Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 172029

Area Forecast Discussion
National Weather Service New York NY
329 PM EST Sun Dec 17 2017

A warm front will approach tonight. Weak low pressure will then
track along the front near or over Long Island on Monday. A
cold front approaches on Tuesday, passes through the region
Tuesday Night, with high pressure then building in for the mid
to late week. A frontal system will likely affect the region for
the weekend.


Canadian high pressure was nosing southward into the eastern
half of the forecast area this afternoon. A large dewpoint
gradient exists as a result of this, with dewpoints near 8
across eastern Connecticut to around 25 in New Jersey.

Areas of rain were evident over northern Ohio in the vicinity of
an approaching warm front. The radar composite indicated the
atmosphere was saturating southwest of the forecast area.

There features are expected to combine to produce a light
precipitation event over the area from late tonight into early
Monday. The thermal structure of the airmass suggests snow at
the start then sleet and finally freezing rain and rain. A
Winter Weather Advisory has been issued for areas where light
freezing rain is expected, as even trace amounts can be
hazardous and require an advisory.

Along the south shore of Long Island and into the New York City
area, a developing light onshore flow looks to allow for the
transition to skip the freezing rain and go right to rain.
These areas were therefore excluded from the advisory.

There are a couple of ways the forecast can go wrong. The first
is that precipitation just doesn`t materialize, as there is not
a lot of strong lift with the system. The second is that the
precipitation ends before the warm nose allows for the
transition to freezing rain. The greatest threat for this
occurring is along and west of the Hudson River where the
precipitation should end quicker, and across eastern
Connecticut, where the cold air may just remain entrenched.

Any precipitation ends Monday morning, with temperatures rising
into the upper 30s and 40s as heights increase and southwest
flow develops.


Westerly flow and lingering clouds will serve to limit low
temperatures. With a lack of deep moisture and lift, the
forecast has been kept dry. The Superblend was used for
temperatures in order to account for the better low level mixing
due to the wind.


Models continue in good agreement with a broad but shallow northern
stream trough approaching the NE on Tuesday and sliding through New
England on Wednesday. At the surface, the primary low pressure/s
will track through Quebec/Ontario with a trailing cold front
approaching the region Tue, and crossing Tue Night. Models have
continued dry with this frontal passage for the local region with
the best forcing remaining north, and confluent upper flow over the
region. The one thing to watch is late Monday Night into early
Tuesday morning, the NAM is hinting at some stratus/drizzle with low-
level saturation in the waa pattern. At this point, a low prob of
occurrence, but if anything develops there would be a threat for
light freezing rain across interior as cold air will have a tough
time scouring out until daytime in this setup.

Upper trough swings through the NE Tue Night into Wed with a
glancing shot of modifying arctic air in the wake of the cold front
for Wed. Upper flow flattens for Thursday, and then becomes ridged
heading into Friday ahead of developing Western/Central Us
troughing. At the surface, high pressure builds in from the west Wed
into Thu, with dry and seasonably cold conditions.

Models continue in agreement with a pattern shift to a more
longitudinally oriented upper flow heading into the weekend, as
several pieces of northern stream energy continue to dig a deep
longwave trough into the Western/Central US. This may bring the
coldest air of the season into the Central US and Rockies US by
around Christmas.

The noticeable trend versus 24 hours, is that the troughing has
shifted a bit westward, with strong Western Atlantic/East Coast
ridging. There is still agreement on energy across the 4 corners
are mid-eek shearing towards the NE for the weekend, resulting in a
a frontal system affecting the region for the weekend. If this trend
holds, unseasonable warmth would be expected up the East Coast for
this Christmas weekend. Quite a bit of model divergence thereafter
on how this troughing and arctic air moves eastward for next


High pressure over New England will move east into tonight as a warm
front approaches from the SW. The warm front gradually moves north
of the area on Monday.

VFR through this evening. Ceilings will then lower to MVFR and
possibly IFR overnight. The exact timing of these conditions may be
off by 1-3 hours from the current forecast. There will be also be a
light wintry mix, especially after 06z. This should taper off from
west to east 11z to 14z.

Winds will continue to become light and variable at all terminals by
this evening. Winds may briefly become WNW Monday morning before
backing towards to the SW in the afternoon.

.Monday Afternoon...Mainly VFR with a low chance for MVFR.
.Tuesday...MVFR possible in the afternoon. SW G20-2KT possible.
.Wednesday-Thursday...VFR. NW winds G20-30KT probable Wednesday.
.Friday...MVFR possible in chance of showers.


Winds and seas will remain below Small Craft Advisory levels
through Monday as weak low pressure tracks in the vicinity of
Long Island. Westerly flow may increase to 25 kt by late Monday
night, especially on the ocean.

Moderate potential for SCA conditions developing Tue in
strengthening SW flow and building ocean seas ahead of approaching
cold front. Widespread SCA expected Tuesday Night through Wednesday
with tight pressure gradient and caa in wake of cold front. Marginal
gale gusts are possible Tuesday night into Wednesday.

Conditions then should gradually fall back below SCA Wed night into
into Thu, with relatively tranquil conds continuing into Fri as high
pressure builds towards the waters.

Next chance of SCA appears to be ahead of an approaching frontal
system for the weekend.


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.


CT...Winter Weather Advisory from 10 PM this evening to 7 AM EST
     Monday for CTZ005-006-009-010.
     Winter Weather Advisory from midnight tonight to 9 AM EST
     Monday for CTZ007-008-011-012.
NY...Winter Weather Advisory from 10 PM this evening to 7 AM EST
     Monday for NYZ067>071-078-079-177.
NJ...Winter Weather Advisory from 10 PM this evening to 7 AM EST
     Monday for NJZ002-004-103>107.


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