Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 280109
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 PM AKDT Fri Mar 27 2020

.ANALYSIS AND UPPER LEVELS...
A strong blocking ridge is centered over the north central Pacific,
with a trough over mainland Alaska. The jetstream extends northward
up the west side of the ridge to the Aleutians, then turns eastward,
with zonal flow from the eastern Bering Sea to the Gulf of Alaska.
A gale force low over the northern Bering Sea is tracking eastward
toward the west coast of Alaska. The original frontal system from
this low has weakened considerably as it pushed inland across
Southwest Alaska and into the western Gulf today. What remains is
some areas of light snow, particularly along the west side of the
Alaska and Aleutian Ranges, with some light snow on the Kenai
Peninsula and rain down over the Gulf. Another round of snow is
making its way onshore along the Kuskokwim Delta coast as the low
center approaches. This is accompanied by strong and gusty winds
due to cold air advection and instability, leading to areas of
blowing snow and reduced visibilities.

Cold air advection extends south of the low center (over the
northern Bering) eastward across the Alaska Peninsula into the
Southwest Gulf. This is producing strong gap winds (up to storm
force) along the Alaska Peninsula. Meanwhile, due to the
combination of Arctic air in the Copper River Basin and tight
pressure gradients from there down to the Gulf, gap winds are also
affecting portions of the Matanuska Valley, the Valdez area, and
the Copper River Delta.

&&

.MODEL DISCUSSION...
Models are in better (but not perfect) agreement with the many
short-waves transiting the eastern Bering Sea, southern Alaska,
and Gulf of Alaska tonight through Saturday. Therefore, confidence
in the snow accumulation forecast for both Southwest and Southcentral
has increased.

&&

.AVIATION...
PANC...A short-wave crossing the Alaska Range this evening will
clip Anchorage, with light snow likely at the terminal this
evening through the overnight hours. With weak forcing and low
level westerly flow (favoring snow along the mountains), expect
ceiling/vis to bottom out at MVFR. There is also a chance that
most of the snow falls east of the terminal and VFR conditions
prevail for much of this time. No matter what, offshore flow will
develop during the day Saturday causing ceilings to lift. The
passage of an upper short-wave tomorrow afternoon combined with
cold air advection will lead to increasing northerlies, likely
becoming gusty after 00Z Saturday.

&&

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

Elevated winds along the northern Gulf coast continue to
gradually weaken as a surface low drifts toward the southeastern
Gulf. A weak frontal boundary associated with an upper level
trough has moved in over Southcentral, producing mostly virga this
afternoon. Snow becomes increasingly likely this evening, with
accumulations on the order of trace to 2 inches possible overnight
into Saturday morning. The upper level low then deepens and moves
southward over the Gulf, leading to another outflow wind event
Saturday night. As the low pulls away, cold Arctic air will be
drawn down from the north, lowering temperatures and allowing
northerly winds to pick up Saturday night. Areas including Seward,
Palmer, Valdez, and the Copper River Basin will once again see
elevated gusty winds Saturday into Sunday. Skies are expected to
clear for the rest of the weekend and into early next week as
colder northerly flow persists.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Monday)...

A low moving into Southwest Alaska from the northern Bering is
causing snow to redevelop across the Kuskokwim Delta this
afternoon. The low will track southeastward across the Kuskokwim
Delta and Bristol Bay tonight into Saturday morning. 1 to 3 inches
of snow are possible overnight tonight across many areas of
Southwest Alaska. Behind the low, strong north to northwesterly
winds following the low will cause snow to blow around, which may
temporarily reduce visibilities to one half mile at times. The
Winter Weather Advisory for the Kuskokwim Delta therefore remains
in effect until 7am Saturday morning. During the day Saturday, the
snow will largely end from northwest to southeast, though upslope
snow showers over the Kuskokwim Mountains and Alaska Range will
persist throughout the day. The primary effect of the weather on
Saturday will be colder air building southward across Southwest
Alaska and gradually clearing skies. Due to the strong late March
sunshine, it will likely take until Saturday night for the really
cold temperatures to be felt.

Polar high pressure builds into Southwest Alaska on Sunday and
Monday. This will lead to temperatures falling to between zero and
10 above for low temperatures in most locations, while highs
during the day rise to near 30, once again due to the strong late
March sunshine, which should be abundant both days under sunny
skies.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Monday)...

A low over the northern Bering will move into Southwest Alaska
tonight, as high pressure builds over the western Bering. A front
moves into the central and western Bering Saturday afternoon,
which will dissipate on Sunday. A second stronger front moves
east into the western Aleutians and western Bering late Saturday
night. It will weaken as it moves to the Pribilofs and eastern
Aleutians on Monday. Most models suggest a weak low develops along
that front near Cold Bay Monday afternoon, but there are some
discrepancies on that.

The strong winds and cold temperatures behind the low tonight and
Saturday will produce areas of heavy freezing spray, which will
spread north to south across the eastern Bering through the Alaska
Peninsula. This could become impactful to shipping in these areas,
particularly south of the Alaska Peninsula, where cold air and
strong winds together have been absent for quite a while.

&&

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

Bering Sea/Aleutians...
A storm system will track across the western to central Bering on
Sunday. There is uncertainty in the exact track of the low center,
but high confidence in widespread small craft winds with gales
likely near the low center. The eastern Bering will likely see
minimal impacts from this, as most model solutions track the low
to the northern Bering, or even into Russia. Forecast confidence
for Monday/Tuesday drops significantly, although it doesn`t look
like any major storm systems will affect the region.

Gulf of Alaska...
There is a high confidence in gap winds along the Gulf coast on
Sunday. These will diminish by Monday and the overall weather
pattern looks quiet.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through Friday)...

Guidance is consistent with the overall synoptic pattern on Sunday
with an upper level trough over the Alaska mainland while upper
level ridging slowly moves eastward across the central and eastern
Bering. This will result in cold temperatures across the
mainland. A high confidence exists with a warming trend in
temperatures across the Alaska mainland in particular Southwest
Alaska Sunday through Tuesday. However, there is low confidence
with respect to the magnitude of the expected warm up as there are
discrepancies between guidance with how fast this ridge is
moving. The GFS appears to have the fastest bias as the GEM and
ECMWF show the upper trough extending into the Ahklun Mountains
while the GFS has the trough only extending to the eastern AKPEN.
More significant differences between guidance arise on Wednesday
with respect to the amplitude of the upper ridge over Southcentral
Alaska. This means that there is a low confidence in the
temperature forecast as the placement of this upper ridge affects
the placement of the surface ridging and thus causing major
discrepancies in the temperature forecast. One area of confidence
for this period is the weather is expected to remain benign across
Southcentral during this time as high pressure will be over the
area. Significant discrepancies continue on Thursday and Friday
and thus the forecast confidence remains very low for this period
at this time.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155
MARINE...Gale 127 132 138 150 185
Heavy Freezing Spray 119 140 160 180 181 185 414
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...SB/ED



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