Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 180834

Area Forecast Discussion
National Weather Service Mount Holly NJ
434 AM EDT Sun Mar 18 2018

High pressure currently over the Midwest will gradually build to the
south and east on Sunday, and will move off the Mid- Atlantic coast
sometime Monday or Monday evening. Canadian high pressure then re-
establishes itself north of the Great Lakes. Meanwhile, low pressure
will move east and emerge over the Mid-Atlantic Monday night. It
then intensifies as it moves out to sea on Tuesday. Another low may
form over the Mid-Atlantic Wednesday or Wednesday night. High
pressure returns from the north to close out the work week. Low
pressure then approaches next weekend.


Quiet, dry weather persists through the near term. The tail end
of a backdoor cold front will push south into the area
overnight but really won`t have much influence on our
temperatures for Sunday as high pressure moves in bringing
sunshine with highs generally reaching the upper 40s to low 50s,
except a bit cooler over NW NJ and the southern Poconos. NW
winds will generally be around 10 MPH or so.


High pressure maintains control over the area through this
period with mainly clear skies leading to another cool night as
lows will mainly be in the upper 20s to low 30s, except colder
across the far north. For Monday, low pressure begins to
approach as it moves east from the southern plain states towards
Tennessee. This will result in mainly clear skies to start the
day giving way to increasing mid and high clouds during the
afternoon, mainly across the southern half of the forecast area.
However, any precip will hold off through the day. Highs will
be mainly in the mid to upper 40s except upper 30s to low 40s
across the southern Poconos and NW NJ.


A very complex setup for the midweek event with multiple waves of
low pressure impacting the Mid-Atlantic region. It may be best
to break the event down into two parts:

Part (1):  Monday night-Tuesday
A primary low west of the central Appalachians transfers its
energy to a coastal low that develops near Norfolk, VA. With the
upper air pattern relative flat, the low should quickly
progress to the E-NE and out to sea. Cyclonic flow ahead of the
850 mb low will induce strong isentropic lift along and north of
the frontal boundary that should aid in precip expanding
northeastward into Delmarva, SE PA and S NJ late Monday night
into Tuesday morning. There will likely be a very sharp cutoff
in the precip on the northern side with drier air originating
from the high centered over Ontario draining southward and with
a relatively zonal steering flow pattern preventing the deeper
lift from expanding much poleward. Significant differences in
the models regarding how far north the steadier precip advances
and where this sharp cutoff of the northern edge sets up. The
typical model biases are evident in the latest runs with the NAM
and SREF wetter/farther north with the precip (heaviest QPF of
1/2-1" falling along and south of the PA Turnpike/I-195
corridors, then amounts dropping off quickly along and north of
I-78) while the GFS is suppressed to the south (QPF amounts of
1/2-1" confined to the S half of DE and adjacent E MD with
amounts quickly tapering off farther north toward the Mason-
Dixon line). Note, there is significant spread among the 00Z
GEFS with several members supporting closer to the NAM. The 00Z
ECMWF was a good middle ground solution between the NCEP
operational models and was blended with QPF guidance from WPC
as well as the NAM (which I think is catching on to the potential
for strong F-gen forcing north of the 850-700 mb baroclinic
zone Tuesday morning).

Despite the above kinematic and moisture (QPF) differences among the
models, guidance is in better agreement with thermal fields, which
gives us higher confidence on ptype. The one caveat with the above
statement is the placement/intensity of mesoscale lift will
influence ptype as dynamical cooling will allow for rain/mix to
changeover to wet snow where areas of strong upward motion and heavy
precip occurs. Thermal profiles from the 00Z operational NAM, GFS,
GEM, and ECWMF generally indicate predominately snow late Monday
night and Tuesday from roughly the Mason-Dixon line northward,
mainly rain across southern DE and a rain/snow/sleet mix in
between. The official snowfall forecast calls for a swath of
2-3 inches of accumulation within 30 miles on either side of
the Mason-Dixon line from late Monday night-Tuesday morning.

