Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 171009

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

Low pressure will remain cutoff over the Canadian Maritimes this
weekend. Meanwhile, an area of low pressure will pass to our south
today, then high pressure builds in from the north and west tonight
and Sunday. A weak cold front progresses southward through the
region Sunday night. An area of low pressure tracking across the Mid
South on Monday will move eastward along the Virginia-North Carolina
border Monday night. This low is expected to deepen as it moves
northeastward off the Mid-Atlantic coast on Tuesday. Another wave of
low pressure may develop along the slow-moving frontal boundary,
passing south and east of the region on Wednesday. High pressure
should start to build in Thursday and Friday.

A low pressure system and an associated upper level trough continues
to linger over Atlantic Canada keeping the area in a broad cyclonic
flow and this will continue to be the trend through today. Most of
the region will see a dry day however there will be a wave sliding
by just to our south and this still looks to potentially clip
southern parts of the Delmarva with a period of light rain this
afternoon...mainly affecting areas south of Dover, DE. Farther north
through SE PA and NJ, morning sunshine will give to filtered sun by
the afternoon as high clouds increase due to the aformentioned
system. Winds will be a little breezy but not as windy as
Friday. Afternoon highs will be mainly in the 40s except some
upper 30s across the southern Poconos.

Weak wave to our south quickly moves offshore through the evening
with an otherwise mainly clear and chilly night on tap as a NW flow
continues to dominate. Lows will be mainly in the 20s except around
30 over much of the Delmarva as well as the Philadelphia metro
area and upper teens over the southern Poconos.

The focus in the long-term period continues to be on the midweek
storm. A positive note is there has been a clear trend in the
guidance over the past couple of cycles, including last night`s 00Z
run, toward a more progressive solution that would favor a
suppressed storm track for the first wave of low pressure Monday
night-Tuesday and even more notably for the second wave Wednesday-
Wednesday night. If these trends were to hold up, the extent,
duration and severity of impacts would be much lower than what
models were indicating 1-2 days ago. Given this storm is still 3-5
days out and the predictability skill for such a complex setup (with
a parade of disturbances in play along with the potential for
phasing to occur) is limited, it`s a bit premature to rule out a
shift back in the other direction that would put us back in play for
a long duration, higher-impact event from late Monday night
through Wednesday night. The bottom line is the poor run-to-
run continuity and lingering spread among the operational models
and the individual ensemble forecast systems hinders our
ability to provide specific details with high certainty, on
things such as rain and snow accumulations, where the rain/snow
line sets up and how it evolves over time, and the magnitude of
winds/beach erosion/coastal flooding.

Keeping the abovementioned caveats in mind, a huge motivating factor
for cautiously maintaining our message for a potentially high-impact
event is the vulnerability of our infrastructure following the
recent Nor`easters this month:  Trees have been weakened/damaged,
restoration efforts are still ongoing, and soils in E PA and NJ are
still very saturated. This all makes our region more susceptible
than usual to another round of power outages and flooding IF a
worst-case scenario would come to we don`t want
to let our guard down too soon.

Concerning the rain vs. snow aspect of this storm... The track of
the primary surface low will likely be south of our region (along
the VA-NC border) while a secondary coastal low develops near
Norfolk and tracks northeastward over or nearby the Gulf stream.
Synoptically, this would be a favorable pattern for snow across the
region. However, thermal profiles look to be marginally supportive
of accumulating snow (more likely a rain-snow-sleet mix) at least
initially, especially in the I-95 corridor and coastal plain, given
the lack of cold air from a storm system of Pac NW origin and the
influences of a high sun angle in late March. A transition to wet
snow would be possible at some point as (1) Canadian high pressure
building eastward allows cold air to drain southward into the region
and (2) the coastal low deepens off the Mid-Atlantic coast while the
upper low reaches our longitude. The latter would favor strong
dynamical cooling although it is highly conditional on deep ascent
and heavier precip rates setting up over our region- which is
becoming increasingly doubtful given the latest trends.

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.


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

Today...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.

Tonight...VFR. NW winds less than 10 knots.


Sunday and Monday...VFR. NW winds of 10 kt or less on Sunday may
briefly back out of the W-SW ahead of a surface trough Sunday
evening, then become N-NE late Sunday night-Monday morning. High
forecast confidence.

Monday night and Tuesday...Onset of precip and MVFR/IFR conditions
likely to occur late sometime Monday night or Tuesday morning. At
this point, a wintry mix (rain, snow, sleet) would be favored for I-
95 terminals, rain farther S/E (ACY, MIV) and snow or dry conditions
farther N/W (ABE/RDG). Confidence on ptype and amounts of each
ptype are still low at this point. NE winds increase and become
very gusty with the strongest winds expected near the coast (e.g.,
ACY) on Tuesday.

Tuesday night through Wednesday night...Following the first part of
the event Monday night-Tuesday, a second wave of low pressure could
prolong the precip, IFR conditions and gusty NE winds. However,
there is less support for this scenario from the latest model
guidance with the potential for drying conditions, especially
along and north of PHL. Forecast confidence is low.


Winds should diminish below SCA levels by morning over the southern
Delaware Bay as well as DE coastal waters so SCA expires at 6 AM for
these areas. Otherwise, expect marginal SCA conditions to continue
through the day for the NJ coastal waters. The winds here could
briefly subside below SCA levels this morning before increasing
again this afternoon.

Heading into tonight, winds should diminish below SCA levels for all
areas by early this evening so SCA over the NJ waters ends at 6


Sunday and Monday...Winds and seas below SCA criteria. A W-NW wind
direction on Sunday becomes N-NE Sunday night and eventually E on

Monday night through Wednesday...NEly winds should quickly
strengthen late Monday night and Tuesday to gale force while seas
build as a low pressure organizes to our south. NEly gales and
high seas will likely continue through Wednesday. A brief period
of storm force winds would be possible if the low deepens fast
enough but that is still highly uncertain at the point given the
southward trend with the storm track.


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


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