Small Craft Advisory 847, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
555 AM AKST Fri Nov 6 2020

.ANALYSIS AND UPPER LEVELS...

Overall zonal upper level flow across Southcentral with a
shortwave moving across the Fairbanks area. Satellite shows high
clouds streaming eastward along the upper level jet from the Kenai
Peninsula and southward with lower clouds seen across the rest of
Southcentral. The cold airmass continues over Southcentral with
temperatures in the mid to upper teens from the Mat- Su Valleys
and Anchorage area with the Kenai in the low 20s north and upper
20s near Homer and elsewhere along the coast, to the single digits
across the Copper River Valley. While warmer air is over Kodiak
Island with upper 30s to low 40s. Occasional light snow flurries
continue from Anchorage to the MAT- SU valleys and the Copper
River Basin.

Farther west a trough east of the Kamchatka Peninsula will
continue to push warmer air into Southwest Alaska with
temperatures currently in the upper 30s with mid 40s across the
Aleutians. Currently very little precipitation is being observed
at our observation sites, however minor shortwaves embedded in
the larger upper- level trough and southwesterly flow will produce
a few showers across the Bering and Aleutians today.

&&

.MODEL DISCUSSION...

Still some model uncertainty with the low pressure moving towards
Kodiak Island Sunday morning. The GFS has been the most consistent
so we have opted to use it as the model of choice for this
forecast period. The NAM is slightly faster but takes a similar
track from near Kodiak Island to eastern Prince William Sound by
Sunday night while the preferred GFS solution puts the low in the
center of Prince William Sound by early Monday morning. The EC is
a bit of the outlier with the main low moving into Bristol Bay
with a weaker triple point forming east of the Kenai Peninsula
Sunday. The ensembles also show better agreement with GFS.

Confidence is still low, but is increasing as GFS and ensembles
are showing better agreement. However, these model differences
still pose challenges to how much precipitation, wind speeds and
precipitation type. Current thinking with the GFS solution is for
several inches of snowfall in most areas with the exception of
Cordova and Kodiak where temperatures are expected to be warm
enough for rain. Advisory to warning level snowfall is becoming a
better bet in many areas with the current track of this system.

&&

.AVIATION...

PANC...Lingering moisture in the Anchorage bowl behind a
dissipating frontal system will continue to generate periods of very
light snow that may mix with light freezing drizzle for the first
few hours of the morning. The light precip, ceilings as low as
500-1000 ft, and perhaps areas of fog will result in LIFR/IFR
conditions initially. Current thinking is for conditions to
gradually improve with lower visibility associated with
snow/drizzle abating early this morning and ceilings lifting to
MVFR then eventually VFR ceilings by late this evening. However,
confidence for timing of better conditions is low to moderate with
no obvious mechanism to fully mix out the damp low-level airmass
arriving until early Saturday morning. Ceilings should more
markedly improve on Saturday morning to VFR out ahead of the next
storm system arriving later this weekend. Winds will remain light
and North to Northeast through Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Sunday Night)...

The wave that crossed the area this morning appears to have been
more bark than bite. The 12Z Anchorage sounding offers the reason
why once again most of the area got very little snow from last
night's frontal passage. Significant dry air moved in over the
area behind the front, which dried out the dendritic snow growth
layer. That is a portion of the atmosphere that needs to be fully
saturated for snowflakes to grow. When that layer dried out, the
snowflakes that fell stayed very small. Thus, when they reached
the ground, they did not result in much accumulation. For the next
day or so, the snow drought continues. The rest of today will
feature mostly cloudy conditions.

