Gale Warning 18, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
546 AM AKST Mon Jan 23 2023

.ANALYSIS AND UPPER LEVELS...

The active pattern remains status-quo as a broad upper trough
stretches from the Bering Sea, down to the North Pacific, and
eastward into the Gulf of Alaska. Meanwhile, an upper ridge sits
over Siberia with an upper low over Kamchatka. Within the broad
upper trough are a number of shortwaves eastward and northeastward
through the North Pacific and Gulf. One such such system, well
south of the Alaska Peninsula (AKPEN) and Aleutian Chain is
drawing moisture northeastward from the tropical Pacific and into
the Gulf as evident on Water Vapor imagery. Along with this
tropical connection also comes the advection warmer air. Middleton
Island radar reveals steady precipitation over the northern Gulf
stretching over the coast this morning while Kenai radar reveals
light returns over the western Kenai Peninsula and through Cook
Inlet. Anchorage and Soldotna both observe light snow and lower
visibilities while Cordova reports temperatures in the middle to
upper 30s and rain as shortwaves embedded in the trough continue
to advance northward.

Out west, strong northeasterly off-shore flow continues over
southwest Alaska and especially the Kuskokwim Delta. The pressure
gradient between a low over the AKPEN near Cold Bay and higher
pressure inland remains tight. With this setup, blowing snow
continues to be an issue with Bethel and Kipnuk both reporting
very low visibilities and winds gusting close to 45mph. Wind-
chills also continue to be quite low in the Kuskokwim Delta as
well with apparent temperatures close to -50F. A Blizzard Warning
continues to be in effect for the Kuskokwim Delta for the ongoing
ground blizzard conditions.

Meanwhile, the Bering Sea and Aleutians remain unsettled the
front form the low near Cold Bay pivots towards the Pribilof
Islands from the southeast. Saint Paul and Saint George both
report snow with reduced visibilities. Both locations have
northeasterly winds gusting towards 40mph. A Winter Weather
Advisory for blowing snow remains in effect for the Pribilof
Islands to address this. Dutch Harbor remains under the influence
of the North Pacific low over the far eastern Chain/far southern
AKPEN; they observe rain and light winds. The central and western
Aleutian Chain is between a flat and low amplitude ridge to the
west of Shemya and the low pressure over the southern AKPEN with
relatively benign weather. Gusty northwesterly winds near gale-
force are currently observed at Shemya along with snow showers.

&&

.MODEL DISCUSSION...

Model guidance is in reasonably good agreement with the larger
synoptic-scale features throughout the short-term period. All
guidance continues the generally active weather pattern throughout
the entire domain. There are some mesoscale differences in the
exact track and timing of shortwaves traversing through the broad
southwesterly to southerly flow ahead of a positively tilted
trough which moves slowly eastward from the Bering toward the Gulf
of Alaska through Wednesday night. Most 00z global guidance has
light snow developing across Cook Inlet Monday morning while hi-
res guidance does not have precipitation coming into Cook Inlet
and into Anchorage until late morning or even early afternoon. One
thing to keep an eye on is exactly when the wind direction changes
to south or southeast in the Anchorage region as this will impact
when precipitation-type changes from frozen to liquid. As of now,
most guidance has this change from light northerly winds to
southerly/southeasterly winds occurring sometime Monday evening
into the overnight. A similar setup occurs for Wednesday but there
is still uncertainty with precipitation-type transitions for the
eastern domain due to warm air intrusions and overall areal
extent of precipitation. Another key feature that all global
guidance is picking up on is an eastward moving gale to even
storm-force front in association with a low near Kamchatka. This
looks to potentially impact the western and Central Aleutians
through Wednesday night.

&&

.AVIATION...

PANC...Light snow continues this morning with MVFR conditions
favorable with periodic IFR conditions possible in steadier snow.
The atmospheric column will continue to warm as southerly flow
increases through the day. A period of wintry mix, including
freezing rain is possible late this morning into the afternoon as
warmer air continues to stream northward. Temperatures are then
expected to warm enough to change precipitation from mix to rain
sometime late this afternoon into the early evening hours. There
is still some uncertainty in the timing of precipitation-type
transitions. Aiding with the uncertainty is the introduction of
the Turnagain Arm wind. Most hi-res guidance has the wind bending
over the terminal between late this afternoon and early evening
and continuing through the TAF period; this will aid further in
warming of temperatures. However, if this wind is delayed, it is
possible that wintry mix could hold on and linger a little longer.
Precipitation looks to taper off towards the end of the TAF
period as stronger southeast flow in the middle levels of the
atmosphere looks to keep most precipitation out of the terminal.

&&

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

A surface low is moving near Prince William Sound with an
additional weak cyclonic circulation over the Kenai Peninsula this
morning on satellite. Weak bands of snowfall associated with this
are moving into the Soldotna area and northward into Anchorage
this morning. Precipitation near Whittier and Cordova is reporting
rain. Valdez and Glennallen are seeing snow.

