fxus61 kphi 122000 

Area forecast discussion 
National Weather Service Mount Holly New Jersey 
300 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 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 west to northwest 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 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 


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 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 
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. 


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. 


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. 


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. 


Phi watches/warnings/advisories... 
New Jersey...none. 
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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