Small Craft Advisory 997, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 PM AKST Tue Dec 24 2019

.ANALYSIS AND UPPER LEVELS...

A negatively tilted trough extending from Western Alaska southeast
towards the southern Kenai Peninsula is providing the forcing
required for widespread snow across the Cook Inlet region this
afternoon. Snow reducing visibility to 1 mile at times has been
reported in Kenai, with a brief period of heavy snow earlier this
morning in Homer. Meanwhile the radar looks very convective for
the northern Gulf over to eastern Prince William Sound.
Temperatures have warmed above freezing in Cordova, making the
precipitation fall as rain. From Valdez over to Thompson Pass,
where the precipitation is expected to remain all snow, it is
increasing in intensity. For Southwest Alaska, the weather story
remains the very cold temperatures. 20 below was seen for several
hours this morning in Aniak, with Koliganek and points west
dropping below zero. Some lingering cloud cover remains towards
Iliamna, but that is clearing as the aforementioned trough swings
through. A developing low just south of the Aleutians is bringing
occasionally moderate snow with gusty winds to the western and
central Aleutians and a rain/snow mix in Dutch Harbor. Cold air
continues to pour southward across the northern Bering into the
Aleutians low.

&&

.MODEL DISCUSSION...

The overall large-scale synoptic pattern in the models remains in
good agreement, though the NAM is a clear outlier among the models
in snowfall amounts around Anchorage and the Mat-Su for the
ongoing snowfall. While many of the local-scale models are well
clustered in Anchorage getting a few inches of snow this evening,
the NAM suggests it mostly misses the area. However, the NAM is
clearly poorly initialized, not resolving the snow ongoing across
the Kenai Peninsula. Thus, that solution is being excluded.
Otherwise, a very active weather pattern over the next week
supports that forecast confidence will be below normal, even with
good model agreement, as slight differences in the track of the
upcoming lows will make large differences in the forecast.

&&

.AVIATION...
PANC...Snow will bring conditions down to IFR or LIFR levels into
the evening hours as an upper level wave tapping into Gulf
moisture brings snow to the area. It will gradually diminish
overnight, but snow showers remain possible well into tonight. The
next round of snow tomorrow could begin as early as midday,
though is more likely tomorrow evening and through the overnight.
Visibilities dropping down into the IFR category at times will be
possible once again.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Snow has commenced as a negative shortwave trough lifts northeast
across Southcentral. Out ahead of it, convective snow showers are
impacting the coast (mixed rain in Cordova) with heavy snow
through Thompson Pass. Moderate snow has pushed across the Kenai
and is now moving into interior Southcentral and will continue
through tonight with an additional 2-5" snow, heaviest from
Anchorage to the northern Susitna Valley. Expect lighter but
continuous snow in the Copper Basin through Wednesday. Cold air
advection wrapping around the back of this low will move from
the Barren Islands into Kachemak Bay, up Cook Inlet, and into
Prince William Sound.

Attention quickly turns to a fast moving low tracking into the
Gulf of Alaska Christmas Day. This occluded low will bring storm
force winds to the North Coast with strong northerly winds coming
down Cook Inlet in response to the strong 968 mb low south of the
Kenai Peninsula. Snow will also overrun this down inlet flow, and
with 30-45 MPH winds moving down the inlet, issued a winter
weather advisory for blowing snow along the western Kenai
Peninsula including the Homer Bluff area. Expect potentially
significantly reduced visibility and areas of drifting snow.
Elsewhere, widespread overrunning snow should impact most of
Southcentral Wednesday through Thursday. As the low moves east,
strong cold air advection ensues and will blast westerly and
northerly gap winds through the eastern Kenai, so issued a winter
weather advisory for blowing snow from Seward to Portage Valley
and Turnagain Pass. This event will be longer lasting than the
western Kenai advisory since the offshore winds and longer lasting
snow event will keep things going a bit longer. Cold air
advection dries things out Thursday evening with much cooler air
moving in. Only area where snow will linger is the Copper Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

An upper trough remains situated over western Alaska, with Arctic
high pressure over northern Alaska. The impending bitter cold
continues to be the main story in the forecast. Temperatures will
plummet as the upper trough digs southward and northerly flow
strengthens. A wind chill advisory remains in effect for tonight
through Wednesday evening for the Kuskokwim Delta, including
Bethel. Wind chills down to -50 are possible in the Kuskokwim
Delta. Temperatures will likely drop to 20 to 30 below zero in
the middle Kuskokwim Valley tonight, but surface winds should
stay below 10 mph. Meanwhile in the Greater Bristol Bay area
expect overnight lows tonight of 5 to 15 below zero. Expect cold,
dry weather and clear skies over most of the mainland Wednesday,
with a few exceptions. Areas along the north side of the AKPEN,
including King Salmon, Iliamna, Pilot Point and Port Heiden may
see some snow during the day as a low currently south of the
Aleutians tracks eastward towards the Gulf of Alaska.