An active pattern is expected through a least the first half of
this week. Successive waves lifting north into Southcentral will
bring additional warm air and the potential for precipitation to
become a wintry mix and even rain. The potential for freezing rain
cannot be ruled out amd any rain that falls onto cold surfaces
could may freeze. The transition looks to be possible this
afternoon for the Kenai Peninsula northward into Anchorage and the
Matanuska Valley. There is some uncertainty that remains in the
locations that would remain a little more sheltered from the
southerly winds, and thus warm air that initially pushes through.
By Tuesday morning, most of the aforementioned locations will have
transitioned to rain. Overall accumulations are expected to be
light. Deeper warmth up Cook Inlet, could cause locations up to
Talkeetna to become rain or rain/snow mix. This up inlet flow will
help to enhance precipitation along terrain into the Susitna
Valley where 8 to 20 inches of snow is possible along the Parks
Highway near Denali State Park and Broad Pass. Further east,
Valdez could also see a transition to rain later this afternoon or
evening. Warm air continuing to usher inland through midweek may
even have areas of the Copper River Basin seeing a mix with
rainfall.

&&

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

A strong surface high pressure centered over the Chukchi Sea in
tandem with an area of low pressure along the Aleutian Islands
continue to drive offshore winds across Southwest Alaska. Ground
blizzard conditions are beginning to show some signs of easing
this morning, but periods of quarter mile visibility are expected
to continue along the Kuskokwim Delta coast through at least this
afternoon. Winds will slowly diminish today, before more quickly
weakening by this evening. This should allow much of the blowing
snow to come to an end. Until then, wind chill values will
also continue to drop as low as -50F for the interior Kuskokwim
Delta.

Meanwhile, a weakening disturbance crossing the southern tip of
the Alaska Peninsula will push a front up the peninsula and into
the Bristol Bay region today. Along the AKPEN, from Pilot Point to
south of King Salmon, mixed precipitation is possible during the
daytime hours today as warm air aloft rides over cold northerly
flow at the surface. By this afternoon, light snow is the expected
precipitation type with the warmer air moderating as it pushes
further north. The snow will spread into the Lower Kuskokwim
Valley this evening and persist across the area through Tuesday
morning. By that point, the front shifts further north and west,
over the Kuskokwim Delta where light snow will combine with
diminishing (but still around 25 mph) winds creating the potential
for further low visibilities below one mile. The front lingers
over the Kuskokwim Delta into Wednesday morning as it gradually
weakens.

Broadly speaking, snow accumulations of 3 to 6 inches are
expected through Wednesday morning. Snow amounts have decreased
slightly with the front now likely to cover more area over
Southwest Alaska, rather than stalling over any particular spot.
More snow may be possible Alaska Range Tuesday night with a low
tracking into the western Gulf of Alaska, potentially bringing
another round of snow to the area. Otherwise, light snow continues
across Southwest Alaska into Wednesday night as the trough aloft
reorients and begins to push the weakening front back to the east.

&&

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

Gale-force northeasterlies continue across the Bering Sea north of
the Pribilof Islands, with extreme freezing spray expected along
portions of the ice edge. A stalled front will continue to bring
light snow across much of the southern Bering Sea. The Pribilof
Islands will continue to see potential for blowing snow, where an
advisory remains in effect, especially as snow intensity increases
today with the low center skirting the area. For the Aleutians, a
series of weak disturbances rotating as part of a complex low
will bring periods of rain to Unalaska through Cold Bay into
Tuesday. By Wednesday morning, a storm-force front pushes into the
western Aleutians, bringing a quick round of heavy rain through
Wednesday. As the front pushes to the central Bering/Aleutians,
cold, northerly flow begins to reverse to warm and southerly
across much of the Bering Sea by Wednesday night.

&&

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

Decent model agreement in the early extended period quickly
deteriorates by next weekend. As such, forecast confidence
Thursday and Friday remain mild, however by Saturday and Sunday
low confidence is a resultant of strong model disagreement. The
extended forecast period begins Thursday morning as a weakening
low and front in Southcentral lifts out of the region by Thursday
afternoon. A ridge of high pressure then begins to build over the
central Gulf of Alaska providing a break in active weather for
Southcentral Friday. Out west, lingering showers in Southwest
Thursday will diminish by the afternoon. In the Bering a broad
front advances steadily eastward providing consistent
precipitation as it goes, reaching the Alaska Peninsula by
Thursday evening.

Features discussed thus far has exhibited good model agreement.
However, by Friday afternoon significant discrepancies arise
between the deterministic model runs. Namely, the 12Z Euro model
keeps the axis of the trough positively tilted as it makes it to
the Alaska Peninsula. Then by Saturday morning, the Euro has the
ridge blocking the front from advancing further eastward, then
ejects the boundary through Southwest Alaska into the Interior
Alaska through Saturday afternoon and maintaining a benign pattern
across Southern Alaska with a slight trough lingering in the
Bering Sunday. The 12Z GFS solution is quite different, showing
the front advancing up the Alaska Peninsula Saturday morning, and
pushing the ridge out to the east. This lack of a blocking ridge
in the GFS solution then suggests a series of troughs and frontal
passages throughout Southern Alaska next weekend.

While these solutions are advertising very different outcomes by
the weekend, experience suggests that the GFS rapidly breaking
down the ridge is a bit too hasty, and we typically see a slower
deterioration of ridges in this region. However, details will
become more clear as we approach the time in contention. Regarding
this development, and as always, stay up to date with our latest
forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 155.
         Winter Weather Advisory: 145, 195.
MARINE...Heavy Freezing Spray Warning: 160, 179, 180, 185, 412, 414.
         Gale Warning: 176, 177, 185, 352, 411, 412, 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DAN
SOUTHCENTRAL ALASKA...VR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CQ
LONG TERM...CL