Scientific Forecaster Discussion
fxus61 kaly 150546
Area forecast discussion
National Weather Service Albany New York
1246 am EST Fri Dec 15 2017
seasonably cold temperatures will continue through the weekend.
Moderate accumulations of lake effect snow are likely in the
western Adirondacks Friday night into Saturday. Elsewhere,
mainly light snow showers will be possible Saturday as a cold
front passes through the region.
Near term /until 6 am this morning/...
as of 1240 am EST...lake effect clouds downwind of Lake Ontario
continue to impact portions of the Mohawk Valley, southern
dacks, capital region and upslope areas of the Berks and
southern greens. Most of the snowfall was into the western
Mohawk Valley as trajectories continue to remain west-northwest
with trends shifting northward overnight. So little changes to
the pop/wx grid. Did lower temperatures a couple more degrees as
kgfl was already below zero.
Prev disc...current water vapor imagery shows the mid/upper
level flow becoming zonal in the wake of last night's clipper
system. At the surface, high pressure is analyzed between the
southern Great Lakes and the Ohio Valley. With the loss of
daytime heating, winds have diminished to under 10 mph for most
areas. Winds will continue to diminish through the overnight
hours, especially as the pressure gradient slackens as the high
continues to nose in and most sites will become light or calm by
Lake effect clouds and snow showers have been limited over our
forecast area despite surface to 850 mb Delta-T around 25c given
the ambient dry airmass, but GOES-16 infrared satellite, mrms
imagery, and New York state mesonet webcams indicate light lake effect snow
continues to occur over the western Mohawk Valley and western
Adirondacks. This is supported by high-res models, so we will
continue chance pops for the western Mohawk Valley and western
Adirondacks into the overnight hours. Still do expect any
accumulation to be less than an inch given ambient dry air and
inversion heights only around 1.5 km.
As the surface high continues to nose in, a fairly good setup
for radiational cooling will occur tonight given expected mostly
clear to partly cloudy skies outside of the lake effect belt.
Also, areas in and south of the capital district received a
fresh coating of snow last night, further contributing to
cooling potential. Some uncertainty with wind speeds as the high
remains centered to the south, but generally have gone near or
below MOS guidance for forecast low temps tonight, especially
south of the capital district with most locations seeing mins in
the single digits.
Short term /6 am this morning through Saturday night/...
Friday, a negatively tilted midlevel wave will approach the
region. Low-level winds will back to southwesterly ahead of the
wave, briefly ending any lake effect snow over the local area.
Much of Friday looks to be dry with increasing midlevel clouds
ahead of the upper wave. Models are forecasting cyclogenesis
offshore of the Delmarva, but current indications are that
forcing associated with the cyclone will remain far enough
offshore to spare our forecast area any associated
precipitation. Some snow showers may redevelop over the western
Adirondacks late in the day due to moist upslope flow and weak
DCVA, but dry air is expected to win out over the remainder of
the forecast area. Another chilly day with highs mainly in the
20s, but less windy than previous days.
Friday night, the trough axis pivots through with an attendant
surface low tracking eastward north of the St. Lawrence Valley.
This should allow winds to back such that they align with the
long axis of Lake Ontario for a time late Friday night into
early Saturday. Surface to 850 mb Delta-ts will range from 20-25
c, and deeper moisture associated with the synoptic wave will
enhance lake effect processes, resulting in a period of moderate
to heavy lake effect snow for portions of the western
Adirondacks. However, residence time of favorable wind fetch
appears to be limited as midlevel heights begin to rise as early
as 06z Saturday, eventually allowing low-level winds to veer
northwesterly by Saturday morning and allowing the dominant band
to break up and shift southward. Current forecast accumulations
in the 3 to 6 inch range support holding off on any headline at
this time, although the potential will continue to be monitored.
