Gale Warning 332, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKDT Thu Apr 20 2023

.ANALYSIS AND UPPER LEVELS...

The weather remains clear and quiet across the state today due
to the influence of an upper level ridge high centered over the
western Chukchi Sea. This places the southern Mainland and much
of the Bering Sea in dry northerly to easterly flow. To the far
north, an arctic trough is beginning to dig south over the North
Slope. This feature will usher in a colder airmass and transition
the overall weather pattern back to cooler and for some areas
wetter conditions over the coming week.

&&

.MODEL DISCUSSION...

Models are in good agreement in the immediate short term but
diverge by late Friday night as to the speed in which the trough
can dig south and over Southcentral. This will have some impact
on the precipitation type forecast due to minor timing
differences. It will also impact the timing of low development in
the Gulf, thus impacting timing for some lower end winds through
gaps and passes.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Saturday night)...

Another day of gorgeous weather expected for tonight and Friday
as a ridge of high pressure influences Southcentral. We will
continue to see drier conditions and mostly clear skies, allowing
overnight temperatures dropping below freezing for inland
locations before rebounding above seasonal averages Friday
afternoon.

Heading into the weekend, a pattern change will develop as the
ridge breaks down in response to a deep arctic trough moving in
from the north. An unseasonably cold airmass will filter with
precipitation spreading from north to south. There is some
uncertainty with precipitation type, but current thinking is
snow/rain mix or even all snow through midday Saturday before
shifting to mix or all rain in the afternoon.

The timing of the trough axis will be the key player as recent
model runs have now shifted the orientation and position. This
will leave the best dynamics and precipitation in the Copper River
Basin and surrounding elevation. Coastal mountains also look
favorable for snow showers between Saturday and Sunday as a
surface low develops over the Gulf, with light accumulations
expected. Winds through the gaps and passes will also slowly
increase through the weekend as the colder air is pulled into the
Gulf by the low.

&&

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

Steady, northeasterly flow continues across Southwest Alaska
and will stay until Friday evening as the flow shifts slightly
becoming northerly. Increasing temperatures today will be
continued Friday as the blue skies allow more heating. However,
by Saturday afternoon cold air from the Interior will move in
depressing temperatures this weekend for much of Southwest.
Temperatures Saturday will return to the upper 30s with some
lower 40s, but by Sunday may return to below freezing for the
high temperature across much of the region north of the Alaska
Peninsula. Winds will increase throughout the region, with gusty
winds prominent from Sleetmute to New Stuyahok starting Saturday,
and from Aniak to Bethel Sunday morning. No precipitation is
expected to accompany these cooler temps and gusty winds, with
skies expected to remain mostly clear through the majority of
the weekend.

&&

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

Much of the Bering Sea and Central/Eastern Aleutians will continue
to see fairly benign weather over the next couple of days as the
high pressure situated over the region remains. From Nikolski
east, northeast winds will persist as the northern periphery of
a North Pacific low edges past the region as it approaches the
southern Gulf of Alaska. As such, offshore winds south of the
Aleutian Chain are expected to reach gale-force overnight tonight
and last until Friday evening. Influence of these winds inland is
likely not going to exceed 30 mph easterly gusts near King Cove
and Sand Point. Partly cloudy skies tonight and Friday in these
region will likely clear to mostly clear by Saturday afternoon.

To the west of Nikolski the pattern is fairly different. Sparse
showers can be found from Adak out to Shemya as the remnants of
a stationary front lingers in the area. This fading feature will
likely move out of the area by late tonight, creating mostly clear
conditions across the Western Aleutians Friday. However, by Friday
night, a leading front originating from a Kamchatka low pressure
system will approach the Western-most Aleutians. Gale-force
southeasterly winds will begin across Attu and Shemya late Friday
night after midnight. Winds are currently expected to increase to
storm-force sustained off the western shore of Attu. Gusts may
approach 70 mph off the western coast of these islands by noon
Saturday. The corridor of strongest winds will then shift to the
north, bringing it to the western Bering, however it is possible
that we continue to see those winds trending higher. We will
monitor upcoming model runs and examine the potential for High
Wind criteria for the western-most Aleutians, however currently
the corridor of these strongest winds is expected to remain
offshore. Saturday morning winds for these islands will also be
accompanied by a band of rainfall that will begin to sweep the
Chain, reaching Amchitka by Sunday afternoon. The role the ridge
will play in the eastern extent of this rainfall is being
examined. Stay tuned as we investigate upcoming model runs and
update the forecast.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through Thursday)...

The long term forecast begins on Monday with upper-level troughs
and embedded lows located over the southern mainland and the
western Aleutians. A ridge of high pressure wedged in between the
two troughs over the Bering Sea and Aleutians is gradually being
pinched off to the north. One of the main talking points for the
long term forecast continues to be the pattern shift towards colder,
below-normal temperatures. As the aforementioned trough and low
over Southcentral continue to drop south from the Arctic, it will
allow a very cold airmass to settle into much of the mainland.
For the beginning of the work week, the coldest 850 mb temperatures
are expected across Southwest and north of the Alaska range,
however Southcentral should still experience colder temperatures
as some of the Arctic air filters through the Alaska Range. By
later Monday, a longwave upper trough and embedded low complex
will emerge and then rotate across the Bering, mainland, and Gulf
through the long term period. By midweek, temperatures begin to
moderate across the mainland as southerly flow increases. The
Arctic airmass retreats northward and to the west across the
Bering and Aleutians. For Wednesday and Thursday, there is
potential for heavier precipitation along the Southcentral coast
as a low pressure system and its surface front traverse the
region. Models are currently struggling with the track of this
low, however Southcentral is generally trending towards above
average precipitation for the remainder of the month.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...MF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL
LONG TERM...AF