Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 180025
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
725 PM EST Sat Feb 17 2018

.SYNOPSIS...
Low pressure passes to our south tonight, followed by high
pressure for Sunday. The high then slides offshore through
Monday. A warm front approaches from the southwest Monday night,
then lifts to the north Tuesday, followed by a slow moving cold
front crossing the area during the middle week. High pressure may
briefly build in to close the upcoming work week, before another
frontal system approaches for the weekend.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
No significant changes with this update. Mixed signals from
model guidance continue on northward progress of rain/snow
line, with latest HRRR keeping it safely south of LI, 18z RAP/NAM
riding it to NYC south coast and north between LIE/Sunrise
across LI, while 18z GFS rides it north across NYC/NJ metro and
into coastal CT.

Have maintained continuity with current forecast and middle of
the road NAM solution. KDIX Dual-pol indicating mixing line
across Southern NJ getting towards southern LI/Queens by 02z,
but some erosion of this is indicated as it works north. The
other thing to note is possible coastal front formation just
south of LI, with winds having turned N/NE across NYC/NJ metro
and LI. This could become the battle ground for the rain/snow
line later this evening. So no clear indication to change snow
amounts at this time.

Meanwhile moderate to heavy snow banding beginning to develop
across northern portions of NYC/NJ metro and expanding towards
southern portions of the Lower Hud and SW CT over the next
couple of hours. Snowfall rates of 1 to possibly 2 inches per
hour are possible with rapid deterioration in road conditions.
One inch of snow being reported at CPK as of 7pm, in about 2
hrs.

Otherwise...Low pressure emerges off of the Mid-Atlantic coast
and moves northeast, passing just south of the 40N/70W
benchmark. The storm will pass by relatively quickly as there
will be a lack of downstream blocking to slow it down.

PCPN types, particularly over coastal areas, remain a challenging
forecast with boundary layer temperatures marginally supportive of
snow. This along with a slight increase of QPF for some of the
models complicates headline decisions. Models have generally nudged
north with the track of the low with the 12z suite. Most noticeable
however is that although the NAM has nudged north, it`s thermal
profile in the boundary layer has warmed up significantly over Long
Island and to a lesser extent elsewhere. The rest of the models are
fairly similar to their respective previous runs regarding the
boundary layer temps and temps farther aloft. So with only NAM
having a notably changed, changes to the snowfall forecast will be
greater over coastal sections of the city and Long Island, including
the North Fork. Snow amounts for these areas have been lowered.

For a few hours within both sides of midnight, the strongest
frontogenetic upward lift shifts through the tri-state area with the
heaviest PCPN. Light onshore winds will also be shifting towards
north during this period. Therefore expecting boundary layer and
surface temps to drop a degree or two at this point, shifting the
boundary from all snow to a rain/snow mix farther south. Banding
potential exists, however the window of opportunity for this to
occur seems brief with the quick advancement of frontogenetic
forcing and the orientation of the jet streak aloft. Best chance for
banded PCPN appears to be over Long Island and SE CT, but thermal
profiles at its onset might not be supporting all-snow before
cooling down.

A blend of NAM 2-meter temps with SuperBlend looked good for surface
temps through the event - generally on the colder side of guidance.
Thermal profiles support all snow for all areas north of the city
and most of NE NJ. Mixing with rain could still occur for part of
the CT coast as well as areas within about 10 miles of the city. A
mix of rain and snow is otherwise forecast, and not even out the
question for a complete changeover to rain over parts of Long Island
and southern parts of the city. Should the warming trend of NAM
hold, may very well need to make mention of a complete changeover in
these areas and lower snow amounts.

A winter storm warning remains in effect where it had already been
posted, with the exception of northeastern Suffolk County where a
winter weather advisory is in effect. Did not have enough confidence
to downgrade elsewhere. All areas that had been in a winter storm
watch are now also under a winter weather advisory. The winter wx
advisory over Orange and Putnam Counties remain, however a good
portion of Putnam County might exceed 6 inches.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Ridging at the surface and aloft occurs on Sunday, with high
pressure shifting offshore on Sunday night. Dry weather through the
period with a mostly clear sky. Sided with the colder side of
guidance for temperatures due to a fresh snow pack.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Anomalously strong Bermuda/East coast ridging develops early this
week and continues into midweek, in response to deep western
troughing. The western trough will gradually shear NE in piecemeal
fashion for the late week/weekend, resulting in a transition to a
bit more suppressed but active confluent sw upper flow.

