Gale Warning 675, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKST Sat Nov 21 2020

.ANALYSIS AND UPPER LEVELS...

A strong low currently over the Bering Sea between the Pribilof
Islands and Nunivak Island is slowly moving north-northwestward. A
negatively-tilted upper level trough extends southeastward from
the center of this low to the central Gulf of Alaska. A triple
point low is currently developing along this negatively-tilted
trough near Kodiak Island and is moving northeastward. An unusual
pattern in the jetstream continues with the jet remaining south of
50N west of 140W. The jetstream splits at 140W with the southern
half extending eastward over southern British Columbia and
northwestern CONUS. The northern half extends northwestward to
over eastern Siberia, then over the Arctic Ocean as a large upper
level ridge of high pressure persists across the eastern Alaskan
interior and over the AlCan border.

Closer to the surface, radar imagery shows stratiform rain bands
moving northwestward across the Gulf of Alaska into the southern
Kenai Peninsula and into western Prince William Sound. Radar
imagery also shows stratiform rain moving over the northern AKPEN
into Bristol Bay. Farther inland, colder temperatures are
allowing this precipitation to fall as snow. Observations show a
southeasterly Turnagain Arm wind with a brisk down-inlet flow
which allows these Turnagain winds to not enter the Anchorage
Municipality.

&&

.MODEL DISCUSSION...

Guidance is in better agreement now compared to 24 hours ago with
respect to the placement and intensity of synoptic scale features
at both the surface and in the upper levels. However, confidence
remains average at best due to the nature of the current jetstream
pattern as this pattern is unusual and not common. Due to this
being a rare pattern there is a degree of skepticism on the models
resolving these features at the surface and in the upper-levels.
One noticiable trend in guidance is the 20th/00z runs showed the
GFS as the aggressive outlier with respect to the development of
this triple point low while the rest of guidance did not show a
low as strong. The 21st/00z runs show the remainder of guidance
trend towards the 20th/00z GFS solution while the GFS solutions
remained consistent over the past few model runs.

&&

.AVIATION...
PANC...VFR conditions remain through 22nd/09z and then conditions
become MVFR as visibility will be reduced to 4SM with light snow.
Low level wind shear will continue at PANC as strong southeasterly
winds aloft oppose the weak northwesterly flow at the surface.
This low level wind shear will cease around 22nd/01z as winds
aloft will weaken.

&&

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

An area of low pressure is in the process of rapidly deepening
northeast of Kodiak this morning, helping to spread precipitation
and wind from Kodiak Island through Western Prince William Sound.
The focus for the more impactful snowfall accumulations looks to
remain across the higher elevations of the Eastern Kenai Peninsula
and Western Prince William Sound today. A Winter Weather Advisory
remains in effect for Portage Valley through Turnagain Pass where
gusty winds could cause blowing snow to reduce visibilities into
the early afternoon. These conditions will taper off later today
as lighter accumulating snow spreads into Northern and Eastern
Prince William Sound.

Across the Northern Gulf, a storm force barrier jet has set up
ahead the approaching front, but these winds should rapidly
decrease later this morning into this afternoon as the front moves
ashore and the low near the Barren Islands begins to weaken.
Gale-Storm force winds will continue across the the southern Cook
Inlet and to the east of Kodiak Island in the circulation around
the low until it begins to weaken late this afternoon/evening.

On Sunday this area of low pressure will begin to weaken and
drift northward up the Cook Inlet providing chances for continued
snow showers from Anchorage through Prince William Sound, but
accumulation potential around Anchorage looks fairly low at this
point.

Another storm moves into the region late Sunday with a strong
surface low tracking from the eastern Aleutians to just east of
Kodiak by early Monday. This storm is likely to bring another
wintry mix of precipitation to many of the same areas dealing with
the current snow/rain event.

In addition to the precipitation another round storm force winds
are expected across the Northern Gulf and portions of Southern
Cook Inlet as this appears to be another barrier jet type of
setup. Expect a ramp up to Gales on Sunday night, with these
storm force winds working in on Monday.

