Gale Warning 10, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
416 PM AKST Wed Jan 11 2023

.ANALYSIS AND UPPER LEVELS...

The subtropical jet remains well south of Alaska, serving to keep
the best dynamic forcing over the Pacific and deep lows generally
along the southern periphery of the the forecast area. A look at
satellite imagery this morning shows two very large upper lows,
one just south of the western Aleutians and a second in the
northeast Pacific. A deep occluding low is drifting northward
toward the western Aleutian chain. A leading frontal system is
lifting northward across the western to central Aleutians and
into the southern Bering Sea, producing gale to storm force winds
and moderate precipitation. The airmass just ahead of the occluded
front is cold enough to produce all snow, with Shemya observing
snow and blowing snow this morning. Marginally warmer air is
advecting northward behind the front leading to a transition in
precipitation type from snow to rain. The northeast Pacific low
is not affecting Alaska at the moment, but with a strong
amplified ridge sitting downstream, short-waves east of the low
center are being steered northward toward the eastern Gulf of
Alaska. These will play a role in the forecast for later this
week.

To the north, a vertically stacked low is nearly stationary over
the northern Bering Sea, with two out of phase short-wave troughs
extending southward across the eastern Bering Sea. These troughs
are tracking eastward toward Southwest Alaska along with a low
level cold front. Areas of very light snow extend northward from
the Alaska Peninsula across Bristol Bay and into the Kuskokwim
Valley. Farther east, a short-wave is lifting northward across
Southcentral with the primary vorticity-maxima crossing the Copper
River Basin. This is producing areas of light precipitation,
primarily from eastern Prince William Sound across the Copper
River Basin. A short-wave ridge is building in behind it, with
areas of low clouds and fog, especially in the Cook Inlet/western
Kenai Peninsula region.

&&

.MODEL DISCUSSION...

Models are in good agreement with large scale features through
Thursday. Solutions quickly diverge in the Gulf of Alaska with
track and strength of short-waves and corresponding surface lows.
The GFS is a particular outlier, with a much strong low crossing
the northern Gulf Friday. Will stick with the majority of
solutions which depict weaker lows tracking farther south. Out
west, the first significant difference pops up Saturday with a
developing low tracking out of the Pacific and toward the Alaska
Peninsula. The GFS is an outlier here as well with a faster
further north track. Will once again stick close to the majority
of solutions and ensemble guidance in keeping the low south of the
Alaska Peninsula. Otherwise, the main forecast challenge for the
short-term will be determining extent of low clouds and fog over
southern Alaska as we remain in an overall weak flow regime which
is favorable for these aviation hazards.

&&

.AVIATION...
PANC...Very weak low level east to southeast flow will persist
across the Kenai and Chugach Mountains through tonight. At the
same time, a short-wave ridge will move overhead, increasing what
are already favorable conditions for low clouds to persist on the
Cook Inlet region. The southeast flow could be just enough to keep
low clouds from settling over the terminal, though there will
certainly be some in the vicinity. A weak upper level trough will
approach from the west Thursday morning, bringing a chance of
light snow. If it does snow, expect something similar to what
occurred this morning, with a brief period of MVFR conditions.
Due to weak overall forcing and the presence of lower conditions
in Cook Inlet, overall forecast confidence is low.

&&

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

Most of the snow associated with a Bering Sea trough moving
eastward has ended across the Southcentral mainland. The exception
is some precipitation continuing along Prince William Sound (PWS)
before tapering off early Thursday. Fog is also expected to
persist for areas near Cook Inlet overnight tonight, especially
Anchor Point to Seldovia. As the aforementioned trough moves
further inland, there is a slight chance of snow for the western
Kenai Peninsula, Anchorage Bowl, and Matanuska Valley Thursday
afternoon. A period of snow is possible for the upper hillside
with snowfall amount less than an inch expected. Heading into the
weekend, a ridge of high pressure builds across the mainland for
drier conditions. Model guidance seem to struggle with a series of
upper level waves streaming northward across the Gulf. Unsettled
conditions looks likely to persist, bringing additional round of
rain and snow near the coast.

