Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS61 KPHI 181353

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

High pressure currently over the Midwest will gradually build to
the south and east today, and then move off the Mid- Atlantic
coast tonight into Monday. A weak area of low pressure will push
a dry back door cold front across the area tonight. Canadian
high pressure then re- establishes itself north of the Great
Lakes. Meanwhile, a complex area of low pressure will move east
emerging over the Mid-Atlantic Monday night into Tuesday. This
system may linger off the coast through Wednesday before moving
off to the north and east by Thursday. High pressure returns
from the north to close out the work week. Low pressure then
approaches next weekend.


No changes to the overall forecast, just a few hourly temperaure
dew point adjustements.

A backdoor cold front remains stalled out across the area and
it`s main influence on the area is lower dewpoints north of the
boundary. Otherwise, quiet, dry weather persists through the
near term. Temperatures for today will generally reach 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, with occasional gusts near 20 mph.


High pressure builds to the south of the area through tonight
as a weak low area of low pressure pushes a backdoor cold front
across the area. Even with this front, mainly clear skies are
expected, 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 ECMWF 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.

Today...VFR continuing. Northwest winds 5-10 knots, gusts may
reach 15-20 knots at times.

Tonight...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 range
near the coast). Moderate confidence.

Tuesday night through Wednesday night...Additional precip is
possible, mainly snow (except along the coast). Confidence in
precip occurring is moderate near the coast and decreases
farther inland. MVFR or IFR conditions would be possible if
snow reaches the terminals but confidence is low at this point.
Gusty N-NE winds expected, especially near the coast.

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


Sub-SCA conditions with fair weather through tonight. 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 increases considerably as you head
south toward the southern NJ and DE coastal waters. Gusts around
40 kt are possible. After accounting for a low-bias that the
GFS-based WaveWatch guidance typically exhibits in NEly gales,
wave heights in our southern coastal waters are forecast to
build to 8-12 ft by late Tuesday.

Tuesday night and Wednesday...The potential for gales exists
but it speeds will depend on how close a second coastal low
tracks to our area and how fast it deepens which is still
uncertain. Adjusted wave heights 2 to 3 feet above WaveWatch
guidance with the potential for northeasterly gales to continue.

Wednesday night and Thursday...SCA conditions likely.


Relative humidity values today 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.


An extended period of strong northeasterly winds are expected
to occur from late Monday night through Wednesday with two
coastal storms tracking south and east of the region. Positive
tidal anomalies will increase with each successive high tide as
water piles up along the coast. The threat for coastal flooding
looks to develop as early as the Tuesday evening high tide along
the NJ-DE coast and continue through the Wednesday night high

The degree of coastal flooding will depend on the track and
strength of both coastal lows, which is still uncertain
especially with with the second one Tuesday night into
Wednesday. ETSS, ESTOFS and other tidal prediction guidance from
Stevens Flood Advisory System indicate minor to low-end moderate
coastal flooding is most likely.


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


Near Term...Fitzsimmons/Robertson
Short Term...Fitzsimmons
Long Term...Klein
Fire Weather...
Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.