Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 150928
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
428 AM EST Wed Nov 15 2017

.SYNOPSIS...
High pressure over our area weakens and retreats toward the
Canadian Maritimes later today and tonight. A strong cold front
moves across our area during Thursday, then high pressure builds in
during Friday. A warm front lifts to our north Saturday, followed by
a cold front Saturday night. High pressure then builds to our south
Monday into Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
215 AM ESTF: Clouds thinning rapidly attm so will adjust early
morning lows back down.

The new 330 am fcst is basically published and posted.

Early this morning...monitoring sprinkles offshore and suspect
that they will remain there... second time within about 10 days
that we`ve seen this at night. Patchy fog-frosty where clearing
of thin deck at 5000 ft now occuring.

Today...Its a model rh time section battle between the NAM and
GFS. An inversion near 800 MB caps moisture below it and overall
am thinking the cooler more moist NAM will prevail. So this
forecast was a blend of the 00z/15 GFS/NAM MOS but LOW confidence
on having made the correct decision on more clouds than sun.
For now...am fcstg temps no higher than 2F above the blended
GFS/GGEM/NAM 18z 2m temps. Light east to southeast winds. If
there is a shower along the coast, it should be this Wednesday
morning.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
Cloudy and relatively mild with nearly steady temps, possibly
rising a bit during the night along and east of I-95. Showers
ahead of a cold front are expected to develop, mainly after
06z/16 as per HRRRX NAM/GFS...mostly likely toward 09z/16. That
means a band moving from w to E across PA/MD into our western
zones around 06z and meanwhile showers sprouting ahead of the
main band in DE/NJ. Amounts .05 to 0.25". Less than average
confidence on rainfall ILG/MIV southward but the 00z/EC has .10
or greater over virtually all of our area 6z-12z Thu. The
drivers for the rainfall area area 100m 500mb height falls over
our area ahead of a modeled 160M 12 hr HFC vt max sliding sewd
through sw NYS at 12z Thu, also a nice 850 trough with
considerable absolute vorticity in its axis as it crosses our
area by 12z. That and some pretty good jet cores at 850-40 kt
700mb-55kt aiding the forcing.

This forecast was a 50 50 blend of the 00z/15 GFS/NAM MOS.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Summary...Strong cold fronts move through Thursday and again
Saturday night with milder air ahead of them, followed by
colder air and gusty winds.

Synoptic Overview...A strong upper-level trough moves through the
Northeast Thursday before lifting out on Friday. Some weak ridging
then arrives, but another upper-level trough quickly arrives
Saturday night into Sunday. As more pronounced ridging moves across
west-central Canada, a downstream trough is forecast to begin
amplifying across the Great Lakes and Northeast Tuesday. The flow
aloft overall looks progressive for awhile, however there are
signals that next week may feature more amplification with a trough
settling into the East.

For Thursday...A strong upper-level trough moves across the
Northeast during Thursday. A surface low tracks well to our north,
however a secondary low should develop offshore of Long Island early
then track northeastward. Showers are expected to end early, with
perhaps some lingering across the far north through about early
afternoon. This will depend on the extent of wrap around moisture.
Cold air advection sets in during the course of the day, however
this is initially stronger at 850 mb and above. The result will
be a milder day for much of the area. The more notable cold air
advection closer to the surface is forecast to arrive later in the
day. The forecast soundings indicate deeper mixing occurs during
the day especially in the afternoon. Based on the soundings, there
is about 25-30 knots of wind available to mix down and therefore the
wind gusts in the forecast were increased. The winds will diminish
some at night, however given the continuation of cold air advection
in the lower levels and the pressure gradient remaining tight this
process may be slowed.

For Friday and Saturday...The strong upper-level trough moves away
during Friday with the flow becoming more zonal. High pressure at
the surface builds across our region Friday which will result in
less wind through the day. It is noted that the stat guidance
especially is warmer for Friday, however the modeled 2 meter
temperatures are colder. The latter is rather close to our going
forecast, and therefore we opted to make little in the way of
changes. Clouds increase Friday night ahead of the next incoming
upper-level trough. There is fairly strong warm air advection in
advance of this, allowing a warm front to become better defined as
it lifts northeastward. This should produce a zone of showers or
rain along and to its north, however this appears to be slower in
the guidance overall as we go through Saturday. As a result, we
slowed down the PoP increase from west to east Saturday. Low
pressure is forecast to strengthen as it lifts across the eastern
Great Lakes region Saturday morning and then into Canada. The
associated cold front sweeps through our area Saturday night.
Increasing southerly flow ahead of the cold front Saturday will
drive milder air into the region, then be replaced by west to
northwest winds behind the frontal passage. We continue to bring in
likely to categorical PoPs through Saturday night. Given the timing,
held off in adding thunder.

For Sunday and Monday...While some showers may linger early Sunday
as the cold front shifts offshore, drying is expected. Low pressure
tracks near northern New England Sunday, with a tight pressure
gradient in place along with cold air advection. Sunday is looking
like another breezy/windy day as mixing deepens during the day. The
air turns colder and therefore lake effect snow should develop off
the eastern Great Lakes. It is possible that a streamer attempts to
approach the Poconos later Sunday afternoon and evening. The winds
should diminish at least some Sunday night. High pressure starts to
build in during Monday, however its center is to our south-
southwest. Overall, a cold day is expected Monday.

