Gale Warning 169, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKST Fri Feb 11 2022

.ANALYSIS AND UPPER LEVELS...

A longwave trough extends from the Beaufort Sea southwest to the
western Aleutians with an upper-level ridge axis positioned along
the west coast of the US and Canada. There are multiple upper-
level waves and associated surface features embedded within and
rotating around the base of this trough, culminating a what is an
active and complex weather pattern across not only the Southern
Mainland, but the Bering and Aleutians as well.

The first of these shortwaves is exiting the northern Gulf this
afternoon. An associated surface low is moving east of Middleton
Island with the bulk of the precipitation also now pushing east of
Cordova. The exception to this is continued light snow across the
Susitna Valley and across the Talkeetnas, where the persistent
southwesterly flow ahead of the aforementioned trough is
maintaining an upslope flow. A second wave is approaching the
AKPen with light snow beginning to develop around Bristol Bay.
This feature will move toward Southcentral for Saturday.

There is a third system, a vertically stacked low, over the
eastern Bering with an occluded front sweeping across the
Kuskokwim Delta this afternoon. A resultant tight pressure
gradient between the low and a cold dome of high pressure over the
interior of the state is producing strong northeasterly winds
across across the Kuskokwim Delta, especially along the coast.
The combination of winds and falling snow is helping drive near
blizzard conditions from Kipnuk to Hooper Bay, where some coastal
locations have reported visibility dropping as low as one quarter
mile at times.

Lastly, a second vertically stacked low remains nearly stationary
northeast of Shemya. This system is maintaining an area of gales
over the western Bering as well as snow showers across the western
Aleutians as colder air wraps around the base of the low.

&&

.MODEL DISCUSSION...

Models are continuing to show good agreement despite the rather
complex weather pattern. As such, forecast confidence remains
above average as an active weather pattern takes hold across much
of the forecast domain. Some subtle differences remain in the next
shortwave lifting to the SOuthcentral coast for Saturday, with the
GFS and GEM slightly farther south and east than the EC and NAM.
While the overall sensible weather should be be affected the
position of the shortwave, the core of the heaviest precipitation
does vary slightly. Nonetheless, all model show a general light
snow across Southcentral with heavier precipitation along the
coast. Models are also varying the timing of a shortwave dropping
southeast from the northern Bering. This features will act to kick
the wave over the eastern Bering east, eventually phasing together
as the entire trough axis moves from Southwest Alaska to
Southcentral. Again, the subtleties do little to significantly
alter sensible weather, but the speed at which the trough axis
moves east will determine the when the widespread light snow
tapers off. Right now, the NAM is the fastest, with the other
models only slightly slower. Models are also in good agreement
with the next storm-force low to approach the western Aleutians
from the south for Monday. One difference of note is the timing of
the northward progression of its front. The GFS is the slowest
with the front reaching the Pribilofs by 00z Tuesday. All other
models push the front to the Pribilofs by 18z Monday. Timing will
determine when snow transitions to rain for for the Aleutians and
Pribilofs.

&&

.AVIATION...

PANC...VFR conditions will persist until the next system
approaches around midnight. Around this time, ceilings will lower
as light snow develops over the terminal. IFR conditions are
expected after midnight due to the combination of low clouds and
reduced visibility from falling snow. IFR conditions are expected
to persist through late Saturday afternoon or evening. Light
northerly winds will persist through early Saturday morning. Winds
are then expected to become southerly while remaining light.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

An upper level ridge currently over Southcentral exits to the
northeast through the night tonight, followed swiftly by an
incoming upper level trough extending off a Bering low. Northerly
flow at the surface will continue across Anchorage through this
evening before switching to southerly, allowing light snowfall to
filter into the area, snow accumulations of 2 to 4 inches is
possible and snow should taper off Saturday afternoon. Colder air
advecting in behind the ridge brings another shot of snow showers
to the Homer and Homer Bluffs regions tonight through Saturday
morning. Southwesterly flow pattern is a good set- up for upslope
precipitation development across Hatcher Pass and parts of the
Susitna Valley, namely Talkeetna north through Broad Pass. This
event will be a long duration snow fall, spanning through the
weekend, with possible snow accumulations of 7 to 9 inches for
Palmer, Talkeetna, and Chulitna, and higher amounts across the
higher elevations. Additionally, the eastern portions of the
Copper River Basin get caught up in the upper level waves moving
through tonight and tomorrow morning. Anticipate light snowfall
from Chitina to Gulkana and areas further north up to Paxson.

Additionally, the bombardment of surface fronts along the North
Gulf Coast will keep continual precipitation (moderate to heavy at
times) to Cordova and Valdez, as well as the higher elevations
through Saturday night.

