will promote radiational
cooling especially through the second half of the night.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Sunday through Monday night:

A wintry mix event is currently modeled across northern portions of
the region with moisture overruning a warm frontal boundary.
Overall, this system will be weakening as it comes northeast into
our region. Through this event, thermal profiles look to support
mainly rain across the Philadelphia area and locations to the south,
while locations further north have modeled soudings which would
yield a mixture of snow and freezing rain. This is noted in a warm
nose advecting northeast at about 925 mb throughout Sunday night
with 925 mb temperatures rising to between 1 and 2C with surface
temperatures near or below freezing. QPF does look light and under
1/10th of an inch on most models and ensembles at this time.
Generally opted temperatures Sunday toward the modeled two meter
temperatures then incorporating some of the MET/MAV/ECMWF temperature
guidance Monday and Monday night. Temperatures should rise into the
40`s for most of us with lows Monday night in the 30`s.

Tuesday and Tuesday night:

Our region will be between a high pressure offshore and an
approaching cold front from the west. This will result in southerly
winds and warming temperatures. Enough moisture and lift could be
present ahead of the front for some scattered rain showers. However,
the signal for precipitation is noisy on the various ensembles and
fairly limited so we will maintain slight to low chance pops. Right
now the temperature forecast is on the warmer end of the ensemble
envelope with highs well into the 50`s, nighttime temperatures should
fall back into the 30`s.

Wednesday through Thursday night:

After the frontal passage, we return to northwest flow with cold
air advection into the region. Another period of gusty winds is not
out of the question. With high pressure building in both days should
be mostly sunny. Right now the wind direction off the lakes looks
unfavorable for any increase in clouds or snow showers up in the
southern Poconos. Bufkit analysis shows the potential for some gusts
from the west at around 20 mph Wednesday afternoon. High
temperatures should fall back into the 40`s with lows in the 20`s,
opted to go on the cooler end of the ensemble guidance given the
cold air advection into the region.

Friday and Friday night:

A warm frontal boundary lifts northward across the region as a low
pressure system tracks into the Great Lakes. Another surge of
southerly flow will come into the region leading toward a moderating
trend with temperatures. This warm front will also bring some
showers with it.


Longer term:

A quick look into Christmas week does still provide the chance of a
white Christmas with an active weather pattern that looks to turn
colder thanks to a EPO which is falling several standard deviations
below normal. The - EPO will discharge cold from AK southeast into
the United States just before Christmas. However, the colder air can
dive into the western United States first leading to warmer
conditions and a typical ridge in La-Ninas over the Southeastern
United States. Exactly how this plays out is still outside the
forecast window and a lot of possibilities are in play. Be mindful
of any weather information you see about Christmas can still change
quite a bit over the next few days.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR. There is a small chance that snow showers along
and north of I80 this afternoon, could move as far south as
KABE. If this happens, there could be brief MVFR conditions.
However, this is very unlikely so have not included it in the
TAF. Winds generally out of the west around 10 kt with gusts
up to 20 kt. High confidence.

Tonight...VFR conditions are expected overnight. A cold front moving
through by 06Z will result in northerly or northwesterly winds, but
speeds should be 10kt or less. After 06Z, winds will likely become
light and variable. High confidence.

Outlook...

Sunday: VFR, winds under 10 knots. High confidence.

Sunday night: Sub-VFR possible with patchy light rain, mixed
precipitation possible at KABE, KTTN and KRDG. Winds under 10
knots. Moderate confidence.

Monday and Monday night: Mainly VFR, winds generally under 10
knots. Moderate confidence.

Tuesday through Wednesday: VFR. An isolated shower can not be ruled
out on Tuesday. Southwest wind gusts around 15 knots then shifting
to northwest and increasing to 20 knots on Wednesday. High
confidence.

&&

.MARINE...
1130 AM Update...
After a brief lull this morning, winds will increase again this
afternoon across the NJ coastal waters and DE Bay. Across the DE
coastal waters, winds have subsided, and if this trend continues,
plan on dropping the SCA by 18Z. Elsewhere, the SCA continues as
posted.

Previous Discussion...
Winds gusting above 25 kt are expected on all the waters
through at least the morning hours. On the Delaware Bay and
Delaware and southern New Jersey coastal waters, winds may drop
below small craft advisory earlier than previously expected,
possibly as early as this afternoon. However, I did not have
enough confidence to change the end time on the small craft
advisory.

On the northern and central New Jersey coastal waters, though there
may be a brief lull in the winds late this afternoon, wind gusts
above 25 kt are possible behind a cold front this evening.

Outlook...

The main period for concern is northwesterly winds Wednesday
afternoon which may approach or briefly exceed 25 knots which may
warrant an SCA. Data is indicating seas stay below five feet
throughout the outlook period. High confidence.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 1 AM EST Sunday for ANZ450>455.
     Small Craft Advisory until 6 PM EST this evening for ANZ430-
     431.

&&

$$
Synopsis...Gaines
Near Term...Franck/Johnson
Short Term...Johnson
Long Term...Gaines
Aviation...Franck/Gaines/Johnson
Marine...Franck/Gaines/Johnson



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