Gale Warning 352, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 PM AKDT Tue May 9 2023

.ANALYSIS AND UPPER LEVELS...

A large vertically stacked low is positioned south of the Alaska
Peninsula, with a digging trough to the south over the northeast
Pacific. An occluded front wraps around the surface low from the
Alaska Peninsula to Bristol Bay and across Kodiak Island and the
western Gulf. Precipitation and winds are both light for the
portion of the front over Southwest Alaska. Much better forcing
exists out ahead of the upper trough over the Gulf, with a
southerly jet streak leading to a long fetch of moisture transport
and a much stronger surface occluded front. Gales are being
observed ahead of the front across much of the northern/northwestern
Gulf waters. Gusty easterly winds also extend across Prince
William Sound and onshore of the eastern Kenai Peninsula, with
localized gap winds through the typical areas. Steady rain covers
all of the Prince William Sound region, with heavier rain across
western Prince William Sound due to the upslope flow. The rest of
Southcentral is mostly dry, but cloudy. Kodiak island is in a
break from the rain, but a new surface low has formed along the
front ahead of the upper trough, with another round of rain headed
for Kodiak.

Another vertically stacked low is tracking eastward along the
western to central Aleutians, bringing light rain and Small Craft
Advisory level winds. Otherwise, weather across the Bering Sea is
quiet.

&&

.MODEL DISCUSSION...

Models are in good agreement with key weather features over the
next few days and overall forecast confidence is high.

&&

.AVIATION...

PANC...Pressure gradients will continue to favor a northerly
component to the surface wind, with the Turnagain Arm wind
curving southward down the Inlet as it exits Turnagain Arm.
However, an upper-level trough moving north across the interior,
combined with a weak surface trough north of the Alaska Range,
may alter the pressure gradient from Anchorage north just enough
to induce a light southerly wind late this afternoon through early
evening, before shifting back to a northerly wind. There may also
be some minimal directional low level wind shear when surface
winds at the terminal are northerly (+-20kts) due to expectant
southeast winds around the 2000 foot level this aft/eve.
Otherwise, expect VFR conditions to persist thanks to downslope
flow off the Kenai and Chugach Mountains.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Thursday night)...

A broad trough of low pressure remains over the western Gulf this
evening. Downstream of the trough axis an occluded front is
lifting into the northern Gulf with widespread small craft
southeasterly winds gusting up to gale force and numerous showers
moving inland of the Kenai Peninsula and Prince William Sound.
Winds will increase in intensity through tomorrow as the trough
continues to deepen over the Gulf. Which should result in
sustained gales over the Gulf and the potential for storm force
winds near the Barren Islands. Inland of the coast, blustery
conditions have persisted through Portage Valley with gusts
ranging from 40 to 50 mph throughout the day. Strong easterly flow
will continue to batter the eastern Turnagain Arm as air is
funneled and accelerated westward through the terrain. Relief
arrives late Wednesday night as the trough begins to pivot further
south and the pressure gradient across the region finally begins
to relax. Otherwise, Anchorage has seen much calmer conditions as
the pressure gradient of Southcentral has resulted in down inlet
flow through Cook Inlet with milder, northerly winds tracking into
the west side of town.

Moderate to heavy rain will continue to be the other main impact
across coastal Southcentral over the next couple of days as the
trough makes little movement and the front hangs up along the
coast. This will see moisture spreading into the coast through at
least Thursday. Lower elevations will see see continued rain while
snow levels between 1000 to 2000 feet should allow for modest
snow accumulations over the eastern faces of the Chugach
Mountains. Snow levels will continue to rise into Wednesday,
eventually climbing to above 2000 feet by Thursday.

Elsewhere, drier conditions can be expected for much of the
Copper Basin as moisture advection from the Gulf is stymied by
the Chugach Mountains and easterly flow limiting. Similarly,
enhanced easterly flow is expected to downslope any meaningful
moisture west of the Kenai Mountains, Chugach and Talkeetnas,
leading to drier and milder conditions into Thursday night. A slow
warming trend is forecast to commence over the latter half of the
week as high pressure over Canada begins to ridge over interior
Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA AND BERING SEA/ALEUTIANS
(Days 1 through 3: This afternoon through Friday afternoon)...

Generally, expect gusty winds and a warming trend across the
region. For the Alaska Peninsula and Eastern Aleutians, lingering
precipitation through Friday morning. For the Western and Central
Aleutians/Bering, light precipitation tapers off tonight but
returns on Thursday and Friday.

Diving a bit more into the details, a broad, upper level low south
of Kodiak Island continues to be the dominant weather-maker for
Southwest Alaska and the Eastern Aleutians in the coming days. As
a shortwave transits around the upper level low, its associated
surface low (with MSLP as low as 986 mb) will move just south of
the Alaska Peninsula through Wednesday. The resultant tightening
pressure gradient will lead to gusty easterly to northeasterly
winds, with the strongest winds expected in Bristol Bay and along
the Alaska Peninsula. Gusts may reach up to 40 mph. Some
uncertainty remains with how well winds will mix down from aloft,
as well as the potential for terrain channeling. The current
forecast package represents a middle of the ground solution, with
winds in Cold Bay and King Cove perhaps leaning on the higher end
of model guidance. In addition to gusty winds, expect steady
precipitation along the front becoming more showery as the low
occludes. Greatest precipitation amounts are expected in the
Eastern Aleutians and Alaska Peninsula. With the surface low
weakening and continuing to dive south, much of Southwest Alaska
will likely see clearing skies as we head into Friday. With
clearer skies and warm air aloft, diurnal heating seems likely to
push Friday's highs into the high 50s and low 60s.

For the Western and Central Aleutians/Bering, light precipitation
will taper off tonight as a low currently near Shemya dives
southeastward and weakens. Then, a surface ridge will likely bring
lighter winds and marine stratus through Wednesday night. For Thursday,
a front moves into the Western Aleutians/Bering before shearing
apart. With Thursday's front weakening more than previously
expected, precipitation chances and amounts have been dropped from
the previous forecast package. By Friday afternoon, an incoming
shortwave will bring in a tropical plume of moisture and help
reinvigorate the front.

&&

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

Beyond Saturday, general model consensus suggests an unsettled
pattern for Southcentral, and an active pattern for the Aleutians
and Southwest which host the passage of several more low pressure
systems. Synoptically features as just discussed have fair model
agreement with some discrepancies regarding the precise timing of
shortwaves leading to moderate forecast confidence. One such
source of model disagreement lies in timing the passage of
easterly waves coming into Southcentral over the weekend.
Additionally, the wind fields represented by the models have very
little agreement over Southcentral and the northern Gulf of
Alaska. It is expected generally that Southcentral will be seeing
an unsettled pattern with the potential for precipitation across
the northern Gulf of Alaska coastline. Features over Southwest
Alaska and the Bering are larger more synoptic features, and the
agreement on system progression there is stronger.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
LONG TERM...CL