For Tuesday...Renewed upper-level trough development is forecast to
take place into the Northeast. This drives low pressure once again
near and north of the Great Lakes with a trailing cold front
approaching from the west. As the flow backs in response to this
system, some warming will be draw northward. The main forcing with
this system may end up lifting to our north, therefore continued
with a dry forecast at this time.

&&

.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Early this morning...VFR CIGS generally based near 5000 ft with
spotty MVFR CIGS along the coast. CIGS may become sct clouds,
especially vcnty KABE/KRDG/KTTN by 10z. This per the sliver of
clear sky as seen on satellite imagery is developing slowly west
through e MD e PA and nw NJ.  Light north to northeast wind.

Today...Confidence lowers on the scenario but we are are not
using the NARRE which is off to a poor start in our forecast
area. Reliance will be on a visually blended GFS/NAM Tsection.
That will mean generally VFR conditions...probably cigs based
near 4000 or 5000 ft with a possibly MVFR CIG here and there,
especially late in the day when MVFR CIGS may become much more
widespread. Light northeast wind to start becoming southeast
this afternoon. Confidence in CIG forecast height is well below
average, and for winds is above average.

Tonight...VFR or MVFR CIGS to start, then degrading to probable
IFR conds toward 09z/16 in showers stratus fog with stratus fog
contg even after showers end vcnty Philly westward near 11z (no
mixing). Confidence above average on sub MVFR conditions late
at night. Light southeast wind becomes light southwest or west
toward 11z/16.

OUTLOOK...
Thursday...Areas of MVFR conditions possible early with showers,
otherwise VFR with clouds thinning with the passage of a cold front.
Southwest winds 5-10 knots, become northwest and increasing to
around 15 knots with gusts to 25-30 knots in the afternoon. The
winds gradually diminish some at night. Moderate confidence.

Friday...VFR overall, then clouds increase and lower some at
night. High confidence.

Saturday...A period of MVFR/IFR conditions with showers, especially
in the afternoon and at night. Some improvement in the conditions
should occur late at night as a cold front moves through.
Moderate confidence.

Sunday...VFR overall. Some flurries or snow showers possible in the
afternoon mainly north and west of ABE. Northwesterly winds 15-20
knots with gusts up to 30 knots. Moderate confidence.

&&

.MARINE...
Small craft advisory for the Atlantic waters from Cape May to
Fenwick Island through 6 pm today as seas will likely stay
elevated through this afternoon, especially with a modeled
slight increase in northeast wind this morning. Winds may
briefly gust to 25 kts, especially off the Delaware coast, but
the main threat will be elevated seas of around 5 to 6 feet.

There is a change of sprinkles or a light shower through much
of the period, generally from the southern New Jersey coast
southward.

Tonight...No marine headlines but rereviewing for the 330 am
fcst.

OUTLOOK...
Thursday...The winds shift to the west and northwest as a strong
cold front moves through. This will result in deeper mixing with
time and gusts 25-30 knots especially in the afternoon.
Northwesterly gale force gusts are probable Thursday night, and
therefore a Gale Watch has been issued for all zones. The extent of
the winds will depend on how quick the colder air arrives, enhancing
the vertical mixing.

Friday...Gale force gusts within a northwesterly flow may linger
Friday morning, however the winds are expected to be diminishing.

Saturday and Sunday...Gale force gusts from the southwest are
possible Saturday afternoon and evening, then gale force gusts from
the west and northwest are possible later Saturday night and
Sunday.

&&

.CLIMATE...
November will probably average below normal throughout our
forecast area. It is expected that the last 16 days will average
at best normal, more likely...below normal...though how much
below normal we`re unsure. I cannot at this time see the departures
listed at the end of this section, warming...and suspect they`ll
run colder by a degree or 3.

The pattern as signaled for many recent days by the NAEFS/GEFS/ECEFS
is basically a trough constantly reloading in the northeast USA
through the end of the month.

So the positive departure excesses from September 14 through
November 6 are history. What we`re experiencing now is what fall
is supposed to be like...chillier. And its probably going to be a
big difference when compared to last November which averaged 1
to 3 degrees above normal.


The numbers below represent the departure from average for the
first 14 days of the month and those departures will probably go
more negative by time the 30th arrives.

ACY -0.2
55N -1.2 (ACY Marina)

ABE -1.0

ILG -0.9

MPO -3.4

PHL -1.5

RDG -2.7

TTN -1.5

GED +1.0


Basically that says the coldest departures were across
the northwest part of our forecast area and the mildest
departures from normal, along our southeast edge.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Thursday evening through late Thursday night
     for ANZ430-431-450>455.
     Small Craft Advisory until 6 PM EST this evening for
     ANZ451>453.
     Small Craft Advisory until 6 PM EST Thursday for ANZ454-455.

&&

$$
Synopsis...Gorse
Near Term...Drag
Short Term...Drag
Long Term...Gorse
Aviation...Drag/Gorse
Marine...Drag/Gorse
Climate...Drag 428



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