Small Craft Advisory 845, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
435 PM AKST Thu Nov 5 2020

.ANALYSIS AND UPPER LEVELS...

Clouds are on the increase over Southcentral this afternoon in
advance of a front draped across the Kuskokwim Delta and sliding
east. The cold airmass in place over Southcentral is also slowly
moderating; however, temperatures across the region are still
quite cold for this time of year. As of this afternoon,
temperatures range from the teens and 20s from the Mat-Su Valleys
to Homer and elsewhere along the coast, to the single digits
across the Copper River Valley. Cold outflow winds remain this
afternoon, with weak troughing lingering over the northeastern
Gulf, but are diminishing, as the trough departs in advance of the
approaching frontal system.

Farther west, snow and gusty southerly winds are occurring over
interior Southwest Alaska ahead of a warm front. Behind the front,
temperatures have climbed into the middle to upper 30s, with the
snow changing to rain from Dillingham to Hooper Bay. This front
is in association with a vertically stacked low lifting northeast
along the Russian coast. Shortwaves embedded in the larger upper-
level trough and southwesterly flow is aiding in the instability
and resulting in convection across the Bering this afternoon.
Lightning has been observed over the central Bering this morning
with a couple of strikes this afternoon near the Pribilofs.

&&

.MODEL DISCUSSION...

Models remain on track through the short term with the
progression of the front and the associated shortwave moving from
Southwest to Southcentral Alaska later this evening. The GFS is
still indicating the least amount of snow with this system, with
the NAM and hi-res guidance more bullish. A middle ground approach
was favored given the very dry air in the lower half of the
troposphere.

As previous discussions have stated, model uncertainty remains
quite high for the next system coming in this weekend. The GFS
is still holding onto a deeper triple point low forming near
Kodiak and driving it to western Prince William Sound. The NAM
was on board with this solution, but the recent 18z run has
trended farther west and keeps the initial low moving across
Bristol Bay stronger, similar to the EC. The EC was beginning a
trend toward a weaker SW AK low and a farther east solution, but
has drifted back to previous solutions in its recent 12z run. The
Canadian is somewhere in the middle. These differences in track
and strength of the lows will yield significant variations to the
overall weather forecast depending on which model is correct (wind
speeds and precipitation type). Currently, the forecast reflects
a GFS and NAM blend, as these models have been the most
consistent. However, confidence remains quite low with this
upcoming system.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist for most of the
day. This evening, a weather system will move into Southcentral
AK. This will bring light snow and MVFR conditions for both
ceilings and visibility through the night. Snow will clear in the
early morning hours allowing visibility to improve, but MVFR cloud
cover will remain through the TAF period.

&&

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

The beginning of a more active weather pattern across Southcentral
is here. Clouds have been increasing and lowering throughout the
day as a frontal system is approaching the region. Not much has
changed from the previous forecast with this upcoming frontal
system for tonight. A few things of note: the front will continue
to weaken and shear apart as it moves eastward and becomes
farther away from its parent low center which is up near eastern
Russia. Next, plenty of cold air is in place for this to remain
all snow for Southcentral. Thirdly, latest sounding data (vertical
profile of the atmosphere) shows the atmosphere really moistening
up especially about a mile above the ground (mountaintop). Below
that, a great deal of dry air continues to be in place. This dry
air can be one of the ingredients that lead to the lower end of
the snowfall amounts predicted to verify. As the flakes are
falling later this evening, some of this moisture will go into
saturating the column of air which would cut into the expected
snowfall amounts. Any dry air is expected to have been overcome by
9pm this evening which is when we expect a few to couple hours of
steady light snow. The steady light snow will begin to gradually
taper off after midnight to the early morning Friday with just
some snow showers remaining, especially across the Talkeetna and
Chugach Mountains. Therefore, a widespread 1-3 inches is expected
for west Anchorage, the western Kenai Peninsula, the Mat Valley,
and from Wasilla west. The Anchorage Hillside, Palmer and the
eastern Mat Valley continue to be expected to see anywhere from 2
to 5 inches of accumulation especially due to upslope enhancement
against the mountains.

Some residual light snow showers or flurries are possible
throughout the day Friday across valley areas but the better
chance remains in the mountains. Parts of the Talkeetna Mountains
will have the chance for snow continuing into Saturday as upslope
flow remains in place Friday night into early Saturday there.
Otherwise, intervals of clouds and sun is expected for much of
Southcentral in the valley areas of the Kenai Peninsula, west
Anchorage, and the western Matanuska/Susitna Valleys Friday with a
mostly cloudy sky for Friday night into Saturday.

Attention then turns to the next more impactful weather system
that will be moving northward with an origination point to the
south of the Aleutians. As this low moves into Bristol Bay on
Saturday, a great deal of uncertainty remains across the model
guidance on what happens after this low moves inland. One model
solution would suggest a great deal of warm air overspreading much
of Southcentral leading to possibly an all rain event. The other
guidance shows a secondary or triple point low developing just
southeast of Seward and then moving into Prince William Sound.
This solution would lead to colder air to be drawn south across
Southcentral potentially leading to a moderate to heavy snowfall
event. Nonetheless, until the model guidance comes into better
agreement on track and if a second low will develop, precipitation
type continues to be a big concern for much of the area. There is
also the potential for the Turnagain Arm wind to develop,
potentially leading to windy conditions across the Anchorage
Hillside and across the Chugach Mountains. There may also be a
barrier jet across the northern Gulf Coast early Sunday into
Monday morning. Right now, high end gale force winds are in the
forecast but the potential for storm force is there. Details on
this system will continue to be fine-tuned over the next few days,
therefore, continue to check back for the latest updates.

&&

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

A front will quickly depart to the east this evening, giving way
to partly clear skies overnight. Despite this, southwesterly flow
will keep lows above freezing over much of the Kuskokwim Delta and
Bristol Bay coast. The general pattern will continue through
Friday night. Most areas are expected to remain dry, with a few
rain/snow showers possible north of Bethel on Friday afternoon
under an upper level shortwave.

A low will approach the Alaska Peninsula Saturday morning.
Precipitation will develop from south to north as the center
moves over Shelikof Strait. While the exact track is still
uncertain, generally all rain is expected along the coast (water
temperatures still in the 40s), rain transitioning to light snow
farther inland and all snow (possibly significant amounts) over
the Middle Kuskokwim Valley.

&&

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

Southwest flow will persist through Friday. Scattered showers
should remain north of the Pribilofs along an upper shortwave.
Likewise, a stream of moisture extends from just south of Kodiak
Island, westward to Unalaska. This will gradually track northward
as a low approaches the Alaska Peninsula. A second wave of
precipitation will quickly follow as the circulation passes south
of Sand Point and into Shelikof Strait. This will gradually taper
to scattered showers and end by Sunday as a weak ridge moves in
from the west. A front will bring rain and gusty southwesterly
winds to the far western Aleutians into Sunday night.

&&

.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 hours 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 hours 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...Coastal Flood Warning: 155.
MARINE...Gales: 132, 150, 155, 172, 179, 180, 181, 185, 411, 412,
413, 414.
FIRE WEATHER...NONE.

&&

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