High Wind in Alaska, November 1 to 2, 2020

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Sun Nov 1 2020

.ANALYSIS AND UPPER LEVELS...

A deep, positively tilted upper trough continues to bring very
cold air and gusty northerly gap winds across Southern Alaska.
The strongest of these can currently be found near Seward,
Whittier and Valdez. Other locations are expected to increase as
the trough becomes cut off and forms a deeper closed low. The
aforementioned strong northerly flow is inhibiting fog
development over Southcentral, however weaker winds and low
dewpoint depressions below 1000' is leading to low status across
the interior Kuskokwim Delta and Middle Kuskokwim Valley. There is
likewise a potent snow squall extending from southern Cook Inlet
to just east of Kodiak Island. This should remain offshore.

The only notable surface feature is a ~990 mb low tracking
eastward on the Pacific side of the western Aleutians. Its
associated front is bringing rain and gusty southeasterly winds
generally west of Adak. Zonal flow dominates the remainder of the
Bering/Aleutians, with presently quiet conditions and the usual
low marine stratus persisting.

&&

.MODEL DISCUSSION...

Synoptic models remain in good near-term agreement. Per usual, the
high-res models are preferred for many of the expected strong
northerly gap wind areas tonight through Monday. There is slight
disagreement regarding the placement of a low moving near the
Panhandle and how elongated it will be, however the effects on its
west side are well-agreed upon, namely northerly gales developing
tonight over much of Cook Inlet. Agreement deteriorates with the
formation of a second low in the northern Gulf on Monday morning.
At this time a NAM/RGEM/ECMWF leaned solution is preferred as the
GFS is noticeably farther south, not even matching with its own
ensemble. The overall circulation will persist in the Gulf into
Tuesday, thus the gusty northerly outflow winds will continue.
Temperatures will remain well below average across much of the
state.

&&

.AVIATION...

PANC...VFR conditions will persist. Gusty northerly winds will
develop this afternoon, potentially gusting to 30 kts by this
evening.

&&

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

A strong polar shortwave is moving through Southcentral today.
This wave will cut off and stall over Prince William Sound Monday
into Tuesday.

The combination of the strong upper support from the approaching
shortwave, a very cold airmass parked across the Copper River
Basin, and an increasing surface pressure gradient due to a
deepening surface low in the Eastern Gulf of Alaska will
increase the gap winds especially for this evening into early
Monday. Gusts up to 80 mph are likely for Thompson Pass, and we
have upgraded our High Wind Watch to a High Wind Warning. Later
on Monday, the surface low in the eastern Gulf of Alaska moves
interior and weakens, causing the pressure gradient to ease a bit
which should cause wind gusts to slightly decrease. Early Tuesday,
models are indicating the development of another, weaker surface
low in the northern Gulf somewhere between Cordova and Yakutat,
and the track/intensity of this low will determine how strong the
gap winds will remain for late Monday into Tuesday. In addition to
the wind, a low forming near the Gulf coast may bring in some
snow showers along with it, and if that were to occur, blowing
snow could become a concern. Confidence in this period is low due
to poor model agreement on the positioning/intensity of this newly
developing northern Gulf low.

In addition to Thompson Pass, very gusty winds are expected across
the Matanuska Valley this evening into tomorrow as the cold
airmass and high pressure over the Copper River Basin sends wind
down the Glen Highway. We have issued a High Wind Watch to
account for this. Gusty winds are also likely to generate blowing
dust. Given the very cold airmass in place, areas seeing these
gusty winds will also see wind chills falling below zero.

For the Cook Inlet, Kenai Peninsula and Kodiak, a piece of energy
rounding the base of the upper low should help bring a period of
gusty northerly winds from late today through Monday, with some
storm force northerly wind gusts possible in the Cook Inlet
tonight.

Finally, the Anchorage Bowl will see winds pick up this
afternoon, with some gusts over 30 mph looking likely by later
tonight into early Monday. These gusty winds should then gradually
subside Monday evening.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2): Sunday and
Monday...

The temperatures at 850 mb range from -12C to -20C over Southwest
Alaska at the onset of the forecast period. This pool of cold air
resides over the region the next 48 hrs. The axis of the arctic
trough is positive-tilt and spans from Healy to Lime Village to
Goodnews Bay. The models show a secondary weak shortwave behind
the main trough axis and the potential for pockets of light snow
or flurries is there for today and Monday. Over Bristol Bay,
moderate to fresh northerly winds combined with the frigid
temperatures will be conducive to light freezing spray for K-Bay,
Bristol Bay and areas south of the AKPEN.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2): Sunday
and Monday...

