Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 151926

Area Forecast Discussion
National Weather Service Mount Holly NJ
326 PM EDT Thu Mar 15 2018

Low pressure remains over the Canadian Maritimes through Saturday,
then weakens as it moves eastward. Meanwhile, a cold front moves
through the region tonight followed by high pressure slowly building
toward our area late Friday. 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 later Sunday and Monday
tracks eastward, and should result in another coastal storm for
Tuesday into Wednesday before moving to off the New England coast


Weak trough out ahead of a cold front moving through western and
central NY and PA will pass through the region this evening. Latest
KDIX radar showing weak returns across the region, but with dewpoint
depressions generally around +15F, not much of that is reaching the
ground. Mainly virga drying up in the air, and otherwise, some
sprinkles and/or flurries. Going into the evening, as those dewpoint
depressions fall, think we should get some rain and/or snow showers,
but not much more than a few hundredths of an inch or so of liquid
QPF, and no snow accumulation for most of the region. For the
Poconos, however, expecting snow showers, perhaps a snow squall, but
not as heavy as compared to Wednesday and Wednesday night. An inch
or two of accumulation is possible across Carbon and Monroe

With loss of diurnal heating, any rain and/or snow showers will
taper off fairly quickly after sunset. Some snow showers may persist
over the southern Poconos through tonight with minimal additional
snowfall. For the rest of the area, skies clear out this evening,
and then mid and high clouds return late tonight as that cold front
passes through the region prior to daybreak Friday.

Tight pressure gradient resulting in 15-20 mph winds with gusts up
to 30 mph will relax this evening, and winds diminish to 10 mph or
less by midnight tonight.

Another cold night with lows generally in the mid and upper 20s, and
perhaps a few degrees cooler in the mountains, and a few degrees
warmer along the I-95 corridor and in southern portions of the


The cold front passes through the region Friday morning and a deep
upper trough remains in place over the Northeast and Mid-Atlantic. A
reinforcing shot of cold air builds south into the region, resulting
in temperatures several degrees colder than today, and around 8-12
degrees below normal. A tight pressure gradient develops again on
Friday, resulting in NW winds 15-20 mph with 30-35 mph gusts in the

Wind chills Friday morning will be quite cold, generally in the
single digits in the Poconos, and otherwise in the teens and low
20s. Afternoon wind chills will not be much higher, generally in the
teens in the Poconos, and otherwise in the 20s and low 30s.

Generally dry and sunny for much of the region, but cannot rule out
some snow showers in the Poconos.


Summary...Colder conditions compared to average overall, although
some moderating may occur over the weekend. Potential for coastal
storm development remains for the Tuesday to Wednesday time frame,
however the evolution/track and therefore any impacts remain
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 weakening short wave
trough is forecast to slide eastward across the Mid-Atlantic
Saturday. A potent short wave or closed low is then 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 the
features offers uncertainty. It does appear that the primary low
eventually gives way to redevelopment somewhere off the Mid-Atlantic
coast, however this placement will depend on the track of the short
waves and their amplification of the trough aloft. A turn closer to
the coast with the surface low or farther offshore will also depend
on the extent of upstream blocking. 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 to a storm.

For Friday night...Cyclonic flow continues as a strong upper-level
trough remains in place across the Northeast. The pressure gradient
however is forecast to relax as a weak area of high pressure starts
to extend into our area, therefore this combined with decreased
mixing will allow the winds to diminish. This will set the stage for
decent radiational cooling as any high cloudiness should off until

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 and shift northeastward some 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 quickly slides by to our south.
This looks to run across an area of confluence, therefore it should
be weak and quick moving. There is enough guidance that indicates
some light QPF across portions of Delmarva to perhaps up to Cape
May, NJ and therefore increased PoPs some for a time. There should
be a sharp northern cutoff from any precipitation. The clouds will
thicken as well with this most noticeable across the southern zones.
It appears that the precipitation type is light rain, although given
some low-level dry air initially some brief ice pellets or wet snow
cannot be ruled out at the onset. Some warming should materialize
Saturday especially across the northern half of the area where more
sunshine should occur, however a weak cold front shifting
southeastward should still keep it on the cooler side for Sunday. We
are expecting much less wind especially on Sunday.

