Winter Weather Advisory 33, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
600 PM AKDT Wed Mar 31 2021

.ANALYSIS AND UPPER LEVELS...

An upper level trough stretches south across central Alaska into
the Gulf, bringing rain and snow to the favored areas around PWS.
Further west, weak upslope flow combined with weak instability
brought snow showers to eastern portions of the Valley and the
Anchorage Bowl, mainly along the Hillside.

A ridge of high pressure stretches north from the North Pacific,
across the western Aleutians into the western Bering. Between the
two features, strong northwesterly winds are ushering in cold air
across the central and eastern Aleutians as well as the AKPEN.
This is also causing gales and areas of freezing spray in the
eastern Bering, as well as from the Kennedy Entrance south to
around Castle Cape.

&&

.MODEL DISCUSSION...

Models are in good agreement on the main features on the synoptic
scale through Saturday. The low pressure trough that moved across
south central Alaska will interact with the low pressure
circulation just east of the Prince William Sound causing a bit of
a resurgence or slight deepening before it moves southeast of the
forecast area by Thursday night.

A ridge of high pressure will continue over the North Pacific
into the Bering over the next few days, slowly moving east through
the end of the week and over the weekend. Between the two main
features, especially across and just east of the AKPEN, expect
strong winds ushering in colder air, leading to gale to possibly
pockets of storm force winds and heavy freezing spray.

&&

.AVIATION...
PANC...VFR conditions and light winds are expected at the terminal
for the TAF package.

&&

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

A vertically stacked low near Prince William Sound (PWS) will
continue the long duration snow event over the eastern coastal
areas and surrounding high elevations. The low is expected to
weaken as it migrates south and east of the central Gulf coast,
thus ending snow showers in the immediate coastal areas by
tomorrow afternoon. With temperatures above freezing and aided by
relatively warmer SSTs offshore, immediate coastal spots (e.g.,
PACV) may see rain only with snow in the mix. Moist, cyclonic
flow will keep the chances of scattered snow showers on the
backside of the low coupled with colder air advection in the
surrounding high elevations and offshore waters before tapering
off altogether by Friday night. In the wake of the low,
northwesterly gale to storm force winds will develop through the
usual gaps and passes with the threat of heavy freezing spray
west of the Barren Islands, including Kamishak Bay, and Shelikof
Strait late tonight and into Friday. Additionally, upper level
support with cold air advection will develop outflow winds will
also flow through usual gaps and passes over the southern
mainland through Friday night.

Radiational cooling will allow temperatures in the low-lying areas
and surrounding elevations to rapidly drop anywhere in the single
digits to the teens later tonight. With temperatures being below
normal for this time of season, our high March and April sun will
be the only source for warmth for the time being. Additionally,
there is chance for patchy fog to develop later tonight over
western Kenai Peninsula with lingering moisture left in the wake
of the aforementioned low.

&&

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

Northerly flow is bringing cold and dry conditions across the
Southwest, and will continue to do so through Thursday night.
After this, a front associated with a low in the northern Bering
will approach the Kuskokwim Delta coast from Thursday evening
through Friday morning. The Winter Storm Watch for potential
blizzard conditions remains in effect for this system, although,
recent model runs have blowing snow a possibility without a
blizzard. We will continue to hone in on this feature as the
situation develops. After this, expect a change of precipitation
type from snow, to rain and back to a wintry mix of precipitation
as the warm center of the low traverses the area through Saturday
night.

&&

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

Strong gusty winds continue to persist through bays and passes in
the eastern Aleutians and the Alaska Peninsula this afternoon.
These are anticipated to drop off later this evening. Concerns for
heavy freezing spray continue through tonight due to these strong
northerly winds for waters south of the Alaska Peninsula and
southwest of Shelikof Strait. Conditions should dry across much of
the Bering and the Aleutians tomorrow afternoon, before a front
moving across the northern Bering brings another round of active
weather across the area through Saturday night.

&&

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

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 with seas subsiding to 10 feet on Sat.

Gulf of Alaska:
Northwest gales will continue 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. Seas
generally 10 to 15 feet or less through Sat.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

By late Saturday/ early Sunday, the amplified pattern begins to
break down as a shortwave trough drops south over the northern
Bering, advancing a surface low pressure system inland over
Western Alaska near Norton Sound. In response to the digging
trough, the ridge will flatten out and shift east, with its axis
somewhere near the AKPEN or Kodiak Island. The movement of these
upper-level features will provide a more zonal west/east flow
across the state. Shortwaves will track quickly through the west
to east upper-level flow, yielding lingering chances of snow
showers into early next week, especially for higher elevations.
This flow will also keep the core of cold air farther north,
allowing for temperatures near or slightly below normal across the
southern mainland.

A more potent, fast-moving shortwave will move from the western
Aleutians Sunday to the northern Gulf for late Monday. This
feature will bring rain and snow from Bristol Bay to coastal
Southcentral Alaska through Monday. It looks as if this feature
will stall over the northern Gulf as a second shortwave drops from
the Bering into the central Gulf. It is looking more likely that
these shortwaves will help to re-amplify the upper-level pattern
with the long-wave trough over the state and a ridge extending
from the western Aleutians north to the Bering Strait. This
pattern would keep conditions winter-like into next week, with a
northerly flow resulting in drier and cooler weather across the
inland areas of the southern mainland and snow showers along the
coast, as well as over the eastern Bering/Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch: 155.
MARINE...Storm: 130.
Gale: 131-136 138 150 171 172 174 176 178 185 352 411 412.
Heavy Freezing Spray: 130 138 139 150 160 180.

&&

SYNOPSIS AND MODEL DISCUSSION...NR
SOUTHCENTRAL ALASKA...MF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...KZ/TM