Again gale force winds have been detected by ASCAT (Advanced
Scatterometer) data with the low south of Attu Island. This system
has a back-bent occlusion on the latest satellite imagery. The
expectation is that the surface low will stay south of the chain.
The pressure gradient will be enhanced with this system and gusty
winds will continue to materialize. As the low moves east-
southeast the wind direction will re-orient. Additionally, this
system has modified tropical air entrained in it. The Western and
Central Aleutians can anticipate precipitation today and tomorrow.
Initially today expect rain but then possibly a rain/snow mix.

&&

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

A reinforcing shot of cold air will dive southward across western
Alaska Tuesday, then across the Alaska Peninsula Tuesday night and
Wednesday. Gale force winds are likely across the Southwest Alaska
marine zones, while gusts on the south side of the Alaska
Peninsula could reach storm force. Some of these stronger winds
will shift into the Gulf on Wednesday, then slowly diminish
Wednesday night through Thursday.

Meanwhile, a deep low will track across the Kamchatka Peninsula,
pushing a strong front eastward across the Bering Sea and
Aleutians Tuesday through Wednesday and reaching the Southwest
Alaska coast on Thursday. The forecast largely reflects gale force
winds with this front. However, it is looking more and more like
there will be some storm force winds as the front crosses the
western to central Bering Sea.

&&

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

The long term forecast begins midweek with a persistent upper-
level, long-wave trough over the state with an upper-level ridge
cresting over the far western Bering and Aleutians. At the
surface, this translates to a strong ridge of high pressure
extending across interior Alaska and a surface trough stretched
out over the northern Gulf. This pattern has resulted in generally
clear skies and frigid temperatures due to the light northerly
flow ushering in arctic air and deepening the column of cold air
over much of the Southern Mainland. This has been the dominant
weather feature over the past couple of days and will remain the
trend into the latter half of next week. By Wednesday, a shortwave
diving south over the eastern Bering will track over the AKPEN,
resulting in an increase in snow showers from coastal Southwest
Alaska to the eastern Aleutians. A surface low is also expected to
spin up near Kodiak Island which will help enhance both the
thermal and pressure gradients, leading to an increase in
northerly gap winds along the AKPEN. This feature will slide
quickly to the south, entering the far southern Gulf by Wednesday
evening. However, the ridge over the interior will strengthen,
leading to the potential for increased outflow winds along the
Southcentral coast through late Thursday.

Guidance, however, is suggesting a pattern change may be in store
following the passage of the aforementioned shortwave.
Specifically, the long wave pattern looks to amplify and progress
eastward by the weekend as a deepening upper-level trough digs
over west Asia and shifts the upper-level ridge over the Aleutians
and trough over Mainland Alaska east. At the surface, this shift
will result in the surface ridge over the Aleutians sliding south
and east, allowing a deep low to spin up off of Kamchatka and lift
a front over the Bering toward Southwest Alaska by late Thu. This
solution would also increase the southwesterly flow and introduce
an increased chance for mixed precipitation along the Southwest
coast with snow over the interior. Guidance then suggests that
this front, or remnants of it, may attempt to work into
Southcentral by the end of the week as the ridge once over the
Aleutians moves to the Gulf and the ridge over the interior
weakens and moves farther east over the Yukon and Northwest
Territories. There is a bit of disagreement within the models,
however. First, the EC is not as amplified as the GFS and weakens
the Kamchatka low and associated front more quickly, with the
front breaking apart before it has a chance to move inland.
Second, as the ridge moves into the Gulf, there are strength and
timing discrepancies with the potential of another storm system
spinning up and moving toward the AKPEN for early next week.
Nonetheless, the shift in the pattern looks to bring a more
southerly flow, warmer temperatures, and increased chances for
mixed precipitation to Southwest Alaska by the end of the weekend.
As for Southcentral, there is also a chance for increased cloud
cover and precipitation by the end of next weekend. However, there
is much less confidence on this as any storm system would have to
break down the cold ridge currently in place. Models are notorious
for trying to break these stagnant ridges down too quickly.
Nevertheless, the shift in the pattern would result in a decrease
in the northerly flow and a slow warm up in temperatures both
aloft and at the surface across Southcentral by the end of next
weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 120 121 125 127-132 136 139 140 150 174 176
                       177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...BG/MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...SB/MK/TM