Small Craft Advisory 820, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 AM AKDT Wed Oct 28 2020

.ANALYSIS AND UPPER LEVELS...

A weak upper low in the western Bering is steering an associated
surface front eastward, which is currently bringing light rain
extending from the Pribilofs to the central Aleutians. Subtle
shortwave troughs are likewise bringing scattered
showers/flurries to the Bristol Bay region. More zonal 500 mb flow
is seen farther east across Southcentral and the Copper River
Basin. The only moisture observed is associated with a nearly
stationary weak surface low over the northern Gulf, bringing
scattered showers along the coast. Fog and/or low stratus
continue this morning over Anchorage and the Mat-Su, particularly
over valleys and lower terrain.

&&

.MODEL DISCUSSION...

Model agreement remains good in the short term. The stalled Gulf
low mentioned above combined with relatively higher pressure to
the the north will continue to bring weak northerly outflow winds
across the region. These are expected to increase Thursday as all
models indicate a stronger trough descending into Southcentral
from the northwest. This is faster than it was yesterday and thus
any moisture associated with the elongated surface Gulf low
should remain over the Copper River Basin. Confidence is good with
regards to overall cooler temperatures, but still average closer
to the end of the week.

&&

.AVIATION...

PANC...Fog is expected to generally remain over the terminal
through the morning. While it may briefly become more patchy, the
overall synoptic pattern supports its persistence. This is
expected to somewhat break up during afternoon hours, however
conditions will be very similar to yesterday and thus periods of
lower ceilings/visibility will remain possible through Thursday
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Thursday night)...

The winter cool-down will continue over the next couple days as
arctic air filters southward across the state. Dense fog will
persist through about midday across much of Anchorage, the
Matanuska Valley, and the southern Susitna Valley. There really
won't be much in the way of wind, but there should still be enough
sunshine to help erode some of the fog and improve visibilities
tonight. Of course, visibilities will then likely head back down
at sunset. The best opportunity to get rid of the fog will be
Thursday afternoon and evening as a short-wave trough moves
through and causes a marginal increase in low level winds.

Meanwhile, a stationary low over the northeast Gulf along with
weak low level cyclonic flow will maintain mostly cloudy skies
across eastern Prince William Sound and the Copper River Basin
along with some areas of light snow, primarily east of the
Richardson Highway. For all other areas of Southcentral and
Kodiak expect a couple nice days with abundant sunshine. The
weak advection of cold air combined with surface pressure
gradients along the coast will maintain weak gap winds in
places such as Resurrection Bay and Passage Canal.

A large and strong storm system will track along the Aleutian
chain the next couple days, then begin to impact Kodiak Island
Thursday night. Warmer air will arrive with the leading warm
front, but it appears the air mass will initially be cold enough
to support all snow. However, as northeasterly winds strengthen
expect temperatures on the east side of the island to jump, with
Sea Surface Temperatures are in the mid 40s. This would lead to a
quick transition to rain/snow mix or even all rain.

&&

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

Southwest Alaska is off to another quiet start this morning with
no significant weather features impacting the region. Fog
development is possible over the Kuskokwim Delta this morning and
possibly tomorrow morning with clouds clearing. Temperatures will
continue to fall over the next few days as a polar airmass works
its way across Southwest Alaska.

Late Thursday into Friday a low south of the Aleutian Chain moves
eastward. The associated front will move over the Alaska
Peninsula, but there is still some model uncertainty as to whether
precipitation makes it into interior Bristol Bay or not. As of
now, models agree that any precipitation should stay south of
King Salmon. Stronger winds along the coasts associated with this
system and cold air could bring light freezing spray along the
coastal waters Thursday night into Friday.

&&

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

A front over Pribilofs this morning will continue to weaken by
the afternoon when attention quickly shifts to the western
Aleutians with an approaching North Pacific low. This system will
bring widespread precipitation and storm force winds, with
strongest winds west of Adak through Thursday morning. Winds will
weaken slightly Thursday afternoon as gale force winds spread
across the Bering.

This system will quickly reintensify late Thursday into Friday as
a colder air mass moves southward over the eastern Bering and
Aleutians, bringing a second shot of storm force winds with this
system. Winds will gradually weaken throughout the day Friday as
the low track back into the North Pacific through Saturday
morning.

&&

.MARINE (Days 3 through 5)...

Northwesterly storm-force winds are likely across the southern
Bering as well as the central and eastern Aleutians Friday as
storm tracks south of the Chain. Storm-force gusts remain likely
across the bays and passes of the eastern Aleutians and southern
coastal AKPEN into Saturday. Winds and waves over this area will
diminish by Sunday as a gale-force front looks to move over the
western Aleutians.

For the Gulf, a low will spin up east of Kodiak Island with the
potential for southerly gales over the northeasterly Gulf by late
Friday into Saturday. Northwesterly gales are likely late
Saturday into Sunday across the northern Gulf, especially through
bays and passes, in the wake of the low.

&&

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

Models are coming into better agreement with the development of a
~990 mb low in the western Gulf on Friday, moving into the
northern Gulf to the Southcentral Coast by Saturday morning. With
colder air in place, this system is likely to produce snow from
parts of Prince William Sound north to the Copper River Basin by
Saturday. There is still uncertainty in the track of this system,
however, and any westward wobble would mean the potential for snow
for a greater portion of the Kenai Peninsula as well as the
Anchorage Bowl and Mat Valley.

This system looks to then looks to move east as a ridge settles in
over the interior. The result would be a return to a cold,
northerly flow with gusty winds along the gaps of coastal
Southcentral through early next week. There is plenty of
uncertainty as to the strength of this ridge and whether a couple
upper-level disturbances may produce some scattered snow showers
across parts of Southcentral, especially over the mountains, next
week.

Out west, scattered snow showers are expected across Southwest
Alaska Saturday as a weak disturbance drops south. Lingering wind
gusts across the eastern Bering/Aleutians and AKPEN will also be
trending downward as a low south of the region shifts east. A
front looks to clip the western Aleutians by Sunday as high
pressure sits over the rest of the Chain. Another round of snow
showers is possible by midweek across Southwest as another system
drops southeast from Russia.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory: 101 111 145.
MARINE...Storm Warning: 177.
         Gale Warning: 170 173-176 178 179 411 412
FIRE WEATHER...NONE.

&&

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