Gale Warning 703, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
552 AM AKST Sun Nov 29 2020

.ANALYSIS AND UPPER LEVELS...

Low pressure over the Gulf of Alaska is allowing for a showery
pattern of rain and snow across the North Gulf coast to start off
the day. Shortwaves embedded in the associated upper level trough
are also helping to keep an active pattern across Southwest Alaska
with this pattern. Radar imagery shows snow showers across
interior Bristol Bay, which are starting to taper off this morning
as the low continues its track eastward over the Gulf.

Upper level ridging is present over the central and eastern
Aleutians, leaving a quick break in the weather early today before
the next system moves in. Farther west, a 960 mb low is just
starting to track into our forecast area. Its front is bringing
widespread rainfall and storm force winds to the western
Aleutians, with hurricane force winds already observed south of
our forecast area on the southern side this system.

&&

.MODEL DISCUSSION...

Models continue to be in good agreement in the short term with
synoptic features. However, they are still struggling with
mesoscale and local effects, including snow showers across the
region, leading to adjustments for each forecast. The biggest
change in the forecast has been to QPF amounts along the North
Gulf Coast starting Monday night with models coming into better
agreement on much higher amounts of precipitation than previous
forecast packages.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist throughout most
of the day. Chances for snow showers and MVFR conditions will
increase late this afternoon into the evening hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Monday Night)...

A low over Prince William Sound is actively transferring its
energy to a new center near the Barren Islands this morning. This
should allow precipitation rates to gradually diminish along the
coastal mountains today. The new low center will be a slow-mover,
gradually drifting east to near Montague Island this evening
before opening up into a trough near Prince William Sound tonight.
With this pattern evolution, the primary effect will be to reduce
the chances for impactful snowfall over western Prince William
Sound, including Turnagain Pass and the Seward Highway. Meanwhile
steady snow will continue over the mountains near Cordova and
Valdez due to upslope from the steady onshore flow of plentiful
Gulf moisture. Some of that moisture has been creeping northward
into the Copper River Basin as well, with some light snow expected
for much of the day across the basin. When the second low reaches
Prince William Sound tonight, the snowfall rates will once again
increase along the mountains near Cordova and Valdez, but
relatively little will spread much further west than Whittier on
the western side of the sound. The precipitation will taper to
showers Monday morning. Through Monday morning, all inland areas
with the exception of the Copper River Basin will remain dry.

The next storm system, currently moving across the Western and
Central Aleutians will be a big weather maker for Southcentral.
The leading front will bring strong winds and occasionally heavy
rain to Kodiak by midday on Monday, persisting throughout the day
before tapering Monday night. The front will lift north all along
the Gulf Coast Monday night. This front will have lots of moisture
with it, resulting in occasionally heavy precipitation rates. It
will also have abundant warm air associated with it, as the parent
low stays west over Bristol Bay Monday night. Steady rain, perhaps
briefly beginning as snow, will move into Seward Monday afternoon,
with occasionally heavy precipitation overspreading all of the
Gulf Coast mountains Monday night. Snow levels could climb as high
as 2,000 feet Monday night. Thus, expect rain at most low
elevation locations, but also inland along the Seward Highway
through Turnagain Pass and Portage Valley. The warmer air will
also get into Anchorage Monday night, but with strong
southeasterly winds, much of any precipitation will be downsloped.
Nevertheless, expect above freezing temperatures making for very
slippery travel conditions across much of Southcentral. 40s are
quite possible for much of the Matanuska Valley on Tuesday from
the Knik River Valley wind. Precipitation amounts nearing 4
inches through Tuesday could also introduce flooding concerns in
the Seward area as much of that falls as rain. Meanwhile, upslope
flow in the western Susitna Valley threatens additional heavy, wet
snow amounts from Monday night through midweek possibly being
measured in feet. Finally, the front could elevate winds to 50 to
60 mph along Turnagain Arm and higher elevations Monday evening.
Though since it will be quickly moving inland, those winds are
not expected to last for very long.

