Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
310 PM akst Sat Jan 24 2015
Analysis and upper levels...
a high amplitude trough is progressing slowly eastward from southwest
Alaska into the western Gulf and western portion of southcentral
Alaska. The trough is just beginning to show signs of developing a
negative tilt...as rising heights downstream over western Canada
and a strengthening upper jet between the trough and ridge cause
the trough to begin to lift northeastward. Most of the accumulating
snow has ended from the western Kenai to Anchorage and mat-su.
Radar does show some weak returns as cold air advection with the
upper trough maintains some very low level lift...resulting in
very light snow or flurries. Strong gap winds have developed over
the western Gulf and Kodiak Island with the onset of cold air
Strong lift out ahead of the trough combined with upslope flow is
producing moderate to heavy precipitation over much of the Prince
William Sound region...with marginally lighter precipitation over the
Copper River basin. Of special note...precipitation started the
morning with above freezing temperatures and rain and snow. The
intensity of the precipitation has helped cool the column and
change precipitation to all snow. Therefore...now expect Valdez to see a
good dump of snow out of this storm.
Behind the trough southwest Alaska has settled into a true Arctic
arm mass for the first time this winter. The cold spotty this
morning appears to be Aniak with a temperature of 29 degrees below
zero. Upstream of this an upper level shortwave is crossing the
Bering Sea bringing some light snow to the region. Meanwhile a
mature low over the northwest Pacific is tracking eastward pushing
an occluded front northward toward the Aleutians. The air mass in
place right now is cold enough to support all snow as
after a few days of dealing with key differences in model guidance
the latest solutions are in very good agreement as things wind
down over southcentral Alaska tonight into Sunday. Attention
shifts to the North Pacific where minor differences arise as a
trough pushes northward into the central to eastern Bering Sea
Sunday night and Monday. The bulk of precipitation will initially be
focused out ahead of a warm occlusion. As this front weakens a
series of upper level waves moving through the trough will help
focus precipitation...and models do not agree on strength or
placement of these waves. With a deep Arctic air mass in place
out ahead of the front there will be a threat for severely
reduced visibilities in blowing snow from Bristol Bay to the
Pribilof Islands...depending on the extent and intensity of the
precipitation. Forecast confidence is generally high...though it
drops significantly with details out west as we head into early
Short term forecast southcentral Alaska (days 1 and 2)...
as the upper trough begins to lift northeastward this afternoon
and evening moderate to heavy precipitation will continue from
eastern Prince William Sound to the portions of the Copper River
basin. The primary short-wave trough will exit to the northeast
on Sunday...but cyclonic flow will persist on the south side of
an upper low which forms over interior Alaska. This combined with
cold air advection will cause snow showers to linger in these same
areas. Meanwhile...cold advection behind the trough combined with
upper level subsidence will produce strong outflow winds across
the region. Will increase winds in the Seward/Resurrection Bay and
Whittier/Passage Canal areas with good upper level support and the
coldest air mass of the entire winter moving in.
Temperatures will plummet over the next 36 hours as the Arctic air
mass settles in. With cold advection continuing through the day
Sunday expect temperatures to hold steady or continue to fall
during the daytime hours. The cold advection will largely come
to an end by Sunday night allowing winds to die down. This will
likely be the coldest night for much of the region. Of note...west
Anchorage (where the official climate station is located) is
expected to observe the first sub- zero temperatures since
Short term forecast southwest Alaska (days 1 and 2)...
in the wake of the upper trough cold advection will come to end
tonight and low level winds will continue to slacken. Thus...tonight
will be colder than Friday night. A very gradual moderation will
then begin Sunday into Monday as the trough over the North Pacific
lifts northeastward toward southwest Alaska. There is a trend in
the guidance toward moving precipitation into Bristol Bay faster with
arrival sometime on Monday. Pressure gradients will tighten out
ahead of a surface occluded front...so whenever snow does move in
there could be some issues with reduced visibilities in blowing snow.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the short-wave bringing snow to the central Bering will flatten
out this afternoon and evening as heights rise ahead of the Pacific
trough. Thus...don't expect any clouds or precipitation to survive beyond
the eastern Bering Sea. The Pacific trough and surface storm
system will gradually push precipitation and warmer air northward across
the Aleutians and southern Alaska Peninsula into the southern
Bering Sea. The Pribilof Islands will remain solidly on the cold
side of the surface occluded front. Therefore...if and when steady
snow reaches the pribilofs blizzard conditions are likely. Will
issue a blizzard watch beginning Monday when most models indicate
a higher potential for steady snowfall.
Long term forecast (days 3 through 7)...
there is an upper level blocking pattern setting up with an
amplified positive tilted ridge building into the western
Aleutians/Bering Sea by Tuesday. This ridge will extend through
the western Aleutians northward into the eastern siberian region.
In the Southeast Alaska area...there is another amplified ridge
building along the Southeast Alaska coastline. This ridge is
extending into the southcentral area and into the Bristol Bay
region. There is an upper level closed low between these two
ridges located south of the central/eastern Aleutians in the North
Pacific Ocean. The southwest Alaska area will have offshore flow
with the northeasterly winds setting up by Wednesday. There will
be scattered rain/snow showers along the southern Mainland along
the eastern Kenai Peninsula extending into Kodiak Island. The
shortwaves will rotate around the complex low in the North Pacific
and push through the eastern Aleutians. The temperatures along the
central to eastern Aleutians extending to the western Gulf and the
coastlines will see a warming trend. This will coincide with the
complex North Pacific low as it gradually moves north towards
Kodiak Island by Wednesday with continued southerly flow in the
Gulf heading into the weekend.
public...blizzard watch 195. Snow Advisory 141.
Marine...Gale Warning 129 130 131 170 179.
Heavy freeze spray 121 126 127 128 129 130 131 138
139 140 141 150 160 165 180 185.
Synopsis and model discussion...sb
southwest Alaska/Bering Sea/Aleutians...sb