Small Craft Advisory 20, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
530 PM AKST Sun Jan 6 2019

.ANALYSIS AND UPPER LEVELS...
Across the western Bering is a vertically stacked low that is
sliding to the northwest and slowly weakening. To its east is a
weather front that is moving across the western Aleutian chain
where storm force winds and showers will work east. Southwestern
Alaska is being impacted by another front that stretches from the
Kamchatka Peninsula, towards St. Matthew Island and then through
Bethel and Dillingham. Light to moderate snow showers moved
through the YK delta this morning and are moving into Bristol Bay
this afternoon. Mainland Alaska is being dominated by an expansive
area of high pressure that remains. GOES 17 imagery is showing an
ever persistent stratus deck that seems to ebb and flow over Cook
Inlet and the adjoined arms. These clouds have acted like a
blanket over much of the Anchorage Bowl, protecting the areas from
radiational cooling and the coldest temperatures of the season.
This morning, the Mat-Su saw lows dip from -11 in Palmer to
-26 in Willow. Meanwhile Anchorage saw it's first sub-zero degree
 temperature of the year when the cloud deck slightly cleared
 around the airport. Across the Copper River Basin are mostly
 clear skies and continued cold. Clouds are only present in the
 valleys below 2000 feet, where a strong inversion is helping to
 lock the clouds in.

&&

.MODEL DISCUSSION...
Model guidance seems to be in good agreement for the first 36 to
48 hours. Differences start to show as we move into tomorrow
evening, but the majority of the differences revolve around the
movement of Pacific lows and will not impact the weather across
the Alaska region through much of the week. The biggest area that
all guidance has struggled to deal with is the moisture in the
low levels across the Cook Inlet region. Model soundings as well
as high resolution model guidance has really struggled with the
sharp inversion that has set up below 1000 feet. Above it is
warmer and drier air, and that has continued to lock in the
clouds.

&&

.AVIATION...
PANC...Fog and stratus (IFR to LIFR ceilings and vis) will remain
the primary issue. The 00Z Anchorage sounding shows the inversion
has dropped closer to the surface. Thus, unlike last night which
primarily features a low ceiling, expect reduced visibilities will
settle in sometime this evening. An approaching trough will bring
mid to high clouds in tomorrow afternoon which should be enough to
finally clear out most of the fog and stratus.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With a cold upper level low centered over the Yukon Territory and
extending an upper trough to the west across interior Alaska, a
cold and dry air mass remains firmly in place across Southcentral
Alaska. Offshore flow continues across the northern Gulf coast,
Prince William Sound and eastern Kenai Peninsula with very cold
temperatures inland. The combination of strong northeasterly gap
winds and cold temperatures are bringing very cold wind chills to
Thompson Pass. A flat ridge over the western Alaska range will
shift slowly east tonight through Monday diminishing offshore
winds slightly. A weak surface low will pass just to the east of
Kodiak Island on Monday and then turn northeast to reach the
northern Gulf Tuesday. In addition to bringing snow showers to
coastal areas, isolated to scattered snow showers may spread
further inland into Southcentral Alaska reaching the Chugach and
Talkeetna Mountains Tuesday and Copper River Basin Tuesday night.
Offshore flow will increase again on Wednesday as another upper
trough approaches the area from the west.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Remnants of an old front are pushing through Southwest Alaska
bringing light snow to the area as it passes tonight. Weak ridging
will then move in over the area allowing for the opportunity of
fog to develop by early Monday morning. Then Monday night another
front will approach from the Bering that will bring more snow to
the area into Tuesday evening. Tuesday night high pressure will
build in over the eastern Bering helping to shift flow across
Southwest Alaska to an offshore, northeast wind for Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The front that is pushing through the central Aleutians will
continue its easterly track through Monday making it through the
eastern Bering Monday evening. The biggest challenge with this
system is how much warm air will be associated with it and how
that will impact precipitation type. The majority of the guidance
leans toward there being enough warm air to have a mix of rain and
snow as the front moves through the Pribilof Islands Monday.

Another front will move into the central Bering/Aleutians Monday
night and then move slightly east before stalling just west of
the Pribilof Islands Tuesday evening. This setup will allow for
southwest winds over the western Bering and northeasterly winds
of the eastern Bering. The front will then dissipate Tuesday
night. On Wednesday flow over the western Bering will become
more southerly as ridging builds in over the eastern Bering.

&&

.MARINE (Days 3 through 5)...
Beginning Wednesday evening, fairly benign weather over the
Bering will give way to deteriorating conditions through the day
on Thursday as a nearly 950 mb North Pacific Low approaches the
Western Aleutians. East to northeast winds will ramp up through
the end of the week, with a large swath of gale force winds
expected over the western half of the Bering and Aleutians, mostly
to the west and south of the Pribilofs. Guidance has been fairly
consistent with this storm system so overall confidence is high.
Winds look to remain at or below upper end gales, though some
embedded storms could be seen over the western Aleutians if the
low is stronger and/or tracks further north. There is a trend of
decreasing winds by the weekend as the low tries to shift east,
though there is a fair amount of model disagreement at this point.
One other issue to consider is the potential for widespread
freezing spray over most of the Bering by Saturday as very cold
air spills south from the arctic. This of course will depend on if
the North Pacific Low stays south and east of the Bering,
allowing more cold air to spill south, or north of the Aleutians,
which would keep most of the cold air confined to the Northern
Bering.

Over the Gulf of Alaska, high pressure will continue to keep the
storm track to the south and west, thus expect generally good
conditions through the week. Localized higher winds may develop
out of bays and passes along the North Gulf Coast due to northerly
outflow winds.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The long term forecast begins Thursday as very cold and dry
conditions dominate the southern mainland. A deep upper level
arctic low will wobble south from the North Slope down over the
central mainland. This will allow frigid arctic air to spill south
across most of the southern mainland through the end of the week.
Current model guidance shows 850 mb (~5000 ft) temperatures
dropping to near -25C on Thursday. This would easily lead to the
coldest temperatures of the season for most locations across the
southern mainland. This looks to be short lived though as a strong
North Pacific Low just south of the Aleutians tries to push north
but gets blocked by the Arctic low. The low looks to weaken as it
elongates out into multiple low centers south of the Aleutian
island chain and AKPEN by the weekend. Models begin to fall out
of agreement with the timing and placement of these lows, though
there is a general consensus that warmer southerly flow will
return to the Gulf of Alaska with increasing chances of
precipitation along coastal areas.

Out west, the strong North Pacific Low will lead to stormier
conditions over the Western half of the Bering and Aleutians by
the end of the week, with widespread gale force winds expected
over the region. Cold arctic air will try and push south across
most of the area by the weekend, though this is all dependent on
how far north the low pushes into the Bering, or if it stays south
of the Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131.
MARINE...Storm Warning 175 176 177 178.
         Gale Warning 170 172 173 174 179 185.
         Heavy Freezing Spray Warning 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
MARINE/LONG TERM...KVP