Skipping ahead to the next storm, much of the area will get a much
better chance of getting the first real accumulating snow of the
season. Unlike the last big storm, where cold air was completely
absent, this time there is no shortage of that, as we all know
from the past few days. The biggest forecast question is the track
of the storm. Most of the models keep some of the energy west of
the Aleutian/Alaska Ranges for a time, then transfer that energy
in the form of a developing triple-point low to the Gulf. The EC
solution is by far the latest with this energy transfer occurring,
taking the stronger low through the Kuskokwim Valley before
developing a much weaker low south of Seward. This solution would
bring a warm event to Southcentral, with most areas seeing lots of
rain. Fortunately, almost all of the other models depict that
transfer happening much sooner, with the stronger low in the Gulf
developing near Kodiak Island, then tracking south of Seward and
into Prince William Sound. This track would keep most of the Cook
Inlet region on the "cold" side of the low. However, regardless of
the low track, it will bring a substantial amount of warm air
north along with it. Thus, some areas, particularly along the
coast from Seward to Whittier and Valdez will be rain for a bulk
of the event. Time will tell how high snow levels rise in Thompson
Pass as well. For now most of the event in Thompson Pass is
expected to stay snow, with Sunday being the best chance of rain
mixing in there.

Regardless of track, plenty of cold air will persist in the
Susitna Valley for this event. The plentiful moisture will result
in heavy snow for much of the Valley. A Winter Storm Watch has
been issued. Special Weather Statements have been issued for
Anchorage, the Mat Valley, and the Western Kenai Peninsula.
Precipitation type will be the primary challenge, which will
depend on the track of the low and how much warm air moves in on a
period of gusty southeasterly winds ahead of the low Saturday
night. Snow will be measured in feet in Turnagain Pass, resulting
in very hazardous to impossible travel along the Sterling and
Seward Highways in the mountains. Snow will linger in the Copper
Basin into Sunday night, where several inches of snow are expected
as well.

&&

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

Drier condition across the Southwest today with persistent
southwesterly flow through tonight, keeping temperatures above
freezing today and tomorrow. For tonight, the coasts will remain
above freezing with temperatures at or just below freezing
further inland. By Saturday, a North Pacific low approaches the
AKPEN. Models are disagreeing on the track of this low, with the
NAM/GFS giving it a more easterly track towards Shelikof Strait,
and the EC/Canadian tracking it north through Bristol Bay.
Precipitation type with this low is very path dependent and the
current forecast follows the more easterly path. Thus, rain is
expected along the coasts by Saturday afternoon, and snow further
inland across the Kuskokwim Valley Saturday evening into Sunday.
Snow accumulations, should this system stay on the current track,
may be significant enough to warrant an advisory with anywhere
from 5-8 inches expected in some areas.

&&

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

Widespread small crafts and persistent southwesterly flow
dominates much of the Bering through tonight. A band of moderate
to heavy rainfall skims the southern edge of the Alaska Peninsula
before tapering off to light showers. Widespread light rain
showers are expected elsewhere through tomorrow night. A second
system approaches the Western Bering on Sunday, bringing more
rain and another round widespread gales with possible storm force
gusts.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: A gale force low will move across the Gulf
Sunday. Wave heights at their worst Sunday will range between 15
to 18 feet. Gale force winds are expected through Monday and as
the low fills the pressure gradient will slack off.

Bering Sea: Sunday a new low will emerge from the North Pacific
with a surface low of 984 mb near Attu Island. 24 hrs later, the
models diverge on the track of the surface low, however, the
ECMWF brings takes the low north of Attu and into the Bering. By
late Monday the surface low is much stronger and in the North
Bering. The model consensus is that this system will produce gale
to storm force winds.

&&

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

The longwave pattern has a trough entering western Alaska, a
glimmer of a ridge axis over the Western Bering and a trough over
Kamchatka. Twelve hrs later, the leading trough is bisecting
Mainland Alaska, the ridge is over the Pribilof Islands and the
trough has made headway into the Western Aleutians. This is a very
progressive and dynamic pattern. The amount of shortwave energy
associated with each upper level trough is stunning. Both of these
lows are juiced. The primary forecast challenge will be pinging
into the precipitation types (rain vs. snow vs. rain/snow mix) and
as always...the timing of the the low tracks. Long story short,
Mainland Alaska will get back to back lows.

&&

.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...PJS

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1000 AM AKST FRI NOV 6 2020

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

0959 AM     SNOW             3 S KENAI               60.52N 151.21W
11/06/2020  E0.3 INCH        KENAI PENINSULA    AK   TRAINED SPOTTER

            SPOTTER REPORTED 3/16TH INCH STORM TOTAL AS
            OF 8AM.

&&

MTL

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