Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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833
FXUS61 KPHI 150208
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
908 PM EST Sun Jan 14 2018

.SYNOPSIS...
High pressure centered over southeastern Canada will continue to
nose southward toward the Mid-Atlantic states through Monday. A
clipper low is expected to move east across the Great Lakes Monday
night and Tuesday, then redevelop off the northern Mid-Atlantic
coast Tuesday night. The low will strengthen as it tracks east of
Cape Code on Wednesday. Another area of low pressure is expected to
form off Cape Hatteras late Wednesday and then move out to sea
Thursday. High pressure builds in from the Southeastern states late
in the week before progressing offshore next weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
High pressure ridge axis is situated north to south across the DE
Valley as of late this evening. Aloft, a short wave is crossing
the central Appalachians, with modest pressure falls off the VA
Capes, as winds veer in response to the north-northeast across
our area. Meanwhile, low-level moisture offshore in the form of
stratocumulus clouds was beginning to make the turn to the north-
northwest towards the coast. This trend will continue overnight,
with clouds overspreading the area from southeast to northwest.

Cross-sections indicate the layer below 5 kFT becoming saturated,
with additional high-level moisture in association with the south-
west upper jet. At the surface, an inverted trough develops near
the NJ coast late tonight. This will enhance low-level convergence,
with weak lift noted (~5 Microbars per second) near this feature.
Some light precipitation is possible, and model thermal profiles
indicate predominantly snow as the type. There is a lot of dry air
to overcome, so the greatest chance of snow showers is along the
coast.

Most of the model guidance suggests at most a couple hundredths of
an inch of liquid precip along the coast, yielding a dusting of
snow to some areas. For now, have included a slight chance (20 pct)
of snow showers from lower Monmouth south through Cape May County,
generally after 8Z.

Previous Discussion...
High pressure continues to lift through the northeast and into
southeastern Canada tonight. Our area will be on the southern
fringe of the high as it noses down the east coast. Dry
conditions are expected through tonight, although clouds are
expected to increase overnight. Another cold night is expected
as cold air will filter down across the area from north with the
high pressure nosing across the region. There will a steady
light breeze for most places through the night, but it is not
expected to be strong enough to create low enough wind chills
for any advisories at this time.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
On Monday, the high pressure to our north continues to lose its
effect on our area through the day. In fact, a weak surface trough
may develop across the ocean to our southeast and push in across our
southern areas by the morning hours. Meanwhile, a short
wave/vorticity impulse is expected to move across the area during
the day along with an increase in low level moisture that is
expected to develop within the lowest 2,000-4,000 feet. This may all
combine to help create some light precipitation across the southern
half of the area on Monday. There remains dry air at the surface, so
the precipitation will likely not accumulate, so only flurries, or
sprinkles for the warmest areas, are currently expected. If this
light precipitation develops, it is expected to develop south of an
ACY-PHL-RDG line from south to north through the morning into the
afternoon. Otherwise just an increase in cloud cover is expected
through the day.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Monday night through Wednesday...A clipper low is forecast to
move east across the Great Lakes Monday night into Tuesday, then
redevelop off the northern Mid-Atlantic coast (near the
system`s triple point) late Tuesday. The low is then expected to
deepen as it tracks northeastward off the New England coast on
Wednesday.

A band of light snow/snow showers ahead of the cold front that
accompanies the clipper low should begin to overspread portions of
NE PA Monday night or Tuesday morning, then expand S/E into the
Delaware Valley/I-95 corridor Tuesday afternoon-evening. The
boundary layer could initially be warm enough during the afternoon
in the I-95 corridor for a rain/snow mix at the onset but
evaporative cooling should result in a quick changeover to snow. The
light snow will eventually progress downstream into the coastal
plain Tuesday night-Wednesday. The latest guidance indicates the
secondary low becomes organizes a bit too late and is too
progressive to put most of our area at risk for enhanced snowfall
that could develop near a sharpening inverted trough that extends
northward from the surface low. The 12Z ECMWF, which is a western
outlier, bring a period of moderate snow to our far northeastern
zones Tuesday night.  While the potential exists for heavier snow to
expand back into northern NJ Tuesday night, went with a more
conservative forecast for snowfall amounts than the ECMWF given the
low confidence (though the 12Z ECMWF was blended in with what turned
out to be primarily WPC guidance for QPF). Snowfall accumulations of
1-4" are expected for areas north of I-78 and less than 1" elsewhere
to the south. Since these amounts are spread out over a 36-hour
period, do not anticipate reaching Winter Wx Advisory criteria
(unless the 12Z ECMWF winds up being closer to reality).
Nonetheless, some roads, especially in NE PA and N NJ, could become
snow covered and slippery given the cold temperatures in place.

