Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 150129

Area Forecast Discussion
National Weather Service Mount Holly NJ
929 PM EDT Wed Mar 14 2018

Low pressure will remain over the Canadian Maritimes through Saturday,
then weaken as it moves eastward. Meanwhile, high pressure will
extend southeastward from Canada across the Great Lakes and Ohio
Valley, maintaining a northwest flow across our region into the
weekend. Low pressure in the Central Plains later Sunday and Monday
tracks eastward, and this may result in another coastal storm in
the East Tuesday into early Wednesday.


For the 930 PM update, drier air is starting to work into the
area, but the main upper level shortwave trough is propagating
over our region now, allowing for a few more hours window of
opportunity for snow showers to continue across the southern
Poconos and NW NJ. For the most part, expect total accumulations
to be less than one inch, except in the the higher terrain of
the southern Poconos, where we`ve had a report of 3 inches so

500 mb trough lies along the Northeast U.S. coastline and will
gradually flatten out late tonight. Meanwhile, surface high pressure
over the Midwest will track east into the Gulf Coast states and high
pressure over central Canada will build to the south and will
approach the Northern Plains states.

The pressure gradient will decrease slightly overnight. That,
combined with decoupling will allow the gusts to decrease into
the late overnight hours.

Another cold night on tap with lows in the teens in the Poconos,
otherwise, in the mid 20s for most of NJ and southeast PA. For the I-
95 corridor, lows will be near 30s, and in the upper 20s in the


Upper trough continues to flatten out and lift to the northeast on
Thursday. Several shortwaves will continue to dive into the base of
the trough, and this may spark off some more rain and/or snow
showers Thursday afternoon. PoPs are low, and not expecting much in
the way of QPF or snowfall, but another quick 1/2"-1" is possible in
the Poconos.

Another relatively cold day with highs in the 30s in the Poconos,
otherwise, in the low to mid 40s for southeast PA and most of
northern NJ, and in the mid to upper 40s for southeast NJ and the
Delmarva. These temperatures are around 7 degrees below normal for
this time of year.


Summary...Chilly conditions continue, although brief warming may
occur over the weekend. Potential for coastal storm development in
the Tuesday to early Wednesday time frame, however the evolution
and track and therefore any impacts are uncertain at this time.

Synoptic Overview...An upper-level trough in the East delivering
colder air is forecast to relax some during the course of the
weekend, however an active pattern continues. A potentially potent
short wave or closed low is forecast to slide east from the Central
Plains Monday, then as stronger energy drops southeastward from
southern Canada a larger trough may begin to develop into the East.
Surface low pressure is forecast to be associated with the first
impulse, however the evolution of this feature offers more
uncertainty. It appears that if low pressure tracks farther south
then it continues as is to off the Mid-Atlantic coast later Tuesday
(colder solution). If it tracks farther north, this would offer more
secondary development just off the Mid-Atlantic coast with potential
for some warmer air getting drawn in for a time. This will also
depend on the extent of blocking upstream. The evolution aloft and
the surface low track will be crucial in determining precipitation
types/amounts and therefore any impacts. This is several days out,
therefore lots of opportunity for the guidance to change however the
signal continues to point toward a storm in the Tuesday time frame.

For Thursday night and Friday...Cyclonic flow continues as a strong
upper-level trough remains in place. Some strong short wave energy
is forecast to move through, although it looks to become more and
more channelized during Thursday night. Some instability driven
snow/rain showers may linger into Thursday evening before
dissipating. Friday should be a dry day, however with the presence
of the trough in place it will continue to be on the chilly side.
Low pressure sitting near the Canadian Maritimes combined with high
pressure extending southeastward from the Midwest will result in a
tight pressure gradient. This will result in winds increasing and
becoming gusty (to around 35 mph) during Friday as deeper mixing
becomes more established. The winds will then diminish Friday night
as mixing wanes and the pressure gradient relaxes some.