&&

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

Warm southerly winds accompanying the next frontal system from the
Bering will cause temperatures to rise in the upper 30s on Mon.
The potential for steady rain is favorable over much of Bristol
Bay and the Alaska Peninsula depending how far the blanket of
warm air advection spreads out to farther inland on Mon. The
P-type could be a challenge for some areas along the lower
Kuskokwim Valley and parts of the Kusko Delta between Mon morning
and Mon night. In the meantime, an upper low embedded in the
longwave trough will eventually enter the Kusko Delta coastline
Sat evening. This latter energy combined with another separate
longwave from the south of AKPEN will allow for another round of
light snow across the region into Sun morning.

Along the Kuskokwim Delta coastline, blowing snow continues, and a
Winter Weather Advisory remains in effect for the Kuskokwim Delta
west of Bethel until 9 PM Fri night. Should reduced visibilities
of 1/4 mile or less persist tonight, this product would be updated.
Stay tuned!

&&

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

A large frontal system approaching the Aleutians Sun morning will
sweep a widespread area of gales and potentially storm force
winds, and high seas of more than 25 ft to the north and east of
the western Aleutians, and up to 33 ft along the south of the
western and central Aleutians Mon morning. This system will drive
warm south-southwesterly winds across the Bering/Aleutians,
causing tropical moisture and potentially steady rain to push
across the region Sun morning into Mon night. Also, gap winds will
continue for much of the Aleutians into Mon.

&&

.Marine (Days 3 through 5: Monday through Wednesday)...

Gulf of Alaska:

High pressure moves across the Gulf Monday. Forecast confidence
is good. A front enters the Western Gulf late Monday. High end
Southerly gales spread across the Gulf, becoming Westerly on
Tuesday and diminishing. Seas building to 20 feet near Kodiak
Island and move across the Gulf through Wednesday. A new front
approaches the Western Gulf for Wednesday with increasingly gusty
Southerly winds.

Bering Sea/Aleutians:

A well developed North Pacific low moves across the Western
Aleutians Tuesday and back into the North Pacific for Wednesday.
Forecast confidence decreases with the low developments near the
end of the period. Widespread high end Easterly gales with areas
of storm force winds extends over the Aleutians and Bering and
move to the West with the low. Seas to 30 feet over the Western
and Central Aleutians and Bering, subsiding Wednesday. A well
developed North Pacific low moves across the Southern Alaska
Peninsula for Wednesday. Winds increase to Easterly gales across
the Eastern Bering and Bristol Bay, and Southerly gales on the
Pacific side of the Eastern Aleutians and Alaska Peninsula,
subsiding Wednesday. Seas building to 20 feet ahead of the low on
Tuesday, subsiding Wednesday.

&&

.Long Term Discussion (Days 4 through 7: Tuesday through Friday)...

Model guidance remains honed in on the upper level pattern
through the long term forecast period. Though, of course, subtle
differences do arise which will alter how the weather at the
surface behaves. Model guidance continues to remain in agreement
across the board on an upper level ridge over the eastern North
Pacific with a fairly strong upper level low especially on Tuesday
remaining in place south of the Western Aleutians. This low will
slowly weaken through the week as a weak upper level ridge takes
its place. An upper level trough remains in place across the North
Slope keeping the deep Arctic air in place here. As a result,
models remain in decent agreement on a low pressure system moving
northward from the North Pacific Tuesday night into Wednesday that
may bring another round of precipitation to much of Southern
Alaska mostly in the form of snow with snow changing to rain for
places like Kodiak. It should be noted that the EC and Canadian as
well as their ensembles are mostly in lockstep with each other
and east with the system while the GFS is stronger and further
west for the time being and is therefore the outlier at this point
in time.

Beyond Wednesday through the end of the period, model guidance
starts to strengthen the upper level trough some across the North
Slope. The GFS and its ensemble members keep the trough over the
North Slope with a magnitude similar to that of the EC but more
over central interior Alaska. The Canadian and its ensembles were
weakest with this trough but in a similar place to the EC.
Therefore, the sensible weather at the surface is uncertain as the
EC would suggest a colder regime with most systems remaining
across the Gulf with little push north. However, the GFS and
Canadian suggest the opposite basically keeping the pattern
currently in place with periodic rounds of precipitation. As a
result, low confidence in the forecast past Wednesday. Overall,
temperatures during this time frame are expected to be hovering
right around normal to slightly above normal.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 155.
MARINE...Gales: 174, 175, 176, 177, 178, 185.
         Heavy Freezing Spray: 179, 185, 411, 412, 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...MK/MV