Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 170505

Area Forecast Discussion
National Weather Service Mount Holly NJ
105 AM EDT Sat Mar 17 2018

Low pressure remains over the Canadian Maritimes through
Saturday, then weakens as it moves eastward. Meanwhile, high
pressure slowly builds to our south and west tonight and
Saturday. A weak area of low pressure will quickly slide to our
south Saturday, then high pressure arrives for Sunday. Low
pressure in the Central Plains late Sunday and Monday tracks
eastward, resulting in a coastal storm for Tuesday and Wednesday
which then moves northeastward to off the New England coast
during Thursday. High pressure should start building in during

High pressure will extend from the Great Lakes to the Middle
Atlantic coast overnight. As a result, we are expecting a mostly
clear sky with perhaps some stratocumulus lingering over parts
of the Pocono Region and far northern New Jersey.

The west northwest wind is forecast to diminish. Minimum
temperatures are anticipated to favor the 20s in our region.

High pressure over the Mid-Atlantic moves out to sea Saturday
morning, and another area of high pressure builds into the Great
Lakes. Meanwhile, a weak wave of low pressure will approach the
Mason-Dixon line and then pass off the Mid-Atlantic coast
during the day. Most models have some kind of precip developing
over southern portions of the Delmarva Saturday afternoon.

GFS is the most robust in terms of QPF with up to 0.20" inches,
and most other models are generally at or under 0.10" across
southern portions of eastern MD and southern DE.

With the cold airmass moving offshore, temperatures warm a bit
compared with today, and will generally top off in the mid to
upper 40s, and in the upper 30s to low 40s in the Poconos and
northwest NJ.

Summary...Less wind on Sunday however colder conditions compare
to average look to continue. Coastal storm development looks
likely next week, however the details/impacts are still

Synoptic Overview...An expansive upper-level trough in eastern
Canada and the eastern U.S. is forecast to shift a little more
northeastward through Sunday. A potent short wave/closed low is
forecast to slide east from the Central Plains Monday, then as
stronger energy drops southeastward from southern Canada a
larger trough is forecast to develop into the East. Surface low
pressure is forecast to be associated with the first impulse,
which then redevelops off the Mid-Atlantic coast. The model
guidance is in good agreement of a storm off the East Coast,
however the track and therefore impacts are less certain. The
placement will depend on the track of the short waves and their
phase/amplification. The evolution aloft and the surface low
track will be crucial in determining precipitation types/amounts
and rates and therefore any impacts. Despite the uncertainty
with the details, there continues to be a strong signal of a
storm next week although the departure of the storm has slowed.

For Saturday night and Sunday...A weakening short wave moving
through a confluence area in the Mid-Atlantic to start Saturday
night quickly shifts to the east. As a result, any light rain
across portions of southern Delmarva ends very early and clouds
decrease. Otherwise, the persistent trough aloft in the East is
forecast to shift a bit farther northeastward during Sunday.
This will allow weak high pressure at the surface to build into
our area, resulting in less wind and more sunshine Sunday.

For Monday...High pressure centered near south-central Canada
is forecast to extend southeastward over our area to start
Monday, which should setup a cold air damming scenario and
potentially deliver drier low-level air. Significant energy
ejecting eastward from the Central Plains will drive surface low
pressure into the lower Ohio Valley by late Monday. Clouds are
expected to increase mainly at night and some large scale
forcing may start to arrive toward daybreak Tuesday in our
southern areas. As a result, we maintained some slight increase
in PoPs later Monday night mainly across our southern areas.

For Tuesday through Thursday...As significant energy rolls
eastward, the upper-level trough is forecast to amplify quite a
bit into the East Tuesday and Wednesday as the northern and
southern streams phase. The complexity of this involves
interaction between the southern and northern streams as well as
relative small scale short waves in both streams. All of this
energy results in low pressure redevelopment off the Mid-
Atlantic coast. While the details and therefore impacts are
still uncertain, there continues to be a strong storm signal for
this time frame. The ensemble guidance still shows the
variability regarding the overall evolution/track of surface low
pressure. We continued to follow closer to WPC which indicates
a secondary low development in the Carolinas early Tuesday
morning. The slowing of the storms departure looks to be due to
some more blocking in place along with a more sharpening of the
trough aloft. At least some guidance has a more strung out
surface low and this may be due to all of the energy involved,
therefore having trouble focusing on where the main surface low

The entire evolution, strength and track of the features at the
surface and aloft will impact the thermal fields and thus
precipitation types, amounts and rates (embedded stronger
forcing). Given some timing uncertainty with the details, we did
not go higher than likely PoPs at this point and did continue
with higher chance PoPs through Wednesday night, then PoPs lower
for Thursday. The hazards that need to be ironed out are, heavy
precipitation (rain and/or snow) and flooding, strong winds and
coastal flooding (due to potentially long duration east to
northeast winds). We used a model blend of partial thicknesses
to derive the precipitation types, however this is highly
subject to change. The system should be moving to a position off
the New England coast to start Thursday, with improving
conditions across our area.

For Friday...As a strong upper-level trough shifts eastward,
short wave ridging is forecast to approach from the west. This
will drive surface high pressure into our area during Friday
with no precipitation currently anticipated along with a
continuation of chilly conditions.


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

Overnight...VFR with a mostly clear sky. West to northwest wind
around 5 to 10 knots.

Saturday...VFR with increasing clouds. Cloud bases are expected
to be mainly above 10,000 feet. West to northwest wind around 8
to 12 knots with gusts of 15 to 20 knots.

Saturday night and Sunday...VFR. North-northwest winds less
than 10 knots, becoming west-northwest Sunday.

Monday...Clouds increase mainly at night, with possible
MVFR/IFR conditions developing toward daybreak Tuesday if
precipitation arrives. Low confidence regarding any sub-VFR

Tuesday and Wednesday...Widespread MVFR/IFR conditions with
rain/snow and potentially gusty east-northeast winds. Low
confidence continues with the details.



1 AM Update...Small Craft Advisory has been allowed to expire
for the northern Delaware Bay as winds have diminished below SCA

Previous Discussion...

Small Craft Advisories remain in effect for tonight for the
ocean waters for winds continuing to gust to 25-30 kt. Seas
remain elevated at 4-6 feet as well.

Wind gusts on upper DE Bay will diminish to sub-SCA criteria
around midnight, and conditions subside to sub-SCA criteria on
DE ocean waters and lower DE Bay by daybreak Saturday.

Wind gusts may briefly subside to sub-SCA conditions on NJ
ocean waters Saturday morning, but pick up in the afternoon back
to 25 kt or so. Will run SCA through the day Saturday, despite
a potential lull in SCA conditions in the morning for the NJ
ocean waters.

Saturday night and Sunday...The conditions should be mostly
below Small Craft Advisory criteria, however a northerly wind
surge with gusts 20-25 knots may briefly occur Saturday night.

Monday...The conditions are anticipated to be below Small Craft
Advisory criteria, although winds should start to increase from
an easterly direction later Monday night.

Tuesday and Wednesday...Winds and seas increase as a coastal
storm tracks off the coast. The magnitude of the winds and seas
will depend on the track and strength of the coastal storm,
however there is an increased chance for a period of gale force
wind gusts.


MARINE...Small Craft Advisory until 6 PM EDT this evening for
     Small Craft Advisory until 6 AM EDT early this morning for


Near Term...Fitzsimmons/Iovino
Short Term...MPS
Long Term...Gorse
Marine...Fitzsimmons/Gorse/Iovino/MPS is the U.S. government's official web portal to all federal, state and local government web resources and services.