Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service New York New York
1238 am EST Tuesday Mar 3 2015

high pressure continues to build in overnight...drifting
over...then east of the region on Tuesday. Low pressure will track
well north of the area Tuesday night and Wednesday...with a warm
front lifting towards the area Tuesday night...followed by a cold
front Wednesday. The cold front slides to the southeast Wednesday
night...with waves of low pressure tracking along it into
Thursday. Canadian high pressure then builds in through Thursday
night...then slides to our south through Friday night. A cold
front approaches from the west Saturday...then crosses the area
Saturday night.


Near term /until 6 am this morning/...
high pressure over the Ohio Valley continues to build east. The
gradient will relax through the night...and northwest winds will
continue to diminish.

Mainly clear skies on tap for the overnight with possibly some
cirrus late. Winds will be light to calm. Combined with the
snowpack...thinking is that areas especially away from the city
will drop a few degrees lower than a mav/NAM MOS blend. Kept
overnight lows a few degrees colder than the colder mav MOS for
these spots. Even though temperatures are running higher than previous
forecast...think that combined snow pack...light winds...clear
skies will promote rapid radiational cooling before daybreak.
Therefore...only increased low temperatures by a degree.


Short term /6 am this morning through 6 PM Wednesday/...
warm air advection begins as the high pressure center moves offshore. With the warm air advection
aloft...clouds will quickly lower and thicken through the day. As
has been the case this winter...overrunning precipitation probably arrives a
little sooner than prognosticated by the models. Will therefore maintain
probability of precipitation increasing to likely/categorical across the area by Tuesday
evening...followed by categorical probability of precipitation Tuesday night...then probability of precipitation
lowering through the day with a cold front passage.

For quantitative precipitation significant differences among NAM/GFS/ECMWF/Canadian
although NAM is somewhat a wetter outlier. Went fairly close to rfc
guidance...which seemed to be close to a blend of these.

For temperatures through this event...didn't stray too far from what
was already in the grids. Thinking is that similar to the past
couple events...surface temperatures will likely be lower than shown by
mav/NAM MOS...with suny sb hi-res WRF closer to what will probably
happen. Aloft...wouldn't be surprised to see a colder trend by
models as we get closer to the event. But even so...thinking is that
the maximum temperature in the elevated warm layer will be plenty warm enough
for an eventual transition to brief sleet and freezing rain after
starting as all snow...then changing plain rain. By the time some of
the northwest suburbs surface temperatures rise above freezing...probability of precipitation should be on
the downward trend. Icing concerns will therefore be the greatest

Expecting generally 1 to 3 / 2 to 4 inches of snow across the area
along with a trace up to a tenth of an inch of ice. Only spot where
icing is not forecast is eastern Suffolk County...which is the only
area that will not have a Winter Weather Advisory with this update.
The advisory starts late Tuesday afternoon and continues into late
Tuesday night into Wednesday morning. Plain rain is expected Wednesday
afternoon anywhere precipitation is falling. Highs on Wednesday in the upper
30s to around 40.


Long term /Wednesday night through Monday/...
there is the potential for warning level snowfall across the entire
tri-state from Wednesday night into Thursday.

There is general model agreement that the cold front crossing the
area Wednesday night becomes anafrontal in nature and that at least
one wave of low pressure develops along the front near the
middle-Atlantic coast Wednesday night and tracks northeast into
Thursday. This is in part in response to a northern stream trough
which tracks from the Midwest Wednesday night to the northeastern US
by Thursday evening.

There are still differences in the details...including exactly how
far significant quantitative precipitation forecast gets pushed into northwest portions of the County Warning Area...and
how fast the precipitation turns over to all snow Wednesday night.

The NAM is the slowest to cool the low levels Wednesday night...but
has had a warm bias of late...the GFS/European model (ecmwf) are the fastest...with
the CMC somewhere in between. Have used a GFS/ECMWF/CMC blend for
thermal profiles with this system...and followed fairly close to HPC
quantitative precipitation forecast - refer to the hydrology section of the afd for details.
Generally expect rain to change to snow from northwest to southeast by around
midnight...maybe a tad slower over the Twin Forks and NYC. The
heaviest quantitative precipitation forecast and hence snowfall though will occur from around
midnight Wednesday night through Thursday morning in response to the
region being in the right rear quadrant of an around 200 knots 250 hpa
jet. Snow tapers off from northwest to southeast late Thursday morning/afternoon.

As noted in the opening to this section...have the potential for at
least 6 inches of snow across the entire area from Wednesday night
into Thursday - with the best chance over NYC/NE New Jersey/Long Island and
southeast CT. This threat will be highlighted in the severe weather potential statement.

