Gale Warning 639, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
543 AM AKST Sat Nov 7 2020

.ANALYSIS AND UPPER LEVELS...
Satellite shows an upper level trough near the western Aleutians
and a ridge over the northeast Pacific with a low pressure system
approaching the eastern Aleutians. This low is spreading clouds
north and east across the AKPEN and into Southcentral. Radar shows
a few showers across the Kuskokwim Delta and Bristol Bay as the
low pressure moves northward.

Stratus and some patchy fog from the eastern Kenai up to Anchorage
and the MAT- SU valleys with some occasional snow flurries across
this area. Temperatures across the Kenai and up towards Anchorage
and the MAT- SU valleys are generally in the 20s with the coldest
air in the Copper River Basin in the single digits to well below
zero. While most coastal locations and Kodiak are in the upper 20s
and 30s.

&&

.MODEL DISCUSSION...
Models are coming into better agreement with increasing confidence
for the forecast. The main low pressure moves towards Bristol Bay
Saturday and then inland Sunday Morning, while a triple point
forms Sunday morning across the Shelikof Strait moving near the
eastern side of the Kenai and then towards Prince William Sound by
Sunday afternoon. Most of the models show this general track,
however the GFS is the outlier with it forecasting the triple
point low to become the dominant feature keeping the
precipitation across Anchorage and the MAT valley all snow. All
the other model tracks will allow for snow to develop initially
and then change to rain for the Kenai into Anchorage and the MAT
Valley while northern parts of the Susitna and Copper River Basin
will stay cold enough with this low track to remain all snow. As
the low tracks east the cold air will move back into Southcentral
but most of the moisture will have moved east so very little snow
expected for Anchorage and the MAT valley as it gets colder Sunday
night and Monday morning.

Otherwise models are in fair agreement with the next low pressure
system moving into the western Bering Sea Sunday and Monday.

&&

.AVIATION...
PANC...Stubborn low ceilings hovering in the IFR range should
improve to MVFR levels late this morning to early afternoon as
mixing increases and the lower atmosphere dries out to some degree
ahead of the next approaching system. Conditions will take a turn
for the worse by the late afternoon as the system approaches from
the southwest and as light snow overspreads the area, and
ceilings/visibilities should drop mainly back to IFR through the
evening. Strong southeast winds aloft will result in substantial
LLWS of 40-50 kts or higher with increasing northerly surface
winds around 12 kts contrasting with roughly 50 kt southeast winds
at 2000 ft AGL by this evening. Snow and likely IFR conditions
will continue until warming low-level temps change precip to a
rain/snow mix, likely just before 12 AM AKST Sunday, when
visibilities should come back to MVFR. Currently expect a
transition to all rain early Sunday morning with continued MVFR
ceilings and visibility along with gusty north to northeast winds
and LLWS lasting through Sunday morning.

&&

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

A strong North Pacific low moving into the area will make for very
active weather across Southcentral this weekend. First with the
active products, a Winter Storm Warning is in effect for the
Susitna Valley for up to 20 inches of snow through Sunday
afternoon, mainly from Talkeetna north and west. A High Wind
Warning is in effect for Turnagain Arm and higher elevations for
wind gusts to 75 mph tonight.

The models are agonizingly slowly coming into better agreement
with the track of the storm. The track makes all the difference in
the world as to what the impacts from the storm will be. The broad
solution is for a bit more westerly track of the low. This means
the predominant center of the low will track north over Bristol
Bay and the Kuskokwim Valley through Sunday morning, with a later
transition of its energy to a triple-point low south of Seward.
This means we are now expecting more warm air and more wind to
impact much of the Southcentral than previous forecasts. Thus,
broadly speaking, forecast snow totals have been lowered for
lower elevation locations, and winds have been increased quite a
bit for the higher elevations. Updated Special Weather Statements
detailing the current forecast have been issued for Anchorage,
the Mat Valley, the Kenai Peninsula, and Prince William Sound.

