Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 200916
AFDOKX

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

.SYNOPSIS...
An upper level disturbance approaches today and 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/...
Subsidence inversion around 925 mb continues to lock about a 1kft
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 begins to erode a bit late this morning and early
afternoon so this should help diminish some of the low clouds.
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/...
A shearing shortwave will move ne of the region Sat with ridging
moving in at the surface and aloft. Weakening shower activity should
be pushing east Sat morning...but lingering cloud cover is likely
with weak waa under subsidence inversion. Temps likely run well
above seasonable in the upper 40s to lower 50s...with potential for
mid to upper 50s for NE NJ/NYC metro with breaks of sunshine.

Main focus is on a storm forecast to take shape as strong Pacific
jet energy crashes onshore the California coast Fri/Sat...allowing
an upper level system to close off over the southern plains this
weekend and then negatively tilt up the east coast Monday into
Tuesday. The result will be strong low pressure tracking through the
southern states this weekend...and then tracking up the eastern
seaboard towards the area Mon/Tue. Models in general agreement with
the synoptic fields...but spread exists in timing/intensity of the
closed upper low as it move up the coast...which is playing out in
model spread on track/speed of the surface low Mon/Tue.
SBU ensemble sensitivity points towards a strong wave packet
emanating from shortwave energy developing off the eastern Asia
coast as the determining factor. Have stayed close to ensemble
means with forecast...with general trends of low pressure
tracking a bit farther se and slower than 24 hours ago.

Potential exists for multiple impacts including strong winds, heavy
rain (wintry mix NW) and minor to locally moderate coastal flooding,
with likelihood of occurrence in that general order. In terms of
heavy precip...PWATs of 3-4 stds above normal are being signaled as
the system taps into tropical connection to the Caribbean and
Eastern Pacific. The potential for 1 to 3 inches of qpf with higher
amounts continues.

Based on strong dynamics and se trend towards a track over or just
se of LI...potential for thermal profiles to diabatically cool to
near freezing and result in a rain/snow mix well N&W of NYC for a
portion of this event...with even a period heavy wet snow across NW
hills. Still too much uncertainty at this point to talk
amounts...but at least a low potential exists for an accumulating
wet snow for the NW hills.

In terms of winds...GEFS signaling an easterly
llj 4-5 std above normal. EPS probabilities for wind gusts over 50
kt continue to run in the likely range Mon/Mon night for the
immediate coastline, and the extended 00z/12Z GFS/ECMWF MOS guidance
indicate sustained winds 25-30 kt along the coast and in the higher
interior elevations. This signals moderate potential for wind
advisory conditions for the coastal and low potential for high wind.
See coastal flood sections for potential coastal flood impacts.

Gradually improving conditions Tue aft into Tue night as the upper
low pivots ne and low pressure tracks up the New England coast.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
A disturbance moves across tonight with high pressure returning for
Saturday. MVFR will be persistent throughout much of the period and
with rain arriving this afternoon through the evening, there will be
an increasing potential for IFR/LIFR tonight 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.

     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: Unsure of 15-20Z VFR period in TAF. This could
have intermittent MVFR.

KLGA TAF Comments: Unsure of 15-20Z VFR period in TAF. This could
have intermittent MVFR.

KEWR TAF Comments: Unsure of 15-19Z VFR period in TAF. This could
have intermittent MVFR.

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

KTEB TAF Comments: Unsure of 15-19Z VFR period in TAF. This
could have intermittent MVFR.

KHPN TAF Comments: Unsure of 15-20Z VFR period in TAF. This
could have intermittent MVFR.

KISP TAF Comments: Unsure of 15-19Z VFR period in TAF. This
could have intermittent MVFR.

.OUTLOOK FOR 09Z SATURDAY THROUGH TUESDAY...

.Saturday-Saturday night...MVFR or lower possible early Saturday
morning, followed by VFR for the late morning through Saturday night.
.Sunday-Sunday night...MVFR or lower in -RA. IFR in any -SN inland.
Moderate rain develops at night, mixing with snow for interior. NE
gusts 20-25 kt by late Sunday afternoon and evening. NE gusts 30-40
kt Sunday night.
.Monday-Monday Night...IFR with moderate to heavy rain and strong E-
NE winds. NE gusts 35-45 kt.
.Tuesday...Rain tapering off with improving ceilings. Gusty NW
winds. Gusts up to 20-25 kt.

&&

.MARINE...
Easterly swell continues on the ocean waters with significant wave
heights between 4 and 6 ft. They should subside over the next few
hours, but will monitor trends to see if some of the SCA needs to be
extended past 11z.

Otherwise, a relaxed pressure gradient over the waters will keep
winds and seas below SCA levels through Saturday night.

An intensifying storm moving up the coast early next week will
likely bring easterly gales to all waters from Sunday night into
Monday night...with storm force wind gusts may be possible on the
ocean and eastern most Long Island Sound. If these winds come to
fruition...15 to 20 ft waves possible over the ocean waters...with
4 to 8 ft on LI sound.

&&

.HYDROLOGY...
No significant precipitation is expected through Saturday night.

Potential for significant rainfall of 1 to 3 inches, with locally
higher amounts with a coastal storm early next week. If high end
rainfall amounts are realized...this will present a threat for minor
areal flooding...including small stream flooding. Rises on larger
rivers would be likely...but at this point the potential for minor
flooding appears slight.

&&

.TIDES/COASTAL FLOODING...
A strong coastal storm may cause minor to locally moderate coastal
flooding if its highest surge coincides with the early morning high
tide cycles either Mon or Tue, which is the higher of the two daily
high tide cycles. Stevens ensemble guidance is showing potential for
surge building to 3 to 4 ft on Monday.

Surge needed to cause minor coastal flooding is about 1.5-2.5 ft on
the Long Island South Shore back bays, and 2.5-3.0 ft elsewhere.

Surge needed to cause moderate coastal flooding is about 2.5-3.0 ft
on the Long Island South Shore back bays, and 3-4 ft elsewhere.

The other concern will be for significant beach erosion from long
period easterly swells of 10-15+ ft Mon into Tue. Localized washover
cant be ruled out if higher surge values are realized.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...Small Craft Advisory for hazardous seas until 6 AM EST early
     this morning for ANZ350-353-355.

&&

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



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