Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 110250
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
950 PM EST Sat Dec 10 2016

.SYNOPSIS...
Large high pressure in the Ohio Valley moves to the mid Atlantic
states by Sunday morning. Low pressure crossing the Great Lakes
region Sunday night pushes a warm front into our area. This will be
followed by a cold frontal passage Monday afternoon as the low moves
to Maine. A strengthening cold front is expected Wednesday. Strong
high pressure builds east through the end of the work week bringing
our coldest air of the season so far Thursday and Friday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Snow showers subsided earlier this evening and winds are begining
to diminish across the region as high pressure settles overhead.

The increase in cloud cover from the west was delayed a bit, and
this leads to slightly lower temperatures. This is reflected in
the lastest ESTF update.

Previous discussion...
Snow squalls generated by cold air moving over the relatively
warmer waters of the Great Lakes will continue through the late
afternoon hours over parts of the southern Poconos...Lehigh
Valley...and northwest New Jersey. Most areas that get any snow
will experience a dusting to less than one-half inch...but this
will be enough to cause slippery conditions on untreated roadways.

W TO NW winds are expected to diminish quickly around sunset as
mixing subsides and as high pressure becomes centered over the mid-
Atlantic region tonight. As the northwest flow begins to shut down,
snow streamers from the Great Lakes will also shut down, bringing
an end to lake effect snows over the northern portions of our
CWA. Under clear to partly cloudy skies...radiational cooling
will bring temperatures down to the low teens in the north and low
to mid 20s south.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
A warm front associated with a low pressure system moving north
through the Great Lakes region...coupled with cold air still in
place from a high pressure system moving off the mid-Atlantic
coast...is expected to produce snow over the northern third of the
CWA beginning Sunday afternoon. Temperatures are expected to be cold
enough for all snow in the Poconos...Lehigh Valley...and northwest
New Jersey. However...snowfall totals through 6 PM Sunday should be
limited to around one inch or less given the limited moisture at the
front end of this event. A Winter Weather Advisory has been issued
for snow and ice accumulations expected during the overnight hours
Sunday night into Monday morning...generally north of the I-78
corridor. A few snow showers...perhaps mixing with some light
rain...could occur as far south as the Philadelphia metro
area...with little to no impact expected. Winds are expected to be
light and variable to start the day...but a southerly flow in the
5 to 10 mph range is expected to develop during the afternoon
hours. High temperatures will range from the mid to upper 20s in
the far north to around 40 on the Delmarva.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
**Potential for three snow/ice events between now and next Saturday
  for e PA and the north half of NJ**

The La Nina Ohio valley storm track appears to be in place and this
winter gets off to a good start over northeast PA and northern NJ.
Snow cover should be expanding nicely across the northern tier of
the USA the next week or two and that probably means down into a
portion of our area as well... which will also allow for the easier
southward advance of sub normal cold. This December looks to be
far different than last years extreme record warmth as a more
realistic winter develops in our area.

Hazards: The main story (aside from any remaining snow squall
streamers into the I-78 -I80 region tonight), will be a snow and
ice advisory hazard for portions of e Pa and northwest NJ Sunday
afternoon into early Monday. There is less certainty regarding
another wintry event Tuesday night or Wednesday, and then there
should be a warm air advection snow or ice event (to rain) developing
next weekend.

Impacts: Sunday afternoon-evening and/or Monday morning commute.
slower travel due to snow or the results of mixed pcpn.

500 mb: A nice little short wave moves from the Great Lakes Sunday
night to Maine by late Monday. A very cold for the season vortex
in central Canada (-3 sd) moves to eastern Canada by Friday, then
strong ridging develops along the east coast next weekend as a
longitudinally large positive tilt trough develops in the western
USA, permitting an extensive warm air advection pattern (aloft) to
occur in the mid Atlantic states.

Temperatures: Calendar day averages Monday will average about 5
degrees above normal, near or slightly above normal Tuesday, cooling
below normal Wednesday with calendar mins possibly occuring at 1159PM,
then average around 10 degrees below normal Thursday and Friday,
and still a few degrees below normal next Saturday.

Temps could run continuously at or below freezing along and northwest
of I-95, including New Brunswick, Trenton, Philadelphia and Wilmington
from approximately 10 PM EST Wednesday through 10 AM Saturday.

Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/10
GFS/NAM MOS guidance Sunday night-Monday night, thereafter the 12z/10
GFS MEXMOS Tuesday, and then the 15z/10 WPC guidance Tuesday night-
Saturday, at times blended with the 12z/10 GFS MEXMOS guidance or
the long term temps were adjusted because of the 12z/10 ECMWF 18z
2m temp fields.

The dailies...

Sunday night-Monday morning event: Mixed wintry pcpn much of I78
north Sunday night (periods of snow) into early Monday morning
(ice), then ends as above freezing temps rain showers forenoon.

South of I-78 there is quite a bit of uncertainty regarding amounts
of any precipitation when temperatures and thermal profiles would
allow wintry precipitation.

These patterns without a blocking high to the north, usually yield
some snow and ice but not likely to have warning amounts, except
if more than .50 inches of water equivalent frozen-freezing pcpn
can occur. The Great Lakes short wave induces a strengthening
southwesterly 850MB jet in the Ohio Valley and also Va Sunday
evening that begins merging into a sw 60 jet core near Baltimore
MD around 06z/12 and then Monday morning is just east of NJ. By
then, the wintry pcpn damage is done.

