Scientific Forecaster Discussion
Area forecast discussion
National Weather Service New York New York
920 am EST Tuesday Dec 10 2013
weak low pressure passing south of Long Island today will bring a
moderate snowfall. High pressure will then build from the west
tonight...followed by an approaching cold front on Wednesday.
The front will move across Wednesday night...with high pressure
building in from the south and west through the end of the week.
A coastal storm will likely impact the region this weekend...with
high pressure returning early next week.
Near term /until 6 PM this evening/...
temperatures have not fallen off as far early this morning over
NYC metropolitan and Long Island as expected...so have introduced a brief
period of rain mixing with the snow at the onset at these
locations. Given increasing dewpoint depressions as dewpoints
continue to slowly fall across the region - would expect surface
layer to quickly fall to at or below freezing - supporting all
snow. Because any mix will happen while the precipitation is
light - it should have no impact on expected amounts. Therefore
snow accumulations have not been changed.
Latest radar trends show area of heavier snow stretching from northwest
Virginia into S central PA. This should reach western zones by middle
morning. Have noticed virga filling in ahead of this band...so
there is still some potential that the precipitation could begin
falling sooner than the arrival of this band.
Msas analysis at 10z had a 1013 low over western NC...with
pressure falls indicating it will track to the east-northeast then NE this
morning. The low will deepen as it tracks NE in response to an
approaching 700-500 hpa trough. Consensus of the models takes the
low to the southeast of the 40n/70w benchmark by early this afternoon. So
this...coupled with ongoing low level cold air advection will
yield a p-type of snow across the entire County Warning Area...with the possible
exception of maybe 1-2 hours this morning over the Twin Forks
where some rain might be mixed in before temperatures wet-bulb to
Temperatures are critical to this forecast - expect generally slowly
falling temperatures this morning once the precipitation begins...and
this will continue into this afternoon with the wet-bulbing of
temperature - especially noting that lower dewpoint air is being
advected in the lower levels. This will place northern zones in the
middle to upper 20s and southern zones around 30. This temperature fall
off will allow for ratios of around 12:1 across southern areas and
Rap/NAM/GFS/sref/ECMWF all show an area of fairly strong
frontogenesis from 700-500 hpa crossing the area this morning to
early afternoon...with the core if it tracing from just west of NYC on
NE into S CT.
Given the ratio and the banding - appears even with quantitative precipitation forecast around 1/3
of an inch across southern zones + and around .2" across northern
zones...expect a 3-5" snowfall across the entire County Warning Area with locally
higher amounts under stronger snow bands. The snow will move through
the region fairly quickly...mainly over a 6-8 hour period from
As a result have expanded the Winter Weather Advisory to the entire
County Warning Area and adjusted its timing. For NE New Jersey and the western lower Hudson
Valley - it is in effect from 11z-21z. For NYC/Long Island/eastern
lower Hudson Valley/S CT the advisory will be in effect from
12z-22z. The heaviest snow will occur during the snow bands with the
aforementioned 500-700 hpa frontogenesis maximum from 14-19z from SW
to NE across the area.
Short term /6 PM this evening through 6 PM Wednesday/...
conditions quickly clear out across most of the area this evening as
the low pulls out with weak ridging building in aloft the exception
will be lake effect clouds setting up across far northwest areas. Blustery
winds tonight across all but western interior areas in the wake of
the storm...with gusts of 20-30 miles per hour. Used the minimum of mav/met/NAM
2-meter temperatures for lows - with values around 10 degrees below
normal. Did not undercut guidance with snow cover due to winds being
a limiting factor. Wind chills overnight will range from 5 to 15
degrees - coldest north of NYC and warmest over eastern Long Island.
Weak ridging exits to the east Wednesday with a west-southwest flow setting up
aloft. This should keep most if not all lake effect flurries/-shsn
to the north of the area. Mixing will be limited to around 925-900 hpa
so highs will struggle to reach the 30 degree mark in most
locations...with coastal southeast CT and the Twin Forks struggling to
reach freezing. These highs are consistent with a blend of mixing
down from 925 hpa...except 900 hpa well inland...with NAM 2-meter
temperatures and a blend of mav/met guidance...and are around 15
degrees below normal. Wind chills Wednesday will mainly be in the
teens...with some around 20 across coastal southeast CT and the Twin Forks.
Long term /Wednesday night through Monday/...
the 500 mb flow across the eastern US will start with a broad trough and
become zonal by the weekend. Meanwhile a split flow out west will
phase with time as the northern branch digs from the Pacific northwest to The
Rockies and plains states. This phasing energy will be the area to
watch through the remainder of the week as signals for a coastal
storm this weekend are apparent.
At the surface...a cold front will traverse the area Wednesday night. Deep
layered moisture is lacking...therefore expect it to pass dry. A Few
Lake effect flurries may make it into far northwest Orange County. Cold
weather will continue through the end of the week with highs
struggling to reach the freezing mark on Thursday (h85 temperatures around
-15c). Sided with the colder guidance with the snow pack expected to
remain on the ground. Lows Wednesday night will generally be in the lower
teens inland to around 20 in metropolitan NY/NJ. Thursday night will be a tad
colder with less cloud cover with teens at the coast and single
digits inland. Wind chills both nights will drop into the single
digits across the area...and could fall below zero Thursday night across
interior zones. Conds will remain dry through Friday with high pressure
nosing in from the S and west.
