Storm Warning 177, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKST Wed Dec 14 2022

.ANALYSIS AND UPPER LEVELS...

The pattern remains amplified with a dome of high pressure south
of the Gulf of Alaska. The upper level trough currently extends
across the Bering/Aleutians. The jet core is over Southwest Alaska
with a magnitude of 150 knots. On satellite imagery there is a
deep fetch of moisture south of the Alaska Peninsula that extends
to the Central Pacific. This moisture fetch has been tracking
towards Anchorage. Early this morning, on the King Salmon radar
there was a light band of snow this morning near the Barren
Islands.

&&

.MODEL DISCUSSION...

Models are in decent agreement through 48 hrs. The GFS, Canadian
and NAM have initialized well this morning. The Anchorage Bowl and
much of Southcentral can expect another round of snow. There are
some minor differences with the end time of the snow. After this
snow event, then Southcentral will move into a colder and drier
pattern.

&&

.AVIATION...

PANC...Pacific moisture is headed to PANC. Conditions will
fluctuate between VFR and MVFR as ceilings and visibility will be
reduced at times due to snow and/or BR.

&&

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

There are a few differences with the front over the Gulf of
Alaska that is creeping northward today and will bring heavy
snowfall tonight as compared to yesterday. The biggest difference
is that it looks like the whole system is a little farther south
which will impact both expected sow amounts and precipitation
types. There was some thought yesterday to areas of mixed
precipitation (rain and snow) and pockets of freezing rain. This
looks less likely today with Anchorage, the Mat-Su and Copper
River Basin all staying firmly entrenched in snow as the
precipitation type. The tricky call will be with the potential for
some rain around Kachemak Bay today and tonight as well as
farther inland in the eastern Kenai Peninsula tonight. Southerly
winds crept into Kachemak Bay overnight last night which turned
precipitation to rain, but with northerly winds moving in this
morning it may turn back to snow again today. It may end up being
mostly rain, with some periods of snow mixed in.

After this system moves through the winds will turn to the north
across the area Thursday and clearing skies from west-to-east will
occur. This will usher in colder temperatures as well for Friday.

&&

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

Cloud cover and precipitation chances increase today as a plume
of moisture streams northward from a low in the North Pacific. A
Winter Weather Advisory remains in effect for Bristol Bay as snow
accumulations with this storm could climb into the double digits.
Snow amounts have dropped with this forecast package as
precipitation amounts are trending lower. The biggest challenge
with this system will be precipitation type, especially from King
Salmon south to about Pilot Point. As surface temperatures climb
near freezing, this area could see a wintry mix. Increasing
temperatures could lead to rain mixing with snow, which would lead
to lower snow amounts than forecast. Depending on how cold surface
temperatures are and how far inland warm air ekes aloft, we could
also see a brief period of freezing rain mixed with snow. As this
portion of the Alaska Peninsula is right at the cusp of where warm
air advection is expected, this continues to be difficult to
pinpoint. However, current thinking is that precipitation should
stay mostly snow.

Quieter weather will prevail for most of Southwest by Thursday
morning as this plume of moisture moves east into Southcentral and
higher pressure builds. Expect falling temperatures, decreasing
precipitation chances, and increasing winds as a front sets up
over the Eastern Bering Sea and offshore flow commences. The
exception will be the southern Alaska Peninsula, where
southeasterly flow associated with this front will lead to warmer
temperatures and lingering snow/rain chances.

&&

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

The big story for the Bering Sea and Aleutians through Friday
will be strong winds and freezing spray. The major players will be
a North Pacific low lingering south of the Eastern Aleutians and
a front pushing into the Western Bering and Aleutians. From today
through Thursday evening, the North Pacific low will induce an
area of northerly gales along the Central and Eastern Aleutians.
Cold, offshore flow from Southwest Alaska will lead to heavy
freezing spray in the Eastern Bering. At the same time, the front
over the Western Bering and Aleutians will bring a swath of
southerly gale to storm force winds until it weakens midday on
Thursday. By Thursday afternoon, these features will merge to
produce a broad area of southeasterly small craft winds across
much of the Bering Sea. Expect temperatures to warm into the high
30s and low 40s by Friday afternoon with this warm air advection.

