Area Forecast Discussion
Issued by NWS Upton, NY

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245
FXUS61 KOKX 130015
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
715 PM EST Tue Dec 12 2017

.SYNOPSIS...
High pressure builds in tonight 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.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
At 23Z the cold front was just east of the CWA and with cold
advection winds have begun to gust. The drier air behind the
front was taking a bit longer to move in and have adjusted dew
points upward through this evening.

Late tonight 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.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
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.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Deep low pressure tracks toward the Canadian Maritimes tonight
as a cold front moves east of the region.

Mostly rain showers will be scattered about initially but, by
mid to late evening, there will be a mix of rain and snow
showers. Mainly dry conditions are expected overnight into
Wednesday. For KSWF, most showers if they occur would be snow
showers. Mainly VFR for the TAF period but any of these showers
will present a brief period of MVFR visibilities.

W-NW flow continues tonight near 15-20 kt with gusts developing
around 25 kt, increasing late this evening and overnight to
30-35 kt range, mainly for the city terminals. Outside of city
terminals, wind gusts late this evening and overnight mainly in
25-30 kt range. Gusty west winds 20-25 kt will gust 30-35 kt
for Wednesday.

    NY Metro Enhanced Aviation Weather Support...

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

KJFK TAF Comments: MVFR possible this evening. Timing of gusts
in 30-35 kt range could be off by 2-4 hours.

KLGA TAF Comments: MVFR possible this evening. Timing of gusts
in 30-35 kt range could be off by 2-4 hours.

KEWR TAF Comments: MVFR possible this evening. Timing of gusts
in 30-35 kt range could be off by 2-4 hours.

KTEB TAF Comments: MVFR possible this evening. Timing of gusts
in 30-35 kt range could be off by 2-4 hours.

KHPN TAF Comments: MVFR possible this evening. Timing of gusts
in 30-35 kt range could be off by 2-4 hours.

KISP TAF Comments: MVFR possible this evening. Timing of gusts
in 30-35 kt range could be off by 2-4 hours.

.OUTLOOK FOR 00Z THURSDAY THROUGH SUNDAY...
.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 G15-20 kt 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.

&&

.MARINE...
No changes to the winds and seas at this time.

It appears that gale force gusts, mainly on the ocean waters,
could arrive a few hours before the time the previous gale
warning was set to begin. Have therefore decided to start the
gale warning on the ocean starting this evening. For the other
waters it will be a close call. Perhaps occasional gusts to 35
kt late at night, but not enough confidence to bump up the
timing of the start of the warning. SCA conds therefore prevail
tonight with gales following through 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.

&&

.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...Small Craft Advisory until 6 AM EST Wednesday for ANZ330-335-
     338-340-345.
     Gale Warning from 6 AM to 6 PM EST Wednesday for ANZ335-338-
     345.
     Gale Warning from 6 AM Wednesday to 6 AM EST Thursday for
     ANZ330-340.
     Gale Warning until 6 AM EST Thursday for ANZ350-353-355.

&&

$$
SYNOPSIS...GC/JC/JE
NEAR TERM...JC/19
SHORT TERM...JC
LONG TERM...GC/JE
AVIATION...JM
MARINE...GC/JC/JE/19
HYDROLOGY...GC/JC/JE
EQUIPMENT...



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