Heavy Freezing Spray Warning 128, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKDT Sun Mar 21 2021

.ANALYSIS AND UPPER LEVELS...

An increasingly quieter pattern is progressing across most of the
forecast area this morning. Weak and mainly zonal flow continues
across much of the state in the upper levels, with the exception
of a ridge extending over the western mainland from points north
of the Y-K Delta. A series of shortwave troughs is moving within
the zonal pattern farther south across parts of the Bering and
Gulf. Several weak lows are oriented in a similar fashion at the
surface, with one center located south of the Pribilofs and
another over the eastern Gulf. The remnants of yet another low are
producing areas of snow showers and regions of low stratus over
Southwest Alaska, and all of these features are drifting slowly
inland and northeast.

Snowfall that impacted parts of the Kenai Peninsula yesterday
evening has long since ended as the associated system departs
farther into the eastern Gulf. Precipitation falling mainly in
the form of snow is also ending over Kodiak Island as the flow
across the island becomes westerly, helping to dry out the lower
levels via downsloping near the lee faces of the steep island
terrain. Aside from a deck of stratus lingering over the Copper
River Basin and parts of the Mat Valley, skies are becoming
increasingly clear across much of Southcentral.

&&

.MODEL DISCUSSION...

Good agreement continues to prevail at the synoptic level for
nearly all features of interest through midweek. This supports
very good forecast confidence for the next few days. One point of
disagreement stems from the interaction of a positively-tilted
shortwave crossing the AKPen with another wave in the North
Pacific on Monday night. Models show differing timing for phasing
of these two waves, resulting in different timing/location for the
development of a surface low over the western Gulf. That said,
models agree the low will be weak and remain over open Gulf
waters, and this should therefore remain a low-impact system.
There could still be some forecast issues with determining minor
snowfall amounts, if any at all, over parts of Kodiak Island
depending on how north/south the low tracks from Monday night into
early Tuesday.

&&

.AVIATION...

PANC...VFR conditions and light north winds will continue.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Tuesday night)...
A fairly benign weather pattern will lead to mostly dry and cold
conditions over the next few days. First off, a trough over the
Interior will drop southward toward the Copper River Basin today,
leading to an increase in offshore flow, with weak cold air
advection down into the northern Gulf. This will combine with
tight pressure gradients in the wake of a low exiting across the
eastern Gulf to produce gap winds along the coast. While the
short-wave will exit eastward to the Yukon tonight, pressure and
temperature gradients will maintain gap winds tonight before
weakening Monday morning. Otherwise, expect a couple of dry days
region-wide with abundant sunshine and continued chilly (sub-
freezing) temperatures.

A trough crossing the Bering Sea will be in a weakening state as
it reaches Southcentral on Tuesday. There is a chance of snow,
particularly on the western slopes of the Kenai, Chugach, and
Talkeetna Mountains thanks to week upslope winds. However, if it
does snow on Tuesday, it won't add up to very much (most likely a
dusting to one half inch). Meanwhile, a separate trough over the
North Pacific will be pulled northward as the south end of the
Bering trough approaches, lifting a weak low and frontal system
northward toward the southern Gulf. The troughs will ultimately
phase and lead to deepening of the low, but by this point it will
be over the eastern Gulf. Therefore, expect impacts of this
system to remain primarily over the Gulf, with perhaps some light
snow reaching as far north as Kodiak Island and the north Gulf
coast. All of this will exit to the east Tuesday night, with
clearing skies. Gap winds along the coast will pick up once
again as pressure gradients tighten between the exiting low
and arriving ridge.

&&

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

Scattered snow showers will persist through this morning under a
weak surface low. Fog also remains possible, though widespread
lower visibilities are unlikely given the already low cloud cover
across the region. Skies will briefly clear this afternoon under a
weak upper ridge. Snow showers will again develop over interior
areas early Monday under a deep upper trough. This will also lead
to much cooler temperatures. Dry conditions are expected for
Tuesday under the trailing ridge. The next frontal system
approaches the Kuskokwim Delta coast Tuesday evening bringing the
potential for snow and blowing snow along the coast.

&&

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

Benign conditions are generally expected today. A series of
shortwave troughs will bring isolated to scattered rain/snow
showers to portions of the Aleutians. Gusty northerly winds will
develop Monday morning between low and high pressure systems over
the central/eastern Aleutians. These will be particularly strong
out of bays and passes on the Pacific side of the Chain. Scattered
showers are also possible along the northern fringes of a low.
Quiet weather will return late Monday into Tuesday under an upper
ridge. A front will track west to east across the Bering Tuesday
afternoon, extending from the Pribilofs to the western Aleutians,
leading to rain/snow and gusty southwesterly winds.

&&

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

Gulf of Alaska: Expect areas of gale force winds from the Alaska
Peninsula west of Sitkinak. There should be areas of gale force
gap winds through the Barren Islands and Sitkinak Wed and Thu.
Seas building to 14 feet.

Bering: Gale force winds out of bays and passes along the Pacific
side of the Alaska Peninsula and eastern Aleutian coasts are
expected for Wed. A gale (and potentially storm) force front
moves across the Bering from west to east Tue through Thu. Seas
over 20 ft are likely with this front.

&&

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

At the onset of the forecast period, a highly amplified pattern
is expected on Wednesday with a trough over the Western Bering
and ridge over SW Alaska and Southcentral. There will be a weak
frontal boundary at the surface on Wednesday that will bring a
batch of moisture to the Central/Eastern Bering and the Y-K Delta.
The jet core will be over Talkeetna with an estimated magnitude
of 160 kts. The temperatures for Anchorage and the Mat-Su valleys
will finally get above freezing in the afternoon. The models are
advertising actually bringing a system from the Central Pacific on
Friday to Kodiak, Anchorage and the Prince William Sound.
Meanwhile, a second system will emerge from the NW Pacific and
move into the Western Aleutians also on Friday and Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 155,170,172-174,176,178,411.
 		 Heavy Freezing Spray: 160,180,185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MK/PJS