Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 101231
AFDOKX

Area Forecast Discussion
National Weather Service New York NY
731 AM EST Sat Dec 10 2016

.SYNOPSIS...
A trough approaches today from the north and west and weak high
pressure then builds across tonight. The high slides offshore
Sunday as low pressure over the Midwest tracks northeast. The low
will track across the Great Lakes Sunday night into Monday,
sending a warm front through the region. A cold front will follow
for Monday afternoon into Monday evening. High pressure will then
briefly build in for Tuesday before another southern branch low
possibly impacts the area for the mid week period. An arctic cold
front then follows behind the system Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
The forecast is on track this morning. Only slight adjustments
were made to temperatures, dewpoints, and cloud coverage to better
match observed trends.

Today will feature increasing clouds and cold air advection as a
shortwave approaches from the west. At the surface, pressure
patterns convey an inverted trough approaching. The timing of
highest positive vorticity advection and the trough will enable for
an isolated snow shower or snow flurries this afternoon into early
this evening.

Used colder blend of guidance for temperature featuring the
ECS/NAM12 with highs maxing out 10-15 degrees below normal.

For tonight, the atmosphere will become drier with more subsidence
as weak high pressure moves across the region. With this high being
weak, going into daybreak Sunday, there will be mid to high level
clouds moving in ahead of the next low pressure system. Used a less
vast spatial variance with lows consisting of GFS MOS.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
High pressure slides offshore Sunday with an approaching warm
front. The warm front moves across Sunday night into Monday with a
subsequent cold front crossing Monday afternoon into early Monday
evening. Main upper level shortwave with more amplified pattern
and right rear quad of upper level jet moving in Sunday night into
Monday.

Dry conditions will be in place initially Sunday but the
precipitation chances will rapidly increase from afternoon into
evening. Initial precip type expected to be snow with cold airmass
in place and wet bulb cooling. Then a transition to rain will take
place from south to north Sunday night with some freezing rain across
the interior.

Monday the transition to rain continues for interior locations and
the rest of the event is expected to be mainly rain. The rain tapers
off Monday evening after the cold frontal passage. A more westerly
flow will advect in drier air.

Temperatures Sunday used the lower ECS and MAV guidance considering
limited vertical mixing. Used raw model temperatures of ECMWF and
NAM12 for Sunday night and Monday but for Monday blended it with
GFS MOS. GFS MOS was used for Monday Night. Expect non-diurnal
trend to temperatures Sunday night with temperatures slowly
rising.

Potential for interior locations to have 3-5 inches of snow and
some locations near interior NE NJ and Lower Hudson Valley could
see a period of freezing rain with light ice accumulations
possible of under a tenth of an inch. Elsewhere, looking at near 1
inch of snow or less with mostly rain anticipated. Bulk of precip
is Sunday night into Monday.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Upper air pattern will feature an anomalously high amplitude ridge
over Alaska, which will allow the polar vortex to drop southward
into central and eastern Canada by midweek. This will open the
door for the coldest airmass of the season for the second half of
the week. Prior to the time, an active southern branch of the jet
will take several pieces of energy from a shearing upper low
dropping southward across western Canada and into the Pacific
Northwest quickly eastward.

At the surface, this will be reflected as area of low pressure that
will track northeast across The Great Lakes and into eastern Canada
Sunday night into Monday. A warm front will approach from the
southwest during the same time, passing through the region on
Monday. This type of fast flow with the low tracking to the
northwest of the area is typically not conducive for a significant
snowfall event, especially at this time of year due to a return
flow developing off of warmer waters. The high is also transient
in nature and builds quickly off the coast on Sunday. There are
though differences amongst the models in how quickly the cold air
erodes over the region Sunday night into Monday morning, with the
operational NAM the most aggressive, and the GFS and ECMWF cold
enough to support several inches of snowfall across the interior.
There is also another potential wrinkle with weak low pressure
forming to the south along the warm front Monday morning. This
could potentially hold in the cold air a bit longer. The exact
timing of when the boundary layer warms sufficiently for a
changeover to rain is critical and allows for a fair amount of
uncertainty in the forecast. For the time, have taken a multi
model ensemble approach, discounting the warm NAM, for a Ptype
forecast. This results in a quick changeover to rain at the coast
Sunday night, and then a northward transition across the Lower
Hudson Valley and interior southern Connecticut near daybreak. A
deviation by 1-2 hours with strong overrunning precipitation can
have a significant consequence. At this time,the forecast calls
for 2 to 4 inches across northern portions of the Lower Hudson
Valley and interior southern Connecticut, with less than an inch
at the immediate coast. The trailing cold front passes through in
the late afternoon/early evening Monday with conditions drying
from west to east. Total liquid equivalent looks to be 0.50 to
0.75 inches.