A few other points of note: on the back side of the storm as the
center moves east along the North Gulf Coast Sunday night,
somewhat colder air will move in to the area. Very strong pressure
rises along the Kenai Mountains will cause strong westerly winds
through Whittier Sunday night. With most of the snow through
Thompson Pass falling on southeasterly winds through Sunday, once
the winds turn around to northerly Sunday night into Monday,
blowing snow could become a worsening problem, even as skies clear
and the falling snow ends. Gusty southeasterly winds through the
Copper River Basin will warm temperatures and could result in rain
mixing in with periods of dry weather in Gulkana during the day
Sunday. Finally, heavy snow across the Denali Highway region of
the Copper River Basin could result in over a foot of new snow
accumulation from Sunday and potentially lasting into Monday
morning.

&&

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

A low out of the North Pacific reaches the AKPEN this morning,
taking a more northerly track than previously suggested. This will
keep the low center west of the Alaska Range, pushing much warmer
air into Bristol Bay. As a result, temperatures will remain above
freezing for the next couple days for majority of the Southwest,
with the exceptions being the higher elevations. Additionally,
precipitation today through tomorrow will likely remain snow for
the higher elevations, rain along the coasts, and a rain/snow mix
elsewhere.

A second frontal system approaches the Southwest coast by Monday.
Southwesterly flow will dominate the coastal areas with gusty
southerly winds across the Kuskokwim Delta. A widespread band of
light showers will also push across the Southwest mainland.
This pattern is ideal for coastal flooding potential for the
Kuskokwim Bay. Surges are currently forecast to be ~2 ft above
Highest Astronomical Tide by Monday afternoon. This will continue
to be closely monitored.

&&

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

An active pattern approaches the Western Aleutians Sunday morning.
A weak surface low moves across the Bering, diminishing quickly
with minimal impacts aside from some coastal small craft winds. A
second low follows closely behind and tracks just to the north of
the Bering. An associated front travels eastward towards the
Southwest Coast through Monday, bringing widespread gales and
periods of storm force gusts. A band of moderate to heavy rainfall
is expected for the Western Aleutians, with light rain elsewhere.

&&

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

Gulf of Alaska:

Starting Sunday evening into early Monday morning, storm force
winds will emerge to the east and northeast of Kodiak, near
Barren Islands, and offshores. These winds are well associated
with a frontal system inland, but as the frontal zone weakens
Sunday night, the storm force winds will also diminish to gales by
Monday morning. Essentially, gales offshores will subside to
below 34 kt Monday afternoon, so will hazardous wave heights by
Monday evening.

Bering Sea:

On Sunday evening, the potential for storm force winds are
expected to extend up to 320 nm southeast quadrant of the low over
the Bering, and up to 150 nm south of the same system. Wave
heights will peak to 30 feet near the center of the low Sunday
night. The areal coverage of developing storm force winds will
gradually reduce Sunday night along the southeast quadrant of the
low, then essentially taper off to gales by Monday mid-morning.
As the low weakens while trekking northeastward toward Yukon Bay,
so will the winds and seas.

&&

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

There is high confidence that the pattern will remain active
through the forecast period. Synoptically on Monday there will be
ridging over the Gulf and southern Mainland with a trough over the
Bering Sea. At this time a moderately strong surface low is
expected to be over the northern Bering. For Southwest Alaska
this should mean snow transitioning to rain as the warm front
moves north throughout the day on Monday. On Tuesday a complex
upper level trough moves east into the Bering from the Kamchatka
Peninsula. Models are in poor agreement in regards to finer
details of the resulting surface low, or low complex, but impacts
indicate widespread rain across Southwest Alaska on Tuesday and
early Wednesday. There is moderate agreement that a warm front
will push into Southcentral on Wednesday with models in
disagreement in regards to precipitation type. Most models
indicate widespread warm air and therefore rain but a transition
to snow on Thursday is possible if a triple point low forms in the
Gulf. On Friday the low complex is expected to be southeast of the
AOR, which will result in the return of cold air to most of the
region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...CB/CJ