Small Craft Advisory 906, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
546 AM AKST Thu Nov 26 2020

.ANALYSIS AND UPPER LEVELS...

A closed upper level low continues to be anchored over eastern
Russia with a negatively tilted trough extending into the Gulf of
Alaska. A ridge is currently positioned over the Alcan border.
This synoptic pattern allows for strong cold air advection over
the north Pacific. Embedded within and south of this flow is a
train of shortwaves which continue to bring active weather across
the Aleutian Chain. These waves then curve north into Southcentral
upon interaction with the North American ridge. Thus, the pattern
of active weather is expected to continue across the domain.

On Satellite, three features that will impact the forecast in the
short term can be easily identified. Over Southcentral the
remnants of a shortwave that brought wind, rain and snow to the
region is moving north into the Susitna Valley and Copper River
Basin. The second notable feature on satellite is the leading edge
of a cold airmass moving east over the north Pacific as a north-
south oriented front to the south of Kodiak Island. This is the
baroclinic zone that is expected to materialize into a few weak
low pressure systems moving into the Gulf and Southcentral later
today. Lastly, a large cloud shield and developing comma shaped
storm system can be seen to the south of Shemya.

The Kenai and Middleton Island Radars have both been
steadily clearing out this morning although a few lingering snow
showers exist over the Anchorage Bowl, Mat-Su Valleys and Prince
William Sound. Over Southwest Alaska, an upper level shortwave is
producing snow showers from King Salmon to Bethel. For the Bering,
cold air advection continues to produce scattered snow showers and
open cell cumulus clouds.

&&

.MODEL DISCUSSION...

Overall model agreement remains fair to good in the short term.
Models have somewhat common solutions for the low expected to move
into Southcentral later today; however, minor differences in the
low track and potential deformation band could have large
variation on snow totals for the population centers. Over the
Bering models continue to disagree on the position and strength of
the stronger of the two lows. Slight deviations in the track to
the north or south on Thursday night could mean the difference
between hurricane or storm force winds for Adak and Atka.

&&

.AVIATION...
PANC... A few remnant snow showers will continue through the early
morning hours, but drier air moving in along with clearing skies
from the south will end the snow threat for now for the rest of
the day. Tonight, a compact low will move into Southcentral from
the Gulf. There remains considerable uncertainty with the exact
track of the low, which will make quite a bit of difference as to
how much snow will fall tonight and for how long. The forecast
takes a middle-ground approach to the snowfall intensity. It is
very possible that there is less snow and conditions are better
than indicated, but also possible that a band of snow sets up over
the area which will make conditions worse.

&&

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

Happy Thanksgiving! The mixed rain and snow event that has been
impacting Southcentral is finally winding down this morning. The
last of a snowband over northern Anchorage to JBER is dissipating.
Skies are clearing somewhat over the Kenai Peninsula, with the
clearing approaching Anchorage and the Mat-Su. Today will feature
a mix of sun and clouds in between systems, with temperatures
steady or gradually falling through the day. This is due to a
colder air mass working in combined with the partly cloudy skies
allowing more heat to escape than the weak late November sun can
replace.

Later on today, a small but potent low will move northward from
the North Pacific towards Southcentral. The low will pass near or
over Kodiak. From there, the models already begin to disagree on
the track of the low tonight. Most of the latest models bring the
low up Cook Inlet, though a few take some semblance of the low
further east into Seward or far western Prince William Sound near
Whittier. Since this low isn't associated with a warm air mass,
there remains relatively high confidence that snow will be the
main precipitation type, except right at the immediate coast. The
track of the low will determine who sees how much snow and for
how long. Thus the track uncertainty is causing significant
uncertainty in the snowfall amounts. Since the low will be
relatively fast moving, and most likely to track up Cook Inlet,
most areas should hold below 2 inches of total accumulation
overnight tonight. However, some of the model solutions continue
to suggest that a more persistent band of heavier snow will
develop west of the low track. If the low tracks up Cook Inlet,
this will most likely be over the western Susitna Valley, but
depending on smaller details of how the surface low interacts with
the upper level wave, some heavier snow could set up over
Anchorage and the Mat-Su as well. This would cause larger snowfall
amounts locally than is advertised. However, it appears that
since most of the snow will be associated with the low itself,
most areas should see relatively equal amounts of snow.

On Friday, as the low dissipates, a corridor of moisture is likely
to set up over the central Gulf, extending north into eastern
Prince William Sound. This could lead to higher snowfall amounts
from Cordova north through Valdez and Thompson Pass. Some of the
remnant moisture could extend north into the Copper River Basin as
well. This is not associated with an organized front, so wind is
not expected to be a concern during the snow in any of those
areas. Much of the rest of Southcentral will get another brief
break in between precipitation events.

Late Friday into Friday night, a strong low over the Bering will
extend an occluding front into the Gulf. It will first move into
Kodiak Friday afternoon. Then as the front elongates over the
Gulf, it will move eastward along the north Gulf Coast through
Saturday morning. The front could cause localized storm force
winds in the usual barrier jet areas south of Cordova, and across
the Barren Islands, but for most areas gales are expected. The
southeasterly flow associated with the front will focus the
precipitation along the coastal mountains, with Anchorage and the
Mat-Su sheltered from most, if not all precipitation through
Saturday morning. It will have somewhat warmer air with it, so
precipitation type concerns will be present just inland from the
coast, including Portage and Turnagain Pass. Gusty winds through
the gaps are expected as well, but are not expected to produce
warning level winds at any time.

