Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 130540

Area Forecast Discussion
National Weather Service New York NY
1240 AM EST Wed Dec 13 2017

High pressure builds in overnight and Wednesday. A low
approaches late Wednesday night and moves offshore Thursday.
High pressure follows through early Friday. Low pressure
develops southeast of the region late Friday and moves offshore
Saturday. High pressure follows Sunday. Low pressure approaches
Monday through Tuesday.


With the initial arrival of the cold air, and onset of cold
advection, wind gusts have peaked as high as 35 kt since 00Z.
Gusts will then be 25 to around 30 kt through the overnight as
cold advection weakens a little, and winds at the top of the
mixed layer remain 25 to 30 kt.

Overnight an upper trough, strong shortwave and surface trough
shift in. Lift and CAA along with moisture from the ocean, LI
sound, and even Lake Erie could produce snow showers over the
area. Wind chills late tonight fall into the teens.


Wednesday begins with an upper low over the Northeast and a
deepening surface low entering SE Canada. There could be some
snow showers over LI and SE CT in the morning before the upper
low shifts east and overall lift diminishes. Dry, cold and windy
otherwise. 850mb temperatures will fall to around -14 to -15C,
but partial sunshine and a strong downsloping wind should
somewhat temper the cold air mixing down from aloft. Highs
expected to range from the upper 20s well inland to mid 30s for
coastal sections. Wind chills start around 10 in the morning and
average around 20 in the afternoon. It appears that wind gusts
should fall short of advisory levels. Model consensus has the
top of the mixed layer at 40-45 kt for a few hours during the
midday hours so it would take just about complete efficiency for
advisory-level gusts to mix down to the surface. Sustained
winds could get close to advisory criteria for some coastal
spots, but thinking is that this would not be a widespread event
for any given forecast zone.


Amplified progressive pattern will under go some
deamplification through the period. A series of shortwaves pass
through roughly every 36 to 48 hours. The first feature will
yield low pressure off the Mid Atlantic coast Thursday morning.
Due to fast progressive flow and a lack of blocking, expecting
this system to remain weak and not sufficiently develop until it
is way offshore late Thursday. A light snowfall is therefore
expected as any accumulations at this point looks to be light.
The least chance of seeing minor accumulations at this time
appears to be for far northern zones. Plenty of cold air in all
levels, thus p-type is expected to be all snow.

Recent model data has trended offshore for the most part with
potential system for later Friday and Friday night. Thinking the
new 12z Canadian which shows a decent low spin up closer to the
coast is an outlier at this point, but later shifts will have
to monitor model trends in case less energy gets sheared and the
system stays in tack. A reinforcing shot of cold air will come
in behind the departing low Friday night into early Saturday.

Then the pattern undergoes deamplification with more of a semi-
zonal look. Shortwaves will approach with similar timing as
mentioned earlier, but should track further north as there
associated frontal boundaries swing through. With some soft
ridging temperatures overall should rebound to more seasonable
levels for the second half of the weekend. A cold front will
swing through sometime on Monday, but there are timing
differences among models, which should be expected with very
progressive nature of the flow. With warmer temperatures late in
the period for Tuesday p-type may be wet along the coast, with
a wintry mix inland with the system to remain relatively weak.
No arctic air on the map late in the period as air of more
pacific origin gets involved, paving the way for a stretch of
more seasonable temperatures late in the period.


Deep low pressure tracks toward the Canadian Maritimes tonight
as the cold front continues to move farther out into the

A few widely scattered snow showers will be possible tonight but
looking for mainly dry and VFR conditions. However, there may
be some MVFR ceilings which will scatter out overnight.

Brief W-NW gusts near 35 kt have been observed, but expecting
most gusts to settle near 30 kt for overnight and pick up again
for Wednesday, with gusts near 35 kt and perhaps a few gusts up
to 40 kt.

.Wednesday night-Thursday morning...MVFR/IFR in snow, mainly
overnight Wednesday night through early Thursday morning. W
winds 15-20 kt gusts 25-30 kt Wednesday evening. W winds then
decrease to 10-15 kt gusts to 20 kt overnight Wednesday night.
.Thursday afternoon-Friday...Mainly VFR. NW wind G15-20kt into
early Thursday night, then winds diminish.
.Friday night...MVFR/IFR possible in snow.
.Saturday-Saturday night...VFR. NW winds G20-25kt during day,
G15-20KT during evening.
.Sunday...MVFR or lower possible in rain/snow.


With the onset of cold advection wind gusts on the ocean waters
have reached 33 to 40 kt. Gale force wind gusts will continue
through the remainder of the overnight as cold advection
continues. Winds may diminish a few hours late tonight as winds
at the top of the mixed layer subside.

A gale warning will remain in effect overnight on the ocean
waters, with a small craft advisory on the remainder of the
forecast waters. Gale gusts become likely on all the forecast
waters Wednesday.

Looks like NY Harbor, Western LI Sound and the South Shore Bays
will be falling below gales by Weds evening, so have shortened
the warning to cover through the afternoon. SCA conds will
otherwise prevail for those waters. The rest of the waters have
been kept under a gale warning through Wednesday night, although
some of these zones could get canceled at some point during the
night. Therefore any advisories should be down for Thursday
morning as pressure gradient relaxes. With low pressure
developing and offshore seas building Thursday night seas may
come up enough that a Small Craft Advisory may be needed,
especially for southern and eastern marine zones.

Another low pressure system 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. Small Craft Advisories may very well
stay up through Sunday into Sunday night with a sufficient
gradient on a SW flow, especially for eastern most zones.


No significant hydrologic impacts expected.


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


MARINE...Small Craft Advisory until 6 AM EST early this morning for
     Gale Warning from 6 AM this morning to 6 PM EST this evening
     for ANZ335-338-345.
     Gale Warning from 6 AM this morning to 6 AM EST Thursday for
     Gale Warning until 6 AM EST Thursday for ANZ350-353-355.


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