Gale Warning 100, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 AM AKST Thu Jan 25 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Generally quiet weather will continue for Thursday across much of
southcentral Alaska. Satellite imagery shows the area on the bottom
periphery of a broad mid/upper-level trough across the state. Nearly
clear skies overnight have resulted in another night of excellent
radiational cooling and temperatures dropping below 0 for much of
the area, with locations in the Copper River Basin and Susitna
Valley below -20. Nighttime microphysics imagery shows that fog/low
stratus has developed in some places such as the Copper River Basin
and along portions of Anchorage, as well as localized spots along
the western Kenai Peninsula. The primary forecast challenge in
regards to today's weather will be possible dissipation of the fog
and how that'll relate to temperature trends throughout the day.

Active weather enters the forecast period as we head towards the end
of the week. On Friday, the the aforementioned upper-level troughing
across Alaska starts to amplify, with the mean trough axis extending
across the western portion of the state. Meanwhile, a low across the
northern Pacific will begin to pivot northward towards the Gulf of
Alaska. As very cold air spills out of SW Alaska and into the Gulf
of Alaska, an intensifying frontal/baroclinic zone will set up
across the Gulf of Alaska. This frontal orientation along with a
deepening and slow-moving longwave mean trough axis centered across
the western side of the state will set up prolonged southerly
flow and a pattern for several storms affecting the eastern
portion of mainland Alaska.

Models have gradually become better aligned and have entered
"believable"-territory with regards to Friday-Sunday weather. Can't
rule out some light snow for either the Copper River Basin and
Prince William Sound on Friday as models hint at saturating
dendritic growth zone (in the low-levels thanks to the pre-existing
cold air) coincident with a weak trough axis moving through. The
main show begins Friday night as southerly flow deepens in the
central/eastern Gulf thanks to the amplifying pattern. Upper-level
divergence underneath the naturally divergence region of the right-
entrance region of a supergeostrophic jet streak, strong warm air
advection, and plenty of moisture advection with a southerly fetch
deep into the North Pacific will combine to create a widespread and
fairly substantial precipitation. Confidence is highest that the
most precip (/snow) will occur along and east of the Richardson
Highway from Valdez northward through Glennallen and Paxson.
Several feet of snow is becoming likely for Thompson Pass area,
with several inches elsewhere along the highway.

Model spread, while getting become better, is still large enough
such that there remains high uncertainty right along the frontal
zone. About a third of models/ensembles bring precipitation along
the Seward and southern/western Glenn Highway while about two thirds
of models remain dry. Of the models showing precip, about a third of
those are showing Anchorage and the Mat Valley getting 6-12 inches
late Friday through Saturday. So could we get quite a bit more snow
here in the Anchorage bowl? Is it likely? Not appearing so at this
very time... but slight changes in storm track would mean quite the
difference in this case.

Perhaps the bigger story for Anchorage and much of Alaska is the
persistent cold weather that is likely to get even colder. Lows
below 0 (and largely below -10) are likely to continue for most
areas outside of the immediate coast for the next several days.

-Brown

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...

Upper level high pressure is currently over the Western Aleutians
and continues to slide west as a strong low lingering just south
of the Aleutian Chain weakens through early Friday. Over the
mainland, zonal flow predominates through Friday morning. Skies
will remain mostly clear across Southwest into portions of
Southcentral and the eastern Bering sea as cold air remains in
place with offshore flow.

An upper level trough will develop over the mainland interior
dipping into Southwest Friday afternoon. This will open the door
for more cold air advection and increasing winds to the end of
this week. Winds may become more northerly as the trough deepens
southward, and increase up to gale force. With the cold air
advection, winds could become gusty through susceptible terrain
gaps. Confidence remains low with regards to precise wind speed
and temperature values due to variations between model runs,
however, there is moderate to high confidence for frigid
temperatures and higher wind by Friday. Wind chill temperatures
over much of Southwest is likely to dip into the minus 40 to minus
50 degrees Fahrenheit range. In the lower Kuskokwim Valley wind
chill temperatures may reach minus 50 to minus 60 Fahrenheit.
Heavy freezing spray will consequently return toward the end of
the week with ice accretion of at least 4 cm per hour near the ice
edge in the eastern Bering beginning as soon as after midnight
into Friday morning, potentially lasting into next week.

As cold air moves over relatively warmer sea surface temperatures
and winds increase from the north, ocean effect snow showers
could develop across the Bering Sea and becoming more prominent as
yet another embedded trough develops in the north Bering and
reinforces cold air southward for Saturday. Blowing snow and
reduced visibility could become an issue with the snow showers
moving across the Pribilofs, the eastern Aleutians, and the Alaska
Peninsula.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through
Tuesday)...

The beginning of the weekend will see a trough developing over
central/western Alaska. With the north-south alignment of the
trough, hazardous cold temperatures are possible along the western
portion of the trough, extending into portions of southwest
Alaska. These conditions would persist through the weekend, but
are possible to extend into early next week. With an increasing
pressure gradient forming through the Bering, strong gap winds
across the western Aleutian islands are currently projected during
the weekend.

Through the end of the weekend into early next week, uncertainty
remains on the potential of shortwaves forming in the eastern Gulf
of Alaska, as well as their track. Presently, a series of
shortwaves are expected to provide increased precipitation along
the Alaska Panhandle and will stretch as far west as the eastern
portions of Prince William Sound. Previous model guidance had the
majority of the heavy precipitation primarily along the Alaska
Panhandle, but there are some trends that are starting to suggest
more precipitation closer to Southcentral. Increased model
agreement closer to the event will help with the confidence of
this forecast in the coming days. Additionally, regardless of the
exact location of the trough and shortwaves, gap winds are likely
with this system. The main region of increased gap winds will
extend from the eastern Bering to the western portions of Cook
Inlet and the Alaska Range. Overall, the positioning of the trough
over Alaska in the coming days will play a significant role in
the placement of strong winds and heavy precipitation throughout
the weekend and early next week.

BS

&&

.AVIATION...

PANC...Nearby fog and low stratus will result in intermittent
IFR/LIFR ceilings and vsbys through late morning. Categorical
improvements to ceilings and vsbys expected through the day,
however, intermittent fog will be possible once again late
tonight through early tomorrow morning. Winds will remain light
and variable.

&&