Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 120613
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
113 AM EST Tue Dec 12 2017

.SYNOPSIS...
Low pressure continues to approach overnight and this passes to
the north Tuesday. This intensifying low pressure system will
move into the Canadian Maritimes Tuesday night into Wednesday
night. A weak Alberta Clipper passes to the south on Thursday,
followed by a low pressure system passing off the coast Friday
night into Saturday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Only minor changes made to reflect current conditions.

Current radar imagery continues to show returns over Northeast
New Jersey and the Lower Hudson Valley, much of which is virga.
However, KSWF reported snow just before 06Z, so the lower to
middle levels are moistening. Many areas inland have risen to
around or just above freezing. However, there is still a good
spread between temperature and dew point for evaporational
cooling to occur. So, it seems reasonable to continue mentioning
a slight chance for snow inland, increasing to chance after 08Z.
Some flakes may mix in over northern portions of New York City,
but that is only a slight chance. By the time POPs increase to
chance, it will be warm enough to support all rain.

Shortwave tracks across IL and OH toward the region just ahead
of vigorous shortwave diving out of Canada toward the western
Great Lakes region. At the sfc, two low pressure centers
approach, passing across the Great Lakes region toward western
NY State by Tuesday morning. A warm front/WAA out ahead of the
lows/upper shortwaves will result in an increase in clouds, and
precipitation chances increase overnight.

Cold air initially across the northeast will warm as the night
progresses. Sfc temperature trends starting to level off with
temperatures in many locations holding steady. Expect the lows
to have pretty much already been established. Temperatures are
still expected to slowly rise overnight as south flow begins to
pick up.

At this time, precip looks to hold off until overnight per most
operational models solutions, and many GEFS members. Across the
interior, a few hours of snow is possible, with very low
chances toward the coast for either a wintry mix or plain rain.
Timing and placement will be the key as low levels warm ahead of
the lows.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/...
Upper trough closes off, as sfc low deepens across NY State and
into New England during this time frame.

QPF looks light, with lower amounts or even trace amounts near
the coast, and higher totals as you head north across the
interior. Would expect interior portions of southern CT, and the
lower Hudson Valley of southeast NY to observe higher qpf up to
a 1/4 of an inch liquid equivalent.

Early in the morning, a snow or rain snow mix will quickly give
way to plain rain as WAA continues, and the lower levels of the
atmosphere warm. With increasing southerly flow, temperatures
warm through the 40s, and coastal locations should warm to
around 50. Will lean toward the warmer guidance, and would not
be surprised to see some temps slightly higher than guidance.

Any precip tapers off as the day progresses, and drier air moves
in behind the low/front by evening thanks to NW flow. With the
upper low in the vicinity, a few snow showers are possible
overnight, but this should be isolated. Gusty NW flow and CAA
will commence.

Temps fall to the upper teens/20s across the region. Quite cold.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Wednesday begins with an upper low over the Northeast and a
deepening surface low entering SE Canada. There could be some snow
showers over eastern 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 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 in the single digits in the morning and average around 20 in
the afternoon. For now it appears that wind gusts should fall short
of advisory levels, but probably not by much, so it`s possible that
an advisory would be needed as the event draws closer. Some coastal
spots might even meet criteria based on sustained winds.

Winds subside Wednesday night and low temperatures will average 5-10
degrees below normal. A clipper low is progged to pass south of us
on Thursday. Models show some track and timing differences as well
as available moisture. Snow from this system may however stretch
north into our region, so will introduce low chances for this into
the forecast for southern sections. Like most clippers, this has the
potential for only light accumulations.

The active weather pattern continues on Friday with an upper trough
axis digging into the Ohio Valley and models showing low pressure
strengthening off the Mid-Atlantic Coast. The trough becomes
negatively tilted Friday night, but not before the low is pushed SE
of the 40N/70W benchmark. Upper jet winds/streaks are not currently
progged in a position to help expand the PCPN shield NW towards us
as they did with the storm this past weekend. Shortwaves embedded
within the longwave upper trough however may help spread PCPN closer
to us. Looks like this would be an all-snow event with best chances
of snow Friday afternoon and night. Too early to go with anything
higher than chance PoPs for this potential event.

