Area Forecast Discussion
Issued by NWS Upton, NY

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323
FXUS61 KOKX 170920
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
420 AM EST Wed Jan 17 2018

.SYNOPSIS...
Low pressure tracks from south of Long Island into the Gulf of
Maine today. High pressure then builds in from the southwest
through Thursday, then to the south through Friday, and then
moves into the western Atlantic this weekend. A warm front will
slowly lift to the north on Monday, then a cold front will pass
through on Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Models remain fairly consistent on large scale details, so
general idea of a 700-500 hPa northern stream shortwave lifting
NE through New England helps to lift a coastal low NE, as it
slowly strengthens remains on track. With this general agreement
on large scale, have focused on Hires models (WRF-ARW, WRF-NMM,
HRRR, RAP, 3km NAM, HREF) for the details today.

For now no changes are being made to winter related headlines,
with only main change to snow totals being to increase over NE
NJ, where there is some increase in confidence of a change over
to all snow by 12Z, so this should allow for 1-3 hours of
moderate snowfall there.

Otherwise, main concern is where exactly does any banding set
up. Most of the hires models suggest to the N/W of NYC, however
there are a few that suggest could occur over northern portions
of NYC including various runs of the HRRR and the 00z HREF. As a
mitigating factor though, the NAM/GFS/ECMWF all have a 500 hPa vorticity
minimum moving over NYC and vicinity around 12Z. If this is correct,
this could serve to significantly limit snow totals during this time
frame. Latest radar trends do show precipitation currently struggling
to develop over eastern portions of NE NJ and NYC - so do see signs
of vorticity minimum`s influence.

The question remains can low level frontogenetic forcing over
come the impact of the vorticity minimum. Noting some stronger
returns all around the relative minima in reflectivity, there
are some indications that this could happen. Given mixed signals
in the data, will continue to monitor, but am prepared to make
short fused adjustments to winter headlines and snow amounts if
the evidence clearly points towards more snow over parts of NE
NJ/NYC/coastal SW CT.

So for now Winter Storm Warning remains for Orange/Putnam
Counties with 4-7 inches expected, and a Winter Weather Advisory
remains up for interior S CT and NE NJ and the remainder of
interior Lower Hudson Valley for 2-4 inches of snow (locally up
to 6 across interior S CT).

Highs toady should occur this morning, then low level cold
advection in the wake of the coastal low should lower
temperatures into this afternoon. As a result, cannot rule out
all locations possibly ending as a brief period of snow.

Highs today should be a few degrees below normal.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
A closed 700-500 hPa low tracks across the Mid Atlantic States
tonight, then to the SE of Long Island on Thursday, as northern
stream shortwave ridging builds into the area. Given that the
best forcing stays to the S tonight, and subsidence from the
ridging on Thursday, it should be dry both periods. AS far as
cloud cover goes, should see decreasing cloud cover tonight from
W to E, then mostly sunny sky conditions on Thursday.

Lows tonight should be up to 5 degrees below normal and highs on
Thursday around 5 degrees below normal.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Surface high pressure will build to our south through Friday. An
upper trough passing through Thu night into Fri morning should bring
an increase in clouds but no precip. Temps on Friday should be
slightly above avg.

As low pressure passes to the north and high pressure to the south
on Sat. expect a brisk W flow to develop, especially along the
coast. These winds should weaken on Sunday as the low continues
east. GFS and ECMWF disagree a little as to whether the flat upper
ridge to the south or the flat trough to the north will prevail, but
at any rate temps will continue to trend above avg into the weekend,
with highs 45-50 if not a little higher.

An amplifying but progressive upper trough entering the Plains
states late this weekend will send sfc low pressure NE toward the
upper Great Lakes. A leading warm front will approach our area late
Sunday night into Mon, and its progress may be delayed by high
pressure to the NE. There could be some spotty wintry precip inland
late Sunday night into Mon morning, otherwise light rain into Mon
evening. Followed slower model trend with a cold front trailing from
the Great Lakes low, with its approach late Mon night into Tue. We
could be setting up for a moderate to locally heavy rain event
during this time as the low taps Atlantic moisture via a
strengthening LLJ, with both it and difluent flow aloft likely
providing enhanced lift, and a possible triple-point low passing
nearby and concentrating low level convergence.

