Small Craft Advisory 7, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 PM AKST Mon Jan 2 2023

.ANALYSIS AND UPPER LEVELS...

A trough is well entrenched over Alaska, with a large cyclonic
circulation centered in the vicinity of the Alaska Peninsula.
Numerous weak to moderate short-waves and lows are embedded
within this circulation. One such short-wave is tracking
northwestward across Southcentral and westward into Southwest
Alaska. The bulk of precipitation is falling along the Gulf
coast and Prince William Sound regions and along the east side
of the western Alaska Range. Strong low level level flow and
accompanying downslope drying is preventing much precipitation
from making it over the mountains. Steady light precipitation is
falling across much of the Bristol Bay region thanks to a surface
low sitting nearby and the approaching short-wave. Showers can
be found back over the Central to eastern Bering Sea and Aleutians.
Warm air which moved onshore with yesterday's storm system in the
Gulf is leading to a precipitation-type of primarily rain along
the coast. Many inland areas of Southcentral are right on the
edge of rain and snow, with a prominent warm nose aloft seen in
the Anchorage sounding. Much colder air is sitting over the
Bering Sea where mostly snow is falling. Down over the northeast
Pacific a gale force low has formed ahead of a sharp short-wave
and is tracking northward toward the Gulf. This will be the next
weather maker for the Gulf and Southcentral on Tuesday.

Meanwhile, way out west a high amplitude low wave-length ridge
extends from the western Aleutians northwestward across the
western Bering Sea. A strong upstream trough is digging right
toward the ridge, with a leading occluded front tracking slowly
toward the western Aleutian chain.

&&

.MODEL DISCUSSION...

Models are generally struggling with handling of the many short-
waves embedded in the upper level flow along with extend of spill-
over precipitation along the lee of the coastal mountains. Thus,
the precipitation forecast will remain a challenge as long as this
pattern remains in place. Fortunately, most of the features are
weak and precipitation light and low impact. Guidance has
converged on a common solution, with a deeper low tracking
westward across the Gulf on Tuesday. This will result in some
large changes to the forecast for the Gulf waters and into
Southcentral. A very large spread in solutions develops with the
handling of a second low crossing the far eastern Gulf on
Wednesday. The remnants of this low could rotate westward into
Southcentral or remain east of the area. Chaos also reigns out
west, where a series of lows will impact the western to central
Bering and Aleutians in coming days. While no single storm looks
particularly strong or large in size, a cold and unstable airmass
will likely lead to small areas of strong winds and moderate to
heavy snow showers.

&&

.AVIATION...
PANC...A short-wave and inverted trough tracking west of Anchorage,
will combine with pressure rises to help bend some gusty Turnagain
Arm winds into the airport this afternoon and evening. The
arrival of a low in the Gulf will then switch winds back to light
northerly overnight. As the Gulf low passes just south of the
Kenai Peninsula Tuesday evening, strong dynamical forcing combined
with a brief weakening of downslope flow along the front range of
the Chugach may be just enough to allow precipitation to reach
the ground in the form of light snow. While ceilings will lower
late Tuesday, expect VFR conditions to prevail.

&&

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

Multiple shortwaves rotating around a large, complex low in the
western Gulf of Alaska continue to traverse across Southcentral
Alaska this evening. Southeast flow aloft will continue to produce
upslope showers along the north Gulf coast, while downsloping in
the lee of the coastal mountains will keep inland locations mostly
dry through Tuesday. The western Gulf low will begin to move into
Southwest Alaska tonight, allowing for a brief period of low-
level southeasterly winds to develop this evening, with a
Turnagain Arm wind expected to brush Anchorage, allowing
temperatures to warm above freezing for a time. Temperatures
should cool below freezing again tonight, most likely after
midnight when the southeasterly winds are forecast to diminish.
Some breaks in the clouds will aid this cooling across
Southcentral as the bulk of energy with the departing low pushes
into interior Alaska. Meanwhile, upslope showers will continue
along the north Gulf coast, with rain falling at sea level and
snow at elevation.

