Gale Warning 590, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Wed Nov 4 2020

.ANALYSIS AND UPPER LEVELS...

The well-advertised upper-level low that has remained parked over
Southcentral Alaska the past few days is finally beginning to lift
northeast and weaken. Nonetheless, this feature continues to
support plenty of Arctic air at the surface. Low temperatures
this morning once again bottomed out between -15F and -15F in the
Copper River Basin, with lows dropping between -10F and +10F for
most other locations. An upper-level shortwave is also dropping
south of the AKPEN this afternoon. Northerly winds continue to
gust across Bristol Bay and the AKPEN with occasional snow
showers. An extension of this wave extends northward and is
tracking east over the Alaska Range. Evidence of this can be seen
on IR satellite imagery as a band of mid-level clouds sliding
east. Farther west, an upper-level ridge is moving east over the
eastern Aleutians/Bering in response to a deep upper-level low
along the Russian coast with embedded shortwaves moving over the
western Aleutians/Bering.

At the surface, the upper-level setup translates to a surface
ridge extending across interior Alaska with series of compact
meso-lows spinning up over the northern Gulf. Gusty outflow winds
are increasing this afternoon due to the combination of a strong
pressure and thermal gradients. A surface low is also deepening
well south of Kodiak Island, but is helping to enhance the gusty
northerly winds and snow showers across the marine zones south of
the AKPEN. Elsewhere, warmer temperatures and southwesterly gales
are occurring over the central Aleutians/Bering along the back
side of the ridge. Storm-force winds and rain continue across the
western Aleutians/Bering in advance of a surface front.

&&

.MODEL DISCUSSION...

Models continue to perform well in the short term. There are
discrepancies; however, guidance is keying in on the significant
features. Thus, hi-res guidance was favored for the high winds
near Valdez and Thompson Pass, and the increase in gusts through
the coastal Southcentral passes. Models are also performing well
with the evolution of the front moving across the Bering and
onshore over Southwest Alaska by Thursday. Models also continue to
advertise widespread light snow for Southcentral late Thursday
night into Friday. Model uncertainty creeps in by the weekend as a
north Pacific low lifts to the AKPEN. Deterministic GFS and EC
runs, as well as their respective ensembles, have flip-flopped on
this solution, alternating between the main low lifting north to
the Y-K Delta and remaining the dominant feature and the dominant
low being a triple-point low spinning up near Kodiak Island and
moving toward Prince William Sound. If the Y-K Delta low remains
the dominant feature, this would likely bring a southerly flow to
Southcentral and introduce warmer temperatures and precipitation
type issues for many locations. If the Gulf low is dominant, this
would likely keep a northerly component to the upper-level wind
field. This would keep colder air in place and increase the
likelihood for widespread snow this coming weekend. Bottom line,
is that there remains high forecast confidence through the rest of
the week. That confidence, however, drops below average by the
weekend.

&&

.AVIATION...
PANC...VFR conditions will persist. Light northerly winds will
continue through most of the day. However, gusts out of the north
will redevelop this evening and linger through the early morning
Thursday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (through Saturday)...
The next few days will feature a transition from one air
mass/upper level pattern to another. The first feature of interest
is an upper level jet/trough over the mainland. Coincident with
Arctic air in place over the Copper River Basin, these conditions
will promote very strong winds for the Valdez/Thompson Pass and
Copper River Delta. A high wind warning will begin tonight,
through Thursday morning.

The upper level pattern begins to shift on Thursday as a ridge
builds in from the Bering Sea, associated with a strong Kamchatka
low pressure. The front extending from this system will move
overhead the Cook Inlet region beginning Thursday afternoon. Look
for light snow to develop mid to late afternoon and continue
through the early morning hours. Light snow will linger along the
Chugach from Palmer through the Hillside of Anchorage. In general,
expect anywhere from 2 to 5 inches with highest amounts at higher
elevations. This front will bring some relief from the below
normal temperatures we've been experiencing over the last week.
The front exits quickly on Friday. The next system to keep an eye
on rolls in over the weekend, but the details are very uncertain
at this point. Stay tuned.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)... The
overall forecast remains mostly on track with the incoming front
is still expected to bring Coastal Flooding, a rain/snow mix, and
strong winds to the region by late tonight.

From the Coastal Flooding perspective, guidance has been very
consistent by indicating storm surge of 3-5 ft above highest
normal tide from Toksook Bay to Hooper Bay over the last 24 hours,
allowing for a Coastal Flood Warning to be issued earlier today.
Afterwards the guidance has trended higher in Kuskokwim Bay as
well due to small changes in when the peak surge is expected in
the tidal cycle. Because of this trend, the higher surge values
were extended into the southwestern exposures of Kuskokwim Bay in
the latest warning.

