Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 120553
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1253 AM EST Tue Dec 12 2017

.SYNOPSIS...
High pressure over southeast Canada moves east and offshore
tonight. Low pressure moving through the Great Lakes will pass
north of the area on Tuesday and drag a cold front through the
region by Tuesday evening. High pressure builds in from the west
on Wednesday, and then weak low passes through on Thursday. A
coastal low may affect the region on Friday, and then high
pressure returns for the weekend.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Warmer air is making progress into the area at this time, with
areas outside of the urban corridor and the beaches in New
Jersey the most stubborn to see much of a temperature rise.
Forecasting hourly temperatures has been a real struggle the
past few hours with the heterogeneous nature of the surface
temps owing to local effects and variable cloud cover combined
with the progression of the warmer surface air to the south of a
warm front moving through the region.

Main updates at 12:30 am included tweaking temperatures to match
current trends and lowering dew points north of the Mason-Dixon
Line, in some cases by 4-6 degrees per hour. This dry air is not
allowing much of the snow developing aloft to reach the surface
in the far northern CWA (where elevated isentropic ascent is
stronger), but have seen a couple of observations of light snow
reaching the surface (generally in Sussex County). However,
this first band is moving out at this time. A second band
appears to be developing to the west (far south-central PA
southwestward), though surface obs suggest this is virtually all
virga at the moment.

The latest HRRR and NAM Nest indicate more substantial
precipitation reaching the northwestern CWA in the 08Z to 12Z
time frame, which is when the light snow (or wintry mix just
south of the Poconos) begins. The NAM continues to indicate a
narrow corridor between the snow and the rain with thermal
profiles favorable for a mixture of the two plus some ice
pellets. Included mention in the forecast in this area
(generally northwest of Reading/Allentown and southeast of the
Poconos), and temperatures in the colder valley locations will
struggle to warm during this time. The cooling processes via
wet bulbing effects may counter the warm advection in place to
see temperatures flirt with freezing at the surface during the
onset of the precipitation in the Lehigh Valley and
northwestward. Not expecting anything more than minor/localized
travel impacts, but will monitor this closely as the
precipitation begins to develop. Made tweaks to PoPs/QPF based
on latest hi-res output, though the changes were minor overall.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TODAY/...
1230 am update: I made some fairly substantial changes to the
forecast for this period. The biggest change was to PoPs, with a
narrower zone of high-chance to likely PoPs (generally north of
I-80) during the morning and slight-chance to chance PoPs in a
north-south corridor progressing through the area during the
late morning and early afternoon hours. Regarding the former,
hi-res model consensus keeps most of the accumulating
precipitation in the Poconos and adjacent areas to the south
during the morning hours before pulling away as the attendant
surface low in the Northeast makes continued progress eastward.
The cold front will progress through the area during this time,
with a band of showers likely developing/progressing just in
advance of the front. There is pretty strong consensus for this
scenario, based on the WRF-ARW/NMM, HRRR, RAP, and NAM/NAM Nest
output at/after 00Z. Temperatures should be near or above
freezing everywhere after 12Z, but thermal profiles remain quite
favorable for snow in the Poconos, so I modified Wx grids
considerably in this area to suggest less potential for a wintry
mix in this region (despite the near-surface air possibly being
a couple degrees above freezing).

As cold air infiltrates the region after the frontal passage,
thermal profiles become well-mixed, and this looks to be a
favorable environment for snow showers. The WRF-NMM is
particularly aggressive bringing these into sections of the area
during the afternoon, but I was more hesitant based on the
remaining guidance. Nevertheless, should see a window during the
afternoon where isolated/scattered snow showers spread
southeastward into eastern PA and northern NJ, at least. I am
somewhat concerned that this environment is at least somewhat
favorable for snow squalls given the favorable low-level
thermodynamic environment and strong low-level winds. Will
assess the latest guidance and make any necessary adjustments to
the grids in the next few hours. For now, showed a more rapid
progression of snow shower chances from northwest to southeast
in the aforementioned areas during the afternoon, with chances
highest north of I-78. Additionally, this is when the most snow
accumulation in the Poconos should be expected given the healthy
snow ratios and falling temperatures. Current total
accumulations are quite similar to the previous forecast -- with
all locations expected to be below advisory criteria at this
time.

