Area Forecast Discussion
Issued by NWS Upton, NY

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268
FXUS61 KOKX 201655
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
1155 AM EST Fri Jan 20 2017

.SYNOPSIS...
An upper level disturbance weakens as it moves across tonight.
High pressure briefly returns on Saturday. Deepening low pressure
moving across the southern states on Sunday will reach the
southern Mid Atlantic region by Monday morning, while high
pressure builds southeast into the Canadian Maritimes. The low
will intensify into a significant coastal storm as it moves up the
coast toward the area Monday through Tuesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

Minimal changes made to the forecast to reflect current
temperature/dew point trends. 17Z radar imagery showing
drizzle/light rain farther northeast than any hi-res or
deterministic model prediction. 12Z RAOB from OKX shows very dry
air above the inversion still in place, so expect any
precipitation that reaches the ground to be very light. Despite
the dry air, have adjusted chances of precipitation upward this
morning to account for any light rain that may fall before the
steadier precipitation later today as moisture slowly increases.

Subsidence inversion around 925 mb continues to lock about a 1 kft
layer of moisture in the form of stratus. Main challenge this
morning is if some of these clouds erode or continue to hang on.
Trends have pointing to the clouds hanging on into the morning as
depicted by thermodynamic profiles on the last several runs of the
RAP and HRRR. Latest forecast calls for mostly cloudy to overcast
skies this morning.

The inversion may begin to erode a bit by afternoon. However,
middle and high level clouds will be increasing ahead of the
approaching southern stream shortwave energy. This will lead to a
mostly cloudy sky condition this afternoon. A few breaks in the
clouds should help temperatures rise into the middle and upper 40s
this afternoon.

Best lift associated with the southern stream shortwave energy
arrives between 3 and 6pm across NYC metro, NE NJ, and Lower Hudson
Valley. Have increased pops to categorical here with a fairly sharp
gradient of lower further east as best lift and moisture lie in a
narrow corridor. High probability of measurable precipitation on
the latest ensembles and deterministic models supports categorical
pops. Temperatures warm enough for plain rain, which will be on
the light side.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Band of light rain progresses east this evening. Lift weakens as the
shortwave weakens and gets trapped under buildings heights aloft.
Pops diminish from west to east through night. A few hundredths to
a tenth of inch total forecast with this system. Low level
moisture lingers through the night and with weak flow some patchy
fog is possible. Temperatures will not be able to fall too much
due to linger clouds with lows forecast above freezing across the
CWA.

High pressure returns on Saturday. Mostly cloudy conditions continue
in the morning, but should see enough breaks in the afternoon for
some sun. With ridging building overhead, the airmass is quite warm.
Highs should reach the lower and middle 50s. The only negating
factor to seeing these temps would be if clouds hang on longer
through the day, which some of the higher res model profiles
indicate as a possibility.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Focus is on a storm forecast to take shape as strong Pacific jet
energy about to come onshore the California coast closes off over
the southern Plains Saturday night, then intensifies on Sunday and
moves up the coast on Monday, as downstream blocking via a full
latitude ridge over the western Atlantic and a closed low east of
Labrador prevent its escape out to sea. One leading low could
approach the Mid Atlantic region on Sunday and bring us some light
rain, then as the closed low becomes negatively tilted over the
Southeast states, the primary low should quickly intensify as it
rides up the coast. As this low moves up the coast, a sharp
pressure gradient between high pressure moving southeast from
Quebec toward the Canadian Maritimes will produce strong winds
along the coast and in the higher interior elevations, and cold
air moving southward with the high looks to make it into the area
from late Sunday night into Monday, with a period of wet snow
across the interior before warmer air moves in aloft. A surface
low track closer to the coast than the ensemble means, and slower
than operational models, is preferred attm.

Potential for strong, possibly damaging easterly winds, continues
to increase. EPS probabilities of sustained 40 mph winds and 60
mph wind gusts continue to remain high for the coastline and now
also for the highest interior elevations, and its probabilities of
45-mph wind gusts are over 50 percent over most of the area. Also,
both GFS and ECMWF MOS continue to predict increasing potential
for sustained winds over 30 mph Mon night into Tue morning,
perhaps as high as 40 mph at the peak of the storm across Long
Island/coastal CT/NYC metro.

A prolonged moderate rainfall of 1-3 inches remains likely as the
storm maintains a tropical moisture connection to the Caribbean
and Eastern Pacific. The rain could become heavy Mon
afternoon/evening especially east of NYC, and maybe even
throughout the region if the low tracks just a little farther
west. With colder air from the Canadian high filtering down into
the region, a period of wet snow is likely across the interior
from late Sunday night into Monday before warmer air moves in
aloft ahead of the low. Amounts should be light, but but as with
several other events this season, could be substantially higher in
the hills of Orange/Putnam/Rockland, and also farther north in
most areas just to the north.

