Small Craft Advisory 157, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 PM AKST Sun Feb 20 2022

.ANALYSIS AND UPPER LEVELS...

The large upper level ridge continues to build over southwest and
Southcentral Alaska ahead of a deep surface low in the central
Bering Sea. This is pushing a lot of moisture and warm
temperatures into Southern Alaska. This moisture can be seen on
the CIMSS MIMIC satellite imagery caught up in the jet streak
that is running over and around the top of the ridge.

Clear skies yesterday evening allowed for the development of fog
over a good portion of Cook Inlet. The high-level clouds overnight
and then mid-level clouds today trapped the fog by keeping the
sun from being able to burn off the fog. Enough warming this
afternoon has lifted the fog to stratus, but it will be tough to
fully get rid of before the next system moves in tonight.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement, but uncertainty persists
with the the smaller scale features. The biggest question mark
is for Kenai northward through the Matanuska Valley where wind
direction will dictate temperatures, which will in turn impact the
precipitation type and amounts. At this time, the Southeasterly
winds shown on the NAM are expected to develop in the Anchorage
area. This will likely cause some cross-barrier flow with less
precipitation though it will mostly be rain. The GFS still wants
to keep northerly flow at the surface which would favor snow, but
this set up does not indicate that will be the case. Hi-resolution
models were likely overdoing the winds through Turnagain Arm in
previous model runs. The pattern suggests this and the fact that
they have been projecting less wind with progressive model runs
also indicates this is likely the case.

&&

.AVIATION...

PANC...With the fog and stratus now mostly cleared out of the
region, expect VFR conditions until precipitation moves back in
during the day Monday. Precipitation will start as snow, but the
low level airmass will warm through the day thanks to
strengtheningsoutheast downslope flow, so ultimately expect a
change to rain. Of course, surface temperatures will be slow to
rise, so there could be a brief period of freezing rain. The
strong southeast flow should keep ceilings/vis in the high end
MVFR or low end VFR category.

As for winds, expect low level wind shear to develop Monday
morning as persistent northerly winds at the surface combine
with strengthening southeasterly winds aloft. As a weak low moves
northward across Southwest Alaska Monday evening, conditions will
be favorable for Turnagain winds to bend into the terminal. While
the exact timing of these winds is uncertain, they are most
likely from 03Z onward Monday.

&&

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

An upper level ridge continues building over Southcentral
overnight. The axis of the upper level ridge is expected to pass
over Anchorage and the rest of Southcentral early Monday morning
before heights begin to fall as the next system approaches. There
will be some embedded weak upper level shortwaves in the flow
aloft that will continue to send some mid to high level clouds
through the area for the remainder of the daylight hours and
overnight. In between these, expect some sunshine till dawn and
then some moonlight overnight in between these upper level waves.
Expect temperatures area-wide to be in the low to mid 20s which is
normally typical of the daytime highs for this time of the year.
The Copper River Basin will be the coldest spot overnight with
single digits, though, high clouds moving overhead may prevent
temperatures from falling all that much. If the cloud cover is
just thick enough, temperatures could potentially fail to drop out
of the teens or hover around 10 degrees. Basically, tonight is
the quiet before the storm.

Heights begin to fall across Southcentral early Monday morning as
deep, moist, southwesterly flow develops from Kodiak Island north
all the way to the Susitna Valley and the northern Copper River
Basin. Precipitation will begin to overspread Kodiak Island
including Kodiak City early Monday morning with mainly rain
expected here. Moisture continues to stream northward with
precipitation overspreading the western Kenai Peninsula northward
to the Susitna Valley after daybreak Monday. The Susitna Valley
will remain mostly snow for this event where a Winter Storm Warning
is in effect from Monday afternoon through Tuesday morning for 8
to 16 inches of snow with the heaviest along the Parks Highway
from Talkeetna north. Snowfall rates are expected to increase
Monday afternoon and continue through Tuesday morning. A very
rigorous and strong upper level shortwave will move north along
the Aleutian/Alaska Range which will enhance the snowfall rates
late Monday afternoon through the overnight hours. Snowfall rates
may approach upwards of 2" per hour at times. The Kenai Peninsula
mainly from Kenai north to Nikiski may remain cold enough for
upwards of 6" or potentially more of snow but this still remains
uncertain. It all depends on just how much cold air is available.
Anchorage and the Mat Valley and interior Kenai Peninsula are
expected to be mostly rain with perhaps some snow. Places like
Valdez and Thompson Pass are expected to remain all snow for this
event with upwards of 12 inches expected. It still remains
uncertain for western Prince William Sound for places such as
Portage and Whittier on precipitation type. Current thinking is
mostly snow, but a degree or two could mean the difference
between rain versus snow.

Also, expect a Turnagain Arm wind to bend into Anchorage Monday
afternoon and continue through the day Tuesday resulting in gusts
as high as 40 mph in parts of town, while the upper hillside and
Turnagain Arm see gusts around 65 mph. The previous High Wind
Watch has been cancelled, as the pattern does not support much
stronger winds than this.

