Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
419 PM akst sun Dec 21 2014
Analysis and upper levels...
an upper level trough extends from the West Coast of Alaska southward
into the North Pacific. The jet remains largely suppressed well
south of Mainland Alaska...with a 200 knots jet exiting Asia and
heading into the longwave trough and a 180 knots jet coming out of
the trough and slamming into the southern Panhandle. A decaying
vertically stacked low in the western Gulf is slowly lifting
northward with numerous embedded features at the surface and also
aloft. This is leading to a more showery pattern across southern
Alaska than has been seen recently. A more focused band of
convective snow showers is persisting from western Prince William
Sound up to Eagle River and into the Susitna Valley. Snowfall
totals have ranged from 4-7 inches near Portage to upwards of 2 inches
on the west side of the mountains. There is another weak
circulation east of Kodiak Island heading northward...with
increasing shower activity already developing across much of the
Gulf of Alaska as this low teams up with colder air moving in at
500 mb to steepen lapse rates across a large section of the Gulf.
Out west...weak high pressure and northerly flow are allowing
cold air to stream southward from the Bering Strait and produce
widespread snow shower activity across much of the Bering Sea and
Aleutians. Meanwhile...a mature storm near the Kamchatka peninsula
of Russia is slinging a front toward the western Aleutians with
winds at Shemya already beginning to increase out of the south.
the models remain in good synoptic agreement...but continue to
struggle immensely with the details of the evolution in the Gulf
and across southcentral during the next 24-36 hours. The main
players are the cold air aloft moving in from the south that will
further destabilize the atmosphere and give an extra shot in The
Arm to snow shower activity. Thankfully this is a broad-scale
feature and is high confidence. Much lower confidence are the
movement and strength of the shortwave moving northward near
Kodiak Island as well as the ability of any localized areas of
convergence to focus this convection (for example near western
turnagain arm). The bottom line today is that the subtle details
in the atmosphere will likely play an even bigger role than
normal in determining where and how much snow falls across
southcentral Alaska over the next 2 days...and out of necessity a
more broad- brushed approach will be employed and radar and
satellite trends will be monitored closely until such a time as
the atmosphere tips its hand about what will happen. As you can
imagine...forecast confidence is below average across southcentral
Out west...the development of a strong low in the western Bering
early next week is becoming the primary forecast challenge. The
GFS continues to be further west than the rest of the global
guidance...and with good run-to-run consistency of the
European model (ecmwf)...will trend the forecast that direction. The primary threat
will be the potential for high winds in the western Aleutians as
the low reaches peak intensity Tuesday morning.
Short term forecast southcentral Alaska (days 1 and 2)...
the forecast remains pretty low confidence over the next two days.
The flow regime does not change much at all. Colder and more
unstable air aloft moves overhead from the Gulf and will be the
focus for snow showers across much of the region. Showers will be
enhanced by a disturbance aloft...currently sitting in the central
Gulf...as it moves northward late tonight into tomorrow morning.
Instability will remain over the area through late tomorrow night
before the flow begins to stabilize into mid-week. Accumulations
will generally remain light but could add up fast for any area
where banding sits overhead for a prolonged period of time. For
the most part any showery activity should stay as open celled in
nature but some banding from low level convergence along Turnagain
Arm is not out of the question.
Short term forecast southwest Alaska (days 1 and 2)...
Northerly flow in cold advection regime will continue through
Monday. Models depict enough low level moisture in the lowest 1k
feet where sheltered areas of southwest Alaska will be capable of
developing some patchy fog...as has occurred today in parts of the
Kuskokwim valley and Bristol Bay. Except for the outer coast of
the Kuskokwim Delta outer coastline tonight...and along the Alaska
and Aleutian ranges...precipitation chances will be zero.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
Tranquil conditions by Bering Sea standards the next day or so
with a minor disturbance bringing some likely rain and snow to
the western Aleutians tonight into Sunday.
The system to watch will begin to make its presence felt Monday
into Tuesday as a low moves across the western Aleutians and into
the western Bering Sea. The European (and to a lesser extent the
global canadian) model is preferred with its more elongated
circulation along its associated weather front. The GFS (with some
support from the nam) keeps the low a bit more west of the
European and Canadian models but also more annular. Regardless of
which model is correct...with the forecasted intensification of
the low there is an increasing likelihood of powerful westerly
winds wrapping around the backside of the low over the western
Aleutians and nearby coastal waters. If the European is
correct...a threat of these stronger winds could also reach the
central Aleutians and its nearby coastal waters.
The preceding weather front will also carry a swath of strong
winds to storm-force along and just ahead of the front as it races
across the Bering Sea...intensifying over the southern and western
Bering Monday night and reaching the pribilofs and eastern
Aleutians during the day on Tuesday. The faster ec was also
preferred over the slower GFS with progression of the front.
Long term forecast (days 3 through 7)...
an active jet stream keeps the weather pattern in motion through
next weekend. Models are finally starting to come into better synoptic
agreement with a closed low center developing Tuesday night in the
western Bering as a longwave shifts east from the Kamchatka
peninsula. This low pressure system and a polar low in the Arctic
forces the weak upper trough...currently over the Gulf of
Alaska...toward Southeast Alaska middle week. The pattern for the
week ahead will mainly be driven by the low in the western
Bering...however the polar low will push a front from the
northwest coast to the southern Mainland Wednesday. The shortwaves
from these two systems will bring snow showers with gusty winds
mainly along the quickly developing low pressure systems driven
from the Bering low. Model confidence is lower than average on the
upcoming shortwaves and thus changes in long range forecast
precipitation and winds can be expected.
Gales 170 172 173 174 175 176 177 185 411 412 413 414
heavy freezing spray...180 181.
Synopsis and model discussion...ad
southwest Alaska/Bering Sea/Aleutians...cirrocumulus