Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
411 PM akst Thursday Dec 25 2014
Analysis and upper levels...
the main upper level trough extends from the North Pole across
the akpen into the North Pacific this afternoon...with a shortwave
trough affecting interior south central and the last of a series
of shortwaves moving into southwest Alaska and Bristol Bay. A cut-
off low remains in place near Kamchatka. The Main Ridge is over
the western coast of North America...with a shortwave ridge moving
over the Kenai Peninsula and another over the central Bering. At
the surface...the decaying front is over the Talkeetna Mountains
and stretches across the Copper River basin to the wrangells...and
a surface trough is over southwest Alaska. Both of these features are
associated with a mix of rain and snow over the Aleutians and
Kenai Peninsula...with significant snow inland...especially over
the Susitna Valley.
models agree that the band of snow will continue into the northern
Susitna Valley this evening before the short wave ridge and decaying
surface front move across the area. The final shortwave/front will
move across the akpen tonight and reach southcentral during the
day on Friday. This will then be followed by the Bering ridge
which will strengthen as it builds east...covering all of Mainland
Alaska by Sat afternoon. Over the extreme western Bering the Kamchatka
upper low and associated surface low will spin in place through
Sat afternoon. An associated weather front will move east behind
the ridge...and will cross the eastern Aleutians and akpen and
move into southwest Alaska by Sat afternoon. The forecast utilizes
the NAM/ec in the east and Gem regional and ec in the west.
Short term forecast southcentral Alaska (days 1 and 2)...
the front has passed leaving most locations in an unstable southerly
flow. The Cook Inlet region is gusting into the 30 miles per hour range which
should hold through the evening before letting up overnight. There
are showers moving into the southcentral from the southwest but
most of them are being blocked by the Aleutian Range. Temperatures
will stay elevated with southerly winds with not much drop off
overnight. A better chance for snow comes tomorrow morning into
the afternoon as the last disturbance in this pattern moves
overhead from the southwest. This disturbance will quickly pass
and bring clearing skies overnight as ridging aloft aggressively
builds into the region.
Short term forecast southwest Alaska (days 1 and 2)...
relatively unstable southwest flow will continue to bring
snow showers (occasionally mixed with rain) that will last into
Friday as ridging will build in this evening and tonight.
Warmer conditions return especially to the Bristol Bay region for
Friday bringing chances for a changeover or mix with rain. South
to southeast winds and temperatures will continue to rise into the
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a strong low pressure system will move across the western
Aleutians into the far southwest Bering and then become stationary
Friday through Saturday. A period of storm-force winds is expected
near Shemya on the back side of this low late tonight into Friday
morning...with some decrease in wind speeds as the low begins to
very slowly fill into the weekend. The associated weather front
will bring strong gale force winds with occasional storm force
gusts through Friday night as it moves north and east across
western and central Bering...then begins to slowly weaken toward
the weekend as it stalls over the eastern Bering near the
A disturbance moving north and east across the central Aleutians
late tonight into Friday will bring a period of storm-force gusts
mainly to the east of Atka before weakening as it becomes absorbed
into the broader flow.
Long term forecast (sunday through thursday)...
A significant shift in the longwave pattern beginning early this
weekend and lasting through the majority of the upcoming week
looks to bring well above normal temperatures to southwest and
southcentral Alaska. An amplifying trough of low pressure
extending from the Kamchatka peninsula to the Alaska Panhandle
will collide with a building northeast Pacific Ridge building
across the Alaska Mainland. Within this amplifying trough...an
area of low pressure originating in the North Pacific south of the
central Aleutians will interact with a moisture rich air mass in
the subtropics and push it towards southwest Alaska by Sunday
morning. This will bring precipitation to southwest Alaska...with
precipitation originating as snow and ultimately transitioning to rain as
the air mass modifies with subtropical air advecting northward.
Temperatures are expected to once again rise well above normal
values...with temperatures in the 40s across the Alaska Peninsula
and parts of the Kuskokwim Delta. Freezing rain will be possible
in the sheltered locations of the lower Kuskokwim valley as the
cooler air will be harder to displace. The rain and snow mix will
gradually taper off by Wednesday as a Second Ridge begins to
develop over southwest Alaska.
Southcentral Alaska will be in store for quite the warm up and
potentially a decent wind event. As the ridge builds across the
Alaska Mainland...the pressure gradient will tighten significantly
across the Cook Inlet and Portage Valley locations. This should
lead to strong southeasterly winds through the valley which would
ultimately impact Turnagain Arm and the Anchorage hillside. While
it is way too far out to estimate at winds speeds...winds between
45 to 60 miles per hour during the beginning of next week cannot be ruled
out. These southeasterly winds will also bring a Chinook to much
of southcentral Alaska. This will aid in warming temperatures into
the upper 30s to middle 40s across Anchorage and parts of the mat-su
valley. Warmer temperatures will be seen along the northern Gulf
Coast and periods of heavy rainfall as moisture from the subtropics
hammers the coastline.
Marine...Storm Warning...173 174 178 185.
Gale Warning...155 165 170 172 175 176 177 179 181.
Heavy freezing spray warning...181 185.
Synopsis and model discussion...ds
southwest Alaska/Bering Sea/Aleutians...cirrocumulus