Confidence: This forecast is still very dependent on how quickly the
warm air will move in, so there is still uncertainty with the exact
timing of the change over, and consequently snow and ice amounts.
We felt it was in everyones best interest to issue an advisory
where confidence for mixed freezing precipitation was 80 percent.

The advisory may yet need to be adjusted based on the updated
model guidance and forecaster expectations.

Monday afternoon/evening: dry air advection is expected on the
heels of the cold front, so precipitation should come to an end
from west to east behind the cold front which should be off shore
by the evening hours. Its still possible a couple of showers could
linger in se DE till around 8 PM Monday.

Tuesday...Dry during the day with weak high pressure interlude and a
decent amount of sunshine through some mid and high clouds. A good
day to catch up on wrapping up outdoor decorations etc before a
southward expansion of frozen ground develops at the end of the
week.

Tuesday night...increasing clouds.

Wednesday...Multi models disagree on the northward extent of a wintry
pcpn event, which would probably be snow here...if it precipitates.
Nothing major foreseen but still worthy to keep an eye on, especially
with sub freezing temperatures immediately following.

Thursday-Friday...Brrr... the real deal cold and I still dont believe
this is pure arctic. I think of this as strong polar outbreak, with
a secondary cold frontal passage early Friday that may be accompanied
by flurries or a snow shower.

Saturday... a bit of snow to ice and then rain along and southeast
of I-95, but a little longer period of snow and ice possible along
and north of I-78.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

VFR conditions are expected through Sunday morning as high pressure
crests over the area tonight...then moves off the mid-Atlantic coast
on Sunday.

W to NW winds 5 to 10 knots early tonight will become light and
variable overnight.

Clouds will lower starting Sunday morning across the region, with
precipitation in the form of snow showers moving into northern areas
during the afternoon...perhaps impacting both RDG and ABE after 18Z
with sub-VFR conditions. The other TAF sites are expected to remain
VFR through a good part of the afternoon. Light and variable winds
in the morning are expected to become southerly in the 5 to 10 knot
range during the afternoon.

Outlook...
Sunday night and Monday...Widespread MVFR and even IFR conditions
at times, especially 06z-18z/12 with low ceilings and reduced
visibilities. Precipitation could begin as snow and may be a wintry
mix for most TAF sites KRDG -KABE and KTTN Sunday afternoon or
night.

All precip should change over to rain no later than 14Z Monday.
Gusty west winds will be possible behind the cold front on Monday
afternoon with conditions probably becoming VFR by late in the
day.

Monday night...VFR. Breezy northwesterly winds possible during the
evening hours.

Tuesday...VFR with sct-bkn clouds aoa 8000 ft.

Wednesday...MVFR or even IFR conditions are possible if wintry
precip moves into the region. Gusty northwesterly winds by night.

Thursday...VFR sct-bkn aoa 4000 ft. northwest wind gusts to 25 or
30 kt.

&&

.MARINE...
This evening...Issued a Marine Weather Statement for northwest
wind gusts up to 25 knots over the DE and NJ coastal waters,
including lower DE Bay, through 11 PM this evening. This was
based on observations of infrequent/marginal gusts of 23-25
knots at 44065, SDHN4, and BRDN1. In addition to the development
of stratocumulus over the waters this evening (indicative of
instability and thus momentum transfer), recent runs of the
HRRR/RAP also showed aforementioned wind gusts via multiplicative
factor of sustained winds, whereas the other guidance did not.

W to NW winds will decrease with gusts up to 20 knots possible
after midnight tonight...becoming south around 10 knots later
Sunday afternoon. Seas are expected to remain well below 5 feet
during the same period.

Outlook...
SCA probable for the Atlantic waters late Sun night-Monday for
seas and probably a period of 25-30 kt waa southerly gusts shifting
to west by afternoon. However, transfer is poor and gales, if any,
should be restricted to the outer coastal waters, if there. We
probably wont know since 44009 conts ots into Feb 2017 and the
stronger wind fields are expected to remain south of 44091 (seas
sensor) and 44065 (wind and seas).

Tuesday...winds and sea should drop below SCA criteria early in the
day, and sub SCA conditions should continue for the remainder of the
day.

Wednesday...Mostly Sub-SCA conditions are expected through the day,
then SCA winds probable Wednesday night behind a cold front with
potential for gale force gusts in the northern Nj waters.

Thursday...At a minimum, widespread northwest flow caa SCA and
probably a gale event of intermittent 34-40 kt gusts all waters
Thursday.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Winter Weather Advisory from 1 PM Sunday to 9 AM EST Monday
     for PAZ054-055-062.
     Winter Weather Advisory from 1 PM Sunday to 6 AM EST Monday
     for PAZ060-061-105.
NJ...Winter Weather Advisory from 1 PM Sunday to 9 AM EST Monday
     for NJZ001-007-008.
     Winter Weather Advisory from 1 PM Sunday to 6 AM EST Monday
     for NJZ009-010.
DE...None.
MD...None.
MARINE...None.

&&

$$
Synopsis...Drag
Near Term...Franck/Miketta
Short Term...Miketta
Long Term...Drag
Aviation...Drag/Franck/Miketta
Marine...Drag/Franck/Miketta


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