Area Forecast Discussion

FXUS61 KPHI 102336
Area Forecast Discussion
National Weather Service Mount Holly NJ
636 PM EST Sun Dec 10 2017
Weak low pressure will pass by well to our north tonight, 
followed by high pressure on Monday. A stronger low will also 
pass by to the north on Tuesday. The associated cold front will 
usher in very cold air for Tuesday night and Wednesday. Another 
weak low may cross the area on Thursday or Friday, followed by 
high pressure and more moderate temperatures heading into next 
Cloudiness will increase tonight ahead of a stout mid- level 
shortwave and accompanying surface trough. Deeper columnar 
moisture should be confined from the Lehigh Valley and southern 
Poconos in PA eastward into northern NJ. There will be 
sufficient lift for the development of precip, but it's expected
to be spotty and light. With the mid and upper levels saturated
with respect to ice, and the atmosphere at or below freezing, 
snow will be the primary precip type. The fast movement of this 
system gives the aforementioned area a 3-5 hour window for some 
spotty, light snowfall. Snow ratios are 10-12:1 and only 
expecting up to several hundredths of an inch of QPF, so a 
dusting to a half inch is possible, generally from Sussex County
NJ southwestward into Berks County PA. Cannot rule out isolated
one inch amount amounts, especially the southern Poconos. 
Given the expectation of a potentially bigger event (see long term 
section below), the snowfall map on our website remains centered on 
that event, and does not reflect any accumulation whatsoever for 
tonight. Tonight's snowfall expectations are far short of advisory 
criteria. Therefore it will be highlighted in the HWO, and if need 
be, handled with a Special Weather Statement (SPS). We held off on 
issuing a SPS for now, given some uncertainty in the temporal and 
spatial extent of the event.
Any snow showers are expected to wrap up by sunrise. Gradual 
clearing will take place from west to east during the morning, 
with additional stratocumulus cloud development during the 
afternoon, especially northwest of the fall line. Temperatures 
will be a few degrees warmer compared to Sunday, but still below
average. In addition, the airmass will be more stable, so 
westerly wind gusts will be capped around 15 mph.
The extended forecast period features a continuing large- scale
trof over the eastern UW, although the trof is forecast to 
become much flatter toward the end of the week and into next 
weekend. There will be a number of shortwv trofs rotating more 
or less rapidly through the main trof, with associated surface 
lows and frontal systems. Timing of the shortwaves becomes more 
doubtful at longer ranges. Also, their rapid movement means 
limited access to moisture and precip production.
One of the more potent systems is forecast to approach the mid-
Atlantic Monday night and then move through the area on Tuesday. Low 
pressure is forecast to intensify as it moves past to the north into 
New England, and its associated very strong cold front will cross 
the area on Tuesday afternoon. Ahead of the front Monday night into 
Tuesday morning, a broad area of UVV is forecast to develop, likely 
leading to some light precip. P-type is an issue, with mostly snow 
over the Poconos and Lehigh Valley, but a mixed bag or just rain 
farther S/E. 2 to 3 inches of snow could accumulate in the Poconos. 
All precip will change to snow by late Tuesday as cold air blasts in.
Tuesday night through Wednesday night will be dominated by very 
gusty NW winds and strong CAA, as the deepening surface low moves 
slowly NE through New England and into Canada. Wind chills in the 
teens and single digits are likely.
A clipper type system is expected to pass near the mid_atlantic 
sometime Thu-Fri, although the timing and track are rather uncertain 
at this time. This system could bring some light snow to the area, 
as temperatures are forecast to remain quite cold. 
Current guidance suggests a moderating temperature trend for next 
weekend with temps perhaps returning to near normal.
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, 
KILG, KMIV, KACY and surrounding areas.
Tonight...Potential for a brief period of MVFR at ABE and TTN due to 
lower ceilings, especially in any snow showers, late tonight. 
Elsewhere, VFR. Winds west-southwest AOB 10 kts. Confidence: average.
Monday...VFR. Southwest winds 10G15 kts. Confidence: above average. 
Monday night...Sub-VFR conditions possible, especially north of
PHL, with a wintry mix of precipitation possible. Winds
generally light and predominantly southerly. Low confidence.
Tuesday...Sub-VFR conditions possible, especially early in the
day, with wintry mix possible generally north of PHL. Winds
becoming west and increasing to 15 to 25 kts with higher gusts
after cold frontal passage. Confidence medium in general
evolution but low on timing.
Tuesday night and Wednesday...Generally VFR with very strong 
west to northwest winds likely. Speeds 15 to 25 kts with gusts 
to 40 kts possible. Medium confidence.
Thursday and Friday...Generally VFR but local MVFR possible in
rain or snow showers. West to southwest winds 5 to 15 kts with 
potentially higher gusts. Medium confidence.
SCA continues through 6AM Monday for the Atlantic Coastal 
Waters. Wind gusts right around 25 kt are expected to continue
through at least some of the night.
On the Delaware Bay, winds have already dropped below 25 kt, so
the SCA was allowed to expire at 5 PM.
Both winds and seas are expected to subside on Monday.
Monday night and Tuesday...Generally sub-SCA conditions Monday 
night, but advisory conditions likely by Tuesday as southwest 
winds increase and seas build. After cold frontal passage late 
on Tuesday, winds will switch to west or northwest and become 
quite strong.
Tuesday night and Wednesday...Gale-force west or northwest winds
quite probable.
Wednesday night...Residual gale conditions becoming advisory-
level conditions during this period.
Thursday and Friday...Sub-advisory conditions are forecast at 
this time.
MARINE...Small Craft Advisory until 6 AM EST Monday for ANZ450>455.
Near Term...Franck
Short Term...Franck
Long Term...AMC