Then, the focus shifts to the next series of lows tracking into
the Gulf. Models are beginning to lock onto a solution with a low
entering the Gulf on Tuesday from the North Pacific and tracking
toward the Barren Islands Tuesday night. The low will bring
another round of heavier precipitation and stronger winds to the
north Gulf coast and northern Kodiak Island as its front arrives
Tuesday evening. Ahead of the low, down-inlet flow should allow
for a period of precipitation to cross the coastal mountains into
the Cook Inlet region. Currently expecting that this would fall as
a light snow, as the air mass associated with this low appears
colder than the current system. As the low begins to pass near the
Barren Islands into Kamishak Bay Tuesday night, pressure rises
behind the system will induce stronger winds into the Kenai
Peninsula and Prince William Sound and another round of
southeasterly gap winds. This could warm temperatures in the lee
of the mountains above freezing again, but by that point
precipitation is expected to have ended, with increased cross-
barrier flow leading to downsloping once again.

The disturbance following the Tuesday/Tue night low has more
uncertainty surrounding it, but its effects are expected to be
primarily coastal, from Prince William Sound eastward on
Wednesday. Going into Thursday, shower activity begins to trend
downward with drier conditions expected as ridging builds in at
low levels across Southcentral.

&&

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

A weakening low pressure system is crossing north of the Alaska
Peninsula (AKPEN) and into Bristol Bay. A series of shortwaves
pinwheeling around the low will maintain snow showers across
Southwest Alaska before tapering off mid-Tuesday. Any snow
accumulation will be generally light with 1-2 inches possible
between tonight and early Tuesday morning. Otherwise, most of the
snow showers will be favored for the upslope areas along the
Aleutian/Alaska Ranges and local mountains. After a brief break of
drier weather on Tuesday, the next storm system tracks across the
western Gulf and into the AKPEN. With decent upper level support
coupled with an inverted trough, another round of snow showers is
likely over the Bristol Bay region from late Tuesday to
Wednesday, especially areas encompassing Koliganek to Dillingham
to Egegik. Snow accumulation will be generally light again, with
higher amounts favored along the coastal mountains. The low is
expected to weaken and lift out of the area for drier conditions
across the mainland, while precipitation continues along the AKPEN
through Thursday.

&&

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

An elongated trough of low pressure extending from Kamchatka to
the western Bering Sea will be the main weather driver through
midweek. For tonight, a frontal boundary will bring a wintry mix
of precipitation and low end small craft winds across the western
and central Aleutians. The front is expected to weaken Tuesday
morning before the next front ahead of a North Pacific low
approaches early Wednesday. Model guidance continue to struggle
with the multiple low centers rotating around the aforementioned
trough. The current forecast package has high end small craft
winds and seas of 10-15 feet for the western Bering and Aleutians
between Wednesday and Thursday. Cold, northerly flow over the
relatively warm waters meanwhile will maintain widespread snow
showers for the rest of the Bering Sea and Aleutian Chain.

$$.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Deterministic guidance is in terrible agreement on any discernible
feature in the long range forecast as the region remains under the
influence of a broad longwave trough with many embedded
shortwaves extending from Kamchatka to Southeast Alaska. More of
an ensemble approach is needed in this scenario with only broad
weather characterizations able to be made at this time. Ridging
building over western Canada will promote storm system movement
into the Gulf, bringing continual warm air and above average
temperatures to Southcentral. Precipitation type will remain a
challenge across Southcentral as there is no good signal of cold
air moving back in aloft, supporting rain, mixed precip type or
wet snow at sea level locations. Most guidance indicates that the
storms will remain coastal, with continual precipitation for the
Mountains. For Southwest Alaska, slightly above to seasonal
temperatures appear likely as they will remain closer to the
colder side of the broad trough. Generally, the models keep the
region drier as well, with a few shortwave ridges moving through.
For the Bering, a large conglomerate of small, and occasionally
strong lows will meander across the chain, bringing below average
temperatures and snow showers.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 120 130 139 351.
         Heavy Freezing Spray Warning: 180 185.
FIRE WEATHER...NONE.

&&

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