Small Craft Advisory 996, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
828 AM AKST Tue Dec 24 2019

.ANALYSIS AND UPPER LEVELS...

An upper level trough west of Adak along with a strong upper level
jet continue to strengthen a surface low south of Atka, bringing
rain, snow and gusty winds to the Aleutians as it quickly tracks
eastward. A weakening low south of the Kenai Peninsula is gradually
moving northward and will lead to accumulating snow across
Southcentral today.

&&

.MODEL DISCUSSION...

Models are in good agreement regarding the deep North Pacific low
tracking south of the Aleutians. It will move very rapidly across
the Chain and as such, although the precipitation type is still
somewhat uncertain, any significant snow/blowing snow on its
backside should be short-lived.

Latest runs of both large and mesoscale models are coming into a
consensus, indicating snow generally beginning in the afternoon
across Anchorage and continuing through the evening. As the snow
will be convective in nature, amounts could vary across town,
though a general 3 to 5 inches is possible. Similar totals are
expected over the Mat-Su, with amounts diminishing from east to
west over the Kenai Peninsula.

&&

.AVIATION...

PANC...VFR conditions will persist through this morning. Snow
will begin this afternoon dropping ceilings and/or visibilities
to IFR conditions. Snow will taper off this evening, though MVFR
ceilings may continue into Christmas morning. Light winds will
persist.

&&

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

A strong negatively tilted trough heads into Southcentral late
this afternoon and moves to over the Copper River Basin late
tonight. This is a little slower than previous runs and will tend
to delay the heavier snow potential to Southcentral. A strong
surface low associated with the upper trough heads into western
Prince William Sound by early evening then dissipates overnight.
Snow will likely spread across the Anchorage Bowl to MAT-SU
Valleys corridor later this afternoon with steadier and
potentially moderate snowfall intensity concentrated toward toward
the evening hours. Snow should taper off in the aforementioned
areas by midnight with steadier snow shifting toward the Copper
River Basin. Coastal locations along the north Gulf will have a
mix or rain and snow.

On Wednesday, a fairly strong low center and its associated
occluded front head into the northern Gulf late in the day.
Kodiak Island will likely see precipitation start as snow early
Wednesday morning and then transition to rain or a mix before
shifting back to snow Wednesday evening as the low passes to the
east.The brunt of the precipitation with this system at this time
looks like it will remain along the coasts/eastern Kenai
Peninsula with lighter amounts inland. This front will bring some
blowing snow potential to Portage Valley to Turnagain Pass late
Wednesday afternoon through evening. The front as it progresses
north Wednesday will also produce widespread gales to storm storm
force winds to the northern and western Gulf marine areas.

&&

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

Lingering snow showers exist across southwest Alaska, remnants of
a low pressure system moving through southcentral AK. Cloud cover
and showers are moving off to the northeast, with clearing behind
them. Northerly flow is also beginning to set up, which is
setting the stage for plummeting temperatures the next couple of
days. The wind chill advisory for southwest Alaska is still in
effect as cold air from the interior still looks to be drawn down
the Kuskokwim Valley the next couple of days. The really cold
arrives later in the week, but winds will be strong enough the
next couple of days to drop wind chills below -40F.

&&

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

The latest round of low pressure is currently moving into the
central Aleutians. Brief but intense snow squalls have been
passing through Adak and Atka. Satellite indicates these are
largely convective in nature(as a squall would be). They have
developed along a line of instability close to the low center,
which is currently just south of Adak and moving rapidly
northeast. A change in the forecast overnight is that this system
is moving faster than initially expected. With snow already
reducing visibility, activated the winter weather advisory early
to start now and end this afternoon. With this system moving so
quickly, it won't have much of a chance to produce heavy
precipitation over any given location for very long, so looks like
the most intense weather will be brief.

After this systems clears to the east, strong northerly winds with
cold air will be behind it. This should create gusty winds across
the Bering and through the island chain, with some embedded snow
showers.

&&

.MARINE (Days 3 through 5)...
(Thursday through Saturday)

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

Winds will increase on Thursday across the western and central
Aleutians as a rapidly deepening area of low pressure approaches
Adak from the south. Sustained middle to hi-end gale force winds
will develop near the island chain, with some storm force winds
possible on the Pacific side, accompanied by seas of 25 to 30
feet. On the Bering side, look for sustained gales and seas of 20
to 25 feet.

The storm then rapidly transitions east Friday and Saturday, as
it gets absorbed into a developing Gulf low, allowing for
improving conditions to commence.

...Gulf of Alaska...