Gradually improving conditions Tue afternoon/night as the low
passes to the northeast.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
A disturbance moves across tonight with high pressure returning for
Saturday. MVFR will be persistent throughout much of today and
with rain arriving this afternoon through the evening, there will
be an increasing potential for IFR and perhaps localized LIFR
tonight into early Saturday with residual low level moisture.

Winds are relatively light through the period at less than 10 kt,
gradually becoming more E-SE this afternoon into this evening and
eventually more southerly later tonight. Winds become variable in
direction overnight before becoming more westerly on Saturday.

 ...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: Mainly MVFR today. Chance that conditions
improve to VFR for a brief period between 15z-20z.

KLGA TAF Comments: Mainly MVFR today. Chance that conditions
improve to VFR for a brief period between 15z-20z.

KEWR TAF Comments: Mainly MVFR today. Chance that conditions
improve to VFR for a brief period between 15z-20z.

The afternoon KEWR haze potential forecast is GREEN...which implies
slant range visibility 7SM or greater outside of cloud.

KTEB TAF Comments: Mainly MVFR today. Chance that conditions
improve to VFR for a brief period between 15z-20z.

KHPN TAF Comments: Mainly MVFR today. Chance that conditions
improve to VFR for a brief period between 15z-20z.

KISP TAF Comments: Mainly MVFR today. Chance that conditions
improve to VFR for a brief period between 15z-20z.

.OUTLOOK FOR 12Z SATURDAY THROUGH TUESDAY...
.Saturday-Saturday Night...MVFR or lower possible early Saturday
morning, otherwise VFR.
.Sunday-Sunday Night...MVFR or lower in light rain, then moderate
rain at night. NE winds G20-30KT by late day, and G30-40KT later
at night.
.Monday-Monday Night...IFR conditions. Snow and rain for the
interior including KSWF, and moderate-heavy rain elsewhere.
ENE winds 25-30KT with G40-45KT.
.Tuesday-Tuesday night...Rain tapering off in the afternoon to
evening with improving ceilings. NW winds G25-35KT, highest in
the morning.

&&

.MARINE...
Seas will remain below SCA levels through Saturday night under a
relaxed pressure gradient.

An intensifying storm moving up the coast early next week will
likely bring easterly gales to all waters from Sunday night into
Monday night. Storm force wind gusts may be possible on the ocean
and the eastern Sound/bays. If these winds come to fruition, seas
as high as 15-20 ft could occur on the ocean waters, and as high
as 6-9 ft on the eastern Sound.

&&

.HYDROLOGY...
A coastal storm could bring rainfall of 1-3 inches early next
week. Most likely time frame of heaviest rainfall is Monday
afternoon/night. If the higher end of these rainfall amounts
occur, minor areal flooding would be likely.

&&

.TIDES/COASTAL FLOODING...
A strong coastal storm may cause widespread minor and locally
moderate coastal flooding with any of the high tide cycles from
Mon morning into Tue morning. Stevens Tech NYHOPS ensemble
guidance continues to show potential for a 3-4 ft surge on Monday
close to the times of high tide Mon evening. Since the amount of
surge needed to produce flooding is large, and since astronomical
tides with the evening high tide cycle are about 1/2 foot lower
than those of the morning cycles, exact timing of the strongest
winds will be critical to the eventual outcome. Because of the
timing uncertainty, right now it is wise to talk in terms closer
to that worst case scenario as is often done with tropical
systems.

Approximate surge needed to cause flooding (in ft). Please refer
to our Web page at http:/www.weather.gov/okx/coastal_flooding
for exact details:

                               Mon/Tue Morning
                            Minor        Moderate
Long Island
South Shore Back Bays:     1.5 to 2.5   2.5 to 3

Elsewhere:                 2.5 to 3     3 to 4

                                Mon Evening
                            Minor       Moderate
Long Island
South Shore Back Bays:     2 to 3     3 to 3.5

Elsewhere:                 3 to 3.5   3.5 to 4.5

The other concern will be for significant beach erosion from long
period easterly swells of 10-15 or more ft from Mon into Tue.
Localized washovers could also occur if higher total water
levels are realized.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...None.

&&

$$
SYNOPSIS...Goodman/DS
NEAR TERM...MMD/DS
SHORT TERM...DS
LONG TERM...Goodman/NV
AVIATION...BC/JM
MARINE...Goodman/DS/NV
HYDROLOGY...Goodman/DS/NV
TIDES/COASTAL FLOODING...



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