Scientific Forecaster Discussion
fxus61 kphi 130251
Area forecast discussion
National Weather Service Mount Holly New Jersey
951 PM EST Tue Dec 12 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
Near term /until 6 am Wednesday morning/...
a cold front has moved through the region this evening. Much
colder air arrives behind the front on gusty west to northwest winds.
Gusts will be 30 to 45 mph overnight. Some gusts in the 45-50
mph range are currently occuring near Delaware Bay and a Wind
Advisory has been issued through the overnight in these areas.
Temperatures will drop through the 30s overnight and bottom out
in the teens across the far north/west areas and low 20s over
Philadelphia and Delmarva areas. Wind chill values will be near
zero far north/west 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.
Strong wind gusts will continue through the day.
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 500 mb shortwave
passes through southeast PA and New Jersey and 125 knots 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. Quantitative precipitation forecast 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 European model (ecmwf) keep the weak primary low across Great Lakes, into
northern New York 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 knots 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
European model (ecmwf). For now, will run with chance pops on Friday for the
whole region, and chance pops for most of New Jersey 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
Another upper level low could then affect the region on Monday,
and then dry weather looks to return on Tuesday.
Aviation /03z Wednesday 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 20 and 35
knots. Clouds this evening scattering out. High confidence.
Wednesday...VFR. Gusty west to northwest winds up to 35 knots.
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 knots become light/vrb late
Wednesday night, then turn northwest 5-10 knots Thursday morning. Moderate
Thursday afternoon through Thursday night...VFR. Light northwest
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.
gale gusts have already begun from the west and northwest on the
coastal waters and in Delaware Bay. Seas will increase to 6 to
9 ft out on the ocean by Wednesday. High confidence.
Wednesday night...gale force gusts come to an end on the ocean
and Small Craft Advisory conditions come to an end on Delaware Bay in
the evening. Small Craft Advisory conditions persist on the ocean. Visibility
restrictions in snow possible late Wednesday night.
Thursday...morning snow showers taper off in the afternoon. Small Craft Advisory
conditions will continue.
Thursday night and Friday...sub-sca conditions expected. Snow
may develop Friday afternoon.
Friday night and Saturday...Small Craft Advisory conditions look to develop
again. Snow possible Friday night.
Sunday...sub-sca conditions expected.
New Jersey...Wind Advisory until 10 am EST Wednesday for njz021>025.
Delaware...Wind Advisory until 10 am EST Wednesday for dez002>004.
Marine...Gale Warning until midnight EST Wednesday night for anz450>455.
Gale Warning until 6 PM EST Wednesday for anz430-431.