Small Craft Advisory 81, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKST Wed Jan 25 2023

.ANALYSIS AND UPPER LEVELS...

The upper-air pattern continues to be amplified as a deep upper-level
low near Kamchatka pushes a storm-force front over the western
Bering Sea and western Aleutians this morning. An upper-level
ridge of high pressure noses into the west-central Bering and
Aleutians downstream of the aforementioned upper low. Another
upper trough/low pressure extends southward from Northwest Alaska
deep into the tropical North Pacific. Embedded in this trough is a
complex low north of Cold Bay moving northeastward towards the
Bristol Bay area. Moisture continues to stream northward from the
tropical North Pacific, into the Gulf of Alaska, and Southcentral
Alaska. A surface low currently lifts northward from the
southwestern Gulf towards Prince William Sound as Whittier and
Cordova both report rain this morning.

&&

.MODEL DISCUSSION...

Guidance continues to show reasonably good agreement with synoptic
features, especially for the eastern domain. Guidance has all come
around to the GFS solution regarding the surface low forecast to
lift northward from the southwestern Gulf of Alaska and into
Prince William Sound (PWS). All guidance has slowed down and
depicts the low in PWS this morning into the early afternoon hours
before weakening and eventually dissipating inland during the
late afternoon. Yesterday, most guidance had this feature already
inland by this morning. After this feature moves on, guidance
strongly depicts a period of quieter weather for the east domain
as ridging works into Southcentral from the Gulf towards the later
half of the work week. Forecast confidence remains generally above
average for the east domain.

Out west, guidance continues to be strong in agreement with a
storm-force front impacting the western Bering Sea and Aleutians
this morning and moving eastward to Southwest Alaska as it
continues to weaken through Thursday night. The forecast with this
particular front will be when it reaches southwest Alaska when
precipitation-type will become a concern towards the end of the
work-week for Bristol Bay and the Kuskokwim Delta/Valley.

&&

.AVIATION...

PANC...Precipitation will arrive this morning as a low pushes from
the Gulf of Alaska into Prince William Sound. Downsloping will not
be an issue with this system as mid-level flow will be light and
with a slight northerly component. Therefore, while precipitation
should mostly be light in nature, MVFR conditions will be
possible, and even likely early in the TAF period with periods of
rain showers, possibly mixed with a little bit of snow through
this afternoon and early evening before precipitation tapers off
and VFR conditions return. There is still some uncertainty with
precipitation-type as the column of air above ground is still
quite warm. Any light precipitation should remain rain, while if
there are a few heavier bursts, the column can cool quicker and
perhaps provide for some snow to mix in with the rain. Southeast
Turnagain winds pick up through the remainder of the TAF period
starting early this evening as these winds should dry things out.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...

A potent upper level short-wave will track inland across Prince
William Sound and the Copper River Basin this morning, bringing
widespread precipitation to the region. With snow levels in excess
of 2000 feet above sea level, expect moderate rain in both Cordova
and Valdez, with heavy snow in the surrounding mountains. The
Copper River Basin will start out as mostly snow early this
morning, but warmer air moving in aloft with the upper wave will
lead to a transition from snow to rain across the southern Copper
River Valley this morning. A trailing surface low over the
northern Gulf will dissipate as it heads inland around midday.
The isallobaric max (area of max pressure rises) behind the low
will head right up the Copper River Valley, leading to gusty
southerly winds. This will further warm temperatures, with many
areas rising above freezing. However, expect the bulk of
precipitation to fall prior to the arrival of winds. Thus, expect
precipitation to fall primarily as snow from around Glennallen
northward. A Winter Weather Advisory remains in effect for the
Copper River Basin through this afternoon.

Meanwhile, precipitation rounding the north side of the surface
low will push westward over the Kenai, Chugach, and Talkeetna
Mountains. The airmass in place is quite warm, as is evident by
the 12Z Anchorage sounding, with a freezing level around 3100
feet. There is some dry air in the low levels, so the column
would cool as precipitation falls into it. Even then, it would
take heavy precipitation and melting of snow to cool the column
enough to change precipitation to all snow. Precipitation looks
light, so thinking any precipitation that makes it over the
mountains this morning will be primarily in the form of rain from
the western Kenai to Anchorage and the southern Mat-Su.
Temperatures will start out near or above freezing which is not
particularly favorable for freezing rain. However, water sitting
on ice and snow covered roads can sometimes be just as bad as
freezing rain.