At the surface, high pressure slides off the coast on Monday, and
then becomes established under the upper ridge by midweek. Meanwhile
a slow moving frontal system will cross through the Mississippi
river valley the first half of the week. Its warm front likely
approaches Monday Night with rain and then gets pushed north of the
region on Tuesday. An unseasonably warm SW flow is likely Tue/Wed
under deep layered ridging, with highs around 70 possible for NYC/NJ
metro and areas N&W on Tue and lower to mid 70s on Wed. Meanwhile
south coastal areas may struggle to get out out of the 50s due to
the cold water temp. Coastal advection stratus/fog and nocturnal
radiation fog is possible during this time as well.

Models in general agreement with the eastern ridging breaking down
enough, to allow a stream of shortwave energy to approach and push
an approaching cold front towards and through the region Wed night.
This likely brings another round of rain to the region Wed night
through Thursday.

Potential for brief ridging Friday, before next shortwave and
associated frontal system affect the region on Saturday with another
round of rain.

Temps during the period expected to average well above average, with
high temps potentially 25-30 degrees above normal Tue and Wed for
NYC/NJ metro and areas N&W..

&&

.AVIATION /01Z SUNDAY THROUGH THURSDAY/...
Low pressure off the DELMARVA deepens and passes south of Long
Island overnight. The low passes well to the east Sunday morning
as high pressure builds to the west.

IFR to LIFR and even VLIFR at times in light to moderate snow.
A rain snow mix is possible along the south shore of Long Island
and into southern NYC, and possibly into southeastern CT. There
is some uncertainty if the rain/snow mix will occur. The
precipitation ends late overnight into early Sunday morning, 08Z
KSWF to 12Z at KGON. Conditions quickly become VFR.

Snowfall rates could reach 1 inch, to as high as 2 inches, per
hour during the heaviest snowfall.

Total snow accumulations:

4 to 6 inches, KJFK, KISP, KSWF, KGON

6 to 8 inches, KEWR, KTEB, KLGA, KHPN, KBDR.

Southeast winds around 10kt will continue this evening, then as low
pressure passes south of Long Island, winds shift to the NE and N.
As the low passes to the east winds become NW 10-15 kt with gusts 18
to 25 kt during the day Sunday. Gusts and winds diminish late in the
afternoon, and become light and variable by 00Z Monday.

.OUTLOOK FOR 00Z MONDAY THROUGH THURSDAY...
.Sunday night-Monday...VFR. Becoming MVFR to IFR in rain and
possibly fog Monday afternoon.
.Tuesday...MVFR or IFR possible in the morning and again at night,
otherwise VFR with SW wind G20KT possible.
.Wednesday...VFR. Chance of showers with MVFR.
.Thursday...MVFR in rain.

&&

.MARINE...
SCA has been issued for the ocean waters for Sunday morning into
early afternoon with increasing winds and seas. There could be some
gusts to 25 kt Sunday morning over the Eastern Sound and eastern
bays, but coverage didn`t warrant an advisory. Sub-sca conds
otherwise tonight and from late Sunday afternoon through Sunday
night.

Generally sub-advsy winds expected Mon through Wed, but a persistent
15 to 20 kt SW flow may have sea building to SCA levels Tue/Wed.

Seas should subside Wed night into Thu as a weak cold front passes
through and winds become offshore.

&&

.HYDROLOGY...
Around 1/2 to 3/4 an inch of liquid equivalent is expected Saturday
night with locally higher amounts. With most, if not all of this
falling in the form of snow, no hydrologic impacts are expected.

No significant hydrologic impacts are expected from Sunday through
the end of next week.

&&

.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 Storm Warning until 7 AM EST Sunday for CTZ005>012.
NY...Winter Weather Advisory until 7 AM EST Sunday for NYZ067-068-
     074-075-079>081-178-179.
     Winter Storm Warning until 7 AM EST Sunday for NYZ069>073-078-
     176-177.
NJ...Winter Storm Warning until 7 AM EST Sunday for NJZ002-004-006-
     103>108.
MARINE...Small Craft Advisory from 6 AM to noon EST Sunday for ANZ350-
     353-355.

&&

$$
SYNOPSIS...JC/NV
NEAR TERM...JC/NV
SHORT TERM...JC
LONG TERM...NV
AVIATION...19
MARINE...JC/NV
HYDROLOGY...JC/NV
EQUIPMENT...


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