Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 130547

Area Forecast Discussion
National Weather Service Mount Holly NJ
1247 AM EST Wed Dec 13 2017

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


1230 am update: Made several changes to the grids, most notably
a big uptick in wind gusts based on recent observations about 5
to 10 mph too low across the board. Model soundings and momentum
transfer calculations are not really catching on to the stronger
gusts this evening, so most of the amendments involved a
positive adjustment of 3-7 kts greater than the most aggressive
hi-res models. Current wind advisory looks good, with
verification already occurring along the immediate coast of
Delaware Bay. However, based on gusts farther northwest the past
couple of hours, the advisory may require some expansion. This
is especially true given that the peak in wind gusts will likely
occur late tonight through tomorrow morning. Will monitor trends
the next couple of hours and make a final decision at 3:30.

Sky cover and temperatures were also adjusted considerably.
Temperatures are much lower than forecast, especially where
cloud cover has thinned or eroded (i.e., especially southern and
western CWA). Cold air advection is winning the battle tonight
over mixing, with the added bone chilling of the strong winds
producing wind chills below zero in the southern Poconos.
Forecast wind chills are around -10 by morning, which is not far
from advisory thresholds.

PoPs/Wx grids unchanged, but coverage of snow showers has been
sparse at best. A perturbation later tonight will likely enhance
snow showers for a time in the southern Poconos, but models have
been overdoing the precipitation during the whole event and do
not see much reason why this trend will not continue.


1230 am update: Adjusted wind grids upward considerably (3-7
kts) most of the day given current trends and a clear negative
bias with the models so far.

Previous discussion below...

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.
Strong wind gusts will continue through the day.


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.


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

Rest of tonight...VFR with 270-310 degree wind directions.
Speeds 10 to 20 kts with gusts 25 to 35 kts. Very high

Wednesday...VFR with 260-310 degree wind directions. Speeds 12
to 25 kts with gusts frequently at or above 30 kts, especially
in the morning. Very high confidence.


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

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.


Gales are in full swing in the marine zones, with Lower Delaware
Bay not far from storm criteria. Current gale warning looks
fine. Did adjust wind gusts a little bit based on current
trends, mostly upward about 1-3 kts through most of the day
tomorrow, based on current coastal and buoy observations. A
strong gale event looks to unfold for a lengthy period.


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.


NJ...Wind Advisory until 10 AM EST this morning for NJZ021>025.
DE...Wind Advisory until 10 AM EST this morning for DEZ002>004.
MARINE...Gale Warning until midnight EST tonight for ANZ450>455.
     Gale Warning until 6 PM EST this evening for ANZ430-431.


Near Term...CMS
Short Term...CMS/O`Hara
Long Term...MPS
Marine...CMS/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.