A couple of upper level short-waves in the vicinity of the Alaska
Peninsula and Bristol Bay this morning, will track northeastward
into Cook Inlet, bringing cold air advection and strengthening
southerly winds. Despite the cooling aloft, the arrival of gusty
winds this afternoon will mix down the existing warm air above the
surface and cause temperatures to once again rise into the upper
30s to lower 40s. Additional periods of light rain are likely from
the western Kenai to Anchorage and the Mat-Su. The exception will
be the northern Susitna Valley where colder air and upslope flow
will bring another round of accumulating snow tonight through
about midday Thursday. The south to southwest low level flow is
also favorable for heavier snowfall in the Hatcher Pass area.
These short-waves will also bring another round of precipitation
to the Valdez/Cordova areas as well as the mountains surrounding
the Copper River Basin.

A building ridge ahead of an Aleutian low will move overhead
Thursday night, bringing an end to precipitation across the
Southcentral. Clouds will diminish, though with low level flow
going slack, low clouds will likely linger in the Cook Inlet
region and Copper River Basin. With a colder airmass in place,
temperatures will drop well below freezing region-wide. A warm
front lifting up the west side of the upper ridge will likely
bring some light rain to Kodiak Island Friday. Otherwise, expect
quiet conditions to persist across Southcentral.

&&

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

A lingering frontal boundary stretches across Southwest Alaska,
with remnant light snow showers persisting through the early
morning hours. Lower visibilities are also likely across the
Kuskokwim Delta from Bethel to Kipnuk into this morning as well.
Precipitation should diminish across the region into the evening
hours as high pressure builds over Southern Alaska. Conditions
become rather benign by Thursday morning, but will only last until
the next front moves against the Southwest Coast Thursday night.
Snow moves back in, beginning at the Kuskokwim Delta before
tracking eastward across the rest of Southwest Thursday night.
Showery conditions are then expected to continue into the
weekend. Warmer air moves onshore on Friday and temperatures are
expected to hover at or rise above freezing for most areas.
Periods of freezing rain or a mix of freezing rain/snow is
possible initially along the coast before pushing further inland
across the Kuskokwim Delta and Bristol Bay Friday evening.

&&

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

A front extending off a Kamchatka low sweeps across the Bering
Sea towards the Southwest Coast through the end of the week. Storm
force winds are likely in the western Bering but will decrease to
gale force as the front progresses. In addition to increased
winds, a band of precipitation and a swath of warmer air follows
the front. Precipitation will likely start as snow along the front
edge before quickly transitioning to rain or a wintry mix as
temperatures rise. The Pribilof Islands will be the main area of
concern as they will likely hold on to some colder air for a
period longer than the Aleutian Chain and blowing snow will likely
be an issue early Thursday morning. Snow should transition to
rain by late Thursday morning for the aforementioned area, but
winds will likely continue to be gusty into the afternoon hours.
Southerly winds out of bays and passes north of the Alaska
Peninsula and Eastern Aleutians will continue into Thursday night.

&&

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

An amplified pattern at the start of the weekend holds until the
the early workweek, when model discrepancies arise as the pattern
progresses. Model confidence regarding the synoptic flow for this
period is strong with lower confidence regarding timing and
intensity of upcoming trough passages. Saturday morning a ridge
will have built over the Gulf of Alaska maintaining an inactive
pattern through most of the weekend in Southcentral. Conversely,
in the Bering multiple troughs aim to make their way through the
Aleutians and Southwest Alaska. The first of which is a trough
that will have already made it to the Eastern Aleutians by
Saturday morning. The progression of this trough however shows
good model agreement that it will be blocked by the ridge out
East, will weaken, and ride the ridge up into Central Alaska by
Saturday late Saturday afternoon. Another front will enter the
Western Aleutians Saturday afternoon and advance eastward,
reaching Atka by Sunday afternoon. The northern edge of a weak
North Pacific low looks to supplement the disturbed pattern across
the Central and Eastern Aleutians for the remainder of the
weekend.

At this point the deterministic model solutions begin to
diverge. The 12Z GFS suggests a slower breakdown of the ridge,
holding off the northward progression of the North Pacific low and
stalling the front that was moving across the Aleutians at the
Central Aleutians and lose its structural integrity by Tuesday
morning. The 12Z Euro solution breaks down the ridge much faster
which allows for the progression of the front moving eastward
across the Chain to merge with the northward pushing North Pacific
low, creating a widespread precipitation event across Southern
Alaska Tuesday afternoon. These very different solutions balance
on a repetitious question for models, how resilient is the ridge?
Typically we see the breakdown of a ridge err on the side of a
slower breakdown. However, details will become more clear with
later model runs. Stay tuned to our latest forecast to obtain the
most up-to-date expectations.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 141, 195.
MARINE...Storm Warning: 178, 411.
         Heavy Freezing Spray Warning: 160, 179, 180, 185, 412,
                                       414.
         Gale Warning: 119, 165, 170, 172-177, 179-181, 185, 351,
                       352, 412-414.
FIRE WEATHER...NONE.

&&

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