Area Forecast Discussion
Issued by NWS Upton, NY

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804
FXUS61 KOKX 301744
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
144 PM EDT Thu Mar 30 2017

.SYNOPSIS...
High pressure slides east across the region today. Low pressure
will impact the area tonight through Saturday. High pressure
builds in for the second half of the weekend. Another low
pressure system may impact the area Monday night and Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Forecast remains on track.

Northern stream shortwave energy slides east today with surface
high pressure building in from the west.

Tranquil and near seasonable conditions today as high pressure moves
across. Increasing high and mid clouds expected in the afternoon
ahead of the next approaching system.

Highs generally in the lower to mid 50s under light winds...deep
mixing and late March solar insolation. If high clouds come in
a little thicker or earlier, temps could run slightly a degree
or two cooler than forecast.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/...
Models in general agreement with a Central Plains closed
low tracking through the Mid-Mississippi River valley today and
into the Ohio Valley tonight. The low then begins to interact
with the northern stream as it slides to the east coast by
Saturday morning. Subtle difference exists in this
interaction...which manifests in some timing and track
differences with the resultant coastal surface low as it
develops over the Virginia Piedmont Friday and tracks just south
of the area Friday Night into Sat am.

The main impact from this system for most of the area will be a
soaking windswept rain developing late tonight into Friday morning
and continuing through Friday night before tapering off
Saturday morning. Ensembles generally clustered between 1 1/4
and 1 3/4 inches of liquid...with locally up to 2 inches
signaled.

Across interior portions of NE NJ/Lower Hud Valley/Southern
CT...thermal profiles will be marginally cold enough to
complicate the forecast late Tonight through Friday Night. Storm
track will be critical...with a farther south track favoring
cooler and more wintry mix potential...while a track over NYC/LI
favors warmer thermal profile and more in the way of plain
rain.

At onset of steadier precip lat tonight/Fri am...evap cooling
will increase the likelihood for a wintry mix of rain/sleet and
snow across interior. Operational models all indicating warming
aloft as Fri morning progresses but to differing degrees. With
that said...March boundary layer warming should allow for precip
in most areas to become mainly rain by afternoon...except
elevations above 750-1000 ft or so.

Late Fri Aft into Fri night...dynamics increase as closed low energy
approaches and frontogenetic banding strengthens over the area
to the north of the developing coastal low. At the same time
low-level winds backing to the ne/n should allow the boundary
layer to slowly cool. The question will be whether diabatic
processes can overcome warming aloft from se llj to bring a
period of heavier sleet...possibly mixed with snow. 90 percent
exceedance snow/sleet forecast covers this low potential for a
period of complete changeover to wintry precip. Another concern
is that temperatures across interior...particularly higher
elevations...could haver around/drop to freezing Fri
night...introducing the potential for freezing rain. Marginal
temp profiles will not support efficient icing...but the threat
of hazardous travel from light icing exists.

Based on above...this is a low confidence snow/sleet accum
forecast for the interior. At this point the most likely outcome
is varying combinations of mixed precip during the period with
a light slushy accum of sleet/wet snow/fzra across the
interior...especially elevations above 750-1000 ft...by the
time everything winds down Sat am.

In terms of winds...easterly winds increase Friday as a tight
gradient develops between high pressure anchored over Se Canada and
developing coastal low pressure. 950 hpa llj appears to peak at 45-
50 kt across LI/SE CT late Friday into Friday Night. Across the NYC
metro and much of the coast...gusts of 30 to 35 mph are likely and
peak gusts to 40 mph possible. The llj peaks across Eastern LI and
coastal SE CT Fri Night...with gusts of 30 to 40 mph likely and a
few peak gusts to 45-50 mph possible. Highest winds will be
along immediate coast.