Upslope snow showers may begin to occur across the higher
terrain of western New England Friday night as well. Elsewhere,
clouds and a moderate wind will keep temps from falling as much
Friday night as they are expected to tonight, although lows
will still be likely below normal. Some lingering snow showers
will possibly spread toward the I-90 corridor and into the
valleys Saturday as the lake effect band shifts southward, but
minimal accumulation is expected as there is no midlevel forcing
with the height rises. A cold and dry night is expected
Saturday night with diminishing winds as high pressure builds
into the region.
Long term /Sunday through Thursday/...
with many differences between the various operational deterministic
model guidance and ensemble solutions, the extended period is a
rather low confidence forecast at this time.
The extended period appears to start off dry, with high pressure
passing by to the north and a sharp gradient in temperatures aloft
located just off to the south over the mid-Atlantic region. With
the nearby high pressure, there should be enough subsidence in place
to keep any lake-effect snow from occurring for Sunday, with temps
below normal (highs mainly in the mid 20s to mid 30s). Skies will
start off the day partly to mostly sunny, but some clouds should be
increasing during the afternoon and evening hours, as the high
pressure area departs and the next system starts to approach from
The 12z European model (ecmwf)/ggem and some members of the 12z gefs suggests that a
quick-moving clipper system may bring some light snow showers for
Sunday night into Monday. However, this would be a fast moving and
northern stream system, so little total precip is expected and snow
accumulation would be very light and fairly spotty. Temps on Monday
will generally be in the 30s.
The next system would be approaching for the middle of the week, but
the models show some differences in its exact track and timing. This
would have implications on exact p-type (snow vs. Mixed precip vs.
Rain) and precip amounts. At this point, will base p-type off a top-
down blend of the GFS and European model (ecmwf) following a typical diurnal temp
trend, although this will ultimately depend on the exact storm track
and timing. At this time, the superblend guidance does suggest
many locations reach the upper 30s to mid 40s by Tuesday afternoon,
so a changeover to rain will likely occur for many spots due to
boundary layer warming.
Behind the storm system, colder air will return to the region,
allowing for any lingering precip to change back to snow before
ending and some additional lake-effect snow for far western areas
for Wednesday, with mostly dry conditions for Thursday. Based off
the superblend guidance, high temps will return to the 30s for
Wed/thurs with lows in the teens and 20s.
Aviation /06z Friday through Tuesday/...
VFR conditions are in place overnight with diminishing winds.
Per GOES 16 data, seems some of the lake effect clouds were
making into the kalb-kpsf with sct-bkn VFR/MVFR decks. As this
band lifts northward, we will keep some of these restrictions
into the tafs at this time. For kpou and kgfl, should remain VFR
due to upstream higher terrain blocking some of these ceilings from
progressing eastward. Winds will become light and variable
During the day on Friday, S-SW winds will generally be around 5
kts with continued VFR conditions. Although it will be nearly
completely clear during the morning hours, mid level clouds will
increase during the afternoon hours, with all sites seeing bkn
cigs around 7-12 kft by late in the day.
Friday night: low operational impact. Slight chance of shsn.
Saturday: moderate operational impact. Breezy chance of shsn.
Saturday night: no operational impact. No sig weather.
Sunday: no operational impact. No sig weather.
Sunday night: moderate operational impact. Chance of shsn.
Monday: low operational impact. Slight chance of rain showers...shsn.
Monday night: moderate operational impact. Chance of rain showers...shsn.
Tuesday: moderate operational impact. Chance of rain...sn.
no hydrologic problems are expected through the weekend.
Temperatures will largely remain below freezing into the
weekend, promoting ice formation on areas lakes and streams.
Any snowfall is mainly expected to be light, although moderate
accumulation of lake effect snow is expected over the western
Adirondacks Friday night into early Saturday, and light to
moderate accumulation is possible over the higher terrain of
western New England as well.
For details on specific area rivers and lakes, including
observed and forecast river stages and lake elevations, please
visit the advanced hydrologic prediction service /ahps/ graphs
on our website.