&&

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

An upper-level low moving north out of Southwest Alaska has left a
trailing weak frontal boundary, producing high clouds and on-and-
off snow showers across Southwest. This feature will shift
northeastward out of the area by Thursday morning. High pressure
will begin to build as these disturbances depart the region and
the previously active pattern will be replaced by one of stability.
Thursday morning, winds will remain generally weak and variable
in direction until Thursday evening when offshore flow develops.
This offshore flow is expected to hold steady until the early
weekend. Temperatures are expected to rise slightly along the
coast Thursday, and lower slightly inland. These temperatures are
not expected to significantly change further as we approach the
weekend. Areas of fog tonight into Thursday morning is expected
for coastal Southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

Currently, a col in the Eastern Bering has kept mostly benign
weather in that region, with the exception of a few snow showers.
The main story however is a North Pacific low pressure system
approaching the Bering Sea. A front along the leading edge of the
low breached the Chain this morning and is currently bringing
gale-force winds and widespread rain and snow to the
Western/Central Aleutians. This front will advance north into the
Bering and eastward along the Chain, reaching Atka early tonight
and Unalaska by late Thursday afternoon. Precipitation associated
with this front will begin as a wintry mix of rain and snow,
transitioning to just rain as the system brings in warmer air
behind the boundary. It is currently expected that the front will
continue advancing north and eastward beyond this point, however
the structural integrity of the frontal boundary will have
weakened significantly by Friday morning. As such, the Pribilofs
and Eastern Aleutian communities such as Cold Bay/Sand Point are
expected to get showers By Friday morning however confidence is
low on intensity and duration. An extended period of gusts, up to
40 mph, in Cold Bay are expected to begin Thursday afternoon with
flow channeling in from the Southeast.

The North Pacific low moving in behind the front is currently
expected to set-up and stall at the tip of the Western Aleutians.
This disturbance will promote unsettled conditions with widespread
precipitation likely across the Western/Central Aleutians that
will persist through the weekend. Winds will relatively calm within
the perimeter of the low compared to current wind speeds.
Precipitation type will likely begin as rain however then quickly
transition to a rain/snow mix as the warm air that was supplying
the low is cut off by colder air from the Bering.

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

A broad upper level trough extends over much of Southern Alaska
through Tuesday, bringing active weather for much of the long-term
period. Shortwaves rounding the base of this trough will lead to
warm and wet conditions for the Gulf of Alaska coast, from the
Alaska Peninsula/Kodiak Island in the west to Prince William Sound
in the east. The exact timing of precipitation and stronger winds
continues to be uncertain with these shortwaves, but forecast
confidence is slowly improving as models begin to hone in on these
features.

Compared to yesterday's long-term forecast, the overall
pattern seems to be advancing a bit more quickly. There also
seems to be improving model agreement that several surface lows
will move into the Gulf of Alaska, increasing confidence that the
Gulf coast will see stormy conditions towards the beginning of
next week. Diving into a bit more detail, there is increasing
consensus for a compact low to move towards Kodiak Island late
Saturday night into Sunday, bringing the potential for a rain/snow
mix along much of the Gulf coast. Precipitation looks to be
reinforced as a shortwave and its associated surface low move
north into the Gulf late Monday night into Tuesday morning. With
deterministic models suggesting this low will be fairly strong
(gale force, with central pressure as low as 965 mb), Eastern
Kenai Peninsula and Prince William Sound will likely see
considerable amounts of snow/rain, but interior portions of the
Southern Mainland seem to remain fairly dry. Conditions look to
improve into Wednesday as the low weakens.

In contrast, quieter weather is in store for the Bering Sea and
Aleutian Islands for the extended period. The main story for these
locations will be small craft to gale force winds along the
Aleutian Chain through Tuesday as a weak low lingers in the
region, along with snow showers and dropping temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 411
		 Gale Warning: 173-178, 412, 413
 		 Heavy Freezing Spray: 180, 185.
FIRE WEATHER...NONE.

&&

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