&&

.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
A developing coastal low will move north towards the region this
morning, tracking just southeast of Long Island early on today, and
then into the Canadian Maritimes this evening.

Conditions have lowered to IFR across city and western terminals
early this morning as precipitation has developed across region.
This will translate east through this morning.

Expect all snow at KSWF with LIFR conds, which may be moderate at
times through 14z. Between 5 and 7 inches of snow expected.

For NYC terminals and KBDR, rain likely mixing with snow towards
daybreak, and then changing to all snow after 11-13z with a coating
to an inch of slushy accumulation expected. For NE NJ terminals, IFR
conditions with a quick coating to 2 inches of slushy snow likely
during the morning push. Low potential for these amounts to be about
one inch higher, with LIFR conds.

KHPN likely to have a mix of rain/snow early this morning, before
changing to all snow with LIFR conds towards daybreak, with 2 to 3
inches of snow likely. Low potential for this amount to be about one
inch higher.

Mainly all rain expected for KISP/KGON with perhaps a brief
changeover or mix before precipitation ends by this afternoon.

Precip taper off from W to E Wed morning into early afternoon, with
conditions improving to VFR, with VFR late across far eastern
terminals.

E/NE winds becoming N/NNW for all terminals towards daybreak. Winds
become WNW with gusts near 20kt late Wed afternoon continuing into
Wed eve push, likely averaging just right of 310 magnetic.

...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: Timing of changeover to snow later in morning
push may be off by +/- 1 hour.

The afternoon KJFK haze potential forecast is YELLOW...which implies
slant range visibility 4-6SM outside of cloud.

KLGA TAF Comments: Timing of changeover to snow later in morning
push may be off by +/- 1 hour.

The afternoon KLGA haze potential forecast is YELLOW...which implies
slant range visibility 4-6SM outside of cloud.

KEWR TAF Comments: Timing of changeover to all snow later in morning
push may be off by +/- 1 hour.

The afternoon KEWR haze potential forecast is YELLOW...which implies
slant range visibility 4-6SM outside of cloud.

KTEB TAF Comments: Timing of changeover to all snow later in morning
push may be off by +/- 1 hour.

KHPN TAF Comments: Timing of VFR conditions may be off by +/- 1 to 2
hours.

KISP TAF Comments: Timing VFR conditions may be off by +/- 1 to 2
hours.  Precipitation may end as a brief period of snow early this
afternoon, but confidence too low to put in TAF.

.OUTLOOK FOR 06Z THURSDAY THROUGH SUNDAY...
.Late Tonight-Thursday Night...VFR. WNW winds G20KT on Thursday.
.Friday...VFR. W-SW winds G15-20KT possible.
.Saturday-Sunday...VFR.

&&

.MARINE...
A moderate pressure gradient over the waters will allow winds on
the ocean waters to reach up to 20 kt with gusts up to 25 kt
through Thursday and over the non-ocean waters up to 15 kt with
gusts up to 20 kt through Thursday. With seas on the ocean
waters forecast to 3 to 7 ft as well, have extended the Small
Craft Advisory (SCA) there through 6PM Thursday.

Marginal SCA conds may continue on the outer ocean waters into Thu
night and Fri. SCA conds are also likely Sat into Sat evening on the
ocean as a moderate W flow develops between low pressure passing well
to the north and high pressure building to the south.

&&

.HYDROLOGY...
No significant hydrologic impacts are expected through at least
the upcoming weekend.

A frontal system passing through Mon night into Tue has potential to
bring over an inch of rain during that time. Mainly nuisance
urban/poor drainage impacts expected attm.

&&

.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...Winter Weather Advisory until 1 PM EST this afternoon for
     CTZ005>008.
NY...Winter Weather Advisory until noon EST today for NYZ069-070.
     Winter Storm Warning until noon EST today for NYZ067-068.
NJ...Winter Weather Advisory until noon EST today for NJZ002-004-
     103>105-107.
MARINE...Small Craft Advisory until 6 PM EST Thursday for ANZ350-353-
     355.

&&

$$
SYNOPSIS...Goodman/Maloit
NEAR TERM...Maloit
SHORT TERM...Maloit
LONG TERM...Goodman
AVIATION...JE/NV
MARINE...Goodman/Maloit
HYDROLOGY...Goodman/Maloit
EQUIPMENT...



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