Since just about all operational models but the GEM show a break in
the precip late in the day Tuesday and Tuesday evening, we will
treat (1) and (2) as separate events with headlines focused on the

Part (2):  Tuesday night-Wednesday
The next wave of low pressure developing on the lee side of the
southern Appalachians will deepen as it moves off the coast
somewhere near the SC-NC coastal border. The 00Z models show
varying degrees of phasing with southern and northern stream
shortwave disturbances, resulting in amplification of the
upstream trough over the Ohio Valley-Mid South regions. The
extent of the phasing and how quickly it occurs will determine
if this second coastal low heads out to sea, passing well to our
S/E (only fringe impacts near the coast) or turns up the coast
(more significant impacts, including strong winds and heavy wet
snow). With support for both of these polar opposite solutions
in the ensemble systems and with high uncertainty in the
forecast, it`s still premature to rule out either scenario for
the second part of this event (i.e., a complete miss or a high-
impact nor`easter). We opted to take a conservative forecast
approach that was somewhere in the middle (closer to the 00Z
ECMWF and WPC guidance).

Note, the official storm total snowfall and probabilistic snowfall
graphics on our winter web page includes the first round (1) of the
event and only a portion of round (2) since it goes out through 8 AM
Wednesday morning. The probabilistic snowfall products probably have
more value than a deterministic forecast at this point because of
the uncertainty in the forecast. If the high-end snowfall scenario
were to verify, there is a potential for extensive power outages
once again given the damage left behind from the previous
nor`easters this month.

High pressure builds in behind the storm later Thursday into Friday.
Cold (5-10 degrees below normal) and dry conditions would be
expected in this pattern.

Yet another storm system could impact the region next next weekend.
There has not yet been a signal for coastal redevelopment with this
system, so the rain/snow line would be determined by the track of
the primary low, which looks to move in from the Ohio Valley


The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Through 12z...VFR with light and variable winds.

Sunday...VFR continuing. Northwest winds 5-10 knots, gusts may
reach 15-20 knots in the afternoon.

Sunday night...VFR with with diminishing winds.


Monday...VFR. High confidence.

Monday night and Tuesday...Onset of precip still looks to be
late evening over the Delmarva as rain. By late Monday night
and Tuesday morning, widespread MVFR/IFR conditions likely from
Phila terminals southward. There is high uncertainty as you go
farther north toward ABE regarding if snow will make it that
far north with a sharp cutoff in precip expected. For MIV and
ACY, a considerable amount of mixing of rain, snow and perhaps
sleet is likely while snow is favored farther northward toward
PHL. NE winds 10-20 kt with gusts 25-35 kt (higher end of the
range near the coast). Moderate confidence.

Tuesday night through Wednesday night...Additional precip
possible with continued MVFR/IFR conditions. Gusty N-NE winds
expected, especially near the coast. Low confidence.

Thursday...VFR. NW winds gradually weaken. High confidence.


Sub SCA conditions with fair weather through Sunday and Sunday
night. West to northwest winds will generally be around 10 to
15 knots with seas 2 to 3 feet over the ocean waters and 1 to 2
feet on the Delaware Bay.


Monday...Sub SCA conditions persisting. NW winds shifting to
east at around 10 knots by late day.

Monday night and Tuesday...A Gale Watch was issued from late
Monday night through Tuesday afternoon for the coastal waters
excluding the northern coastal waters of NJ (ANZ450). Confidence
in winds reaching gale force increasing considerably as you head
south toward the southern NJ and DE coastal waters. Gusts in
excess of 40 kt are possible at its peak on Tuesday.

Tuesday night and Wednesday...The potential for gales continue
Tuesday night into Wednesday but it all depends on how close a
second coastal low tracks to our area and how fast it deepens.

Wednesday night and Thursday...SCA conditions likely.


Relative humidity values on Sunday will drop into the 20s
again, but winds are expected to be less than Saturday. Even
though fuels have been drying, they are expected to remain
above critical levels as well. No enhanced statements are
expected at this time.


MARINE...Gale Watch from late Monday night through Tuesday afternoon
     for ANZ451>455.


Near Term...Fitzsimmons
Short Term...Fitzsimmons
Long Term...Klein
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