Small Craft Advisory 59, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKST Thu Jan 16 2020

.ANALYSIS AND UPPER LEVELS...
The upper level ridge over the west coast of the state is now
becoming a closed ridge in a rex-block pattern. This is lending
upper level support to the winds through the Matanuska valley
and Thompson Pass today, though this support should diminish
overnight as the ridge drifts eastward.

The front in the Bering Sea that brought the strong winds the
past two days has weakened as a low is moving west-to-east
across the western and central Aleutians.

Some weak polar lows continue to develop in the Gulf of Alaska
as a result of the cold outflow winds from along the north Gulf
coast and moving over the relatively warmer waters of the Gulf.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement over the region through the
remainder of the week. While they are in good agreement with the
overall complex low that will move through the Gulf from east to
west later this week, the individual smaller low embedded in this
complex low will have a large impact on the weather in Kodiak.
This is the largest area of uncertainty for the region in this
forecast package.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3;
Tonight through Sunday)...

A tight pressure gradient between an arctic ridge stretched
across the interior of the state and a series of surface troughs
moving east to west across the Gulf of Alaska will continue to
support the strong, northerly gap winds through the nighttime
hours. These winds have also been supported by a vigorous
shortwave diving south over the northern Gulf. As the evening
progresses, however, the orientation of this shortwave will
shift, producing more of an easterly flow aloft. This, combined
with a gradually weakening pressure gradient, will allow the
winds to relax by early morning.

A series of inverted troughs is embedded in the broad, easterly
flow across the Gulf. These features will help to enhance show
shower activity across the Gulf and over Kodiak Island through
Friday night. By late Friday, a surface low will develop over the
central Gulf and quickly intensify as it, too, tracks toward
Kodiak Island. As it does, widespread snow and gusty northerly
winds will develop over the island. The snow is expected to become
heavy at times for locations like Kodiak City through Saturday
morning. The combination of snow and wind gusting to 40 mph will
also result in blowing snow and reduced visibilities. The easterly
flow will help push some warmer air over the eastern half of the
island by late Saturday morning. There is still a bit of uncertainty
as to how much warmer air works in along coast, but it is possible
that the snow does change over to a rain/snow mix before ending
by early Saturday afternoon. A few inches of snow is likely,
however, before any potential changeover. There is also some
uncertainty in the exact timing and total snow amounts. A Winter
Storm Watch has been issued for Kodiak Island for late Friday
night through Saturday morning for this upcoming snow and blowing
snow potential.

The rest of Southcentral will remain clear and cold through
Saturday. Clouds and the chance for snow will be on the increase
along the coast and across the southern Copper River Basin on
Sunday, however, as an area of low pressure lifts northwest from
the Alaska Panhandle.

&&

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

An area of high pressure over interior Alaska will continue to be
the dominant weather feature controlling the weather through
Saturday. This will keep the winds offshore, keeping skies relatively
clear with below average temperatures. On Saturday, easterly waves
tracking across the Gulf will move across Kodiak and into the
Aleutian Mountains east of King Salmon. There is good model
agreement that a large share of the moisture associated with this
will be taken up by the mountains. How much of that moisture makes
it over the mountains into Bristol Bay is a point of uncertainty.
Thus, the threat of snow showers along with mostly cloudy
conditions have been added to the forecast, particularly around
Dillingham, which has an upslope component to the wind in easterly
flow. Regardless, accumulations, if any, will be very minor.
Sunday will be a quieter day regarding precipitation, though
lingering cloud cover is expected. Thus, temperatures should not
move much over the next few days over Southwest Alaska than where
they are right now, with the cold spots remaining the sheltered
valleys in the Lower Kuskokwim Valley.

&&

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

Two weather features are impacting the Bering Sea region this
evening. The main on is a strong low passing south of the central
Aleutians. This is spreading a very wet snow across Adak and Atka.
This low will begin to gradually sag southward away from the
Aleutians tonight into Friday morning, resulting in improving
weather conditions. The other feature is a slowly moving front
located west of the Pribilofs. The associated line of cloud cover
and rain extends from the Pribilofs southward through Dutch
Harbor. The front has begun to retrograde (move westward), as it
weakens, so there should be very gradual improvement in the
conditions across those areas through tonight. The front will
persist as a shell of its former self through the weekend,
reaching the western Bering by Saturday, then moving back eastward
on Sunday. It will be the focus of more cloud cover and light snow
shower activity, but will otherwise be unimpactful. Elsewhere,
easterly winds will persist through Saturday in the eastern
Bering, with westerly winds and snow showers continue over the
western Bering.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Marine weather still looks quiet in both the Bering and Gulf of
Alaska. A weak low pressure system will be present in the Gulf on
Monday, providing some localized areas of small craft advisory
winds on the north gulf coast. Then it moves inland and the Gulf
becomes quiet. The Bering will have light westerly flow, switching
to a light northeast late Tuesday. While low pressure is in the
area, we are confident it will be weak with little impact, so
moderate to high confidence in the marine forecast.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through Thursday)...

The long term forecast starts off with the upcoming pattern shift
already taken place. A long wave, upper level trough looks to
exist stretching southeast across the Bering from Eastern Siberia
all the way down into the Gulf of Alaska. This means generally
unsettled weather for our forecast area to start the week. More
specifically, we are still expecting low pressure in the Gulf of
Alaska and zonal flow in the Bering and Southwest AK.

However, by Wednesday model agreement falls apart. Two of our
heavy hitters - the GFS and ECMWF - have very different solutions
for our forecast area, the Gulf of Alaska in particular. This
means past early next week there is not a good answer for the
forecast and we have low confidence in any solution past Tuesday.
We will have to wait and see what future solutions come up with.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 171.
MARINE...Heavy Freezing Spray Warning 119 127 139 140 160 180 185.
         Gale Warning 120 127 130 131 132 139 176 177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...BB