For Saturday and Sunday...While the overall upper-level trough
remains in place across the East during this time frame, it does
start to de-amplify though during Sunday. Before this though, a
short wave sliding east along the southern periphery of the main
trough aloft looks to produce a weak area of low pressure Saturday
which then scoots to our south. Some guidance however wants to bring
some mainly light precipitation to our southern areas during the day
Saturday. This may end up be pushed a bit farther south given the
cyclonic flow in place, therefore we opted to keep a dry forecast
for now. This energy should at least toss some cloudiness our way
for a time over the weekend especially the southern areas. Some
warming should occur, although a cold front shifting southeastward
may halt that for Sunday.

For Monday and Tuesday...High pressure should be extending over our
area to start Monday, however significant energy ejecting eastward
from the Central Plains will drive surface low pressure into the
lower Ohio Valley by later Monday. This will back the flow aloft
some in the East, however it appears that the surface low will
maintain an eastward track. Clouds are expected to increase and if
timing is correct, large scale forcing starts to arrive overnight
Monday with the increasing chance for some precipitation. The
forecast then becomes increasingly less certain especially
pertaining to the details.

As significant energy rolls eastward, the upper-level trough is
forecast to amplify into the East with the potential for some
phasing. This will take low pressure generally across the Mid-
Atlantic region then northeastward off the coast through late
Tuesday. The amount of short wave energy involved needs to be
resolved by the model guidance, which involves a more organized
system aloft undercutting and moving in tandem with a progressive
ridge moving through southern Canada to its north. While the details
and therefore any impacts are uncertain this far out, there
continues to be a storm signal for about the Tuesday time frame. The
ensemble data shows the variability with this, and whether it is one
main surface low or secondary development occurs as the primary low
reaches the Appalachians. We followed closer to WPC which indicates
some semblance of a secondary low development. The track of this
storm will depend on how much blocking is still in place upstream.
This of course will impact the thermal fields and therefore
precipitation types and amounts. We did bring likely PoPs a bit
farther north for now on Tuesday, then the system should be coming
to an end as we progress through Tuesday night.

For Wednesday...The continuation of an upper-level trough in place,
although the amplitude of this is a bit uncertain. Low pressure is
forecast to be well to our northeast Wednesday, and cyclonic flow
and lingering deeper moisture may result in some snow showers and
more clouds into northeastern Pennsylvania especially. Otherwise, a
potentially gusty northwest wind during the day.


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

Tonight...Mostly VFR conditions expected overnight. There remain
a few clouds around 2500 ft AGL near ABE, but expect these to
decrease through 06Z, so not anticipating MVFR conditions for
them at this time. Westerly winds will slowly decrease from
gusts up to 30 KT this evening, to around 10 KT by day break.

Thursday...VFR with BKN CIGs around 5000 ft in the afternoon. West
winds 10-15 kt with gusts up to 20 kt in the afternoon. ISO-SCT
rain/snow showers possible in the afternoon.

Thursday night and Friday...VFR. West-northwest winds diminishing to
10 knots or less Thursday night, then northwest winds around 15
knots Friday with gusts up to 30 knots.

Saturday and Sunday...VFR overall.

Monday...Clouds increase especially late day and at night, with
possible MVFR/IFR conditions developing overnight if precipitation


Gale Warning remains in effect for all waters tonight and for NJ
ocean waters through Thursday morning as W-NW winds will gust to
around 35 kt. Thereafter, Small Craft Advisories will likely be
needed for at least Thursday as winds gust 25-30 kt on the
ocean. Wind gusts on DE Bay should be around 20 kt.

Thursday night and Friday...Small Craft Advisory conditions mostly
due to gusty northwest winds up to 30 knots, however this should
diminish some during Friday night.

Saturday and Sunday...Diminishing conditions, therefore mainly sub-
Small Craft Advisory criteria.

Monday...The conditions should be below Small Craft Advisory
criteria, although winds should start to increase from the east-
northeast later Monday.


MARINE...Gale Warning until 8 AM EDT Thursday for ANZ450>453.
     Gale Warning until 6 AM EDT Thursday for ANZ430-431-454-455.


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