For Monday and Tuesday...High pressure should be extending
southeastward to 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 a mostly eastward track. Clouds are
expected to increase and if timing is correct, large scale forcing
starts to arrive toward daybreak Tuesday with the increasing chance
for some precipitation. The forecast then remains 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 interaction needs to be
resolved by the model guidance, which also 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 still uncertain this far out, there
continues to be a strong storm signal for the Tuesday and Wednesday
time frame. The ensemble data continues to show the variability with
this, including when and where the secondary low redevelopment
occurs. We followed closer to WPC which indicates a secondary low
development near the North Carolina coast early Tuesday morning. The
track and timing of this storm will depend on how much upstream
blocking is still in place. This of course will impact the thermal
fields and therefore precipitation types and amounts. We did bring
likely PoPs farther north on Tuesday with a generic rain, snow or
rain/snow mix as the precipitation types.

For Wednesday and Thursday...The guidance appears to be slowing down
the features some as an upper-level trough could become more
amplified in the East. This will take low pressure off the coast
northeastward, however some precipitation may linger on Wednesday.
The amplitude of the trough is still lower confidence and this will
determine how quickly the storm exits. Low pressure is currently
forecast to be off the New England coast to start Thursday with
cyclonic low-level flow occurring across the Mid-Atlantic. The
strength of the storm will determine the magnitude of the winds over
our area later Wednesday and especially Thursday.


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

Tonight...Brief -SHSN at KABE/KRDG may result in brief MVFR, even
IFR conditions early this evening. Otherwise, VFR with BKN CIGs
around 5000 feet, scattering out this evening. W-NW winds 15-20 kt
with gusts to 25 kt will diminish fairly quickly after sunset, and
will be 5-10 kt for the overnight.

Friday...VFR. BKN CIGs around 5000 feet possible in the afternoon.
NW winds 15-20 kt with 20-30 kt gusts developing in the late morning
and continuing through the day.

Friday night...VFR. West-northwest winds diminishing to less
than 10 knots.

Saturday and Sunday...VFR overall. Lighter winds especially on

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

Tuesday...Widespread MVFR/IFR conditions with rain/snow possible.
Low confidence with the details.


Strong NW winds continue on the waters tonight and Friday. Confidence
in gales occuring this evening, however, is low enough to warrant
removing the Gale Warning that was in effect for DE ocean and
southern NJ ocean waters. Cannot rule out a few gale gusts, but do
not think they will be widespread enough to warrant a Gale Warning.
Small Craft Advisories are in effect for all waters tonight, though
conditions may briefly subside to sub-SCA criteria on at least
northern DE Bay waters.

On Friday, a tight NW pressure gradient develops once again. Think
max gusts in the afternoon will be 28-32 kt, and although some gale
gusts are possible, not confident enough to hoist a Gale Warning for
Friday afternoon. Will go with a Gale Watch for the ocean waters,
and this will either be upgraded to a Gale Warning or replaced with
a Small Craft Advisory.

Friday night...Small Craft Advisory conditions expected, although
the winds diminish with time.

Saturday and Sunday...The conditions should be mostly below Small
Craft Advisory criteria, however a northerly wind surge with gusts
20-25 knots may 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...Small Craft Advisory conditions are anticipated, however
the magnitude of the winds and seas will depend on the track and
strength of a coastal storm.


MARINE...Gale Watch from Friday morning through Friday afternoon for
     Small Craft Advisory until 6 AM EDT Friday for ANZ450>455.
     Small Craft Advisory until 6 PM EDT Friday for ANZ430-431.


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