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

A very high amplitude pattern is expected to take hold over the
AOR through the extended period. The main feature of note is a
high amplitude, negatively tilted ridge extending from the Pacific
Northwest to the Bering Strait. A trough will persist from the
Kamchatka Peninsula southeast through the Aleutian Chain. Overall
weather will trend from very active that we've seen this weekend
to cold and clear for much of the southern Mainland. This will be
a welcome respite for many communities across Southcentral;
however, with the strong ridging expect temperatures to be below
to well below average for Southcentral, with normal temperatures
over Southwest. There is potential for some strong winds through
gaps and passes as a strong thermal and pressure gradient builds
between the Copper River Basin and the Mat Valley to the west and
Valdez/Cordova areas to the south, but in the absence of a
surface low or strong upper level shortwave diving south or east,
they should remain a light to moderate drainage wind. Over the
Bering, where the warm air is expected to be, rain will be
intermittent throughout the Aleutian Chain as stationary fronts
and surfaces lows move in and out of the region.

&&

AVG Discussion:

Most of the upper level support from today's system will move
through this evening, leaving only upslope flow for continued
light snow on south and western facing slopes, particularly the
Hatcher Pass and Valdez/Cordova areas. There may be little to no
break in snow for these areas as the next system approaches.

The system moving into the area Wednesday is the result of the
interaction of a shortwave trough and upper low moving east from
the Bering and a stream of southerly moisture in the form of an
atmospheric river. The overall trend has been less phasing of the
Bering shortwave with the southerly moisture stream and it's
associated upper level low over the north Pacific. This has
resulted in a colder solution that is more progressive. However,
QPF and snow totals for Valdez and Cordova will be significant.
There is a high probability (>56%) of two inches of QPF and 2+
feet of snow falling over a 24 hour period from Wednesday
afternoon to Thursday afternoon for these areas. We area leaning
between 2.5 and 3 inches total QPF through Thursday afternoon.
Our forecast is for all snow to fall with this system for the
Valdez and Thomson Pass area, but there is a slight chance warm
air could move through the Copper River Basin and move into the
pass from the north. In the Port, southerly winds may also move
into the narrows and warm things up near town. This could
transition things to rain in the lower elevations and near road
level, but should this occur precipitation intensity could
alternatively keep things snow at above freezing temperatures.
Again, the chance of rain is slight but not zero.

For the western areas, southerly facing slopes/ranges will be
favored with up to 1.5 inches of QPF falling over the Wednesday
afternoon to Thursday afternoon timeframe. Winds will transition
from south-southeasterly to southwesterly through the event, with
gusts in the 25 to 35 mph range for most areas. Snow levels will
rise to around 1500 ft for the Kenai Mountains and possibly
through Eastern Turnagain Arm near the end of the event before
cold air advection on the backside moves in and returns snow
levels to near sea level. Generally speaking, other than a dusting
of lighter snow on the top, SLRs will decrease through the event.

-CJ

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 161 145 111 101 131 141 121 161.
MARINE...Heavy Freezing Spray 160 180. Storm 178. Gales 411 413
414 150 155 170 172 173 174 175 176 130 132.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
LONG TERM...CJ

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
950 AM AKST Wed Dec 14 2022

AKZ101-111-150000-
Anchorage-Matanuska Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, and Chickaloon
950 AM AKST Wed Dec 14 2022

...INTERMITTENT FREEZING DRIZZLE AND SNOW THROUGH THE EARLY
AFTERNOON...

Intermittent freezing drizzle and light snow is expected across
the Anchorage Bowl and Matanuska Valley through the early
afternoon hours. Slick roadway conditions are possible on paved
surfaces. A light glaze of ice and up to 0.5 inches of snow are
possible during this time.