Thereafter...northwest flow aloft Thursday night gives way to zonal flow by
Friday night. A northern stream shortwave approaches from the west
Saturday...then passes to the north Saturday night. With best forcing
currently prognosticated to the north...have kept things dry for now.

The European model (ecmwf) and GFS then diverge a bit...with the European model (ecmwf) more
progressive...with zonal flow or weak ridging...while the GFS has a
mean trough setting up over the east. They both agree on a northern
stream shortwave approaching Monday.

Given mixed signal...and general indication that for now more likely
to be dry then wet...went with a dry forecast Sunday/Sunday
night...then slight chance probability of precipitation ahead of the shortwave on Monday.
However...given changeability in models so far this year...would not
be surprised if forecast changes from the above.

What is more certain is that temperatures through the long term
remain below normal...with Thursday-Friday quite cold with
temperatures around 20 degrees below normal.


Aviation /06z Tuesday through Saturday/...
high pressure builds overhead this morning...then moves east. A warm
front approaches tonight...moving across the area late tonight.

VFR with mainly clear skies early this morning. Northwest winds continue to
diminish. Winds become light and variable by 12z with the high
moving overhead. Then winds turn to the southeast by afternoon at 5 to 10 kts.
Middle and high level ceilings will move in toward daybreak.

By late day...expect ceilings to lower to MVFR in light snow. IFR or
LIFR expected tonight...after 23-00z. Moderate snow transitions to a
wintry mix as the evening progresses.

Outlook for 06z Wednesday through Saturday...
tonight...IFR or lower likely. Wintry mix changing over to all rain
from southwest to northeast. Low level wind shear likely after 06z.
Wednesday...MVFR or lower possible in light rain in the morning.
Conditions may improve to VFR in the afternoon.
Low level wind shear possible in the morning.
Wednesday night-Thursday...MVFR or lower possible in any
precipitation...with a transition back to snow.
Thursday night-Friday...VFR. Northwest winds g15-20kt possible Thursday


Small Craft Advisory conds continue for western ocean waters/Li sound/New York Harbor/Li
bays until midnight...and for the central and eastern ocean waters
until 6 am Tuesday. 25 knots gusts will continue during this
time...and then seas will remain above 5 feet from midnight through
daybreak Tuesday.

For Tuesday....a tranquil period with high pressure passing over
the area...leading to light winds. Small Craft Advisory conds then return at least
on the ocean waters Tuesday night and Wednesday morning.

Small Craft Advisory conditions are likely on the coastal ocean
waters from Wednesday night and Thursday...and possible Thursday
night. Small Craft Advisory level seas could linger on the coastal ocean waters east
of Moriches Inlet into Saturday.

Elsewhere...sub-Small Craft Advisory conditions are expected from
Wednesday night though Saturday. There is however...a low chance of
Small Craft Advisory level gusts on the non-ocean waters Thursday night.


between about 1/2 to 3/4 inch of liquid equivalent precipitation is likely
late Tuesday through Wednesday morning. Some of this will fall as plain
rain...therefore the combination of this and some melting snow pack
could cause nuisance urban flooding issues across the metropolitan and
coastal plain Tuesday night into Wednesday morning.

From 1/3 of an inch over far West Orange County to around 1 inch over
Long Island of liquid equivalent is expected to fall from Wednesday
night into Thursday. Most if not all of this will fall in the form
of snow. Given the uncertainty in how fast the rain changes to snow
over NE New Jersey/NYC/Long Island...minor flooding cannot be completely
ruled out at this time.


Okx watches/warnings/advisories...
CT...Winter Weather Advisory from 3 PM this afternoon to 10 am EST
Wednesday for ctz005>008-011-012.
Winter Weather Advisory from 3 PM this afternoon to 6 am EST
Wednesday for ctz009-010.
New York...Winter Weather Advisory from 3 PM this afternoon to 10 am EST
Wednesday for nyz067>070.
Winter Weather Advisory from 3 PM this afternoon to 6 am EST
Wednesday for nyz071.
Winter Weather Advisory from 3 PM this afternoon to 2 am EST
Wednesday for nyz072>075-078-080-176>179.
New Jersey...Winter Weather Advisory from 3 PM this afternoon to 10 am EST
Wednesday for njz002-103.
Winter Weather Advisory from 3 PM this afternoon to 6 am EST
Wednesday for njz004-104.
Winter Weather Advisory from 3 PM this afternoon to 2 am EST
Wednesday for njz006-105>108.
Marine...Small Craft Advisory until 6 am EST early this morning for


short term...jc
long term...maloit
aviation...precipitable water

National Weather Service Glossary of Abbreviations