&&

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

Quieter weather is expected across most of Southwest Alaska and
the AKPEN to start off the weekend. The occluded low that has been
the main weather story the last few days is now centered just
north of Nunivak Island with the front extending from Iliamna to
Aniak. Cold air wrapping around to the south has mixed in with the
front, thus switching all remaining precipitation to snow. This
front is expected to move slowly into the interior while weakening
through Sunday afternoon. For the Southwest coastline, a few
quick snow showers are possible through Sunday due to the cold air
advection to the south of the low center.

Active weather is expected to return to the AKPEN midday on Sunday
as the next system moves in from the north Pacific. There is still
some uncertainty in the forecast where the low will track, which
will determine the magnitude of rain for the AKPEN and snow
accumulation for Bristol Bay on Monday. There is potential for
heavier snow and strong northeasterly winds over Bristol Bay in
some of the modeled low tracks.

&&

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

An occluded low exists over the eastern Bering today and is
expected to move northwest into far eastern Russia Saturday night.
This will lead to a brief respite from the active weather before
the next system arrives near the eastern Aleutians Sunday morning.
With the existing pool of cold air brought in by the low pressure
system this week, a strong thermal gradient will exist across the
low. This is expected to result in rapid storm intensification
with gale to storm force winds on the western side of the low
center late Sunday and into Monday as the low tracks towards
Kodiak Island. Winds are expected to be very strong through the
gaps and passes along the AKPEN as the cold air moves through.
Meanwhile, another strong, yet rapidly occluding low should form
to the east of the Kamchatka Peninsula Sunday night. The low
center is not expected to come into the forecast domain; however,
a large gale to storm force front is expected to sweep east across
the western and central Bering on Monday. With a large southerly
fetch and warm air advection, rain is likely along the front.

&&

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

Gulf of Alaska:

Some uncertainty remains during the 3-5 day timeframe. There is
moderate confidence a front associated with a strong low near
Kodiak Island moves east across the southern Gulf on Monday.
Confidence is increasing that there will be a large area of Gales
with areas of storm force winds over the Gulf waters Monday
through Tuesday morning as the low tracks eastward into the Gulf.
There is still some uncertainty with the track and strength of
this low, however models are agreeing that it will take a track
across the southern Gulf. Fairly good confidence for decreasing
winds on Tuesday as high pressure moves across the Gulf before
another front enters the Gulf Wednesday with Gales possible.

Bering Sea:

Confidence is increasing that Gale to Storm force winds will
occur over the western and central Bering waters Monday as a
front moves across the Bering. Winds will decrease to Gales
Tuesday morning as the front slowly weakens. Late Tuesday into
Wednesday a low will develop south of the Aleutians and move into
the eastern Bering. However low confidence in track and strength
as models are all over the place, it is certain that an active
pattern will continue through Wednesday across the Bering.

&&

.LONG TERM FORECAST (Days 4 through 7...Tuesday through
Friday)...

The amplified pattern early in the week will transition to a more
zonal like pattern across the Bering and Gulf by Wednesday with a
series of shortwaves/fronts riding along the upper level jet near
the Aleutians and tracking into Southwest and Southcentral with
increasing chances of snow by mid- week.

Beginning Tuesday, an upper level low near the Bering Straits with
a front extending into the eastern Bering will move into the Gulf
on Wednesday as an upper level ridge moves east out of the Gulf.
Models also show a low forming along the front near the Aleutians
and tracking towards the eastern Bering and Southwest Alaska
Wednesday, however lots of uncertainty with track and strength of
this low. This front will then move into Southcentral Wednesday
into Thursday increasing chances of accumulating snow. However
there is uncertainty with timing and strength of this front.
Thursday into Friday additional lows and fronts may form near the
Aleutians and track into the Gulf, however models are all over the
place on track and strength of these features. It does look
certain that an active pattern will continue through Friday for
much of the forecast area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 125.
MARINE...Storm Warning 119 120 130 131 139.
Gale Warning 125 132 140 141 172 174 179 181 185 351 352 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...BG
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...RC