Wednesday night...The upper trough progresses downstream toward the
eastern seaboard. Enhanced height falls and forcing for ascent
associated with a shortwave disturbance rounding the base of this
trough will favor secondary cyclogenesis east of the NC Outerbanks.
The forecast for this system continues to be watched closely as the
storm has a potential to deepen rapidly offshore. The track of the
low continues to be in question but its looking more likely that the
storm takes a more progressive, northeastward track vs turning up
more northward the coast. The former, more progressive scenario
would imply (i.e., northwestern edge of light snow near the coast)
or no impact (i.e., snow remains offshore) to our area. What`s
forecast to go on upstream favors the former scenario with a series
of Pac NW shortwave troughs acting to flatten the upper ridge over
the Rockies. This signals a more progressive eastern CONUS trough
that doesn`t have an opportunity to phase with any northern stream
energy digging southeastward from Hudson Bay in time to pull the
secondary low closer to our coastline.

Thursday through Sunday...Broad high pressure builds across the
southeastern CONUS late in the week before gradually moving off the
Southeast coast next weekend. Our region will initially be situated
in westerly flow on the northern periphery of the high that
eventually becomes more southerly as the high starts to move
offshore, resulting in a warm up that starts Friday and continues
into the weekend. The high should keep us dry during this portion of
the long term.

&&

.AVIATION /02Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.

Tonight...VFR through most of the night, with MVFR CIGS expected to
develop late for ACY/MIV toward daybreak Monday. N winds 5 to 10,
becoming NE overnight. High confidence.

Monday...MFR CIGS expected to develop and spread inland through the
morning. A chance of flurries is possible from RDG-PHL-ACY
southward, but not included in TAFS at this time. Northeast winds
around 5-10 knots. Winds may gust in the mid to upper teens,
especially for ACY. Moderate to high confidence.

Outlook...
Monday night...A chance of snow overnight for KRDG and KABE. If
the snow materialize, MVFR or even IFR conditions would be
possible. For I-95 and coastal terminals, stratus could lead to
MVFR CIGs.

Tuesday through Wednesday...Snow likely on Tuesday for KRDG/KABE.
Chances for snow expand to I-95 terminals late in the day Tuesday
and Tuesday night though confidence in timing and coverage of snow
decreases the farther S/E you go. Reductions in CIGs and VSBYs would
accompany snow.

Wednesday night...VFR with snow likely remaining offshore. However,
a slight shift closer to the coast could bring a period of light
snow to KACY/KMIV. Breezy NW winds.

Thursday through Friday...VFR.

&&

.MARINE...
A Small Craft Advisory remains in effect for the Atlantic
coastal waters tonight through Monday. Winds will will drop off
below 20 knots for a period, but are expected to increase to 25
knots later in the night into Monday. Seas may also drop below
5 feet for a period as well, but will range between 4 and 6
feet, and likely increase again late tonight through Monday. So
the Small Craft Advisory will remain in effect for the Atlantic
coastal waters. For Delaware Bay, conditions are expected to
remain below advisory levels tonight through Monday.

Freezing Spray...
Ice accretion rates are expected to increase again across the
lower Delaware Bay tonight. Based on current observations, up
to 0.1 inch per hour rates were observed so far this evening
at CMAN4 and LWSD1. I expect these rates to increase, as the
winds remain elevated out of the northwest, with our nomogram
indicating rates AOA 0.3"/hour between 1Z and 15Z. Therefore,
have issued a Freezing Spray Advisory for the lower DE Bay
for this time frame. Across the upper DE Bay, ice accretion
rates are decreasing as winds decrease, and expect this trend
to continue. So no advisory attm, but will continue to monitor.

Outlook...
Monday night...Winds should quickly weaken. We may need to
extend the SCA into Monday night if seas are slow to subside
below 5 ft.

Tuesday through Wednesday...Winds and seas below SCA criteria.

Wednesday night and Thursday...NW winds increase to 25-30 kt and
seas build to 4-7 ft. Light to moderate freezing spray is expected.

Thursday night and Friday...No marine headlines are anticipated.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EST Monday for ANZ450>455.
     Freezing Spray Advisory from 1 AM to 10 AM EST Monday for
     ANZ431.

&&

$$
Synopsis...Klein
Near Term...Robertson/Franck
Short Term...Robertson
Long Term...Klein
Aviation...Klein/Robertson/Franck
Marine...Franck/Klein/Robertson



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