As for the weekend system...the initial surface low associated with the
southern branch of the jet stream is forecast to develop in the Lee
of The Rockies and move across the Southern Plains Friday and Friday
night. As the northern stream energy phases with it and moves
towards the middle Atlantic coast...the system will strengthen and
track from the middle Atlantic coast just S and east of Li before lifting
towards Nova Scotia.
The devil is in the details and there is a 6-12 hour difference in
the timing between the operational ec...CMC and GFS. The track is
the most consistent aspect of the system and with a decent high to
the NE it should be cold enough at the onset for snow with a gradual
warming of the atmosphere. The timing and extent of the changeover
still needs to be resolved. The GFS is much warmer than the ec to
start which is a known bias of the model. It is likely eroding the
cold air too quickly. This will ultimately have impacts on
p-type...so have low confidence in the details of the current
High pressure then builds towards the area for the start of the new work
week with dry but cold and potentially gusty conds.
Aviation /14z Tuesday through Saturday/...
low pressure passing to the south will bring varying IFR/LIFR
conditions through midday...with improving conditions through the
afternoon into tonight.
Heaviest snow expected into early afternoon...tapering off from west to
east during the 17z-19z time period. IFR ceilings...with varying visible
between 1/4sm-3/4sm possible with the snow.
Light snow with IFR ceilings/visible through the afternoon...ending west to
east between 19-21z. Could see ceilings/visible improve +1-2 hours sooner
than in taf over entire area.
Runway snowfall totals generally 3-4 inches.
Clearing skies and VFR conditions after 21z.
Light west-northwest winds this morning...generally 8 kts or less. Winds
increase from the west-northwest after 21z...with gusts between 20-25 kts
lasting into tonight.
New York metropolitan enhanced aviation weather support...
Detailed information...including hourly taf wind component forecasts can
be found at: http:/www.Erh.NOAA.Gov/zny/n90 (lower case)
Kjfk fcster comments: brief periods of visible down to 1/4sm possible
through 17z with +sn. Winds may be light and vrb through 18z.
Klga fcster comments: brief periods of visible down to 1/4sm possible
through 17z with +sn.
Kewr fcster comments: LIFR conditions may end +/- 1 hour than in
Kteb fcster comments: LIFR conditions may end +/- 1 hour than in
Khpn fcster comments: LIFR conditions may end +/- 1 hour than in
Kisp fcster comments: LIFR conditions may end +/- 1 hour than in
Outlook for 15z Wednesday through Sat...
Wednesday...VFR. West winds g20-25kt.
Thursday...VFR. West-northwest g20kt.
Sat...MVFR or lower conditions with mixed wintry precipitation.
seas at 44017 still at just under 6 feet...so have extended the Small Craft Advisory
for anz-350 until 9 am and allowed for it to expire at anz-353.
After a lull with sub-Small Craft Advisory conditions this morning...wind gusts
increase late this afternoon to Small Craft Advisory levels on the coastal ocean
waters late this afternoon. Since this should be only a couple of
hours before the Gale Warning starts...it will not be of long
enough duration to warrant an Small Craft Advisory. Gusts to gale force are
expected tonight on the coastal ocean waters...with gusts to Small Craft Advisory
levels elsewhere in the wake of the departing low. The Small Craft Advisory level
gusts will likely continue on all waters Wednesday.
As a result have issued a Gale Warning for the coastal ocean waters
for tonight and a Small Craft Advisory for the remainder of the
waters. With confidence not quite at 80 percent...opted not to
extend the Small Craft Advisory into Wednesday on the non-ocean zones.
Small Craft Advisory may need to be continued into part of Wednesday night with winds and
seas diminishing. After a brief lull on Thursday/Thursday night...cold air advection is
expected to bring winds and seas back up with Small Craft Advisory likely need on at
least the ocean waters Friday. Conds then should remain at sub-advisory
levels until the coastal system this weekend although timing is
uncertain at this time. Strong gales are possible on the backside of
will receive from .1 to .4 inches of liquid equivalent
today...almost all of it in the form of snow. No hydrologic issues
are expected. It should then be dry through Friday.
There is the potential for significant precipitation this weekend in
association with a coastal low pressure system.
due to interference issues with the U.S. Coast Guard
emergency broadcast channel...the New York City National oceanic and atmospheric administration all hazards
weather radio station kwo35 remains out of service. The National
Weather Service is working with several U.S. Government agencies
to isolate and resolve this technical problem.
During this time...the transmitter may be returned to service
intermittently for use of the National oceanic and atmospheric administration all hazards weather radio
during potentially dangerous weather situations...for routine
weekly tests of the warning system...and to determine if the
interference issue has been resolved.
CT...Winter Weather Advisory until 5 PM EST this afternoon for
New York...Winter Weather Advisory until 5 PM EST this afternoon for
Winter Weather Advisory until 4 PM EST this afternoon for
New Jersey...Winter Weather Advisory until 4 PM EST this afternoon for
Marine...Small Craft Advisory from 6 PM this evening to 6 am EST
Wednesday for anz330-335-338-340-345.
Gale Warning from 6 PM this evening to 6 am EST Wednesday for