An area of low pressure will move from near Kodiak Island to
Prince William Sound on Thursday, allowing for hi-end sustained
gales and seas of 15 to 25 feet to develop across the Gulf and
near the Barren Islands. These conditions will improve by Friday
afternoon as the system weakens and moves ashore. This will be
short-lived though, as another rapidly deepening area of low
pressure moves north into the Gulf late Friday night and into
Saturday, with sustained gales and an increasing sea state once
again possible.

&&

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

...Southcentral...

An active pattern with periods of snow for the end of the week. A
fairly strong low in the lower to mid 970 mb range in the
northern Gulf will be in a favorable position for a fairly
widespread snowfall to continue (develops Christmas eve
night/early Christmas morning) for most of Southcentral.

Snowfall then winds down by Friday morning, but, this will be
short-lived as another deepening area of low pressure moves into
the Gulf. Where this low goes is still up for debate. The European
model (ECMWF) takes the parent low towards Yakutat, with another
weaker low near Hinchinbrook Island, as an inverted trough
extends back. This solution keeps most of the snowfall in the
Copper Basin and Valdez area.

Meanwhile, the GFS Model (American) tracks the parent low much
farther west bringing the center into the Valdez area, placing
more of the western Kenai into the Anchorage bowl and Matanuska
Valley (perhaps southeastern Susitna as well) into the deformation
snowfall. The Canadian Global takes a middle ground approach but
is closer to the Euro solution. Thus, it looks like most of the
steadier snowfall for Saturday will be confined to the Copper
Basin, with some very light snowfall perhaps extending back to the
eastern slopes of the Chugach Mountains.

Then on Sunday, a third storm system moves into the Alaska
Peninsula, with a warm front approaching the Southcentral Coast,
bringing more snowfall potential as an over-running sets up.

As for temperatures, some colder air does make it into
Southcentral as each storm helps draw down colder air via
northeast winds as multiple lows pass mainly to our south and
east. However, the models have backed off the amount of cold
advection, as 850 mb temps around -15 deg. C are expected compared
to -30 deg. C yesterday. Thus, although it'll still be quite
chilly this weekend, it's not looking as cold as it was yesterday.

...Southwest Alaska through the Bering Sea and Aleutians...

Very cold conditions are still anticipated across southwestern
Alaska through the Pribilof Islands, as cold air advects into the
region from central portions of the state. Here, 850 mb temps
dropping into the -20 to -30 deg. C. range are expected, with the
coldest temps located across the southwest mainland. A
northeasterly offshore flow will keep this region mainly dry,
while snow squalls can be expected for much of the Bering and
Aleutians, as the colder air moves over the warmer sea surface
temperatures, creating sufficient instability for them to develop.

For the Aleutians, a pair of lows will approach the chain while
staying south of there. This places the region in a favorable
over-running setup as elevated warm fronts attempt to approach the
coast. This warm advection helps moderate temperatures some from
Dutch Harbor east, while helping extend the colder air from the
Pribilof Islands across the western half of the region. In either
case though, precipitation will be fairly widespread as each low
approaches.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...PD

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1042 AM AKST Tue Dec 24 2019

AKZ101-111-121-125-131-141-145-250900-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Northeast Prince William Sound-
Copper River Basin-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass, Valdez,
Thompson Pass, Glennallen, Eureka, McCarthy, Paxson, Slana,
Talkeetna, Willow, and Cantwell
1042 AM AKST Tue Dec 24 2019

...ACTIVE AND SNOWY WEATHER PATTERN BEGINS IN SOUTHCENTRAL...

Multiple rounds of snow are expected for much of Southcentral into
the weekend, as a series of fronts and lows track northward
through the Gulf and into Prince William Sound. Warmer air along
the coast may allow for periods of rain for areas such as Cordova.
Elsewhere, snow is expected with each of these lows and fronts.

The first round of snow is already making its way into
Southcentral and will continue into tonight.

The second round will be with a low that moves into Prince
William Sound Wednesday night into Thursday morning. A few inches
of snow are possible with this low for Anchorage, the Matanuska
Valley, and the Copper River Basin. Much higher amounts are
expected for Valdez and Thompson Pass, as the snow lingers through
at least Thursday night.

Round three will begin a series of lows that move into the Prince
William Sound region Friday afternoon into next week. This round
may see snow amounts measured in feet in the mountains around
Thompson Pass and Turnagain Pass.

A result of this more snowy pattern will be that temperatures will
be more mild (but below freezing) and not as bitterly cold as
previously expected.