Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000
FXUS61 KPHI 051156
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
656 AM EST Mon Dec 5 2016

.SYNOPSIS...
A weak disturbance will exit the region this morning. High pressure
briefly builds into the area this afternoon and tonight. An area of
low pressure lifts northeastward through the Mid-South states
Tuesday morning before redeveloping near the North Carolina coast
Tuesday afternoon. The low will pass just to our south Tuesday night
and then out to sea by Wednesday. Another area of low pressure will
progress move across eastern Canada during the second half of the
work week. A cold front with this system will move through the area
late Thursday. The coldest air mass of the season is expected to
arrive behind the front for Friday and Saturday while high pressure
builds in. The high moves off the coast late in the weekend when the
next disturbance approaches from the northwest.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

As of about 700 am, the rain (light snow far north) has pretty
much ended across our forecast area. We have received reports of
0.5 to 1.5 inches of snow from the Pocono region and far northwest
NJ. A lot of low clouds remain over the area however and the GOES
low-cloud imagery shows cloudiness extending all the way back
to the MS valley. Winds will shift to the west later this morning
but not too confident that this will be enough to clear things
out. Still hoping for partly sunny this afternoon.

Previous discussion below:
A band of light precip is moving quickly across the mid-Atlantic
region early this morning, about as expected. Snow appears to be
confined to the Poconos and far northern NJ. Total snow amounts
have been reduced geographically and quantitatively. Areas where
it is snowing could still pick up a quick inch or two. Radar loop
shows the trailing edge of precip already moving into eastern PA
and most precip should end across the forecast area by around 10Z.

The surface low pressure trof and shortwave aloft which caused the
precip will swing E/NE of the area later this morning with high
pressure then building in from the Ohio Valley. Some low clouds will
hang around this morning after the rain ends but by late morning
drier air at all levels is forecast to push in from the west. Thus
skies should mostly clear out by mid-day. With considerable sunshine
and rather weak if any could advection, max temps this afternoon
should be a few degrees warmer than yesterday. There will be a bit
of a westerly breeze, however.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
For tonight, high pressure will move over the area resulting in
light winds and mostly clear skies. Temperatures will cool quickly
under good radiating condition and overnight mins will reach the
mid 30s over Delmarva and upper 20s to low 30s over most of east
PA and NJ. By late night some high clouds will spread over the
area from the SW ahead of the next weather system.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
635 AM...With the current snow exiting the area early this
morning, storm total snow and ice graphics as well as
probabilistic snowfall info for the Tuesday-Tuesday night event
were sent to our winter webpage.

600 AM...A forecast update was sent to correct bad RH/apparent
temperature values (wind chills as low as -120F) in the long-term
portion of the forecast.

Low pressure forming over the northern Gulf of Mexico will track
northeastward through the Mid-South region Tuesday morning. This
system is forecast to transfer its energy to a secondary low that
develops farther east (lee side of the southern Appalachians). The
secondary low should then track up the Southeast coast late Tuesday
and then turn out to sea as it passes just to our southeast Tuesday
night. Models seem to be in very good agreement with regards to
large-scale structure and evolution of this storm as well as
timing/amounts of QPF, which adds confidence to the forecast.
Compared to yesterday`s 00Z runs, models have trended a few hours
slower with the arrival of precipitation on Tuesday as the high is a
bit slower to retreat into New England. Precipitation is forecast to
start in Delmarva early to mid morning but may struggle to advance
northward as it moves into a increasingly dry airmass closer to the
high. PoPs were lowered across the northern third of the CWA on
Tuesday, with precip likely holding off until late in the day.

The Tuesday-Tuesday night system looks to predominately be a rain
event for 90 percent the CWA (the exception is southern Poconos and
far northwest NJ) although the rain could briefly mix with snow
and/or sleet at the onset north and west of the Fall Line. There
were still subtle differences between the models with respect to the
thermal fields, which factor into the ptype uncertainty for these
northern-most zones, particularly along and north of I-80. The most
likely scenario at this is for precip to start out as a wintry mix
along and north of I-80, possibly changing back over to snow in the
evening as precip intensity increases and both evaporative/dynamical
cooling becomes more of a factor. A quick burst of snow could yield
accumulations of 1-3 inches in Carbon and Monroe Counties in PA and
Sussex County, NJ. An elevated warm layer should surge northward in
concert with the arrival of SW low-level jet, turning precip into
all liquid later Tuesday night. The big question is surface
temperatures. Some of the colder model solutions, including the NAM,
keep temperatures near freezing north of I-80 at higher elevations
(above 1000 ft). The NAM tends to handle thermal profiles in the
boundary layer better in these types of setups when the residual
cold-air damming wedge can be stubborn to erode. Forecast
temperatures were taken from a blend of various models/previous
forecast but the NAM was had more weight. The top-down method was
used to determine ptype, resulting in a potential for pockets of
freezing rain Tuesday night in the coldest spots. Using the FRAM
with input from our forecast database, several hundredths of an inch
of ice are possible across the far north. A Winter Weather Advisory
may be needed as we get closer to the event.

