Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 132053 AAC
AFDPHI

Area Forecast Discussion...UPDATED
National Weather Service Mount Holly NJ
353 PM EST Sat Jan 13 2018

.SYNOPSIS...
High pressure over the Midwest tonight will build eastward and into
the Northeast states Sunday and Monday. A clipper system is expected
to track from the Great Lakes region Monday night to New England
Monday night by Tuesday night. An area of low pressure is forecast
to develop off the eastern seaboard on Wednesday. High pressure
builds in from the Southeast U.S. Thursday before eventually moving
offshore Friday into Saturday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
High pressure continues to build toward the area tonight, but will
not fully build across the area. Our area will remain on the eastern
edge of the high, which will keep a steady north-northwest breezy
across the area overnight. Cold air will continue to filter in
across the area tonight, with locations along and south of the I-95
corridor in the low to mid teens, and north of I-95 corridor in the
single digits. The steady breeze and temperatures overnight will
combine to produce low wind chills overnight and through daybreak
Sunday. Wind chills will drop below zero for most places, or single
digits for areas with lighter winds. Carbon and Monroe, Pennsylvania
are expected to drop to -15 degrees or lower wind chills, so we have
issued a Wind Chill Advisory overnight. Dry conditions are expected
tonight as drier air moves in as the high tries to build into the
region.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
A cold but dry day is expected on Sunday as high pressure to our
west begins to lift to our north through the day. The high still
never fully builds across the area, so a 10-15 mph breeze will
continue through the day. It will remain chilly with temperatures in
the 20s and teens. The coldest wind chills will be in the morning,
but it will still feel cold through the day with the added wind.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Sunday night and Monday... High pressure centered over SE Canada
will ridge southward down the eastern seaboard. Dry and cold
conditions will prevail in this setup. Forecast lows Monday night
are in the single digits at higher elevations along/north of I-80
and teens elsewhere. Wind chills will be roughly 10 degrees lower
than ambient air temperatures given the light winds (under 10 mph)
that are expected. Mid to high clouds will likely overspread the
area late in the day Monday. Highs range from the mid 20s in NE
PA/NW NJ to mid 30s from about Phila, S/E.

Monday night through Tuesday night...An alberta clipper low is
expected to track eastward across the Great Lakes Monday night,
before passing to our north on Tuesday. The latest 12Z guidance
continues to advertise a band of snow showers ahead of the attendant
cold front that moves eastward through the area Tuesday into Tuesday
night. PoPs were increased only slightly from previous forecast
given some uncertainty with timing of the cold front and with
uncertainty regarding how much of these pre-frontal snow showers
will be maintained downstream of the Fall Line (some of the guidance
indicates this activity dissipates as it approaches I-95).

Wednesday and Wednesday night...Shortwave energy digging toward the
base of an eastern U.S. longwave trough will lead to some
amplification of the upper pattern. Height falls downstream of the
upper trough should induce secondary low pressure development
somewhere off the Southeast or Mid-Atlantic coast on Wednesday. The
The ECMWF still remains the most bullish with coastal cyclogensis
though the 12Z run has trended more progressive with the upper flow
pattern and thus farther offshore (which is more in line with the
other NWP guidance). PoPs were decreased slightly following the
offshore trend in the track of the low. However, this system still
bears watching, especially considering how models have had a clear
westward bias with coastal storms so far this winter in the medium
range (before eventually trending closer/stronger within 3 days of
the event).

Thursday through Saturday...High pressure builds across the
Southeastern U.S. late in the week. The broad circulation of the
anticyclone will keep our region under its influence during this
time, resulting in a dry period. Temperatures will moderate Friday
and Saturday as winds back out of the W-SW and then S in response to
the high starting to move offshore.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Tonight...VFR. Northwest winds 10-15 knots, with gusts 20-25
knots. Gusts may drop off through this evening and overnight for
most areas. High confidence.

Sunday...VFR. Northwest winds 10-15 knots. Winds are not expected to
be gusty through most of the day.  High confidence.

Outlook...
Sunday night...VFR. N-NE winds 5-10 kt. High forecast confidence.

Monday and Monday night...Onshore flow should allow marine stratus
deck to advect/ expand inland. Probability of MVFR CIGs increases
closer to the coast. There is some indication of IFR CIGs,
especially closer to the coast during the afternoon and night. High
forecast confidence for sub-VFR CIGs for ACY and MIV, decreasing to
moderate confidence for I-95 TAF sites and to low confidence for
RDG-ABE.

Tuesday and Tuesday night...Low clouds may erode on Tuesday. But
with snow showers likely to move eastward thru the area, a period of
MVFR or IFR restrictions is possible. A wind shift from SW to NW
occurs with cold fropa Tuesday night. Gusts to 20 kt possible in
wake of fropa. Moderate forecast confidence.

Wednesday and Wednesday night...Low pressure should develop off the
coast. There is still high uncertainty regarding the track of the
low and how close the western fringe of the precip (snow) shield
makes it. The latest trends is to keep majority of precip offshore.
VFR would be likely if the system stays far enough offshore. Gusty
NW winds. Low forecast confidence.

Thursday through Friday...VFR. High forecast confidence.

&&

.MARINE...
Small Craft Advisory will remain across the waters tonight as winds
are expected to gust 25-30 knots. Winds are expected to drop below
advisory levels on the upper Delaware Bay by daybreak Sunday, and
lower Delaware Bay by the early afternoon. The Atlantic Coastal
Waters may hang on to advisory level winds and/or seas through the
afternoon hours.

A Freezing Spray Advisory is in effect for tonight through Sunday
morning as the combination of cold temperatures and gusty winds are
expected to create periods of moderate freezing spray across the
waters.

Outlook...
Sunday night through Tuesday...Winds and seas are forecast to
remain below SCA criteria. Light freezing spray expected Sunday
night and Monday morning.

Tuesday night through Thursday night...NW winds increase behind a
cold front late Tuesday night. A SCA may be needed with gusts to 25
kt. The likelihood for a SCA increased Wednesday night into Thursday
if an offshore low deepens to our east.

Friday...No marine hazards anticipated.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...Wind Chill Advisory from 9 PM this evening to 10 AM EST Sunday
     for PAZ054-055.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory until 6 PM EST Sunday for ANZ450>455.
     Freezing Spray Advisory from 11 PM this evening to 1 PM EST
     Sunday for ANZ430-431-450>455.
     Small Craft Advisory until 7 AM EST Sunday for ANZ430.
     Small Craft Advisory until 1 PM EST Sunday for ANZ431.

&&

$$
Synopsis...Klein
Near Term...Robertson
Short Term...Robertson
Long Term...Klein
Aviation...Klein/Robertson
Marine...Klein/Robertson


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