Small Craft Advisory 103, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
811 AM AKST Thu Jan 30 2020

.ANALYSIS AND UPPER LEVELS...
An upper level trough across the western half of the state has a
closed center located between Kodiak Island and the southern Kenai
Peninsula. Ridging borders to the east and west of the trough
with another upper level trough farther west over the northern
Kamchatka Peninsula. At the surface, snow showers continue to
develop across the Gulf and eastern Bering as upper level
shortwaves slowly pinwheel around the low center aloft. A few of
the snow showers are making it inland before dissipating. The
gustiest winds this morning are across the Alaska Peninsula where
visibilities in blowing snow are reduced to one half mile at
times.

&&

.MODEL DISCUSSION...
Models are generally in good agreement with the synoptic
pattern through Friday. Only minor changes were needed for the
morning package with higher resolution NAM data favored.

&&

.AVIATION...
PANC...VFR conditions are expected through the next TAF package.
Gusty southeasterly flow tapers off this morning as low pressure
over the Kenai Peninsula weakens.

&&

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

Snow showers will continue across the Gulf and Prince William
Sound as a surface low in the western Gulf continues to wrap in
colder air. Winds across most of Southcentral mainland have
diminished overnight as the strong pressure gradient weakened,
which will continue through the day. In the upper levels, a trough
over the mainland will continue to lift northward tonight through
Friday, helping to spread snow showers inland into the Copper
River Basin.

As the upper level trough slides eastward through Friday, a
progressive wave pattern develops, with a ridge building in behind
the trough. This will help to strengthen the pressure gradient
over the western Gulf. The pressure gradient along with weak cold
air advection moving in will help to increase winds across
Southcentral on Friday. The strongest winds are expected out of
bays and passes of Shelikof Strait as well as Kamishak Bay out to
the Barren Islands, with winds peaking Friday afternoon and
evening. Seward, Whittier, and Valdez are also expected to see an
increase in winds Friday through Saturday as the upper ridge nears
and offshore flow strengthens over Southcentral. After a warm-up
in temperatures region-wide on Wednesday, temperatures will trend
colder over the next couple days.

&&

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

An arctic trough remains over the coastal portion of Southwest
Alaska with a secondary upper level trough centered over the
Alaska Range, Cook Inlet, and Kodiak Island. This second trough
is supporting weak deformation banded snow across the Alaska
Peninsula northward to the Lower Kuskokwim Valley this morning.
This activity will taper off through tonight as the upper low
moves east. Dry weather dominates elsewhere with the exception
being the Kuskokwim Delta coast and Nunivak Island where satellite
imagery tonight is picking up impressive ocean-effect "streamer"
snow showers, along with some reported areas of blowing snow from
Hooper Bay to Toksook Bay. These persistent northerly winds are
also still supporting wind chill values below -40F, so the wind
chill advisory will remain until midday as winds slowly taper and
temps slowly recover. Temps drop right back tonight, but winds
will also be much lighter.

&&

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

Widespread snow showers continue to move across most of the Bering
Sea as two regimes are in place. One is a continued area of
northerly cyclonic flow over the Eastern Aleutians which is
supporting snow squalls and blowing snow from False Pass to Port
Heiden. West of the Pribilof Islands, a trough axis has shifted
the flow direction to westerly, which is also warmer than the
aforementioned deep arctic air mass. The trough axis will slowly
shift east and strengthen, and will bring enhanced snow showers
from Adak/Atka east to the Pribs and Dutch Harbor Thursday night
and Friday. Snow squalls will weaken considerably by late morning,
so the winter weather advisory for the Alaska Peninsula will end
later today.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...
Gulf of Alaska: Strong northwesterly flow is expected to continue
for the Western Gulf through Saturday. This will create gale
force winds, especially through gaps and passes. Saturday night
into Sunday, this flow will diminish easing winds and seas. A
shift to southerly flow can be expected Sunday. We have a moderate
amount of confidence in the forecast. We are confident in
northwesterly flow and the pattern shift Sunday, but wind speed
may need to be adjusted a bit.

Bering Sea: A low in the Western Aleutians Saturday will bring
strong gales and a few areas of storm forces winds to the area. It
will weaken as it moves east through Monday, allowing winds and
seas to calm to small craft advisory by Monday. There is moderate
confidence in this forecast. High confidence with low pressure in
the Bering, though wind speeds will likely need to be adjusted.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through Wednesday)...
The upper level long wave trough currently over Alaska still
looks to shift east on Saturday. This will allow the ridge that is
currently over the Bering Sea and Aleutians to move east over
Southcentral AK. Over the weekend this high pressure will keep
cold air in place over Southcentral AK that will have moved in on
the backside of low pressure currently over Southcentral AK.
Meanwhile out in the Bering, a long wave trough will be digging
south with an anchoring low over the western Aleutians, meaning
unsettled weather in the Bering.

By early next week, movement of this high and continued
development of the long wave trough in the Bering will cause a
significant pattern shift for our forecast area. On Monday, the
axis of the high will move east to be located over Western Canada,
with low pressure over the central Bering. This will create stormy
conditions for the Bering and Southwest Alaska and broad southwest
flow over Southcentral, which should warm temperatures quite a
bit. Models are actually in good agreement for this pattern change
to occur. They have differences of exact location of the high and
low, but all are capturing the same general synoptic flow. This
gives us an above average confidence level in the long term, but
as usual figuring out specific weather systems will have to wait
until the forecast is in the short term.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 181.
         Wind Chill Advisory 155.
MARINE...Storm Warning 130.
         Gale Warning 131 138 150 155.
         Heavy Freezing Spray Warning 130 131 132 136 137 138
                                      139 141 150 155 165 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...AH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...BB