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FXUS61 KPHI 130251

Area Forecast Discussion
National Weather Service Mount Holly NJ
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 


A cold front has moved through the region this evening. Much 
colder air arrives behind the front on gusty W to NW 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 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.


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.

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. 
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.


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 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 Wednesday for NJZ021>025.
DE...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.



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