Once the low progresses into the Gulf, cold air advection will
strengthen over the Southwest Mainland, and temperatures will drop
even further Thursday. That said, with a diminished pressure
gradient once the low clears to the east, this will no longer be a
favorable pattern for hazardous wind chills.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A low currently roughly 80 miles southeast of Seguam continues to
be the focus this evening. The low will continue to bring gales
to the southern Bering and central/eastern Aleutians into
tomorrow, with an embedded swath of storm force winds. In
addition, there is concern about snow impacts in the
central/eastern Aleutians up to the southern AKPEN. The winter
weather advisory for blowing snow in the central Aleutians has
been extended until late tonight. A deformation band has developed
on the back side of the low, which is causing precipitation to
persist longer than initially expected. Looking farther east,
we've issued winter weather advisories for blowing snow in the
eastern Aleutians up the AKPEN for tonight/ Wednesday morning.
This includes (but is not limited to) Unalaska, Akun, Cold Bay and
King Cove. Confidence is good in forecast wind speeds, but not
great with precipitation type. The track of the low so close to
the chain really presents a challenge, with very cold air blowing
south over the Bering (though SSTs continue to be over 40
degrees) and warm moist air being drawn in from the south. It's
possible that precipitation type will switch to and from snow over
the islands...but given some strong winds even a wet snow will
create visibility impacts and hazardous travel conditions.

The low will exit to the Gulf Wednesday afternoon, and in its wake
expect strong gap winds south of the eastern Aleutians/AKPEN. A
pair of lows approaches the Aleutian chain from the southwest by
Thursday, bringing another bout of gales and wet weather.

&&

.MARINE (Days 3 through 5)...
(Friday through Sunday)

...Bering Sea and Aleutian Coastal Waters...

An active pattern is in store for the Aleutians coastal waters,
with multiple areas of low pressure affecting them. The first
storm stalls south of Amchitka on Friday, with another rapidly
deepening low tracking from well south of Cold Bay towards Kodiak
Island. These two storms act in concert to spread sustained gales
across all of the Aleutian chain and into the southern half of the
Bering Sea. Seas will be in the 15 to 20 feet range on the Bering
Side and 20 to 25 feet on the Pacific side.

These winds and waves subside some for Saturday, before another
storm approaches the east central Aleutians from the Pacific,
while remaining well offshore. Northeasterly winds on the
backside of the low will spread sustain gales across all of the
Aleutian chain and the southern half of the Bering, with seas
building to near 20 feet across the southwestern bering and west
of Nikolski for the Aleutians. Some sustain storm force winds may
develop as well for the east central Aleutians, depending on the
track of the low (how close to the coast does it get and how
quickly does it deepen).

...Gulf of Alaska...

A trio of storms will move across the Gulf, with a storm on Friday
bringing widespread sustained gales to all of the Gulf and perhaps
localized storm force winds possible along the northern Gulf
Coast and near the low center south of Kodiak Island. Seas of 15
to 20 feet may develop for the outer waters. Then, a second storm
moves in quickly on its heels for Saturday, with sustain gales and
a similar sea state. A third storm again moves into the region,
with high-end sustained gales and perhaps sustained storms near
the coast, with an increasing sea state expected, perhaps in the
20 to 30 feet range for the offshore waters and 15 to 20 feet
closer to the coast.

&&

.LONG TERM FORECAST (Days 3 through 7)...
(Friday through Monday)

This looks to be possibly one of the most active pattern in many
years for the region, as the current model suite shows several
storms affecting the region in the extended period, and that's on
top of the first two moving through before it. These will bring
multiple rounds of accumulating snowfall to all of Southcentral,
with some pretty impressive totals possible for some coastal
locations if the current model projections don't change (we all
know they usually do, as the further you go out in time, the more
the model variability increases).

An important feature to watch in later model runs is an unusually
strong mid-level polar low off the North Slope that just sits
there while deepening to 477 dm. That in turn allows for a second
strong mid-level low on the order of 495 dm to take up residence
in the Bristol Bay region. These two systems then act in concert
to funnel very cold air from the Arctic into southwest Alaska,
with the Alaska Range keeping it just north and west of
Southcentral. This in turn allows for a third trough further south
in the north Pacific to feed off this funneling of colder air
aloft to help induce cyclogenesis (low pressure development), with
these storms intensifying as they track just along the baroclinic
zone aloft (just east of the trough axis), while the degree of
the cold air poised just north and west of the Alaska Range
prevents the storm track from moving further west or ingesting
warmer air from regions further south. This is because as the low
approaches, it does ingest colder air on the backside of each low,
as the air will be -30 deg. C at 850 mb (~4400 feet) across
southwest Alaska. This air modifies some, but stays cold enough to
allow any over-running precipitation to remain as snow.

So now on to the details. Our first storm (for the extended
period) moves from the north Pacific on Friday, to western Prince
William Sound Saturday morning. This storm will be in a deepening
phase, helping to draw abundant moisture northward.

Quickly on its heels is a second storm that moves from the
southwestern Gulf Saturday morning, to western Prince William by
late Saturday evening, with more abundant moisture spreading
northward.

Then on Sunday, another fairly strong low moves from Kodiak
Island, to yep...you guessed it...western Prince William Sound
with a more pronounced precipitation shield ahead of it, due to a
stronger warm front aloft.

A fourth storm in a deepening phase will then track into the
Alaska Peninsula on Monday, with more over-running precipitation
moving into the region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 121 125 181 185 187.
         Wind Chill Advisory 155.
         Flood Advisory 145.
MARINE...Storm Warning 119 130 170 173 174 176.
         Gale Warning 120 121 125 131 132 136 137 138 139 140
                      150 155 160 165 171 172 175 177 178 179
                      180 351 352 413 414.
         Heavy Freezing Spray Warning 160 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...PD

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