This front will initially bring another round of snow and blowing
snow to the Kuskokwim Delta starting early Thursday morning.
However with strong warm air advection, southerly winds, and very
little existing transportable snowpack, this is expected to be
very brief and not reach advisory criteria. As the front tracks
inland, rainfall should quickly shift over to rain throughout the
Kuskokwim Delta and much of Bristol Bay by Thursday afternoon.
Further inland over the Kuskokwim Valley conditions should remain
cool enough to support a few inches of fresh snowfall, but likely
not more due to the decaying nature of the front.

After a brief break in the active weather on Friday, this very
active pattern for the region is expected to continue into the
weekend as the next system approaches the AKPen for Saturday.
There still remains some uncertainty in the low track of this
system, but there is good confidence that this system will bring
rain, snow, and gusty east winds to most areas through Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and through 3)...
A storm force front will continue tracking eastward through the
area into Friday, bringing continued strong winds and rain to much
of the region. There is an outside chance of seeing a few warning
level (75 mph) gusts as this front moves through the Eastern
Aleutians and Pribilof Islands, but with strong warm air advection
these are not expected to be widespread or long lasting enough to
warrant a warning.

The next North Pacific low will then approach the AKPen/Eastern
Aleutians on Saturday. Impacts from this system should generally
be confined to the Eastern Bering, however some of the guidance
continues to follow a farther west track towards Dutch Harbor. At
this point this more westward track is not included in the
forecast, but will continue to be monitored in the coming days.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Latest guidance continues to show the Saturday to
Monday timeframe being very active across the western Gulf. A gale
force storm system is expected to begin impacting the area
Saturday morning with winds increasing from southwest to northeast
throughout the day. Gale force winds are expected through Monday
before slowly diminishing below gale force Monday
morning/afternoon. There also may be a brief period of storm force
winds through the Barren Islands into the western Gulf Sunday
afternoon/evening but this is less certain. The exact track and
strength of the low will determine if the winds reach storm force
from the Barren Island into the western Gulf.

Bering Sea: Saturday is expected to be the beginning of another
active period for the Bering Sea with a gale force storm system
beginning to impact the region Saturday morning. This initial low
continues moving towards the Pribilofs Saturday evening with
another low on its heels. Beyond Saturday night, the details of
what exactly happens and how this low develops/moves diverge
among the model guidance. Either way model guidance shows another
storm force storm system moving into the Bering Sea.

&&

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

The long term forecast period continues to look active for much of
southern Alaska including the Bering Sea and Aleutians. For the
Bering Sea and Aleutian Chain, a parade of storm systems is
expected to keep the area unsettled for the duration of the this
time frame. A low is expected to move from the southwest corner of
the Bering northward towards Saint Lawrence Island sometime
Tuesday afternoon. This will yield to a great push of warm air
across the Bering and into Southwest Alaska leading to
precipitation type issues. With this push of warm air, depending
on how much cold air is able to hold on at the surface, a mix of
snow, sleet, and freezing rain and even plain rain is possible.
Accompanying the unsettled weather will be an increase in winds
across much of the Bering Sea, Aleutians, and Southwest Alaska.
Storm force winds are possible through the Aleutian Chain and into
the Bering Sea.

Farther east, models continue to remain in disagreement on the
exact track of a low moving northward from south of the Aleutians.
One model (GFS) brings the low northward and develops a triple
point low to the east of the Kenai Peninsula Sunday morning that
then continues into Prince William Sound. The other models (ECMWF,
Canadian-NH) keep one low center that moves northward straight
into Bristol Bay with no triple point low development. If this low
were to take a further west track with no new triple point
development just east of the Kenai Peninsula, then much warmer air
will be able move into Southcentral. This will cause
precipitation type issues. This solution appears unlikely.
However, with the triple point low development which seems more
likely, this could mean an increased chance for snow across the
Southcentral area as cold air will be able to hold on as this low
would pull cold air south into it and the low would be east of the
area placing Southcentral on the northwest side. After this storm
moves out of the region Sunday night into early Monday, a brief
upper level ridge will build across Southcentral that may provide
a respite from the active weather the day prior. This break will
be short-lived though as southwesterly flow in the upper levels
moves back across Southcentral and starts up another period of
active weather for late Tuesday through early Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Warning...155. High Wind Warning...131.
MARINE...Storms: 127, 170, 172-179, 185, 411-414.
Gales: 119, 128, 131, 132, 138, 150, 155, 160, 165, 171, 180,
181, 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
MARINE/LONG TERM...MV