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000
FXAK68 PAFC 200122
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 PM AKDT Tue Mar 19 2019

.ANALYSIS AND UPPER LEVELS...
At the surface and aloft we have a low and trough crossing through
Southcentral Alaska and the Copper River Basin as it weakens. A
strong upper level ridge extends up through British Columbia into
the Yukon Territory. Weak surface high pressure is over the Gulf
of Alaska while wrap around low clouds and cold air is moving into
Southwest Alaska. A moderate low is south of the eastern Aleutians
with a front extending from the low to the southeast to a developing
triple point low about a 1000 nm south of Kodiak. A weak surface
ridge of high pressure is over the western Aleutians and Bering
Sea.

&&

.MODEL DISCUSSION...
The models are in good agreement at the synoptic scale, but there
are differences when you dig deeper into the smaller details. The
most notable difference is with respect to the progression of the
warm air across Southcentral Alaska tomorrow. The Canadian and
GFS are initially faster to push the warm air across the Alaska
Range, though the EC and NAM catch up with them by tomorrow
afternoon. The models align well with the push of moisture across
Southwest Alaska tomorrow and holding cold air along the Bering
Sea coast and a bit inland. The models are also reasonably well
aligned with the amount of snow in the Kuskokwim Delta and
Northwest Bristol Bay lending to greater confidence in the
forecast.

&&

.AVIATION...
PANC...Clouds will increase beginning around 08Z as a front
approaches from the east, though strong cross barrier flow will
maintain fairly high ceilings. Expect the front to move through
dry, with perhaps a few flurries or sprinkles.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Thursday)...

The mainly clear skies Southcentral is enjoying this afternoon
and evening are expected to cloud back up around midnight as a
retrograding front moves westward from the central Gulf and
eastern Alaska into Southcentral. With the initial pass of the
strengthening front, some snow showers and/or sprinkles are
possible after midnight. After that the Turnagain Arm and
Matanuska Valley winds will kick back in and return the area to
a cloudy, dry, warm, and occasionally windy pattern for Anchorage
and the Mat-Su. The onshore southeasterly winds will also ramp up
with time across the mountains, ending the brief break in the snow.
Thus, rising freezing levels will lead to more mountain snow but
valley rain, with Turnagain Pass, the Seward Area, and much of the
rest of the coastal Chugach and Kenai Mountains returning to
periods of heavy snow for the day Wednesday. This will likely keep
the avalanche threat high.

Precipitation rates will diminish Wednesday night into Thursday
morning across the mountains in between systems. However, another
front pushing up from the south will move into Kodiak Island on
Thursday, reaching the north Gulf Coast Thursday night, making for
yet another round of occasionally heavy precipitation in the
mountains.

The retrograding front moving across the Gulf currently is the
leading edge of another unseasonably warm air mass associated
with the very strong western North American ridge. While much of
Southcentral will remain cloudy and wet, the warmer air mass will
continue to promote high temperatures approaching record territory
as the March thaw continues. With mainly clear skies and light and
variable winds for the first half of tonight promoting colder
temperatures along with the snow pack, tonight`s low temperatures
may be the last sub-freezing readings in Southcentral for at
least the next several days.

&&

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

Offshore north to northeasterly flow will continue to increase
over Southwest Alaska this afternoon through tonight in response
to both a west to east running upper level ridge lifting north
across Southwest Alaska and a developing surface low tracking
toward the Southwest Gulf. An westward propagating frontal wave
will rotate into the Greater Bristol Bay area Wednesday morning
and then continue north and west into the the lower Kuskokwim
Valley through Wednesday afternoon and over Kuskokwim Delta
Wednesday evening. The frontal band will slow overnight, stalling
along the northwest Kuskokwim Delta through Thursday morning.

One of the main concerns with this system is snow and blowing
snow developing over the Kuskokwim Delta late Wednesday afternoon
through and persisting through Thursday morning where temperatures
will remain colder and northeasterly winds stronger. Additional
impacts include moderate to heavy snow late Wednesday morning
through Wednesday afternoon over western portions of the Greater
Bristol Bay area from the Nushagak River west combined with breezy
winds. In this location, much off the expected visibility reduction
will be a result of heavy snow fall rates (as opposed to cold dry
snow becoming resuspended) but areas of blowing and drifting snow
are a concern as well despite temperatures being somewhat warmer
than along the Delta.

Along the Bristol Bay coast temperatures will be warming from east
to west through the day Wednesday as the front rotates east,
changing snow over to rain. By late afternoon through the evening
the warmer air should push further north and inland with temperatures
hovering near or slightly above freezing through the night. Further
to the east, along the lee-side of the Alaska and Aleutian Range,
temperatures will warm more quickly with the increasing
northeasterly downsloping gap winds and keep what precipitation
that does fall in locations such as the King Salmon and Iliamna
areas as mainly rain.

&&

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

The upper level flow across the Bering continues to be weak with
cold upper troughing persisting across the western and central
Bering. To the south of the eastern Aleutians and Alaska Peninsula
a vertically stacked low will persist tonight through Wednesday
morning before a developing North Pacific Low grabs it and pulls
it into the western Gulf. A Kamchatka Low will send a weak frontal
system to the Western Aleutians where it will stall and then
gradually shear apart through Thursday.

&&

.MARINE (Days 3 through 5/Friday through Sunday)...

We are still expecting a wide swath of northerly gale force winds
in the Bering through Saturday. Winds should taper on Sunday and
switch to a more southerly direction. Meanwhile in the Gulf of
Alaska, southeast winds will continue as they circulate around a
low southwest of Kodiak Island. Winds should start out as gale
force on Friday and taper off Sunday. Confidence is high in
forecasts for both areas as models seems to be doing a good job
of picking up this pattern.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday through Tuesday)...
Another round of warm and wet weather is in store for southcentral
AK to start the long term. This will be due to another low
pressure system tracking north into the Gulf of Alaska, with the
continued high pressure ridge to the east over Canada sending low
pressure in that direction. A brief break will be in store on
Saturday with another low moving into the Gulf again on Sunday.
The Sunday low should finally be the last of this pattern, as it
appears high pressure moves into the Gulf of Alaska early next
week. Model agreement is good enough in the long term that we are
confident in the overall pattern. However, as with most weather
systems as of late, we will have to wait until the short term to
be able to figure out your local forecast.

The Bering and Aleutians will start the long term with continued
northerly flow, keeping temperatures on the cool side with some
snow showers. Another large low pressure system looks to move
into the western Aleutians/Bering early next week, transitioning
the region to a more southwesterly pattern. Model confidence is
pretty good in the long term, so there is a high level of
confidence this forecast will generally hold true.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
MARINE...Heavy Freezing Spray 180 181 185.
      Gale Warning 119 120 125 130 131 138 139 155
  172 173 174 176 179 180 181 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
MARINE/LONG TERM...BB