Heavy Freezing Spray Warning 9, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKST Thu Jan 26 2017

.ANALYSIS AND UPPER LEVELS...

A powerful shortwave trough is rapidly moving northwest into the
Lower Kuskokwim Valley this afternoon. The associated storm
force warm front has moved inland along the North Gulf Coast.
Southeast wind gusts to near 60 knots were reported across Prince
William Sound into Portage Valley and across higher elevations of
Turnagain Pass. Currently, southeast winds have moved across large
swaths of Southcentral including the Anchorage Bowl and Matanuska
Valley. This "chinook" wind has brought temperatures up into the
low to mid 40s, the warmest temps in nearly two months. Across the
Bering Sea, there is an arctic trough centered over the Western
and Central Bering Sea with strong gale force northwest winds
associated with it. Farther south, a powerful 195 knot Pacific
jet stream is continuing to spin up strong lows along the left
exit region of the jet. The latest in a series of these lows is
developing and moving into the southeast Gulf of Alaska.

&&

.MODEL DISCUSSION...

As usual, the numerical models are in good agreement with the
large scale synoptic pattern, but they are not in good agreement
at all with a series of lows expected to move toward the northeast
part of the Gulf of Alaska as well as the merging of these lows
with an arctic trough over the Bering Sea. These interactions are
important and will impact how much moisture retrogrades west and
interacts favorably with the arctic trough as it swings east this
weekend. This makes the precipitation forecast for inland
locations across all of southern Alaska lower than average,
including for the Anchorage Bowl. For this forecast, a blend of
all the models was favored to take into account the amount of
discrepancy.

&&

.AVIATION...
PANC...East southeast winds across the airport will diminish this
evening along with any low level wind shear. More low level wind
shear will be possible near the terminal Friday evening.
Otherwise, VFR conditions are expected to prevail with on rain and
snow showers in the vicinity after midnight tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Fri and Sat)...
Broad southeasterly flow will continue to keep things warm and
showery through the overnight hours. A broad trough centered over
the Bering will leave the area vulnerable to various shortwaves
riding up from the North Pacific. The flow generally favors
precipitation along the coast with drier conditions on the leeward
side of the mountains. But as we saw this morning in parts of the
Matanuska Borough and Eagle River, some precipitation may be able
to make it over the mountains. The strongest winds have pushed
through most of the area, but things will remain gusty through the
evening hours, especially in favored terrain. The biggest concern
is that later tonight our atmosphere stabilizes enough to allow
radiational cooling. This should lead to many temperatures
dropping below freezing on Fri morning resulting in icy surfaces.
A special weather statement has been issued to address this.

On Friday colder air will start to filter in over the Kenai
Peninsula while a stronger low moves over the Eastern Gulf. The
colder air will lead to increased instability and could make for
some heavy snow showers tracking up the Cook Inlet from Kachemak
Bay. By Fri night, a shortwave within the broad trough will help
to focus these showers and push them through the Anchorage Area.
This should lead to a few inches of fresh snow by Sat morning.
Over the Eastern Gulf, the Fri low tracks quickly northward
bringing some rain to Cordova and light snow to the eastern Copper
River Basin. Another very similar low will follow nearly the same
track on Sat. There is still a lot of uncertainty with this second
system, but if it holds its course, it would bring more rain to
the eastern Gulf and more snow to the Copper River Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The main features continue to be the low off the coast of the
Kuskokwim Delta and the rotation of a low south of the Alaska
Peninsula into southwest Alaska. The low will remain nearly
stationary off the Kuskokwim Delta coast through Saturday morning
before drifting north Saturday afternoon. The low south of the
Alaska Peninsula moves into the Bristol Bay region tonight as the
remnants of a low in the Barren Island move across the Iliamna area.
Therefore higher probability of precipitation and precipitation
amounts were brought into the forecast for the southwest mainland
through tonight. Friday morning will see the flow predominantly
out of the southwest and west resulting in a change over to more
showery precipitation and colder air advecting into the southwest
mainland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
To the west of the low and trough are more convective conditions
accompanied with gusty west and northwest wind. Persistent snow
showers across the Pribilof Islands along with the wind is
resulting in reduced visibility in the snow and blowing snow. Snow
showers and blowing snow are prevalent across the length of the
Aleutian Islands in the west to northwest flow. A low over the
Kurile Islands will rapidly move east along the Aleutians as it
dissipates to a trough through Saturday morning.

The next system to cause an impact is the low and associated gale
force front that will move into the western Aleutians by Saturday
morning.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Saturday night into Sunday the upper low center over the eastern
Bering lifts north and swings a strong shortwave east from the
western Alaska Range and Gulf of Alaska. This system will bring
light widespread snow across most of the southern mainland and
Gulf Saturday night. A surface ridge transverses the region
briefly on Sunday bringing a break in precipitation across the
gulf and Southcentral.

To the West...an active weather pattern amidst the broad longwave
trough over the Bering keeps ongoing precipitation across the
Aleutians and Southwest. A significant storm system brings a warm
front into the western and Central Aleutians Saturday night
through Sunday. Max southeasterly gales winds are expected to
accompany the front along with snow and a brief period of rain as
warm air intrudes into the southern Bering. This front progresses
to the Pribilof Islands, eastern Bering and Alaska Peninsula on
Sunday.

The next system to monitor for gusty winds and snow across all of
the southern mainland and surrounding waters moves into the
western Aleutians Sunday and tracks quickly eastward. This stacked
low pressure system will spread snow across the Bering and
Aleutians on Sunday and then into the Southwest late day. This
system may edge into the western Gulf and Southcentral by Sunday
night however models have not come into agreement with the surface
low track and frontal progression.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 170 179 185 412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
LONG TERM...KH