&&

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

A cold weather pattern is expected for Southwest Alaska as brisk
northeasterly offshore winds continue bringing continental polar
air to the region. Today will be the quietest of the next three
days as a second storm system is expected to move northeastward
along the AKPEN late Sunday night through Tuesday afternoon. There
is a moderate confidence of snow and blowing snow across the
Kuskokwim Delta Monday night. A shift in storm track of 50-100
miles could make a big difference if the Kuskokwim Delta remains
dry and windy or has snow and wind. Visibility could be
significantly reduced across the Kuskokwim Delta if the storm
takes a more westward track. The current projected track of this
system will also make precipitation type a concern along the AKPEN
Monday night as southeasterly flow ahead of the low can bring
warmer temperatures to the AKPEN and cause for the snow to become
mixed with rain. There is a high confidence that freezing rain is
not a concern with this event as all of guidance shows unfavorable
vertical profiles for sleet or freezing rain.

&&

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

A storm system moving into the western and central Aleutians will
bring storm force winds today and tonight. ASCAT data shows
storm force east-northeasterly winds along the northern edge of
this storm system. This storm will begin to gradually weaken late
tonight as it moves towards the eastern Aleutians and AKPEN.
Widespread gale force winds are expected across the eastern
Aleutians from late tonight through midday Tuesday. There is
moderate confidence that snow and blowing snow could reduce
visibility across the western AKPEN as gale force east-
northeasterly winds combined with precipitation are expected from
Monday morning through midday Tuesday. There is moderate
confidence at best in the precipitation type at the onset of
precipitation Monday morning. Despite this location will be
northwest of the low center, sea surface temperatures remain in
the low 40sF which could moderate the temperatures over the land
and keep the precipitation type as rain. The precipitation type is
highly dependent on the storm track where only a 50-100 mile
difference can have a significant impact on the western AKPEN
seeing rain or snow. A more southerly track will favor snow while
a more northerly track will favor rain. Snow chances will increase
Monday night as brisk northerly winds advect a colder airmass into
the area. Snow and strong northerly winds could reduce visibility
at times across the western AKPEN Monday night into Tuesday
morning.

&&

.MARINE (Days 3 through 5 - Tuesday through Thursday)...

Gulf: A well-developed low moves over the Western Gulf through
Thu. Its model track and development are mixed with the wrapped
low as it weakens. However, widespread southerly high-end small
craft winds and waves with local areas of gale force winds move
over the Gulf through Thu. These winds will diminishing to
southwesterly small craft over the Gulf and local southeasterly
small craft winds around the Barren Islands, Kamishak Bay and
lower Cook Inlet.

Bering: A well-developed low moves into the Gulf of Alaska weakens
by Thu. The model track and development of the wrapped low vary
through Thu. Northerly gale force winds and waves over the
Central and Eastern Bering and Alaska Peninsula diminish to small
craft winds over the Eastern Bering and West Coast waters. Cold
high pressure dips into the Bering for Thu, but an upper level
trough brings easterly small craft winds to the West of St Matthew
Island Wed and Thu.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through Saturday)...

A weakening low pushes through Cook Inlet on Wednesday, keeping
the Southcentral coastal areas under showery conditions.
850mb temperatures remain warm enough to keep precipitation as all
rain, followed by a brief period of rain/snow mix on Thursday.
Models differ slightly with this low, where the Canadian/ECMWF
dissipate the low as it makes its way onshore, while the GFS
lingers it over Bristol Bay through Friday. This will have an
impact on a second low that enters the Southern Gulf by Thursday
afternoon where the GFS path is pushed more towards the Southeast
Coast. Both the Canadian and ECMWF solutions track this low north
towards the Southcentral Coast with slight differences between
strength and center location. These differences in model solutions
can impact precipitation type and intensity for the Southcentral
Coast for Friday and will need to be monitored over the next few
days. A third system tracks south of the Aleutians late next week
and will bring gusty winds to the central/eastern Aleutian Chain.
Additionally, persistent upper-level troughing lingers over the
Southwest region through Friday. Flow with this will be
predominantly northerly, thus, below average temperatures can be
expected over the Southwest mainland and the AKPEN/eastern
Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storms 131 172-178 351.
         Gales  119 120 132 136-139 150 155
                160 165 170 171 179-181 352 411 413 414.
         Heavy Freezing Spray 160 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ALH
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...KM