Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 211907

Area Forecast Discussion
National Weather Service Mount Holly NJ
307 PM EDT Fri Oct 21 2016

Low pressure will consolidate near southern New England this
evening, then intensify as it tracks northward through late
Saturday. An associated strong cold front will sweep across our area
through this evening. A secondary cold front arrives Sunday night,
then high pressure gradually builds in late Tuesday and Wednesday. A
warm front to our southwest Thursday is then forecast to lift across
our area during Friday.


Area of showers along the NJ coast continues to push offshore. Some
light showers are developing over the coast, but will continue to
push to the east.

A broad area of low pressure lies to the west of the Appalachians,
and extends from northern NY state down into the Mid-Atlantic, and
is associated with a strong cold front that will push through the
Eastern Seaboard tonight.

Ahead of that front, a warm and humid airmass remains in place with
temps in the 70s and low 80s, and surface dewpoints well in the 60s.
A much colder and dryer airmass with temps in the 50s and dewpoints
in the 30s and 40s lies over the Great Lakes and OH Valley, and will
spread into the Northeast on Saturday.

With that humid airmass in place, there are ingredients for
thunderstorms, but the strongest upper level support will pass to
the north of the region. Therefore, not anticipating too many
thunderstorms in the local CWA, but cannot rule out a rumble of
thunder or two. Some of the showers, however, may result in
localized wind gusts of 35-45 mph, as well as brief, heavy
downpours for the rest of the day and into this evening.

Cold front moves across the region tonight, but will probably not
clear the region until Saturday morning. Meanwhile, upper level
trough with strong H5 low and several strong shortwaves will pass
through the region with the front. Showers begin to taper off after
midnight tonight, but with the upper level low still in the area,
will keep likely PoPs in the forecast over northern zones through
the early morning hours.

As the front passes through, winds abruptly shift from the S-SW to
the W-NW, and winds should be NW throughout just after midnight
tonight, and temps fairly quickly fall into the 50s by daybreak


As upper low lifts to the north and east on Saturday, upper trough
remains over the area. Surface low pressure over the Canadian
Maritimes will intensify, deepening from a 990 MB low Saturday
morning to a 980 MB low by Saturday evening. Meanwhile, 1022 MB high
pressure will build into the Gulf Coast states. As a result, a tight
pressure gradient will form across the entire Northeast U.S. Strong
NW winds develop fairly quickly Saturday morning, increasing to 20-
30 MPH winds with 35-45 MPH gusts. Based on latest NAM and GFS
forecast soundings, think wind gusts will fall short of Wind
Advisory criteria, so will not issue a Wind Advisory. Will keep the
mention in the HWO, as it will be close, and cannot rule out a few
45-50 MPH reports Saturday afternoon, especially in the higher
elevations, and near the coasts.

For temps, with strong CAA underway, looking as if the highest temps
of the day will occur just after midnight, as temps will be in the
upper 50s to low 60s. By Saturday morning, temps will be in the
upper 40s to low 50s, and should hold mainly steady from there.


The synoptic setup is comprised of a deep upper-level trough in the
East to start Saturday night into Sunday, however this feature is
forecast to lift out during Sunday. A secondary short wave amplifies
the trough once again Sunday night and Monday as another cold front
pushes through. The trough weakens thereafter with surface high
pressure building in later Tuesday and Wednesday, then another
trough tracks across the Midwest and Upper Great Lakes regions
Wednesday and Thursday before pushing eastward. This should send a
developing warm front northbound across our area Thursday night and
Friday. Overall, it looks to be a cooler time frame given a trough
that wants to remain in the Northeast. We used a model/continuity
blend Saturday night into Monday, then blended in the 12z WPC
Guidance thereafter. Some adjustments were then made following
additional collaboration with our neighboring offices.

For Saturday night and Sunday...A deepening upper-level trough will
swing to our east during this time frame. This will drive
intensifying surface low pressure up across New England and then
into Canada through Sunday. The best low-level cold air advection
wanes through Saturday night, then some moderation is forecast at
least at 850 MB during Sunday. Meanwhile, high pressure centered
well to our southwest will result in a tight pressure gradient. This
will result in gusty winds continuing through this time frame.
However, there should be enough decoupling Saturday night and early
Sunday to allow the winds to diminish some. Despite a strong flow
remaining in place for much of Sunday, the gradual warming aloft
should allow for less mixing overall (not as steep low-level lapse
rates). While breezy conditions are expected Sunday, the peak gusts
are expected to be less than Saturday.

