High Wind in Alaska, December 21 to 22, 2017

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Thu Dec 21 2017

.ANALYSIS AND UPPER LEVELS...
A broad negatively tilted upper trough extends from eastern Russia
southeast to over the northeast Pacific. To its east a relatively
warm upper ridge extends northwest across the Gulf of Alaska to
over the mainland. At the surface, a series of lows, associated
with upper trough, extend from western Bering Sea to south of the
Alaska Peninsula. The most significant of the surface features is
a 980s low heading north toward the Alaska Peninsula and
associated leading front. Strong easterly winds ahead of these
features extend from Gulf, Ak pen, to the eastern Bering/Bristol
Bay. Rain was evident across the AK pen and was approaching Kodiak
Island the the north Bristol Bay coast. Satellite imagery shows a
broad Cirrus shield, associated with mountain wave turbulence and
warm advection extending to the lee of the Ak Pen and Aleutian
Range. Elsewhere, strong and gusty winds with showers were evident
across the central and western Aleutians/Bering Sea.

&&

.MODEL DISCUSSION...
Guidance continues to have a good handle on the synoptics of the
pattern, but struggle a bit with phasing of the Pacific system
into the Bering Sea trough. All guidance phases the system, but
how much and exactly where the resultant surface low sets up are
still in question.

&&

.AVIATION...
PANC...Generally VFR conditions and light surface winds will
persist. Cross barrier flow will increase tonight and bring in
potential for LLWS across the airport complex.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
"Dirty" upper level ridging is building over Southcentral ahead
of a rather impressive warm front which will track across
Southwest Alaska tonight and tomorrow. A very dense cirrus cloud
shield has formed in association with a elongated mountain wave
along the Alaska Peninsula and Eastern Aleutians...with these
cirrus wrapping anticyclonically over Southcentral (hence the
"dirty" upper ridging as that is more commonly associated with
more clear weather). There is also extensive low status and fog
over the Copper Basin associated with a strong surface high and
stagnant air. The main weather feature of interest the next two
days will be a strong storm force front moving into the Gulf of
Alaska. This will bring 3-6" of rainfall for Kodiak Island along
with strong easterly winds. Beyond this, interior weather will be
rather banal given the dry offshore flow and moderate inversions.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There are two storms west of southwest Alaska and their cloud
shields have already merged on satellite imagery resulting in a
broad shield from the interior into the North Pacific. The High
Wind Warning remains in effect as there is good confidence that
the low will bring high wind gusts to the downslope and gap wind
prone areas along the Aleutian Range as well as portions of north
of Dillingham. Warm air advection will accompany this event,
although some places may get a wet snow. Another low is expected
to move into the area Saturday keeping the unsettled conditions in
place.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
There is a broad area of troughing from NE Russia to the North
Pacific. There are several distinct circulations at the surface,
one over Attu Island, one near the Pribilofs, and another south
of the AKPEN. To the east, there is a dome of high pressure over
the interior. With high pressure in place, expect the lows to
follow the path of least resistance. Sensible weather-wise, expect
more gradient winds over the Eastern Aleutians. As this unsettled
pattern evolves, the storm from the North Pacific will become
entrained in the synoptic flow and get pulled northward keeping
the gusty winds in place over the Eastern Aleutians this forecast
period. Given the source region for this storm, expect warm
air to advect into the region. All the while, another low will
trek from Attu directly over the chain with the models spinning up
another low in its wake. Overall, its a very dynamic pattern with
plenty of opportunities for gusty winds and rough seas.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long-term forecast beginning Saturday evening will start off
with ongoing wet and windy conditions stretching from the northern
Gulf area through the southwest Mainland, and into the
eastern/central Aleutians and Bering as a north Pacific low
begins to track into the Alaska Peninsula. There remains some
model differences regarding the timing of the frontal winds and
precipitation, and how far the system will push into the northern
Gulf coast and Cook Inlet region by Sunday. Currently the middle
ground GFS solution was preferred, bringing the rain over Kodiak
Island northward into the Kenai Peninsula by Saturday evening,
then into the northern Gulf coast by Sunday morning. The low then
begins to track north along the western coast Sunday afternoon,
switching the southeasterly flow over the southern Mainland to
southwesterly flow, which will bring colder onshore flow and
showery conditions over the southwest mainland Sunday night into
Monday morning. This will also bring a slight chance of some
light snow to fall over the Cook Inlet region overnight Sunday
into Monday morning.

On the western periphery of this low pressure system, some strong
cold air advection will persist over the Pribilofs and Eastern
Aleutians through the weekend and into the early part of next
week. This will likely bring snow showers and the potential for
blowing snow over these areas Saturday evening through Sunday
morning. A ridge will build in over the western Aleutians/Bering
Monday morning, and quickly push eastward over the central
Bering/Aleutians by the middle of the week. This ridge will be
short lived as the next trough moves over the area Tuesday. This
system could bring another round of wet and windy weather to the
western and central Bering/Aleutians Tuesday afternoon through
the later part of the week. However, previous runs have shown
significantly different solutions, so confidence in the forecast
from Monday through the end of next week remains low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 161.
MARINE...Storm Warning 130 131 132 150 155 160 165 180.
 Gale Warning 119 120 136>138 141 170 172>176 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...PEPE