Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 200535
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
1235 AM EST Mon Nov 20 2017

.SYNOPSIS...
Low pressure over southeast Canada will continue to lift to the
north and east tonight. Meanwhile, high pressure over Texas
builds east through the Gulf Coast states and will move off the
Mid-Atlantic coast Monday night. A cold front approaches
Tuesday night and passes through the region on Wednesday. At the
same time, low pressure over the Southeast approaches the Mid-
Atlantic and moves out to sea. High pressure builds through the
region to close out the week. A cold front passes through the
region Saturday, then low pressure will be over eastern Canada
over next weekend.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1230 am update: Impressive snow showers with some wind gusts 20
to 30 mph and bursts of brief, moderate snow are occurring in
isolated spots generally north of I-78 the past few hours, with
Allentown recently seeing a brief snow shower and visibility
reduced to around 2 miles. Current forecast has this well in
hand, so made no changes to PoPs/Wx grids overnight. Potential
will be waning with time as lift with the shortwave trough
moving through the area progresses rapidly eastward with time.

Sky cover has generally diminished this evening outside of the
northern CWA. Made some adjustments to the grids to account for
this. Winds have also diminished considerably, so have lowered
these for the rest of the night across the area. Still expecting
west winds around 10 kts for much of the area, and there will
be stronger gusts with any remaining snow showers in our
northern areas.

Previous discussion below...

Minor temperature updates. A few adjustments in Pops/Wx to align
with present radar trends. A few reports of snow showers and a
little sleet up north earlier this evening.

Strong northwest flow continues across the area between an area
of low pressure to our northeast across southeastern Canada,
and high pressure to our southwest across the Lower Mississippi
Valley. This tight pressure gradient will continue through
tonight and will keep winds elevated overnight. However, wind
gusts will continue to diminish this evening, and may drop off
for many locations overnight.

With the northwest flow, clouds will continue to to be trapped
under inversion around 4,000-6,000 feet and remain across most
of the area. The thickest clouds cover will be from the I-95
corridor northward.

A short wave/vorticity impulse and an area of enhanced low
level lift and moisture will move across the northern half of
the area later this evening and overnight. The combination of
the lift and moisture associated with the short wave/vorticity
impulse may be enough that may help any lake effect streamers
within the northwest flow may make their way into portions of
our area overnight. There will be a slight chance of isolated
snow showers/flurries later tonight for areas along and north of
the I-78 corridor. The Poconos may have the better shot of
seeing any accumulation if any showers do occur.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/...
Monday is expected to be a fairly nice day. The pressure
gradient relaxes some during the day as high pressure builds to
our south, but there will remain a steady breeze through the
day. Also, winds will be gusty around 20-25 mph through much of
the day as well, but much less windy than Sunday.

There will remain some scattered cloud cover across the area,
especially in the morning hours and areas along and north of the
I-95 corridor. Skies are expected to begin to clear out by he
afternoon. No precipitation is expected through the day on
Monday as any lake effect streamers will get cut off or push
north of our area as the flow turns more west to southwesterly
through the day and moisture/lift becomes limited as well.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
High pressure over the Mid-Atlantic moves out to sea Monday
night and will be off the Mid-Atlantic coast on Tuesday. Return
flow sets up, and highs on Tuesday will be about 3-5 degrees
above normal, topping off in the 40s in the Poconos, otherwise
in the low to mid 50s north and west of I-95, and near 60s in
the Delmarva and southern NJ. S to SW winds increase Tuesday
afternoon to 15-20 mph with gusts up to 30 mph.

High pressure moves out to sea late Tuesday as a cold front
moves through the Great Lakes and Ohio Valley. Meanwhile, low
pressure develops over the Gulf Coast on Tuesday, and that low
moves off the Southeast U.S. coast Tuesday night. Tuesday night
through Wednesday morning, that low approaches the Mid-Atlantic
coast and drifts out to sea. That cold front will pass through
the region Wednesday afternoon. Based on latest model guidance,
will carry a swath of likely PoPs across southern DE and
southeast NJ, and chance PoPs for most areas south and east of
the Fall Line. Cooler temps will move into the Poconos Wednesday
afternoon, but stronger CAA will not be underway until
Wednesday night.

Chilly high pressure builds through the region Thanksgiving Day
with temperatures 6-8 degrees below normal. Highs top off in
the mid and upper 30s in the Poconos, otherwise in the low to
mid 40s.

High pressure reestablishes itself over the area Friday with
highs a few degrees warmer than Thursday.

Low pressure passing well north of the region will drag a cold
front through the region on Saturday, and then low pressure
remains over eastern Canada through next weekend. Unsettled
weather possible.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR conditions expected with west winds around 10 kts.
A small chance for a brief snow shower at KABE with sub-VFR
conditions likely should these reach the terminal. However,
threat should be diminishing with time and is too low for
mention after 06Z. Confidence above average.

Monday...VFR with west winds 10 to 15 kts with gusts to 25 kts.
Confidence above average.

Outlook...

Monday night through Tuesday...VFR. SW winds 5-10 KT Monday
night increase to 15-20 KT with gusts up to 25 KT Tuesday
afternoon.

Tuesday night and Wednesday...Mainly VFR. Rain possible south
and east of I-95 Tuesday night through Wednesday morning with
MVFR or lower conditions. SW winds 5-10 KT become NW Wednesday
afternoon and increase to 10-20 KT.

Wednesday night and Thursday...VFR. NW winds gradually
diminishing. A storm system is currently expected to remain
offshore. Forecast confidence: Medium to High.

Friday...VFR. NW winds 5-10 KT.

&&

.MARINE...
Occasional gale-force gusts continue on the waters, so the
current gale warning has been unchanged with the 1230 am update.
Expecting to replace gale warning with a small craft advisory,
which may extend through Monday night, especially for the
Atlantic waters.

Outlook...

Monday night through Tuesday night...Brief lull in SCA
conditions late Monday night through Tuesday morning. Otherwise,
25-30 KT wind gusts, primarily on the ocean waters, may
possibly over Lower DE Bay Tuesday afternoon.

Wednesday...Lull in SCA conditions possible Wednesday morning.
Otherwise, winds shift to the NW and increase to 15-20 KT with
25-30 KT gusts.

Thursday through Friday...Sub-SCA conditions expected.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Gale Warning until 5 AM EST early this morning for ANZ430-431-
     450>455.

&&

$$
Synopsis...MPS
Near Term...CMS/Robertson/O`Hara
Short Term...Robertson
Long Term...MPS
Aviation...CMS/MPS
Marine...CMS/MPS


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