A high pressure ridge follows for Saturday with dry weather, then
another low moves through the vicinity of the southern Great
Lakes/Ohio Valley regions on Sunday. Global models disagree on its
evolution and track, but will go with a dry forecast through Sunday.
A better chance of PCPN across the entire region appears to be
Sunday night or Monday as the low shifts east.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
Low pressure from the eastern Great Lakes to the Ohio Valley will
bring a warm front through the terminals late overnight into early
Tuesday morning. As the low moves into northern New England,
Tuesday, a cold front will move through during the afternoon.

VFR overnight. MVFR ceilings and visibilities develop early Tuesday
morning as precipitation, light rain, becomes steady, the exception
is KSWF where a period of rain and snow is likely. There is now more
confidence in rain at the terminals as warm air moves in ahead of
the cold front.

After the cold frontal passage snow showers will be possible,
even at the coastal terminals.

Southerly winds develop behind the warm front, then shift to
the west to northwest behind the cold front, and become gusty.
There is the potential for winds to gust as high as 30 KT,
especially Tuesday evening.

.OUTLOOK FOR 06Z WEDNESDAY THROUGH SATURDAY...
.Tuesday night-Wednesday...VFR, with a chance of MVFR in snow
showers. W-NW winds G20-30KT probable.
.Wednesday night-Thursday night...VFR. W-SW winds G15-25KT possible.
.Friday...MVFR possible with snow. W-NW winds G15-20KT possible.
.Saturday...VFR. NW winds G15-20KT possible.

&&

.MARINE...
Seas and winds are expected to remain below SCA criteria
tonight with a weak pressure gradient in place. Deepening low
pressure approaches from the west late tonight and Tuesday,
increasing the pressure gradient between the low center and the
high center out into the Atlantic. South winds increase, and
SCA conditions return during the day Tuesday all waters. Cannot
rule out a gale gust or two during the day, but as a front
approaches, winds may temporarily diminish. By nighttime
Tuesday, westerly winds pick up behind the deepening low as it
passes north across New England.

Gale Watch remains in effect for the ocean waters beginning
Tuesday night for increasing winds. The SCA continues for non
ocean waters, with wind gusts in the 25 to 30 kt range.

A gale watch remains in effect for all waters Wednesday and
Wednesday night with a strong NW flow behind low pressure
strengthening as it heads towards the Canadian Maritimes. SCA
conds will likely continue into Thursday morning for most of the
waters. Sub-SCA conds finally reach all waters by Thursday
night with the pressure gradient weakening. This relatively
tranquil period should continue through Friday. A coastal storm
may then impact the waters Friday night into Saturday with at
least SCA conditions developing.

&&

.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. See our headline news on
the web at: http:/weather.gov/nyc

The Riverhead NOAA Weather Radio Station WXM-80 (162.475 MHz)
is currently out of service. NWS technicians are working with
Verizon to restore service. An estimated time of return to
service is not currently known.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...Gale Watch from Wednesday morning through late Wednesday night
     for ANZ330-335-338-340-345.
     Small Craft Advisory from 6 AM this morning to 6 AM EST
     Wednesday for ANZ330-335-338-340-345.
     Gale Watch from this evening through late Wednesday night for
     ANZ350-353-355.
     Small Craft Advisory from 6 AM this morning to 6 PM EST this
     evening for ANZ350-353-355.

&&

$$
SYNOPSIS...JC/PW
NEAR TERM...JM/PW
SHORT TERM...PW
LONG TERM...JC
AVIATION...BC/19
MARINE...JC/JM/PW
HYDROLOGY...JC/PW
EQUIPMENT...



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