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Area forecast discussion 
National Weather Service New York New York
608 am EST Sat Dec 20 2014

Synopsis...
high pressure remains over the region this weekend into Monday. A
wave of low pressure weakens as it approaches Monday night into
Tuesday. A more significant storm system then affects the area
Tuesday night through Wednesday night...with its effects still felt
on Thursday. High pressure builds in on Friday.

&&

Near term /until 6 PM this evening/...
high pressure to the north shifts east toward Maine. The main
forecast challenge today will be the amount of cloud cover. Some
moisture is being trapped under an inversion near 900mb...and models
in general agreement that moisture increases below this during the
afternoon. Will go with a partly to mostly cloudy forecast. Quantitative precipitation forecast
output also indicates very light precipitation chances over The Forks
region/southeast CT towards sunset...but will maintain a dry forecast for
the daytime. Mav/NAM MOS blend looked good for high temperatures.

&&

Short term /6 PM this evening through Sunday night/...
we will have a chance of precipitation for some areas tonight...but very
light. During the evening...weak lift...mainly over the eastern
areas...will be supplied by a shortwave in the low-middle levels. This
combined with low level moisture in place will bring a chance of
flurries/light snow over Long Island and southeast CT. Then for the
overnight hours...chances of flurries/light snow spread west with
low level moisture advection and increasing positive vorticity advection from the west.
Thinking is that precipitation type would favor flurries/light snow over
drizzle in spite of moisture lacking in the middle levels. Looks like
coldest temperatures in the saturated layer beneath the low level inversion
will be cold enough to support ice crystals. Boundary layer temperatures
would also tend to favor flurries/light snow.

Middle level shortwave passes through Sunday morning with
flurries/drizzle possible...then negative vorticity advection behind it and deep-layered
ridging likely prevents precipitation in the afternoon. High temperatures near
normal. Dry weather continues into Sunday night with high pressure
ridging.

&&

Long term /Monday through Friday/...
weak low pressure forms along a frontal boundary off the Carolina
coast on Monday and begins to head north. Meanwhile...high pressure
centered to the northeast shifts farther east. This will low level
winds here veer to the east and aid low level moisture advection. A
surface trough extending north from the center of the low will serve
as a focus for weak lift and moisture convergence. Low chances of
light rain result...but better overall chances occur during the
afternoon.

The low center weakens as it heads north Monday night and Tuesday.
However...isentropic lift ahead of the low still strengthens...low
level moisture still increases on an east-northeast flow...and the surface
trough will still be in the vicinity. Probability of precipitation for Tuesday might be a
little too high...but they are still reasonable...and currently do
not have the confidence to lower probability of precipitation from the current low-end
likely. Better overall chances of rain occur in the afternoon. Precipitation
type is rain...except a mix of rain and snow late Monday night and Tuesday
morning for the northern fringe zones. Might need to monitor for the
possibility of minor coastal flooding starting with Tuesday
morning's high tide cycle as a persistent easterly flow strengthens.

A stronger storm system then affects US Christmas evening and day.
Models show a strong primary low center moving through the Great
Lakes region on Wednesday before moving into Ontario on Thursday. A
weaker secondary low pressure center or triple point low forms in
the middle Atlantic region before passing over or just north of US
Wednesday night. This will drag a cold front across the area during
the night. Categorical probability of precipitation for Wednesday/Wednesday evening before the cold
front moves through...then just low chances of showers after
midnight...mainly east of the city. Rainfall expected to be mostly
on the moderate side with minor urban flooding possible. For
Thursday...a strong fetch off of Lake Erie could bring a rain or
snow shower to the far northwest portion of the County Warning Area Thursday morning.

Breezy conditions for Wednesday through Thursday. A strong low level jet pushes
through the eastern zones Wednesday aftn/eve...but strong gusts will
probably have a tough time mixing down through an inversion.
Stronger gusts may be realized during Thursday behind the cold front
courtesy of better mixing. Marginal advisory criteria gusts could be
possible Wednesday afternoon through Thursday. Temperatures during this period will be
above average. Highs on Wednesday in the 50s...and with a moderate west-southwest
flow and partial sunshine on Thursday...high temperatures well into the 40s
expected.

High pressure builds in for Friday with lighter winds and closer to
normal temperatures.

&&

Aviation /11z Saturday through Wednesday/...
high pressure builds over the area for this weekend and departs early
in the new week.

Ceilings will vary between scattered-broken 2500-3500 feet as moisture remains
trapped under an inversion. Best chances for VFR ceilings will occur
from around 15z-22z today. Increasing chances for widespread broken-
overcast MVFR ceilings 2500-3000 feet tonight.

North to NE winds at 6-8 knots today will become light/vrb late this
afternoon and through tonight.

Light snow and/or flurries possible for kgon/kisp after midnight
tonight.

Outlook for 12z Sunday through Wednesday...
Sunday...MVFR ceilings...with light snow/rain showers possible for
city/coastal terminals. Light northeast winds.
Monday...MVFR or lower possible in east-southeast flow. Chance of rain.
Monday night-Tue...IFR likely at times in rain...ceilings and visibilities. East-southeast
winds 10-20 knots.
Tuesday night and Wednesday...high impact event possible...IFR likely with
heavy rain and gusty southeast winds 25kt+ shifting to SW late Wednesday.

&&

Marine...
with high pressure building over the waters...can expect north winds 10
knots or less through the weekend along with seas generally 1-2 feet on
the ocean or 1 feet or less on all other waters.

Easterly flow develops on Monday as low pressure passes south of the
waters early in the new week. Small Craft Advisory conds develop on
Tuesday as gradient tightens. Another low develops over the
middle-Atlantic area and tracks north through the waters Wednesday and
Wednesday night. That low rapidly deepens over Canada on Thursday...
allowing the gradient to tighten over the waters. Small Craft
Advisory conds expected through Wednesday...and then gales are
likely Thursday and Thursday night.

&&

Hydrology...
no Hydro impacts expected into early next week.

Around a quarter inch of rain is possible is possible Monday
afternoon through Tuesday as low pressure passes south of Long Island.
No hydrological impacts expected.

Rain on Wednesday/Wednesday night may average 1-2 inches across the region.
Minor urban flooding is possible.

&&

Okx watches/warnings/advisories...
CT...none.
New York...none.
New Jersey...none.
Marine...none.

&&

$$
Synopsis...jc
near term...jc
short term...jc
long term...jc
aviation...mps
marine...mps
hydrology...jc

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