Heavy Freezing Spray Warning 137, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 AM AKDT Tue Mar 30 2021

.ANALYSIS AND UPPER LEVELS...

The pattern remains amplified as the jetstream extends
northeastward from the Sea of Japan through the Kamchatka
Peninsula and then southeastward from the Kamchatka Peninsula
through 45N 171W. The jetstream then extends northeastward to over
Southcentral and Southeast Alaska before extending southeastward
to the Pacific Northwest of the CONUS. An upper level trough
currently over the Bering/Aleutians is moving eastward. An upper
ridge currently extends across the Gulf of Alaska and is moving
eastward.

Closer to the surface, radar imagery shows rain and snow showers
moving across the northern Gulf and Prince William Sound. Snow
showers are also observed moving northeastward along the northern
Alaska Peninsula.

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.MODEL DISCUSSION...

Guidance is in good agreement with the placement and intensity of
synoptic scale features from the surface through the upper levels.
High resolution models were favored to resolve local effects.
Forecast confidence is above average.

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.AVIATION...
PANC...VFR conditions and light winds will persist. Temporary
gusts from the southeast are possible through 16z as Turnagain Arm
winds are likely to approach the airfield but will only be
intermittent.

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.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Upper level troughing continues to dig south and east through
Southwest AK. This trough is well defined on satellite with a
sharp trailing edge in cloud cover associated with an open wave
ahead of it over Southcentral AK. This wave is being sandwiched
between the trough and high pressure south of the Gulf of Alaska.
Troughing has now progressed close enough to Southcentral to push
this wave and its southwesterly flow to the east of the Susitna
Valley, ending the winter weather advisory for the area last night.

Later today, when this trough enters Southcentral AK, an upper
level low will form at the base of it over Prince William Sound.
This upper level low will support the development of a surface low
in the Northern Gulf, stacked generally beneath the upper level
low. This next system is expected to push into Eastern Prince
William Sound. This will create a precipitation event for areas
east of the sound like Cordova. Storm total QPF of around an inch
can be expected in and around Cordova, with high amounts of up to
two inches in some areas of the Chugach. A warmup can be expected
as well. Right now, models generally agree precipitation will not
make it to areas west of the sound, like Anchorage. The GFS is
the outlier here, but we have steered away from it.

After the exit of this system on Wednesday, cloud cover and
showers will linger along the coast. High pressure then sets up
over mainland AK, returning cooler (below average) and drier
conditions to Southcentral AK. Winter is not quite done yet. This
will also induce cold air advection through gaps across the
Alaska and Chugach Ranges and into the Gulf. Places like Kamishak
Bay, the Barren Islands and Shelikof Strait will see the strongest
flow, with gale force winds expected.

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

Upper level troughing continues to move across the Southwest
Mainland towards the Gulf through Tuesday night. Light snow
showers will linger across Southwest, especially along the
upslope (west-facing) of mountain ranges, during this
progression. Behind the trough, flow transitions to northerly,
bringing a fair amount of cold air advection along with it.
Conditions are to become dry and clear behind the trough as well,
allowing for radiational cooling. When coupled with the colder
northerly air, temperatures are expected to drop significantly
during the overnight hours.

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

A weak surface low pushes into the Northern Bering Tuesday night
and looks to move south across the Central Bering, dissipating
before it reaches the Eastern Aleutians. This will keep
precipitation confined to the Eastern/Central Aleutians, as well
as the Pribilof Islands, through Wednesday evening. Light snow
will begin late Tuesday night and transition to rain/snow or all
rain in some areas as temperatures increase during the day
Wednesday.

As a longwave trough moves onshore, winds behind it will be
primarily northerly for the Eastern Bering through the forecast
period. Heavy freezing spray will be a concern for both the
Eastern and Northern Bering and along the ice edge. Additionally,
winds are expected to be stronger through bays and passes south of
the AKPEN. Flow in the Central and Western Bering will begin
northerly and transition to westerly as a surface low enters the
Northern Bering and pushes south. Gales are likely across the
Southern Bering and along the Western and Central Aleutian coasts.

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.MARINE (Days 3 through 5: Thursday through Saturday)...

Bering Sea: Confidence is good for a low to track across the far
northern Bering from Thu to Fri. Southwesterly small craft to
possibly gales will track south of the low across the Bering with
seas building to 15 feet from west to east through Fri. A front
trailing from the low will track across the eastern Bering for
Sat. A region of westerly gales may move with this front through
Sat evening, but otherwise expect winds decreasing to small craft
or lower by late Sat.

Gulf of Alaska:
Northwest gales will continue for Thu out of bays and passes along
the Alaska Peninsula including near Kodiak and Barren Islands,
subsiding to small craft or lower by Fri. Forecast confidence for
Thu and Fri is high. A low is expected to form over the northern
Gulf by Sat, however differences for this low formation are more
significant, and confidence for Sat is lower. Potential exists for
westerly gales increasing in the western Gulf on Sat south and
west of the low, particularly out of gaps along the AKPen.

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...
A low will be traversing the northern Bering near the start of the
period on Friday. The supporting upper level shortwave will begin
to weaken/flatten out a longwave ridge extending from the North
Pacific into the southern Bering and Southwest AK. Model agreement
with the timing and track of the Bering low has improved
significantly over the past day or so, with the consensus being
that the low will move inland near the Y-K Delta by Saturday
morning. This should bring another round of snowfall to much of
the Southwest Friday through Saturday, with more of a mixed
precipitation event possible near the AKPen and Bristol Bay area
where low level temperatures will be warmer. There are some key
model differences with the amplitude of the upper level ridge and
resultant track of the short-wave overtop of it. This will affect
temperatures and precipitation over Southcentral this weekend.
Expect either a glancing blow of precip with warmer temperatures
(ala the ECMWF solution) or more widespread snow followed by
another round of cold air advection (as per the GFS solution).

By early next week, operational model agreement deteriorates once
again, but the general consensus suggests a return to a very
similar synoptic setup compared with the pattern at the very start
of the period (early Friday). Another wave moving across the
Southwest and then Southcentral Saturday to Sunday around a deep
upper level low near the North Slope should result in a new
longwave trough developing over the mainland, with a new low also
forming over the Gulf. Another surge of Arctic air may descend
into much of the southern mainland with this pattern shift,
favoring much colder temperatures returning by the end of the
weekend. Ridging will also build back into place across the
Bering, potentially advancing into the Southwest and AKPen by
Monday. Overall, expect the typical spring weather fluctuations
to continue with the progressive synoptic pattern and decreasing
model consistency going forward into early next week.
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warning 130 160 180 185.
Gale Warning 130 131 132 138 150 155 160 165 170 171 172 173 174 175 176 177
178 179 180 411 412 413 414.
FIRE WEATHER...NONE.

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SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...AS/SB