Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 171636
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1136 AM EST Fri Nov 17 2017

.SYNOPSIS...
High pressure builds over our area during today before quickly
moving offshore tonight. Low pressure strengthens as it moves into
the eastern Great Lakes region Saturday, then to near northern New
England Sunday. A strong cold front sweeps across our area later
Saturday night into early Sunday morning, then high pressure builds
to our south Monday into Tuesday. A cold front moves across the
region Tuesday night, followed by high pressure arriving later
Wednesday and Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Forecast mainly on track. Will adjust hourly grids based on
latest surface obs and visible satellite images.

Generally mostly sunny skies for the rest of the afternoon with
a brisk northwest wind gusting to 25 or 30 MPH at times and
seasonably cool temps...highs generally in the upper 40s to
lower 50s... low or mid 40s Poconos. Several degrees below
normal.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
Mostly clear and a bit chillier than last night with radiational
cooling in the countryside. Bands of cirrus become evident near
or after midnight. Light northwest wind to start becoming light
south toward morning. Lows near normal in the urban centers and
about 5 or so below normal in the countryside.

Forecast basis was a 50 50 blend of the 00z/17 GFS/NAM MOS.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Summary...Strong cold front moves through Saturday night and early
Sunday with potentially strong winds, followed by colder air and
windy conditions Sunday into Monday. Turning milder Tuesday followed
by another cool down Wednesday and especially Thursday.

Synoptic Overview...An upper-level trough arrives Saturday night and
Sunday which takes on a negative tilt across the Northeast. This
will drive deepening low pressure to our northwest, with an
associated strong cold front sweeping across our area later Saturday
night. As a ridge moves across west-central Canada, the next
downstream trough is forecast to begin amplifying across the Great
Lakes and Northeast during Tuesday. The flow aloft looks progressive
at least for a time, however later next week may feature more trough
amplification into the East. This still is less certain given the
parade of short wave energy which may or may not result in the flow
becoming highly amplified. Through this forecast period,
temperatures look to be a bit on a roller coaster.

For Saturday...Warm air advection strengthens in advance of an
incoming sharpening upper-level trough. This will occur as a warm
front lifts to our north. An area of showers or rain will accompany
the warm front, although the strongest and more focused lift is
mostly to our northwest. However, some showers should occur just to
its south into the warm sector for parts of eastern Pennsylvania and
northern New Jersey mainly Saturday afternoon. Low pressure is
forecast to strengthen quite a bit as it lifts across the Ohio
Valley and eastern Great Lakes region, then eventually into Canada.
The associated strong cold front sweeps through our area later
Saturday night. Increasing southerly flow ahead of the cold front
Saturday will drive much milder air into most of the region. The
deeper warm sector may not get all the way to our northern zones,
especially if showers develop sooner.

The flow aloft is forecast to increase substantially Saturday
afternoon and especially Saturday night including a low-level jet,
as the upper-level trough eventually takes on a negative tilt. There
are decent 3-hour pressure rises right behind the cold front along
with a rapid increase in cold air advection at 850 mb. The forecast
soundings indicate as quick cooling occurs aloft combined with some
lingering warmth in the boundary layer, mixing deepens and this
could result in strong wind gusts along and just behind the cold
front. There looks to be little in the way of instability, however
strong frontal forcing may result in an enhanced band of showers
with the front which could mix down some of the stronger winds. We
continue to bring in categorical PoPs later Saturday and Saturday
night from west to east. Given the timing and lack of instability
forecast, we continue to not include thunder. Given the
strengthening wind field and warm air advection, temperatures should
hold steady or even rise some ahead of the cold front.

