Brisk Wind Advisory 68, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKST Sun Dec 27 2020

.ANALYSIS AND UPPER LEVELS...

The dominant feature is a very deep, mid 940 mb low over the
Bering and its associated front. As the low has peaked in
intensity and is now fully mature, jet support has generally moved
away from it. The jet now carves out a deep trough down to 40 N
over the North Pacific before taking a gentle turn towards the
Gulf of AK/Kodiak Island. It is generally moving at about 100 kts
at 300 mb (30,000'). Satellite clearly shows that cold air has
settled within this trough as open cell cumulus are apparent from
Shemya to Bristol Bay.

The front is pushing through Southwest AK and into Southcentral.
Cold air continues to hold firm ahead of the front. Many places
across the northern portions of Bristol Bay and the YK-Delta coast
observed blowing snow with blizzard conditions through the
morning. Even Kodiak saw a brief period of fairly wet snowfall. As
winds slowly shift from NE to S with the front, warm air is able
to mix down to the surface and turn most of these locations over
to rain or at least a rain/snow mix. Southeast winds are starting
to ramp up across the Chugach and Kenai Mountains. Radar is
starting to show the typical downslope hole opening up in response
to this cross-barrier flow. This will be the general case with
this system...moderate to heavy QPF along the coast and the
upslope regions of the Susitna Valley, while things remain mostly
dry for many inland areas.

&&

.MODEL DISCUSSION...

Models are in good agreement with most synoptic features, so
forecast confidence remains moderate. The few big questions
revolve around exactly how much cold air can linger over Eastern
Prince William Sound as this system arrives and how strong the
winds will be across Turnagain Arm. The NAM has been the most
aggressive with both of these components, but it also a marginal
outlier. Thus, this forecast has stayed the course and kept more
of the middle ground approach of the GFS/Canadian models. All
models did agree in higher snow amounts across across the Western
Susitna Valley, so this forecast has trended those upward a
healthy amount.

&&

.AVIATION...

PANC...VFR conditions will persist. The biggest impact will be in
the form of low level wind shear increasing and persisting through
the period. A brief shot of southeast winds are expected to come
off the Turnagain Arm Monday afternoon as a strong shortwave
passes through the terminal.

&&

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

A deep frontal boundary passing through Kodiak Island will move
north toward the Kenai Peninsula through Prince William Sound
(PWS) tonight into Mon morning. Warm air advected by south-
southeast winds from the Gulf will maintain some rainfall
activity mostly along the coastal areas, then transitioning to
snow farther inland and over higher elevations tonight. The
heaviest snow showers will concentrate mostly along the eastern
Kenai Peninsula through much of PWS, with Turnagain Pass likely to
receive up to more than 20 inches of snow tonight. A couple of
shortwaves from the storm over the Bering will increase
enhancement of snowfall in the coming days.

The potential for blowing snow exists in Thompson Pass and
Turnagain Pass. On this forecast package, MOS guidance hasn't been
forthcoming lacking strong indication whether blowing snow will
take place, but it's something worth monitoring for the
aforementioned locations tonight.

The jet stream stays south over the North Pacific, which provides
minimal upper level support to the front. Overall, the front will
bring sufficient, measurable rainfall along the coast with snow
showers inland over the next couple days.

&&

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

Showers remain across Southwest Alaska tonight as a decaying front
associated with a weakening Bering low continues to push farther
inland. Across the Kuskokwim Delta, snow showers continue along
the northern coast, including Hooper Bay. However, conditions are
improving from blizzard conditions earlier today with less
visibility restrictions and lower wind speeds this evening. Along
the Bristol Bay coast, warmer temperatures have helped switch
precipitation over to rain or a mix of rain and snow, which is
also an improvement from this morning. This showery pattern will
continue through Monday as precipitation starts to taper off.

By late Monday afternoon, the next system will work its way
towards the Alaska Peninsula and Southwest Alaska from the North
Pacific. There is still uncertainty in the timing and placement
of this feature as it continues north through Tuesday. Gusty winds
and another round of precipitation can be expected to accompany
this system as it tracks over the AKPEN and along the Southwest
coast throughout the day on Tuesday, with showers lingering
through Wednesday.

&&

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

Widespread gales will continue over most of the Bering through the
overnight hours and throughout the day on Monday as a weakening
low remains nearly stationary over the western Bering. Rain and
snow showers will persist across the Bering and Aleutians before
colder air starts to move in and transition back a rain/snow mix
or all snow late Monday.

By Wednesday afternoon, a strong low will skirt along the central
and eastern Aleutians as it moves eastward, with strongest winds
staying south of the Chain. Meanwhile, farther west a much
stronger system begins to approach the western ALeutians and
Bering, which will be the dominant weather feature to keep an eye
on later this week.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska:
Good forecast confidence on Wednesday/Thursday for no significant
winds or waves. Model guidance shows the potential for a small
area of gale force easterly winds in the southern portions of the
gulf on Friday.

Aleutians/Bering:
Model guidance are in good agreement on a sub 940 mb low lifting
north from the Pacific and moving over the central Bering and
western Aleutians Wednesday night to late Thursday. There is a
risk for more widespread coverage storm force winds than currently
forecasted Wednesday night through Thursday night for the western
and central Bering/Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)...

An active pattern will hold over the central Bering and western
Aleutians Thursday through Friday with a rapidly intensifying (sub
940 mb) low lifting south of the Aleutians and moving over the
Bering. Gusty winds and the potential of heavy precip, including
wintry mix, will cross the Aleutians Wednesday night through
Friday. After Friday, this low weakens and migrates over the
western Bering. Model guidance diverge with the next potential
storm force low paralleling the Aleutians, but unsettled weather
will the continue with the risk for winds and precip over the
weekend.

For Southcentral on Thursday, a trough moves from the western
Gulf to northeastern Pacific with a ridge over mainland AK. A
couple of weak shortwaves passes through the southern portions of
the gulf and AKPEN Thursday through Sunday. Southcentral will be
dry Thursday through Fri, but model guidance diverge with timing
of the next precip late Saturday through Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 176
Gale Warning: 119 120 130 131 150 155 165 170 172-175 177-185
351 352 411 414.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...MF