The easterly winds will result in minor coastal flood concerns for
the Fri night high tide. See tides and coastal flood section.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Good model consensus that low pressure will be tracking south of
Nantucket on Saturday, then further out to sea Saturday night. As
this occurs, light precipitation on the back side of the system will
taper off through the day. Time heights show a lack of deep lift,
and therefore no ice crystals introduced, precluding snow. As a
result, some mixing with sleet was included in the forecast early
Saturday, with mainly rain expected across the majority of the area.
Any precipitation ends by Saturday evening at this time, with fair
weather then settling in for Sunday as high pressure builds in.
Increasing clouds on Monday as both northern stream and southern
stream moisture converge on the region. The ECMWF and GFS then
continue a latitude battle with regard to the southern stream low,
with the GFS about 150 miles north of the ECMWF. As a result of this
uncertainty several days out, the probability for rain has been
limited to the chance category. High pressure builds in in the wake
of the low for Wednesday, then another low may impact the area at
the end of the week. Increasing clouds with a low chance for rain
have therefore been forecast for Thursday. A blend of guidance was
used for temperatures except for Monday night, where the forecast
was weighted heavily towards to raw model data. In this case, the
potential clouds and rain would likely limit the climatology
influence evidenced by MOS.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
VFR conditions are expected through the first half tonight as
high pressure remains over the region. MVFR conditions move in
after 06Z Friday as low pressure approaches. Conditions
gradually fall towards daybreak, with IFR conditions or less
developing after 14-15z in rain. At KSWF, a mix of rain and ice
pellets will be possible.

South-Southeast flow develops this afternoon with wind speeds
remaining below 10 kt. Winds lighten this evening and overnight
with speeds falling to 5-6kt or less. Late tonight, the wind
direction becomes more easterly, and during the morning hours on
Friday, speeds will increase to 10-15kt, with gusts into the 20
kt range.

     NY Metro Enhanced Aviation Weather Support...

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

KJFK TAF Comments: Timing of southerly flow could off by +/-
1-2 hours.

KLGA TAF Comments: Timing of southerly flow could off by +/-
1-2 hours.

KEWR TAF Comments: Timing of southerly flow could off by +/-
1-2 hours.

KTEB TAF Comments: Timing of southerly flow could off by +/-
1-2 hours.

KHPN TAF Comments: Timing of southerly flow could off by +/-
1-2 hours.

KISP TAF Comments: Timing of southerly flow could off by +/-
1-2 hours.

.OUTLOOK FOR 18Z FRIDAY THROUGH MONDAY...
.Friday afternoon...MVFR/IFR conditions in rain in the
afternoon. SE winds G20-25 KT at the coast.
.Friday night and Saturday...MVFR/IFR in rain with gradual
improvement by Saturday afternoon/evening. E/NE winds G20-30KT
at coastal terminals.
.Sunday-Monday...VFR.
.Tuesday...Chance of sub-vfr in rain.

&&

.MARINE...
Winds and seas into tonight as high pressure moves across the
waters.

An approaching low pressure system will increase the pressure
gradient across the waters and expected to bring SCA conds to
the nearshore waters Friday morning and likely continuing into
Saturday. Gale force winds gusts are possible across the ocean
waters Fri afternoon through Friday night...warranting a gale
watch. Ocean seas expected to build to 7 to 11 ft on the ocean
and 3 to 5 ft on LI sound Friday night with good easterly
fetch.

Marginal gales will be possible into Saturday, before
conditions subside to Small Craft Advisory levels by Saturday
night. High pressure building over the waters will result in
relatively light winds Sunday into Monday. Seas at or above 5 ft
may linger into the day on Sunday on the ocean, then all waters
will fall below Small Craft Advisory criteria for Sunday night
and Monday. Winds will increase on Monday night and Tuesday as
low pressure passes near the waters.

&&

.HYDROLOGY...
Between 1 1/4 and 1 3/4 inches of QPF is likely from tonight
through Saturday morning...with locally around 2 inches. While
there could be some urban and poor drainage nuisance flooding,
flood impacts along rivers and streams are not expected. Heavy
rain during the time of high tide Friday Night could exacerbate
roadway flooding in low-lying coastal communities.

Additional rainfall of around an inch is possible Monday night
and Tuesday.

&&

.TIDES/COASTAL FLOODING...
An approaching low pressure system is likely to bring
widespread minor coastal flood impacts to vulnerable coastal
communities along lower NY/NJ harbor, southern bays of LI/NYC,
and western LI Sound during the Friday Night high tide cycle.
Locally moderate flooding is possible along the southern bays
of western LI.

Tidal departures of 1 to 1 1/2 ft are needed to reach minor
flood levels and 2 to 2 1/2 ft for moderate flood levels during
that time.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...Small Craft Advisory from 11 AM Friday to 8 AM EDT Saturday
     for ANZ330-335-338-340-345.
     Gale Watch from Friday afternoon through Saturday morning for
     ANZ350-353-355.

&&

$$
SYNOPSIS...12/NV
NEAR TERM...MD/NV
SHORT TERM...NV
LONG TERM...12
AVIATION...JP
MARINE...12/NV
HYDROLOGY...12/NV
TIDES/COASTAL FLOODING...



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