Meanwhile, winds and decreasing temperatures will be the main
story farther south. Made few changes to these grids from the
previous forecast. Get set for a blustery period starting this
afternoon.

&&

.LONG TERM /TONIGHT THROUGH MONDAY/...
The long-term period continues to show a long-wave trof pattern
over the eastern US with several shortwave trofs rotating
through. The trof is deepest at the beginning period Tuesday
night with a roughly 505dam closed upper low forecast over NY
state. The associated surface low will deepen as it moves
northeast from New England into Canada, maintaining strong NW
winds and CAA over the area through Wednesday. Some snow showers
are possible, mainly over the Poconos.

For Thursday and Friday, temperatures should moderate slightly
but remain well below normal. A couple of fast-moving low
pressure centers could scoot past to our south and possibly
bring some light snow to the area.

The outlook for next weekend attm suggests temperatures
returning to near normal. Saturday looks mainly dry, but by
Sunday another low pressure may approach from the SW and bring a
a mix of rain and snow late Sunday into Monday. Confidence in
this system is rather low.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
Tonight...Generally VFR, though CIGs between 5000 and 10,000
feet will be common. Winds generally light southeast or south.
May see some snow showers approach RDG/ABE late tonight, but
better chances are to the north of these terminals, so no
mention in the TAFs at this point. Medium confidence.

Tuesday...Generally VFR, though brief sub-VFR conditions may
occur with a band of showers that progress just in advance of a
cold front moving through the area during the day. Generally,
CIGs of 3500-7000 feet should be the dominant mode. Winds will
veer from southeast to southwest prior to frontal passage and
gradually increase to around 10 kts, then quickly become west or
northwest and become strong/gusty after frontal passage.
Scattered snow showers are possible during the afternoon,
especially north/west of PHL, and a snow squall cannot be ruled
out. No mention of this in the TAFs for now given low confidence
on occurrence/timing. Low confidence overall in the TAFs, and
several amendments are likely during the day.

OUTLOOK...

Tuesday night through Wednesday...Mainly VFR conditions expected
but possible MVFR at times at RDG/ABE with snow showers. Gusty
west to northwest winds around 15 to 25 knots with gusts around
40 knots possible. Confidence: High

Thursday through Friday...Mainly VFR conditions expected. MVFR
or lower possible in snow showers. West to southwest winds
around 5 to 10 kts. Confidence: Moderate

Saturday...Vfr conditions expected. Southwest winds around 10 kt.
Confidence: Low

&&

.MARINE...
1230 am update: Moved up the timing of the small craft advisory
to 1 am for Delaware Bay and 4 am for the Atlantic waters based
on the latest guidance and current uptick in winds on Delaware
Bay. Southwest winds will become strong/gusty today, with some
gusts approaching gale force by afternoon. Cold front will move
through during the afternoon, with a quick switch to west or
northwest winds, with gusts easily exceeding gale force by
Tuesday evening. Gale watch will likely be upgraded to a warning
later this morning.

OUTLOOK...
Tuesday night through Wednesday...Strong west winds with gale
force gusts likely. Seas will also remain elevated around 5 to 7
feet on the ocean. Seas will start to subside Wednesday night.

Thursday through Friday...Sub-SCA conditions expected.

Saturday...Small Craft Advisory conditions possible. West winds
with gusts around 25 knots possible.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from this evening through Wednesday afternoon for
     ANZ430-431-450>455.
     Small Craft Advisory from 4 AM early this morning to 6 PM EST
     this evening for ANZ450>455.
     Small Craft Advisory until 6 PM EST this evening for ANZ430-
     431.

&&

$$
Synopsis...MPS
Near Term...CMS
Short Term...CMS
Long Term...AMC
Aviation...AMC/CMS
Marine...AMC/CMS



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