Gale Warning 347, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
538 AM AKDT Sun Apr 30 2023

.ANALYSIS AND UPPER LEVELS...

A shortwave trough and attendant surface low continue to lift
north toward Cordova this afternoon with an area of precipitation
extending from Middleton Island north to Eureka. The precipitation
is falling as rain for elevations below 1,000 ft and as snow over
the eastern Chugach Mountains and western half of the Copper River
Basin. For the western half of Southcentral, a transient ridge is
keeping conditions rather benign from the Susitna Valley south to
Kodiak Island, with only scattered cloud cover and temperatures
in the 40s. A second shortwave trough is lifting toward the
western Gulf from the south of the AKPen this afternoon with a
general increase in clouds and southerly winds for coastal waters.

Farther west, a vigorous shortwave south of Adak is phasing with
larger upper-level low dropping south from the central Bering. The
result is the development and strengthening of a surface low south
of Adak moving slowly to the northeast. The associated warm front
is lifting toward the central Aleutians with its trailing cold
front extending well south of the Chain. The colder airmass in the
wake of the low is also evident on satellite with the wide expense
of open-celled cumulus clouds moving south across the western
Bering and Aleutians.

Elsewhere, a wave moving over Southwest Alaska continues to
produce scattered showers from Dillingham north to Bethel.

&&

.MODEL DISCUSSION...

Models are in good agreement over the strength and track of the
low over the central Aleutians as its approaches Bristol Bay
Monday morning. Slightly less agreement exists concerning how
quickly the low center begins to turn north across the western
edge of the state. The timing and orientation of its front as it
begins to occlude and move from the western Gulf Sunday morning to
the northern Gulf on Monday is the least resolved feature. Decent
overall agreement between models lends to above average forecast
confidence through the weekend.

&&

.AVIATION...

PANC...VFR conditions are expected through Tuesday morning.
Sunday afternoon a southeast Turnagain Arm wind develops. This
wind will likely bend over the terminal during the afternoon to
early evening hours before returning back over the arm and down
Cook Inlet. This will result in northerly surface winds and
southeasterly winds aloft, increasing the probability for LLWS
Sunday afternoon through Monday morning.

&&

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

An upper level shortwave over Southcentral Alaska early this
morning is moving northeast and will exit the area by this
afternoon. This short wave is the reason for the snow overnight in
the Copper River Basin and rain in the Prince William Sound area.
There is a break in the clouds right behind this shortwave which
should bring in some sunny breaks today before the next front
reaches the area from the Southwest. This front in already
encroaching on Kodiak Island where it will bring in rain for
today. AS it tracks northeastward, the precipitation will mainly
be over the Coastal mountains with rain at sea level and a
transition to snow from generally between 1000 to 2000 ft above
sea level. This front will also bring some gusty wind through
channeled terrain including Turnagain Arm. These Turnagain Arm
winds are expected to move over the Anchorage Bowl this
afternoon/evening and again Monday afternoon. We are getting to
the time of year where the Turnagain Arm winds will typically bend
over Anchorage in the afternoon and evenings in response to the
diurnal thermal low in the Susitna Valley. Through the summer, it
will generally take a stronger low to bend the Turnagain Arm winds
down the Inlet in the afternoon and evenings though they will do
so overnight and before the daytime heating ramps up.

Back to the weather the next few days: After the initial front
moves across Southcentral tonight and MOnday, the cold, The upper
level low will move into the area from the west. This will bring i
some rain and snow showers with it. The snow showers will mainly
be at elevation with rain at lower elevations. However, some sea
level snow showers during overnight hours from the Western Kenai
Peninsula northward cannot be completely ruled out.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/Bering Sea and Aleutian
Islands (Days 1 through 3)...

A large low pressure system over the Bering/Aleutians will
continue to drift eastward while weakening. That being said,
precipitation will continue across Southwest Alaska with snow in
the higher elevations and rain in the lower elevations and coastal
areas. Winds will gradually ease over the next 24-48 hours as
this low weakens.

&&

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

The long-term period opens with most of the region being
encompassed within a broad trough from the Bering Sea to the Gulf
of Alaska. The result will be continued unsettled weather region-
wide through Thursday. However, there is still uncertainty with
the exact tracks of surface lows and fronts. Deterministic and
ensemble guidance the advertise a sharpening of the trough through
Southwest Alaska and into the Gulf as it takes on a more negative
tilt while a shortwave ridge builds across the central Aleutians.
The GFS and ECMWF guidance are clustered reasonably close with
the evolution of the negative tilted trough and the central
Aleutian ridge for Thursday and Friday. However, the operational
Canadian guidance keeps the developing North Pacific low for
Friday way further to the south; the Canadian ensembles are more
in line with the GFS and ECMWF solutions. Also, the ECMWF and
Canadian guidance, especially the ensemble means, are a little
more amplified with the ridge and trough than the GFS is.
Regardless, precipitation and continued unsettled weather looks
probable for coastal sections of Southcentral Alaska through
Saturday. Meanwhile, there will be a short-lived break for the
central and eastern Aleutians Thursday before the next North
Pacific low and front move eastward for Friday and Saturday. It is
important to note that the ensemble guidance hints at the ridge
hanging around longer than the deterministic suites. Thus, there
is still an abundance of uncertainty with timing and track of this
North Pacific low at the end of the long-term.

&&

SYNOPSIS AND MODEL DISCUSSION...ER
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
LONG TERM...DAN