Small Craft Advisory 1003, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKST Mon Dec 17 2018

.ANALYSIS AND UPPER LEVELS...

The predominate synoptic feature is a long wave trough spread
over western mainland Alaska, the AK-PEN and the eastern Bering.
Cold air continues to advect on the backside of the trough
lowering temperatures and clearing skies over the western mainland.
The cold air advection is also bringing widespread snow showers
across much of the central Bering Sea, and central Aleutians to
the Alaska Peninsula. Northerly flow over the eastern Bering is
increasing winds to gale force through the gaps in the Alaska
Peninsula. The jet stream is meridional in nature, northwesterly
aligned over the western Aleutians with wind speeds approaching
125 kt at 300 mb. In the Gulf there is southerly flow aloft and
widespread weak low pressure at the surface. Most of Southcentral
remains in the cold pool where isolated snow showers continue
through the Anchorage Area, western Prince William Sound and
Susitna Valley. The front that brought significant snowfall to
the Anchorage area is pushing further inland and dissipating
today. The atmosphere has become more dry aloft with low level
clouds dominating Anchorage. The low cloud deck combined with
super cooled liquid water near the surface and a lack of ice
crystals kept the precipitation as freezing drizzle rather than
snow today. Along the northern Gulf coast east of Prince William
Sound, temperatures are warm enough to turn the precipitation
into rain.

&&

.MODEL DISCUSSION...

Models are in fair synoptic agreement out west across the Bering
Sea and Aleutians through Thursday. Models in the Gulf still
continue to struggle with positioning and intensity of surface
lows across the Gulf through Thursday. They however are in
generally good agreement with strong westerly flow setting up
over the western Gulf Tuesday/Wednesday.

&&

.AVIATION...
PANC...Generally stable conditions will continue MVFR and
possibly IFR conditions through tonight. Light snow developing
Tuesday most likely will keep at least MVFR conditions in place.
The threat of light freezing drizzle could continue into the early
evening hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The remnants of an old front that remain along the north Gulf
coast will continue to diminish overnight. However, more
precipitation will be on the way as a weak upper level feature
will help bring in light snow to areas around Anchorage and the
Kenai Peninsula tonight. On Tuesday, a new low will form off a
baroclinic zone in the Gulf that will head north towards the north
Gulf coast. This low combined with upper level flow will help
bring in light snow to areas around southern Southcentral Alaska
Tuesday. On Wednesday an upper level low that was previously near
Kodiak Island will move over the southern Kenai Peninsula helping
to pull more cold air to the Gulf and north Gulf coast and bring
more snow to the coastline.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
A couple more days of cold with generally dry, northwest flow are
in store for the area. A trough of deep cold air remains entrenched
from the Bering Strait through the Alaska Peninsula. The net effect
with be continued cold temperatures which will only be reinforced
by cold, dry northerly moderate winds. Exactly when the coldest
morning will be completely dependent on cloud cover. Most places
have cleared (from lingering low stratus) this afternoon. But
another swath of clouds is tracking into the YK-Delta from the
north. Either way, it looks like most places will reach from
somewhere between 10 to 20 below Tue morning. Right now we are
forecasting Wed to be the coldest with the best potential for
mostly clear skies over the mainland. The exception to this will
be right along the AK Range where some clouds creep back in from
the east by Wed morning.

The only real potential for precipitation will be associated with
a shortwave rotating in from Southcentral AK Wed morning. Some
light snow will be possible mainly from Sleetmute to the east on
Wed. Then a reprieve from the cold air is in store by Wed
afternoon as a front moving across the Bering should finally
start to displace the cold air mass.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue and
Wed)...
We will need to divide the domain into two parts for this forecast.
First, the Eastern Bering remains under the influence of a deep
cold trough and gusty northerly winds. The net effect is
widespread snow showers from the Western Capes through the
Pribilof Islands and down to the Alaska Peninsula and Dutch
Harbor. These snow showers coupled with gusty winds could produce
pockets of blowing snow in all these areas through Tue. Gusts will
continue to be especially strong on the south side of the Alaska
Peninsula as the cold air pushes through the terrain there.

Part two will be the Western Bering as it will be the first to
see the change across the entire area. A decaying warm occlusion
will slide from Shemya through Atka on Tue. This will usher in
some warmer air and a fairly elongated band of precipitation
across the Aleutians. This precipitation should mostly fall as
snow, but it could mix with rain from time to time. The front will
have gale-force southeast winds initially before winds switch to
a SW direction as the front pushes into the Central Bering on
Wed. This front will also bring a gradual end to the cold
northerly winds and snow showers for the Alaska Peninsula on Wed.
By late Wed, westerly flow will push a band of rain and snow from
Dutch Harbor into the Alaska Peninsula.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Thursday starts off the long term forecast with an upper level
trough over Prince William Sound and the northern Gulf and a
broader upper level low and cold air mass stretched across
mainland Alaska. Precipitation along the coastal areas will taper
off through the day and shift offshore overnight as the upper
level support weakens and leaves a remnant surface low drifting in
the Gulf. A low south of the eastern Aleutian Chain Thursday will
make its way east to be located south of the Gulf on Friday. With
the more southern track of this low, impacts will be primarily
limited to the offshore waters. As the low continues into the
southeastern Gulf through the weekend, offshore flow and gap winds
will increase across Southcentral Alaska with skies clearing and
a rather cold air mass settling in over the area. This cold
pattern with strong northerly winds through channeled terrain
looks likely to persist through the first half of next week as
a more southern storm track keeps the strongest storms well south
of the area and directed into the Pacific Northwest.

Looking out west, another front will approach the western
Aleutians and Bering on Thursday. Additional lows spinning up in
response will keep the weather pattern active over the Aleutians
and Bering for Friday and Saturday as well. A smaller frontal
wave in the eastern Bering will bring the potential for some snow
and blowing snow to the western capes and Kuskokwim Delta coast
Thursday through Saturday while the remainder of Mainland
Southwest Alaska remains generally dry. Heading into Sunday, a
low south of the Aleutian Chain will be the main weather maker
to keep an eye on for placement and intensity as its front moves
east towards the AKPEN through Monday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 131 132 138 150 155 175 176 177 178.
         Heavy Freezing Spray Warning 130 138 160 180 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BC/RA
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
MARINE/LONG TERM...JR/ALH