Precipitation will taper off from SW to NE late Tuesday night and
Wednesday morning as the coastal low moves farther out to sea and
a weak trough progresses offshore. Expect some breaks in the cloud
cover to develop Wednesday afternoon with high temperatures
ranging from the mid 40s in NE PA/NW NJ to mid 50s across the
coastal plain in S NJ and Delmarva.

Shortwave energy rounding the base of the longwave trough will move
in from the west on Thursday. Low chances for light showers were
kept in the forecast for Thursday. Thermal profiles support a mix of
light rain/snow in the Poconos and rain elsewhere during the day.
Precipitation could briefly mix with or even change over to snow if
showers lingers into the evening when much colder air moves into the
area. At this time, we are not anticipating much in the way of
snowfall accumulations.

The Mid-Atlantic region will be positioned in between low pressure
over eastern Canada and strong high pressure over the Great Plains
late in the week. will tighten over the region late Thursday and
Friday. A brisk northwest wind develops late Thursday and Friday in
response a tightening pressure gradient. CAA pattern intensifies,
marking the arrival of the coldest airmass of the season so far.
Forecast highs are only in the 30s and lower 40s both Friday and
Saturday. Wind chills look to dip into the teens across most of the
forecast area (single-digits in the Poconos) both Thursday night and
Friday night.

The next storm system could affect the area sometime either Sunday
or next Monday. Even though the low looks to track well to our
north, there may be enough residual cold air in place for a mixed
bag of precip at least at the onset.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

As of 700 am, IFR or LIFR ceilings have developed at all our TAF
sites, even though the rain has ended. The latest TAFs show cigs
improving around midday as winds shift to the west, but confidence
in the this timing is not really high.

Previous discussion below:
IFR conditions have developed at ABE/RDG where rain has changed
over to snow. Radar loop indicates that the snow should end
shortly there, with improving vsbys but probably still IFR cigs.
Otherwise mostly VFR or MVFR conditions are noted. Reduced
cigs/vsbys will likely persist through mid- morning by should
improve to VFR at all sites by mid day as high pressure builds in
from the west. Winds will shift to the W or NW at around 10 kt.
VFR conditions will continue through tonight but winds will die
down as high pressure moves overhead.

OUTLOOK...

Tuesday and Tuesday night...VFR conditions early, lowering to
MVFR/IFR by midday and continues through the night with periods of
rain/fog/drizzle likely. Winds likely become gusty 15-20 kt out
of the northwest late in the day.

Wednesday and Wednesday night...Rain tapers off early in the day.
MVFR CIGs may linger through the day and night.

Thursday and Thursday night...Mainly VFR but SCT SHRA may develop
during the day which could briefly lower conditions in a few spots.
NW wind gusts 20-25 kt develop in the afternoon and evening.

Friday..VFR. Brisk NW winds gusting 25-35 kt.

&&

.MARINE...
Sub SCA conditions are expected through today and tonight. There
will be some restricted visibility in areas of rain and fog early
this morning. The current south winds will shift to the WNW this
afternoon as a low pressure trof swings by and then high pressure
builds in from the west. Winds may gust to 20kt or so this
afternoon and evening but should subside some and veer to the N as
high pressure moves over the area.

OUTLOOK...

Tuesday and Tuesday night...A SCA was issued for our coastal
waters off DE and southern NJ as well as the lower Delaware Bay
for Tuesday afternoon and night. Confidence is high in these
southern zones where onshore flow is expected to be strongest. The
strongest winds are expected to develop late in the afternoon and
continue in the evening. Wind gusts near gale force are possible
for a brief period near or just after after sunset. The SCA may
eventually need to be expanded northward in our coastal waters for
Tuesday night but marginal conditions do not warrant a fourth period
headline.

Wednesday...Winds will weaken but seas remaining at or above 5 ft
may necessitate a SCA during the day.

Wednesday night...Both winds and seas look to drop below advisory
threshold Wednesday night.

Thursday and Thursday night...SCA will likely be needed as NW
winds increase late in the day and into the night.

Friday...Winds and seas exceeding SCA criteria. Winds may get
close to gale force.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 1 PM Tuesday to 6 AM EST Wednesday
     for ANZ452>455.
     Small Craft Advisory from 4 PM Tuesday to midnight EST Tuesday
     night for ANZ431.

&&

$$
Synopsis...Klein
Near Term...AMC
Short Term...AMC
Long Term...Klein
Aviation...AMC/Klein
Marine...AMC/Klein


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