A short wave then amplifies the trough starting Sunday night as a
secondary cold front moves through. The moisture looks to be limited
with this feature despite a strong short wave, however the guidance
shows a tightening thermal gradient near our northern zones ahead of
and with this feature. This should drive a band of frontogenetic
forcing leading to some showers. The coverage and placement of these
showers varies some in the guidance, given the track of the
aforementioned short wave. We will keep some low POPs across the
northern areas for now later Sunday night into early Monday. It will
be chilly to cold Saturday night, then some moderation on Sunday
ahead of the next cold front.

For Monday and Tuesday...An upper-level trough dominates the East
for much of this timeframe, although it is forecast to shift east
during Tuesday. This will result in cyclonic flow of cool to chilly
air across the area. To start off Monday, an initial short wave that
starts the trough amplification moves through and this may result in
some showers early up north. Otherwise, the boundary layer should
become well mixed during the daylight hours with plenty of drying
above the inversion. This should result in less clouds but at least
some stratocumulus should occur especially on Monday. It looks to be
breezy once again especially during peak heating as the forecast
soundings show deeper mixing occurring with 25-30 knots of wind to
mix down. There is a frost or freeze potential for some areas
Tuesday night especially as the winds diminish.

For Wednesday through Friday...As the trough continues to shift east
of our area Wednesday, another trough gradually moves from the
Midwest to the Great Lakes. High pressure briefly over our area
Wednesday will shift offshore. A return flow sets up in its wake,
allowing warm air advection to occur up the Ohio Valley and into our
area especially starting Thursday. This will allow a developing warm
front to our southwest to move into our area Thursday night then
slowly advance northward Friday. The timing and placement of this
warm front will have an affect on cloud cover and any precipitation.
As of now, the main forcing may remain to our west and therefore the
POPs were kept on the low side later Thursday and Friday.


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

VFR to occasional MVFR CIGs through this evening, and then
conditions become MVFR throughout as a cold front approaches from
the west and passes across the region. Showers associated with the
front will reduce conditions to MVFR, but IFR VSBYs are possible,
especially in heavier showers.

Once the front passes through, conditions should improve to VFR.
Winds shift to the NW and increase to 15-20 KT with 20-30 KT gusts

Some lingering showers are possible Saturday morning, otherwise, VFR.

NW winds increase to 20-30 KT with 30-40 KT gusts.

Saturday night and Sunday...VFR overall. Northwesterly winds 15-20
knots with gusts to 30 knots early are expected to diminish some
during Saturday night, then turn gusty again Sunday (not as strong
as Saturday). A secondary cold front arrives Sunday night, with some
showers possible mainly near and north of KABE to KTTN.

Monday and Tuesday...VFR overall. Northwesterly wind gusts up to 30
knots especially Monday in the wake of a cold front, then
diminishing winds at night.

Wednesday...VFR with high pressure in place, resulting in Northwest
winds less than 10 knots.


Showers over the northern ocean waters are diminishing and moving to
the north and east. Meanwhile, additional showers will impact the
waters this evening as a cold front moves across the waters tonight.
A rumble of thunder or 2 is possible, but not expecting widespread
convection. Once the front passes through, winds shift from the S to
the NW.

Winds will increase to 15-20 KT with 20-30 KT gusts, and then
strengthen even more on Saturday. Think the gales will be delayed,
so will go ahead and push the start time of the Gale Warning to 10Z
Saturday, and will carry a Small Craft Advisory from 05Z-10Z
Saturday, for the late night/early morning hours.

Winds increase to 20-30 KT with 35-45 KT gusts on the waters on

Deep mixing within a colder airmass will allow for gale force gusts
Saturday night and Sunday. These are expected to diminish first for
Delaware Bay, then from south to north for the Atlantic Coastal
Waters during Sunday. We did not make any changes to the ending
times of the Gale Warning. The conditions are then expected to be to
Small Craft Advisory levels Sunday night into Tuesday in the wake of
a cold front. The conditions are anticipated to drop below advisory
criteria Tuesday night and Wednesday as high pressure arrives.


MARINE...Small Craft Advisory from 1 AM to 6 AM EDT Saturday for ANZ430-
     Gale Warning from 6 AM Saturday to noon EDT Sunday for
     Gale Warning from 6 AM Saturday to 6 PM EDT Sunday for ANZ450-
     Gale Warning from 6 AM Saturday to 2 AM EDT Sunday for ANZ430-


Near Term...MPS
Short Term...MPS
Long Term...Gorse
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