The upper level shortwave trough passes through Southcentral
Tuesday morning ending precipitation from west to east. Quieter
weather is then expected Tuesday afternoon through Wednesday
before the next potential system later in the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3 - Tonight
through Wednesday afternoon)...

A low currently centered over the Bering Sea has a front extending
over Southwest, and precipitation is just starting to reach the
coast. The biggest forecast challenge with this front has been
precipitation type, especially across the Kuskokwim Delta Coast
where temperatures have hovered right around the freezing mark.
Current thinking is that as this front lifts inland, there will
be a mix of rain and snow, with some pockets of freezing rain
possible. Meanwhile across the interior portion of Bristol Bay,
precipitation will mainly be all rain as widespread southerly flow
in the mid-levels spread in warmer air through the evening. By
Monday morning, an upper level trough over the Bering digs
southward and picks up a developing low over the North Pacific.
This system will track northward over the Alaska Peninsula and
introduce more southerly flow, warm air, and precipitation.
Precipitation type with this second system presents another
challenge, though currently thinking most of Southwest will see
rain or a rain/snow mix at times. Farther north over the Kuskokwim
Delta, snow is more likely as colder air works its way in,
particularly Monday afternoon and evening, but not expecting much
in the way of accumulation, and any snow that does fall will
likely be the heavy, wet kind. Conditions improve briefly for
Tuesday as an upper level ridge passes overhead. By Wednesday,
another front nears the Southwest coast bringing yet another round
of warm southerly flow. This time the orientation of the front is
from the northwest to the southeast, and gusty southeasterly
winds are likely especially along the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3 -
Tonight through Wednesday afternoon)...

A 975 mb low centered over the Bering Sea is gradually weakening
and lifting toward the northeastern Bering. Colder air wrapping
around the backside of this low brought gusty winds to the
Aleutians, particularly Unalaska Island earlier this afternoon.
As this low moves away, the western Aleutians will see a mix of
rain and snow showers during the overnight hours tonight. By
Monday, residual precipitation will move over the Pribilofs
bringing a chance for snow showers, as the colder air wraps in.
All attention then shifts to an incoming front over the western
Aleutians Monday night. This elongated front associated with a
Kamchatka low will move westward across the Chain before stalling
over the Central Aleutians and Pribilofs Tuesday morning.
Meanwhile, another developing North Pacific low lifts northward
over the eastern Aleutians Tuesday night, bringing warm air and
gusty southerly flow. Expect abundant moisture across the eastern
Aleutians and eastern Bering for Wednesday as an embedded upper
level shortwave enhances precipitation. A broad area of low
pressure remains parked over the Bering, keeping the unsettled
weather pattern in place through Wednesday night.

&&

.Marine (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska:
Southerly winds on Wednesday will become more southeasterly
throughout the day. Winds may increase through the Barren Islands
with the potential for some enhancement through Shelikof Strait. A
barrier jet is expected to set up along the Gulf coast beginning
Wednesday night that could last through at least Friday as a low
enters the Gulf on Thursday and tracks north. Seas will build on
Wednesday to 10 to 15 feet with the potential for higher waves
developing along the barrier jet extending from the eastern Gulf
to the Barren Islands during this duration. Another building of
seas in the open waters could occur on Friday up to 20 to 25 feet
with the transit of a low pressure system.

Bering Sea/Aleutians:
A couple fronts will cross through the Bering during the last half
of the week. As winds turn southeasterly on Wednesday, the
potential for gales with frontal passage exists along the eastern
Aleutians through the eastern Bering and along the coastal areas
of Bristol Bay northward along the Kuskokwim Delta and Nunivak
Island. An approaching low towards the western Aleutians could
increase southerly winds Thursday night and Friday night with the
passage of another low. Seas to about 20 feet on the Pacific side
on and 10 to 15 in portions of the Bering expected Wednesday. Seas
will remain elevated on the Pacific side through Friday.

&&

.Long Term Discussion (Days 4 through 7: Thursday through Sunday)...

Looking ahead to the extended forecast, the latest operational
model runs and ensemble means are in good agreement with the
synoptic scale features in the short term, but begin to diverge
toward the end of the period. A broad ridge extends northwestward
from the eastern Pacific to western North America and building
through the Arctic. This will set the stage for a persistent
southerly flow pattern with embedded upper level waves streaming
north into the Gulf and Southcentral. From Thursday to Saturday,
models are keying on two distinct shortwaves and associated
surface lows with conditions being warmer and wetter than normal
along the southern Alaska coast and parts of the interior valleys.
With the exception of high elevation snow, precipitation type
will be mostly rain as a warmer airmass stays in place. Additionally,
depending on the strength of the cross-barrier flow across the
Chugach Mountains, precipitation may be limited in the Western
Kenai Peninsula, Anchorage, and the Matanuska Valley. Out west,
a deep longwave trough over the Bering Sea and Northwest Pacific
will maintain a cool and wet pattern for the Aleutian Islands
through the end of the weekend with minimal impact expected.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 145.
MARINE...Gale Warning 119 125 130 131 141 165 170 172 173 174 175
                      176 177 178 179 180 181 185.
         Heavy Freezing Spray Warning 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...MV
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...VR