Small Craft Advisory 278, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
357 AM AKDT Thu Mar 26 2020

.ANALYSIS AND UPPER LEVELS...

The upper air analysis consists of a building ridge axis over the
Bering Sea with a region of broad northwest flow aloft over the
Southern Mainland. A shortwave trough has passed over Kodiak
Island, and another is tracking east into the Yukon Territory. A
rather stout warm front is moving into the western Bering Sea and
Aleutians, and this is associated with a cutoff upper low south of
Kamchatka. The low level flow field is dominated by weak offshore
flow (mostly northerly flow across the region) and increasing
cold air advection. However, given the relatively flat troughing,
the low levels are having a tough time really drying out,
especially across southwest AK where the ridge axis aloft is
nosing in more. Thus, extensive regions of low stratus and fog are
in place across Southwest and Bristol Bay as well as the Copper
Basin. Rather quiescent weather will give way to increased
unsettled weather as a front moves into Southwest Alaska Thursday
night and Friday.

&&

.MODEL DISCUSSION...
Models are in good agreement with handling of large scale features
The numerical models are in excellent agreement with the large
scale synoptic pattern heading into the weekend. However, they are
in rather poor agreement as a front moves into Southcentral from
Southwest Alaska on Friday evening and Saturday. A series of
subtle upper level shortwave trough features will be moving
through in northwest suppressed flow, and therein lies the
challenge as this will determine the strength of the front and the
amount of precipitation (snow) across Southcentral, as well as
its northward extent. Given the trend towards a stronger arctic
flow from the north, this forecast will favor a more suppressed
model solution with less snow and less of a northward extent.
Thus, the ECMWF and GFS were favored for this forecast.

&&

.AVIATION...
PANC...MVFR cloud CIGS are going to be the main challenge at the
terminal as they move in and out at times through the early
morning. VFR conditions should become dominant as the day
progresses with light and variable winds.

&&

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

An arctic trough moving south from the Eastern Brooks Range will
promote increasing cold air advection into Southcentral today
through Friday morning. The gradient along the coast will tighten
and this will promote gusty northwest winds across western Prince
William Sound with northerly winds across eastern Prince William
Sound in normal favored locations such as Whittier, Cordova,
Valdez, Thompson Pass, Isabel Pass, and parts of the Mat Valley.
Temperatures will also gradually trend back below normal with this
arctic trough heading into the weekend.

Attention turns to a front moving into the Gulf of Alaska on
Friday afternoon into Saturday. This will bring light snow
chances to Southcentral, although the track of the upper level
lows is not favorable as this will be coming in from the west (in
conjunction with dry low level air mass), so snow amounts look
rather light. The only exception to snow will be Kodiak Island
where rain will be the predominant precipitation type.

&&

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

Areas of fog and low stratus this morning will gradually scatter
out and diminish with the afternoon sun. Dry and otherwise quiet
weather will quickly change as a stout weather front moves into
Southwest, bringing widespread accumulating snow to the region.
This front will move in on Thursday evening and spread east to the
Alaska Range through Friday. Expect widespread snow accumulations
with this front of 3-5" of snow, heaviest along the Kuskokwim
Delta. Cold air advection will wrap in behind the front, and with
favorable upper level forcing, expect an additional 2-4" Friday
night into Saturday. Expect the early snow on Thursday
evening/overnight to be rather "wet" and dense, with drier and
less dense snowfall on Friday through Saturday. Some blowing snow
is likely along the Kusko Delta coast into Bristol Bay as onshore
westerly winds will increase to 25-40 MPH.

&&

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

A front approaching the western and central Aleutians right now
will sweep northeastward across the Bering through Thursday
night. As the front runs into colder air over the northern Bering
Thursday, a low will develop along the front which will move into
Southwest Alaska Friday evening. High pressure will move over the
Aleutians through Friday. Behind the low, much colder air spilling
southward will cause a return to freezing spray conditions over
the eastern Bering, and by Saturday on the Pacific side of the
Alaska Peninsula. Meanwhile a weak low will move across the
central and eastern Aleutians on Saturday as a separate front
moves into the western Aleutians Saturday afternoon.

&&

.MARINE (Days 3 through 5/Fri through Sun)...

The main focus for Friday will be a low tracking into Southwest
Alaska with strong northerly flow and cold air advection on the
back side. Confidence is high in widespread gale force winds,
with potential for storm force gusts through gaps of the Alaska
Peninsula Friday night through Saturday. The colder air will also
reintroduce the likelihood of heavy freezing spray across the
eastern Bering and along the Alaska Peninsula. The stronger winds
and freezing spray will also likely move into the western
Gulf/Kodiak Island area as well. There is also a moderate to high
degree of confidence in large short period seas east of Kodiak on
Saturday as the southwesterly seas oppose the northeasterly
currents.

Another storm system will makes its way into the western to
central Bering Sea on Sunday. While there is some uncertainty
in the track of this storm, it looks like winds will top out at
small craft or min gale with this storm.

&&

.LONG TERM FORECAST (Days 4 through 7/Sat through Wed)...

There is a high degree of continuity between guidance that an
upper level trough will move over the western Alaskan mainland on
Friday. This upper trough will result in colder temperatures
across the Alaskan mainland this weekend. Significant differences
between guidance begin to arise on Sunday. The only area where
there is a moderate degree of continuity between guidance is the
upper trough is over the southeastern mainland near the AlCan
border while an upper ridge moves in from the Bering towards the
Alaskan mainland. Temperatures are expected to rebound early to
middle next week. However, significant amplitude differences are
evident with respect to the upper ridge and therefore the forecast
confidence in the magnitude of temperatures rebounding is very
low. For the remainder of the forecast period, confidence remains
very low as significant discrepancies arise between guidance with
respect to the placement of upper level features which in turn
effects the placement of surface features.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter weather advisory 155.
MARINE...GALE: 130 150 155 165 180 181 185 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...Ahsenmacher
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/Ahsenmacher
MARINE/LONG TERM...ED