Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 211736
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1236 PM EST Sat Jan 21 2017

.SYNOPSIS...
Weak high pressure will build across our region today. A strong
and complex area of low pressure will impact the Mid-Atlantic
region Sunday through Tuesday. Multiple waves of low pressure
will likely track far enough to our north to keep conditions dry
for the second half of next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The dense fog has mostly dissipated except one or two spots that
have thicker fog and light fog/mist remains across the
remainder of the area, along with low clouds. The exception to
this is the higher elevations of the Poconos and northern New
Jersey where these spots may be above the cloud/fog top level.
Some clearing has occurred there, and may spread some during the
day, but we are losing daylight hours for heating to take place
and contribute to more mixing. So we do not expect widespread
improvement.

Temperatures where these breaks have/could occur were raised a
few degrees, while the rest of the area remains pretty close to
the previous forecast. Still everyone should be a few degrees
above normal.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
The aforementioned high pres moves off the cst tonight.  Then, a
complex area of low pres over the srn plains and sern CONUS begins
to move twd the region. By late tonight/erly Sun mrng, the guid
begins to diverges.  The GFS and NAM are dry thru the end of the pd.
However, the ECMWF does bring some light precip to the Delmarva and
srn NJ by daybreak Sun. The CMC and SREF bring it even further n
sooner.  For now, will just bring some low chc precip to the srn
areas, with the caveat that it cud be further n and pops may need to
be raised. Nevertheless, qpf shud be light during this time.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
The main focus in the long term is the strong coastal low this is
expected to impact the area late this weekend into early next week.
Overall models from last night`s 00Z runs have come into better
agreement with the track and intensity of the low, but continue to
offer a variety of solutions (which is not much of a surprise given
the complexity of the forecast with so many different players
involved). Accordingly, there is high confidence that the forecast
area will see some impacts from this storm in terms of soaking
rain, strong winds, tidal flooding and beach erosion. However,
uncertainty regarding the timing and magnitude of the above impacts,
as well as the possibility of wintry precip to mix in across far
northern zones, remains until the smaller- scale details are able
to be resolved.

Here is a break down of impacts:

Rainfall...
The faster envelope of guidance indicates widespread rain for most
of the area Sunday. A few of these faster solutions, including the
00Z Canadian, are pronounced with coastal cyclogenesis to our south
along a warm front early on in the event. The coastal low, which
would be located well ahead of the primary low that would likely
still be located back over the Mid South, would result in a round of
heavy rain (0.75-1.5) on Sunday, north of the warm front across
Delmarva and southern NJ. On the other end of the spectrum, some of
the slower solutions, including the 00Z GFS and NAM, hold off the
onset of the steadier rain until Sunday afternoon for Delmarva
region and Sunday night farther north across eastern PA and much of
NJ. PoPs for Sunday were not increase with the early morning
forecast update and were even adjusted downward across the northern
half of the CWA.

PoPs were raised to 100 percent Sunday night into Monday with
periods of rain expected. Although subject to change, we are
currently projecting the a greatest potential for heavy rainfall to
be Sunday evening-Monday morning from the DE Bay region southward
(Delmarva and far southern NJ), late Sunday night-Monday afternoon
farther north across southeastern PA/central NJ, and Monday morning-
Monday evening for northeastern PA and northwestern NJ. This is when
the warm front moves northward through the region and strong
lift/moisture transport off the Atlantic is aided by 65-75 kt
easterly winds with a low-level jet streak.

There is a potential for precipitation to linger into Monday night
and even into part of Tuesday as the closed upper low passes
overhead and the primary surface low transfers its energy to a
secondary coastal low off the Delmarva coast. Models show light QPF
during this part of the forecast but with a well-defined TROWAL
overhead, cannot rule out a continuation of steadier/heavier rain.

We are still thinking storm-total rainfall amounts will generally be
between 1-3. Locally higher amounts of 4+ are possible, which
would lead to more significant flooding issues if it were to
occur. Conversely, rainfall amounts may be under an inch in some
areas if the dry slot from this system is quicker to advance
northward and shut off precip later on Monday. Additional
discussion about the potential for inland flooding are included
in the hydrology section below.

Winds...
The strongest winds with this storm are most likely to occur
sometime overnight Sunday night into Monday afternoon. A tight
pressure gradient is forecast to develop during this time as the low
deepens near the central Appalachians and high pressure tries to
build southward from eastern Canada. A High Wind Watch was issued
for eastern NJ where there is a potential for easterly wind gusts of
50-60 mph. Confidence in high wind criteria being met is along the
coast. Wind advisories may be needed farther inland toward the I-95
corridor.

Wintry Precipitation...
There is a potential for freezing rain in Carbon and Monroe Counties
in PA and Sussex County, NJ on Monday and Monday night as colder air
in the boundary layer drains southward in response to Canadian high
pressure trying to build southward. The current thinking is ice
accretion will be confined to the highest elevations (above 1500 ft)
in these counties. Forecast temperatures weighted more toward the
colder NAM/ECMWF/Canadian solutions, which yield surface
temperatures around 32F (and wet-bulb temperatures not much lower)
along the higher elevation ridges. These marginally cold
temperatures and higher precip rates are not particularly favorable
for a significant icing event. Ice accumulations were capped at one-
tenth inch.

Precip may transition to snow or sleet before ending on Tuesday as
colder air wraps around the backside of H8 low. Will likely need
deep lift to dynamically cool the column sufficiently for a change
over to snow. Low confidence in this happening at this time since
the low is forecast to weaken on Tuesday as it transfers its energy
offshore.

Coastal Flooding/Beach Erosion...
See the tides/coastal flooding section for more details.