For Sunday and Monday...The strong cold front quickly shifts
offshore early Sunday morning with showers ending. Some clearing is
expected, however a stratocumulus deck may occur as strong cooling
aloft works its way downward. Deep low pressure tracks just north of
New England Sunday with a tight pressure gradient in place. A
continuation of cold air advection and strong flow will result in a
windy day as mixing will be efficient. There could be a burst of
wind first thing with the cold frontal passage accompanied
potentially by a band of enhanced showers. For now, kept gusts just
below advisory criteria however it is possible a wind advisory may
eventually need to be issued. The airmass does turn colder with high
temperatures probably occurring in the morning Sunday for most, with
some drop off through the afternoon. Lake effect snow should develop
off the eastern Great Lakes, and it is possible that a streamer
attempts to approach the Poconos later Sunday afternoon and evening.
It is possible a few showers make it farther southeast, however the
main focus looks to be closer to the Great Lakes. The winds should
diminish at least some Sunday night as the pressure gradient weakens
a bit. High pressure starts to build in during Monday, however its
center is to our south-southwest and this will keep a west-northwest
breeze going. Overall, a colder day is expected Monday.

For Tuesday through Thursday...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 drawn
northward. The main forcing with this system may end up lifting to
our north although this is less certain. For now, opted to maintain
a dry forecast despite a cold frontal passage Tuesday night. High
pressure is forecast to gradually build in from the west later
Wednesday and Thursday with a colder airmass settling in.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Today...VFR with few clouds aoa 5000 ft. NW wind around 15 kt
with 25-30 kt gusts, with gust speeds decreasing late in the
afternoon. Confidence high.

Tonight...VFR few clouds aoa 10000 ft with thickening cirrus
arriving toward sunrise Saturday and a light northwest wind this
evening becomes light south toward morning. Very high confidence.

OUTLOOK...
Saturday...VFR during the day for most with clouds lowering some,
however ceilings lowering to MVFR in the afternoon mainly at RDG and
ABE with some showers. A period of MVFR/IFR conditions with showers
at night. South to southwest winds increasing to 10-15 knots with
gusts up to 25 knots, then becoming west to northwest and gusty
toward Sunday morning. Moderate confidence.

Sunday...Improving to VFR early as showers move offshore. Some
flurries or rain/snow showers possible later in the afternoon and
evening mainly north and west of ABE. Northwesterly winds 15-25
knots with gusts to 30-40 knots, diminishing some during the night.
Moderate to high confidence.

Monday...VFR. West-northwest winds 10-15 knots with gusts up to 25
kt, diminishing at night. Moderate confidence.

Tuesday...VFR. Southwest winds 5-15 knots with afternoon gusts up to
20 knots possible. Moderate confidence.

&&

.MARINE...
SCA all waters today and for NJ into this evening for winds
gusting to around 30 kt.

Diminishing northwest winds early tonight becoming southwest or
south toward morning.

OUTLOOK...
Saturday and Sunday...Gale Watch issued for Saturday afternoon
through the day Sunday. Southwesterly winds increase Saturday
especially in the afternoon and evening ahead of a strong cold
front. There is lower confidence though with the amount of mixing
that may occur within the southwesterly milder flow, with higher
confidence of mixing along and behind the cold front for Sunday.
Given the potential and enough confidence overall, will go with the
watch for all zones. The winds should settle down some later Sunday
night.

Monday...Small Craft Advisory conditions due to winds during the day
are expected to diminish at night.

Tuesday...Small Craft Advisory conditions should develop within
southwesterly flow ahead of a cold front.

&&

.FIRE WEATHER...
Dryer air will mix down into our area today and minimum RH will
drop to near 30 percent. This combined with very little rain
the past few days, wind gusts of 25 mph and fine fuels indicating
considerable dryness, suggests concern for enhanced rate of
spread this afternoon for any uncontrolled fires. We will check
with our partners during mid morning with a small chance of a
special weather statement issuance for s and central NJ.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Watch from Saturday afternoon through Sunday afternoon
     for ANZ430-431-450>455.
     Small Craft Advisory until 10 PM EST this evening for
     ANZ450>453.
     Small Craft Advisory until 5 PM EST this afternoon for ANZ430-
     431-454-455.

&&

$$
Synopsis...Gorse
Near Term...Drag/MPS
Short Term...Drag
Long Term...Gorse
Aviation...Drag/Gorse/MPS
Marine...Drag/Gorse/MPS
Fire Weather...



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