Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 131818
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
118 PM EST Wed Dec 13 2017

.SYNOPSIS...
Strong low pressure moves into the Canadian Maritimes today. A
weaker low pressure system tracks south of the area late tonight
and then moves offshore Thursday, followed by high pressure
briefly Thursday night into Friday. Low pressure passes
southeast of the region Friday night into Saturday, followed by
a second wave of low pressure late Sunday and Monday. High
pressure briefly builds over the region Monday night, followed
by another chance of unsettled weather for mid- week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
There have been a few reports of flurries as there is just
enough moisture around 4-5kft. As the surface low lifts further
north, drier air should continue to work in from the west. This
is already being seen on visible satellite imagery across
eastern PA and NE NJ.

An arctic airmass is already in place as observed by the 12z
OKX RAOB. The sounding observed an 850 mb temperature of -17C,
which is about a degree higher than the min observed value of
-18.2C for December 13 per SPC Sounding Climatology page, which
dates back to 1957.

Anomalous upper level trough lifts into the Canadian Maritimes
this afternoon and evening with brief height rises ahead of
another strong vort max swinging across the Great Lakes.

Forecast soundings show mixing to 4-5kft, with winds around 40
kt at this layer. Strong pressure gradient between the departing
low and high pressure to the south will lead to gusts 35 to 40
mph today. A few locations could see gusts up to 45 mph, but not
widespread enough for an advisory.

Have lowered high temperatures several degrees with this update.
Highs will be in the middle and upper 20s for most locations
away from the coast, with highs around 30 across Long Island,
NYC metro, and coastal Connecticut. These temperatures combined
with the wind will make it feel like the single digits this
morning and teens this afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
Winds are forecast to diminish tonight ahead of approaching
clipper low. Mostly clear conditions to start the night will
give way to increasing clouds ahead of the low. Temperatures
fall into the teens to low to mid 20s.

Strong vort max swings around the mean upper trough tonight with
just enough lift to develop light snow overnight from west to
east. Strong lift from the vort max should be enough to
overcome limited moisture. Cold atmosphere in place should lead
to a higher ratio, dry light snowfall into the early morning
hours. Current mesoscale guidance indicates slightly better
moisture near the coast, so amounts could be a bit higher there.
Overall less than inch expected inland to around an inch at the
coast. This will be refined once the full 12z suite of model
data is available.

The light snow ends during just after sunrise with high pressure
building in behind the passing low. High temperatures on
Thursday will be in the upper 20s to mid 30s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
The long term period starts off with weak high pressure over the
region Thursday night into Friday. 13/00z forecast guidance
indicating a shortwave/upper level low diving out of Canada and
moving over the region Friday night into Saturday. As it does, a
surface low develops, passing well offshore. Thinking at this point,
that most of the region stays dry, but can not rule out a slight
chance of some snowflakes across the twin forks of LI and eastern
CT.

Weak high pressure and ridging then briefly builds back over the
region Saturday night into Sunday, allowing temperatures to rebound a
bit back to more seasonable levels. Another weak wave of low
pressure slides across region late Sunday through Monday. Both the
ECMWF and GFS capture this system, however the ECMWF is a bit more
robust with its precipitation. Also, there are some slight timing
issues with just how quickly the system moves into and out of the
region. With the warmer temperatures in place, do expect
precipitation to be either rain or a rain/snow mix along the coast,
with temperatures cold enough for some light snow inland.
Temperatures do warm enough on Monday for precipitation to
become rain everywhere.

Dry conditions then expected Monday night into the first part of
Tuesday. Additional light precipitation is then possible Tuesday
afternoon into Tuesday night as another shortwave slides across the
region. A few left over rain or snow showers will be possible on
Wednesday, however for now will keep conditions dry as weak high
pressure starts to build southwest of the region.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
VFR into this evening as low pressure well to the northeast and
its associated BKN045-050 cigs pull away, then IFR vsby in light
snow expected tonight into mid morning Thu as low pressure
passes just to the south.

W winds 20-25 kt with gusts around 35 kt should continue until
around 22Z-23Z, gradually diminish this evening, and then back
WSW before snow starts. Winds should become mainly light and
variable overnight with the arrival of the snow, then after the
low passes (close to 11Z-12Z Thu NYC metro) become NW and
gradually increase, with gusts close to 20 kt likely at the NYC
metros by late morning.

 ...NY Metro Enhanced Aviation Weather Support...

Detailed information, including hourly TAF wind component fcsts,
can be found at: http:/www.weather.gov/zny/n90

KJFK TAF Comments: Snow accumulation 1-2 inches after midnight
tonight into mid morning Thu.

KLGA TAF Comments: Snow accumulation 1-2 inches after midnight
tonight into mid morning Thu.

KEWR TAF Comments: Snow accumulation 1-2 inches after midnight
tonight into mid morning Thu.

KTEB TAF Comments: Snow accumulation 1-2 inches after midnight
tonight into mid morning Thu.

KHPN TAF Comments: Snow accumulation 1-2 inches late tonight
into mid morning Thu.

KISP TAF Comments: Snow accumulation 2-3 inches late tonight
into mid morning Thu.

.OUTLOOK FOR 18Z THURSDAY THROUGH MONDAY...
.Thursday afternoon and night...VFR with gusty NW winds, just
right of 310 magnetic, diminishing during the evening.
.Friday and Friday night...VFR.
.Saturday-Saturday night...VFR. WNW winds G25KT daytime and
G15-20KT evening.
.Sunday...VFR.
.Sunday night and Monday...MVFR or lower flight cat possible in
snow/rain.

&&

.MARINE...
Gale warning continues across all waters through tonight, with
NY Harbor, Western LI Sound and the South Shore Bays likely
falling below gales by this evening. Small craft advisory
conditions are then expected tonight with a weaker pressure
gradient.

Some leftover 5-6 ft seas continue Thursday night on portions
of the ocean waters, then conditions fall below SCA levels
Friday morning.

Low pressure, well south and east of the region intensifies Friday
night into Saturday as it moves northeast off the coastal waters.
Tightening pressure gradient and building seas will most likely
yield Small Craft Advisories, with possible Gales for the ocean
waters late Friday night and Saturday. Winds and seas then fall
below SCA levels on Sunday.

A series of weak wave of low pressure move across the area waters
Monday-Wednesday of next week. As each of these systems moves
across the area waters, Wind and seas may increase to SCA
levels.

&&

.HYDROLOGY...
No significant hydrologic impacts expected.

&&

.EQUIPMENT...
NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the
air for an extended period of time.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...Gale Warning until 6 PM EST this evening for ANZ335-338-345.
     Gale Warning until 6 AM EST Thursday for ANZ330-340-350-353-
     355.

&&

$$
SYNOPSIS...BC/Fig/DS
NEAR TERM...DS
SHORT TERM...Fig/DS
LONG TERM...BC
AVIATION...
MARINE...BC/Fig
HYDROLOGY...BC/Fig
EQUIPMENT...



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