&&

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

Snow showers will continue across Southwest Alaska, especially
along the coastal areas as cold air advection continues across the
eastern Bering into the Kuskokwim Delta and Bristol Bay.
Temperatures will drop over the next 3 days as a strong area of
low pressure moves northeastward along the southern AKPEN and
stall over the northern Gulf Friday afternoon onward. This will
allow for brisk east- northeasterly offshore flow across the area
and advect continental polar air into the region. As this low
moves south of the AKPEN, precipitation is expected across the
AKPEN and Bristol Bay. There is a high confidence that the
precipitation type will be snow as models consistently have these
areas northwest of the low center which will keep temperatures
cold and allow for snow for the entirety of the event. Snowfall
totals will range from a coating to 2" for most of Bristol Bay.
Higher snowfall accumulations are expected in the Ahklun and
Kilbuck Mountains due to upslope enhancement. Snowfall totals of
3-6" with localized 5-9" possible in the Ahlkun and Kilbuck
Mountains.

&&

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

A rapidly developing low pressure system in the North Pacific will
move into the central Aleutians this afternoon and evening. This
storm system will bring widespread gale and storm force winds to
the Aleutians. The strongest winds will reach Hurricane Force and
will be located across the central Aleutians tonight through
Friday mid-day. Due to the widespread Hurricane Force winds
expected, a High Wind Warning has been issued for the central
Aleutians (Zone 187) from 9pm AKST this evening through 12pm AKST
Friday. Winds will be locally higher in areas prone to funneling
from the west-northwest. Winds will gradually ease across the
Aleutians Friday afternoon onward as this storm system will move
south of the AKPEN and into the Gulf of Alaska at that time. Large
long period seas are expected in addition to these strong winds.
The largest seas will be along the Pacific side of the central and
eastern Aleutians as this is where the Hurricane Force westerly
winds will be located. These seas are expected to build near 40ft
tonight through Friday night and then will gradually abate
thereafter. Guidance is fairly consistent with the placement of
this low but varies on intensity forecasts with the GFS/NAM being
the strongest and ECMWF the weakest. A GFS/NAM blend was favored
as the dynamics of this system favor the stronger solutions due to
the trough-axis that this storm system is developing along is
changing its orientation from a positive to a negative slope which
favors rapid cyclogenesis. Furthermore, the GFS has been
performing the best over the past week with the intensity
forecasts of these lows, another reason this model was weighted
more than the rest of guidance.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...
A large and deep storm system will make its way from the Bering
Sea into the Gulf Friday night through Saturday, with widespread
gales across the region storm force winds likely along the leading
frontal boundary as it crosses the Gulf. As this system weakens in
the Gulf Saturday night, another storm force low will track south
of the Aleutian chain. As this storm approaches the Gulf on
Monday, the low will start to take a more northerly track. Thus,
there is potential for another round of storm force winds across
the Gulf waters as the leading front crosses the Gulf on Monday.
The forecast will stick with gales for now on Monday, but suspect
as we get closer models will settle on the stronger solution.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...
A seasonably strong upper level jet combined with progressive
upper level flow will persist through at least the first half of
next week. There is seemingly no end in sight to the supply of
large deep storm systems traversing the Alaska region. Starting
out on Sunday, one storm will be in the weakening phase over the
northern Gulf and Southcentral Alaska. Another storm will be
strengthening as it tracks just south of the Aleutian chain.
Expect widespread strong winds and moderate to heavy precipitation
along the Aleutians and Alaska Peninsula as this storm continues
eastward Sunday through Sunday night. As this low continues to
deepen, a short-wave ridge will strengthen out ahead of it, which
will then cause the storm to curve northward as it approaches the
Gulf. As a result, expect precipitation and wind to overspread
southern Alaska. Based on a storm track in the vicinity of the
Alaska Peninsula and the western Gulf, warm air will surge
northward toward Southcentral. Thus chances are this will be
primarily a rain event along the coast, with potential for enough
warm air to spread inland and cause precipitation to change to
rain there as well. The effects of this storm look to linger
through Wednesday, with colder air moving in behind it. Stay
tuned, as it's too early to nail down specific impacts from this
storm.

&&

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

&&

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
512 AM AKST THU NOV 26 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0510 AM     SNOW             14 S KNIK               61.26N 149.81W
11/26/2020  M1.9 INCH        ANCHORAGE          AK   OTHER FEDERAL

            1.9 FELL WITH 1.6 REMAINING AFTER SETTLING.

&&

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
513 AM AKST THU NOV 26 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0510 AM     SNOW             14 S KNIK               61.26N 149.81W
11/26/2020  M1.9 INCH        ANCHORAGE          AK   OTHER FEDERAL

            1.9 FELL WITH 1.6 REMAINING AFTER SETTLING.

&&