Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 122000
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
300 PM EST Tue Dec 12 2017

.SYNOPSIS...
A cold front will move across the region this evening, and then
high pressure passes south of the region on Wednesday. Several
weak low pressure systems will impact the area through the end
of the work week. High pressure returns for the weekend, and
then another system will affect the East Coast early in the new
week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
A cold front will cross the area through the evening. Widely
sct showers will accompany the front, but overall the present
depiction on radar is not well defined and weak. Much colder air
arrives behind the front on gusty W to NW winds. Gusts will be
30 to 40 mph overnight. Temperatures will drop through the 30s
overnight and bottom out in the teens across the far N/W areas
and low 20s over Philadelphia and Delmarva areas. Wind chill
values will be near zero far N/W and 5 to 10 degrees most other
areas.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
The core of the cold air will begin moving away Wednesday. The
ridge will cross the region late in the day. A mostly sunny and
cold day is expected across all areas. Highs will reach the
mid/upper 20s north and low 30s elsewhere. Winds will remain
gusty through much of the day, but then begin to taper off late.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
A fairly active pattern is in place for the Long Term as
several weak clipper systems pass through the region through the
end of the work week, and then again for the start of the new
week. Each system will bring a reinforcing shot of cold air that
will keep temperatures below normal into the weekend.
Temperatures may then moderate a bit back closer to normal for
the start of the new week.

The first clipper system approaches Wednesday evening and then
slowly works its way across the region before moving offshore by
Thursday afternoon. The surface low tracks from the Great Lakes
and Ohio Valley Wednesday evening and moves off the Mid-
Atlantic coast late Thursday morning. Fairly potent H5 shortwave
passes through southeast PA and NJ and 125 KT jet streak passes
south of the Delmarva Thursday morning. With overnight lows in
the teens and 20s, there will be plenty of cold air in place to
support an all snow event starting in the evening, and then
really getting going from around midnight through daybreak or so
Thursday. QPF amounts will be light, generally around 1/10",
but temps will be cold enough to support around 1", but in some
cases, up to 2" of new snow Wednesday night through Thursday
morning. Could make for a slippery commute in most areas.

Precip tapers off Thursday afternoon, possibly mixing with a
brief period of rain mainly south of I-195 before ending.

Dry weather on tap for Thursday night as surface high pressure
passes through the region. Cold night with lows in the teens
across most of the area, and in the 20s from around Philly to
Wilmington, and into the Delmarva.

The next clipper system approaches on Friday and slowly passes
well north of the region Friday night. There is some uncertainty
with this one as to where everything sets up. 12Z/12 GFS and
12Z/12 ECMWF keep the weak primary low across Great Lakes, into
northern NY State, and then through northern New England and
offshore by Saturday morning. However, some strong mid-level
shortwave energy passes through the region Friday afternoon and
Friday night, while a 175 KT jet at 300 MB passes through the
Mid-Atlantic and Southeast U.S. This develops a fairly strong
coastal low Friday night, but latest guidance seems to keep the
precip offshore. 12Z/12 CMC-GDPS indicates some possible
moderate banding right over NJ, but that is because it is
farther north with the developing secondary low than the GFS and
ECMWF. For now, will run with chance PoPs on Friday for the
whole region, and chance PoPs for most of NJ and down into
Delaware for Friday night. This would probably be a mostly snow
event, but may start out as rain over southern Delaware Friday
afternoon. Low confidence forecast for Friday-Friday night.

High pressure builds into the region Saturday and moves
offshore on Sunday. Yet another system is gearing up to impact
the region for the start of the new week, but with upper level
ridging passing through the East Coast, this allows for more of
a southwest flow that would usher milder air into the region,
which would allow for a period of rain sometime Sunday afternoon
and night.

Another upper level low could then affect the region on Monday,
and then dry weather looks to return on Tuesday.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR. Gusty west to northwest winds between 30 and 35
knots. Snow showers with restrictions far north/west. Clouds
this evening then decreasing overnight.

Wednesday...VFR. Gusty west to northwest winds up to 40 knots.
Few snow showers possible early across the far N/W.

Outlook...

Wednesday night through Thursday morning...MVFR or lower in
snow showers. Potential for 1-2" snow accumulation on runways by
Thursday morning. West winds 10-15 KT become LGT/VRB late
Wednesday night, then turn NW 5-10 KT Thursday morning. Moderate
confidence.

Thursday afternoon through Thursday night...VFR. Light NW
winds. Moderate confidence.

Friday through Friday night...Primarily VFR conditions, but
there is the potential for MVFR or lower conditions in snow
showers. Low confidence.

Saturday...Mainly VFR conditions expected. West to southwest
winds around 10 knots. Confidence: Moderate

Sunday...VFR initially, then sub-VFR conditions possible in
snow and/or rain showers. Low confidence.

&&

.MARINE...
Raised the Gale flag at noon, with the fropa occurring later
this afternoon and Gale gusts expected right behind it. Sct
showers will accompany the front, but other than that, fair
weather is expected. Seas increasing to 6 to 9 ft out on the
ocean by Wednesday.

Outlook...

Wednesday night...Gale force gusts come to an end on the ocean
and Small Craft Advisory conditions come to an end on DE Bay in
the evening. SCA conditions persist on the ocean. VSBY
restrictions in snow possible late Wednesday night.

Thursday...Morning snow showers taper off in the afternoon. SCA
conditions will continue.

Thursday night and Friday...Sub-SCA conditions expected. Snow
may develop Friday afternoon.

Friday night and Saturday...SCA conditions look to develop
again. Snow possible Friday night.

Sunday...Sub-SCA conditions expected.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Warning until midnight EST Wednesday night for ANZ450>455.
     Gale Warning until 6 PM EST Wednesday for ANZ430-431.

&&

$$
Synopsis...MPS
Near Term...O`Hara
Short Term...O`Hara
Long Term...MPS
Aviation...MPS/O`Hara
Marine...MPS/O`Hara



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