High pressure will briefly follow for Tuesday with temperatures
just below seasonable levels. Another piece of Pacific shortwave
energy races across the country. Global models vary with the
amplitude of this feature with the nearly zonal flow in the
southern branch of the polar jet. The amount of phasing with the
northern branch looks to be the difference. Due to the fast flow,
there is likely to be uncertainty in the magnitude of this system
the next several days. For now. will run with a chance of
rain/snow Wednesday afternoon/night. Arctic air then spills
southward on the backside of the system Thursday into Friday with
daytime highs possibly not getting above freezing with overnight
lows in the single digits to teens. These values are 15 to 20
degrees below normal. To make matters worse, strong NW winds will
follow for Thursday producing very cold wind chill values.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
An upper level disturbance will move across today with BKN VFR
cigs 040-060 this afternoon and early evening. Scattered snow
showers/flurries possible with brief vsby reduction, most likely
18Z-21Z NYC metro and 20Z-23Z KISP. WNW flow left of 310 magnetic
just over 10 kt with gusts 15-20 kt also expected. Expect
diminishing winds and clearing skies thereafter.

.OUTLOOK FOR 12Z SUNDAY THROUGH WEDNESDAY...
***HIGH IMPACT WEATHER LIKELY LATE SUNDAY NIGHT INTO MONDAY
 MORNING***

.Sunday...Mainly VFR. Chance of light snow in the afternoon with
MVFR conditions likely, IFR possible late.

.Sunday night...IFR/LIFR conditions developing, with snow mixing
with and changing to rain at most terminals. LLWS possible late
NYC metros/coast. S winds G25KT late at the NYC metros and KISP.

Here are the most likely precip types and snowfall accumulations:

KISP/KJFK/KLGA/KBDR/KGON: Snow changing to rain by midnight, with
a coating to an inch accumulation.

KEWR/KTEB/KHPN: Snow mixing with and changing to rain late, with
1-2 inches accumulation.

KSWF: Snow with accumulation 4-5 inches.

.Monday...IFR conditions. Snow mixing with/changing to rain in
the morning at KSWF, and rain elsewhere. LLWS possible in the
morning NYC metros/coast. SW winds G25KT at the NYC metros and
KISP, becoming W in the afternoon.

.Monday night...Still a chance of rain with MVFR or lower
conditions early at KISP/KGON. otherwise VFR.

.Tuesday...VFR.

.Wednesday...Chance of rain, changing to snow at night. NW winds
G20KT after midnight.

&&

.MARINE...
Minimal SCA conditions continue on all the ocean waters, with NW
winds gusting up to 25 kt and seas as high as 5 ft. This conditions
should last through this morning, then gradually subside from west
to east this afternoon. There is a chance these conditions could
last east of Moriches Inlet into this evening.

As a low pressure system approaches Sunday night, SW winds should
increase, with SCA conditions becoming likely, and gales possible
late at night into Mon morning. As the system passes, moderate W-NW
flow should bring at least SCA conditions to all waters on Mon.
Ocean seas will take longer to subside, and so SCA conds there
should last into Mon night and possibly tue morning out east.

Low pressure may develop over the Atlantic during mid week, which
could bring at least SCA conditions Wed night behind an arctic cold
front.

&&

.HYDROLOGY...
A half to an inch of rain and/or liquid equivalent is possible
Sunday through Monday.

&&

.EQUIPMENT...
New York City NOAA Weather Radio NWR transmitter KWO-35 is
experiencing intermittent outages.

&&

.OKX WATCHES/WARNINGS/ADVISORIES...
CT...None.
NY...None.
NJ...None.
MARINE...Small Craft Advisory until noon EST today for ANZ355.
     Small Craft Advisory until 6 PM EST this evening for ANZ350.
     Small Craft Advisory until 4 PM EST this afternoon for ANZ353.

&&

$$
SYNOPSIS...JM
NEAR TERM...JM
SHORT TERM...JM
LONG TERM...DW
AVIATION...Goodman
MARINE...Goodman/DW
HYDROLOGY...JM/DW
EQUIPMENT...


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