Gale Warning 339, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
523 AM AKDT Sun Apr 23 2023

.ANALYSIS AND UPPER LEVELS...

High pressure ridging exists across the eastern Bering Sea and
portions of Southwest Alaska. The polar low tracks into the Gulf
of Alaska this afternoon, with the a series of associated
shortwaves moving east to west across Southern Alaska. Satellite
imagery shows clear skies under the ridge, with incoming cloud
cover with the approach of the shortwaves. Further west, a trough
extending off a Kamchatka low encompasses the western Bering Sea
and Western Aleutians. A secondary low develops at the base of the
trough just north of the eastern Aleutians Monday evening that
will track towards the Alaska Peninsula through mid-week.

&&

.MODEL DISCUSSION...

Models are in good overall synoptic agreement through the short-
term. There are some minor discrepancies with the shortwaves
moving across Southern Alaska as the the low moves into the Gulf
of Alaska. Towards Wednesday, models struggle with a developing
complex system across the Bering Sea.

&&

.AVIATION...
PANC...VFR conditions will persist. Winds will continue to be
gusty at times through the day of today.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Sunday through Tuesday night)...

An upper-level low over Kenai this morning will continue to slide
south over the Gulf of Alaska by late Sunday and over the eastern
Gulf for Monday. A colder and drier airmass is moving into
Southcentral in the wake of the upper-level low. Even with this
drier airmass, there is still some moisture advection and enough
instability to provide for areas of rain and snow showers, mainly
along the coast and across north-facing slopes of the coastal
mountains due and upslope flow.

The vorticity advection and upper-level divergence out ahead of
the upper-level low is also allowing for the development of a
surface low south of the Kenai Peninsula. This low will strengthen
through the day, moving south then east by Monday in tandem with
the track of the upper-level low. The combination of cold air
advection and the increasing pressure gradient between the
developing low and building ridge over the interior has initiated
gusty northerly winds across parts of interior Southcentral and
through coastal gaps. These winds will persist through Monday
morning before the gradient relaxes as the low exits the region
and the cold air advection shuts off.

An inverted trough moving toward Prince William Sound and a weak
upper-level wave entering the Copper River Basin will likely
allow showers to continue into Tuesday for these locations.
Elsewhere, conditions are expected to be rather tranquil for most
of Monday into early Tuesday. For Tuesday, an occluded front will
move first over Kodiak Island and the western Gulf, eventually
moving to the Southcentral coast by Tuesday evening. There may be
enough colder air in place for snow at the onset of precipitation
for Kodiak early Tuesday morning before easterly winds usher in
warmer air and changing the snow quickly to rain. A similar set up
is also expected along the immediate coast. With colder
temperatures across the interior of Kenai Peninsula and aloft, a
heavy, wet snow could persist for elevations above 600 ft,
including Turnagain Pass and Summit Lake into Wednesday morning.

&&

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

A winter-like pattern has returned to Southwest overnight as cold
air spills in from the northeast behind a front continuing to push
south into the eastern Bering. Colder than average temperatures
and gusty north winds will continue today off to the west of a
stalled upper level low in the Gulf, with highs mostly struggling
to get above the freezing mark. Winds should begin to relax by
tonight as cold air advection finally begins to let up and as a
dry upper trough moves overhead, but cold temperatures dipping
down into the 10s to 20s will persist. A slight warming trend
will begin by Monday as the upper trough continues to pivot west
and as southeasterly flow begins to pick up ahead of developing
low in the Bering. This same system will mark the next chance for
measurable precipitation, with the center expected to move up the
Alaska Peninsula into Bristol Bay on Tuesday. Low level
temperatures will likely be cold enough for snow to fall across
the northern Alaska Peninsula and parts of Bristol Bay, although
some mixing with rain will be possible during the afternoon and
evening as daytime highs finally push above freezing.

&&

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

A weakening front over the western Bering associated with a broad
Kamchatka low is now approaching Adak, with winds out ahead of the
front still breaching gale force for the time being. Southerly
winds along the front will drop to small craft range as it moves
into the central Aleutians later today and tonight, with low level
temperatures initially warm enough to support mainly rain falling
along the boundary. On Monday, a triple point low will form along
the front southwest of the Pribilofs, with a shield of
precipitation expected to overspread the islands and remain in
place until the evening as the low pulls away and into the Alaska
Peninsula. It's looking more likely low level temperatures will
cool enough for all snow to fall across the Pribilofs near the
low, with warmer temperatures farther south across the eastern
Aleutians keeping precipitation mostly rain. A few inches of
accumulation will be possible across the Pribilofs through Monday
evening, though this will depend heavily on how cold surface
temperatures remain throughout the day. Easterly winds around 20
to 25 kts could lead to some minor blowing snow as well, but it
does not appear likely to cause significant visibility reductions
at this time due to marginally favorable temperatures hovering
right around freezing. A quieter but still unsettled pattern will
continue for the remainder of the period through Tuesday night as
the low shifts into Southwest, leaving behind cool north to
northwest flow and areas of rain/snow showers.

&&

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

Looking ahead in the extended forecast, the latest model runs and
ensemble guidance are in fairly decent agreement with the
evolution and progression of the synoptic features. Starting
midweek, a deep arctic trough and upper low moving in from the
north retrogrades into the Bering Sea, while a ridge builds across
the eastern Alaska mainland. As such, a warm, southerly flow
pattern develops, allowing a series of low pressure systems to
lift northward across Southcentral which will support heavy
rainfall along the coastline. In addition to precipitation, these
systems should lead to periods of gusty winds across portions of
the Gulf of Alaska and Alaska Peninsula. Out west, the upper
trough will usher an unseasonably cold airmass with scattered
showers spreading across the Bering Sea, Aleutian Chain, and
Southwest Alaska through late next week. To the far west, a system
will push its front into the western Aleutians, bringing winds
and rain to the area.

&&

 $$

SYNOPSIS AND MODEL DISCUSSION...KM
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
LONG TERM...MF

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
338 PM AKDT SUN APR 23 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 AM     SNOW             7 MILES SE OF EAGLE RIV 61.23N 149.44W
04/23/2023  M1.7 INCH        ANCHORAGE          AK   PUBLIC

            OVERNIGHT SNOW.

0300 PM     SNOW             UPPER POTTER MARSH      61.06N 149.76W
04/23/2023  M0.5 INCH        ANCHORAGE          AK   PUBLIC

&&