Tuesday night through Friday...
Multiple waves of low pressure will likely track far enough to
our north to keep most of the area dry. The first system will
track northeastward through the Great Lakes and into
southeastern Canada on Wednesday. W-NW flow behind this system
will gradually draw colder air into the region. We may see lake
effect snow/rain showers reach the Poconos Wednesday night
through Friday.

&&

.AVIATION /17Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

The dense fog has dissipated across the TAF sites, but low CIGS
and light fog/mist remain across area. We no longer expect any
improvements, and should keep IFR conditions through the period.

Areas of dense fog will likely develop again tonight, but we`ve
only included 1/2SM in the TAFS at this time as confidence of
1/4SM or lower affecting any individual TAF locations isn`t
strong.

Winds are generally west to southwest around 5 knots or less
today and will become light and variable/calm overnight tonight.

OUTLOOK...

Sunday...Rain spreads in from S to N. Still some uncertainty in the
timing of the rain, but once it arrives, conditions should quickly
deteriorate to IFR. E winds increasing to 10-15 KT.

Sunday night through Monday night...IFR/LIFR likely with periods of
rain. Also, E winds continue to strengthen to 15-20 KT with 25-30 KT
gusts at ABE/RDG, 25-30 KT with 35-40 KT along I-95 terminals, and
the potential for 40-50 kt gusts near ACY late Sunday night through
Monday afternoon.

Tuesday...Lingering MVFR/IFR conditions with showers for at least
the morning. Gradually improving conditions from SW to NE later in
the day.

Tuesday night and Wednesday...VFR. NW winds 10-15 kt with gusts to
20 kt.

&&

.MARINE...
The Dense Fog Advisory has been extended through Sunday morning.
Visible satellite shows the fog expanding eastward, so any fog
over the waters will likely have a hard time dissipating today
or tonight. Otherwise, a quiet day on the waters as seas will
genly be in the 2 to 3 foot range and winds will genly be 10 kts
or less.

OUTLOOK...

Sunday morning...Winds and seas below SCA.

Sunday afternoon through Monday night...Easterly flow quickly ramps
up late in the day Sunday into Sunday night. Expect gale force gusts
to develop overnight Sunday night and storm-force gusts in the in
our northern coastal waters by early Monday morning. The Storm Watch
was expanded southward to include all of the coastal waters of NJ.
The strongest winds are expected Monday morning across the southern
coastal waters and DE Bay and prolonged into Monday afternoon for
the northern coastal waters.

Tuesday...Lingering SCA conditions possible.

Wednesday...Winds and seas below SCA criteria.

&&

.HYDROLOGY...
There is a potential for hydro impacts from the upcoming storm
system Sunday through Tuesday. In short, we don`t think there
will be any river flooding unless basin-wide rainfall averages
climb above three inches. The most sensitive responding rivers
still appear to be the Millstone and the Rancocas.

The entire region has some positive things working in its
favor...precipitation has been below normal the last 30 days,
there`s really no consequential snow cover to speak of, there`s
no ice on area waterways, streamflows are either running at or
below normal as is soil moisture, and last but not least, the
ground isn`t frozen. We think all the above will help mitigate
the flood threat.

We feel the latest MMEFS runs are running a bit hot. They show
some impacts with about 2.50 inches of rainfall. But taking into
account the above mentioned items and the fact that the rains
will fall over a 24 to 36 period, we feel more rain will be
needed for river flooding. MARFC in-house contingency runs agree
with this.

As of now, it appears poor drainage or nuisance flooding is
more likely, not flash flooding or river flooding with storm-
total rainfall amounts will generally between 1-3. There
would be more concern for flooding if we start seeing amounts
greater than 3 inches. With some of the wetter outlier model
solutions showing localized amounts near 4, it is still too
soon to rule out more significant flooding.

&&

.TIDES/COASTAL FLOODING...
It continues to appear as though an onshore flow will develop
on Sunday, then it should strengthen on Sunday night. The
onshore flow is forecast to be strong on Monday morning and it
may begin to weaken gradually from south to north along the
coasts of Delaware and New Jersey on Monday afternoon and Monday
evening as low pressure approaches from the southwest and the
pressure gradient relaxes.

We will continue to monitor the two high tide cycles on Monday.
However, the astronomical tides are rather low at that time so
we are anticipating mostly minor flooding. Any flooding may be
enhanced by the wave action. Breaking waves are forecast to be
in the 5 to 8 foot range. There is the potential for localized
moderate flooding, especially from Long Beach Island up to
Raritan Bay, based on the latest guidance.

The first high tide on Monday occurs in the early morning hours
along the oceanfront and it would require a surge of about 2.0
to 2.5 feet to reach the minor flooding threshold. The second
high tide which occurs in the late afternoon along the
oceanfront would require a surge of about 3.0 feet to reach the
minor flooding threshold.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...High Wind Watch from late Sunday night through Monday evening
     for NJZ012>014-020-026.
     High Wind Watch from late Sunday night through Monday
     afternoon for NJZ022>025-027.
DE...None.
MD...None.
MARINE...Dense Fog Advisory until 10 AM EST Sunday for ANZ430-431-
     450>455.
     Storm Watch from late Sunday night through Monday afternoon
     for ANZ452-453.
     Storm Watch from late Sunday night through Monday evening for
     ANZ450-451.
     Gale Watch from late Sunday night through Monday afternoon for
     ANZ430-431-454-455.

&&

$$

Synopsis...Klein
Near Term...Nierenberg/Robertson
Short Term...Nierenberg
Long Term...Klein
Aviation...Klein/Nierenberg/Robertson
Marine...Klein/Nierenberg/Robertson
Hydrology...Kruzdlo/Klein
Tides/Coastal Flooding...Iovino/Klein



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