Small Craft Advisory 882, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 AM AKST Sat Nov 10 2018

.ANALYSIS AND UPPER LEVELS...

Relatively quiet weather currently will be quickly coming to an
end later this morning as a complex of three separate low pressure
systems begin to merge eventually into a single deep low. The
strongest low, which will become the primary one, is deepening as
it races northward out of the Pacific in the general direction of
Kodiak Island and the Alaska Peninsula. The low in the middle is
the weakest one, and is passing almost right over Dutch Harbor
this morning. The area in between the two lows is getting
southerly winds off the Pacific leading to gusty winds and low
clouds across the Alaska Peninsula, with the high clouds beginning
to move over Southcentral. The third low is over the central
Bering, and its contribution will be to drag much colder air from
out of Russia southward. This interaction of warm air and moisture
from the North Pacific low and the cold air over the western
Bering will become ample fuel for the newly combined low. More
impacts from the low can be found in the regional discussions
below.

Some localized areas of Anchorage, Palmer, Kenai, and Glennallen
are dealing with fog this morning. This fog will cause locally
reduced visibilities and briefly lower ceilings in these areas. An
aspect to the potential hazardous winter weather across portions
of Southern Alaska is a general lack of cold air most anywhere in
the area, as most interior locations are in the 20s to low 30s.
The few locations that are in the teens are widely scattered apart
from each other. All of mainland Southern Alaska's soundings also
show a small nocturnal inversion, but otherwise very shallow and
limited amounts of cold air.

&&

.MODEL DISCUSSION...

Model disagreement today is very high concerning the track and
interactions of the various synoptic features affecting the area.
While all of the models show the three lows merging into one
strengthening low just off the Bering coast of Cold Bay by
tonight, their agreement essentially ends there. This is for a
multitude of reasons. How the low center interacts with the
infusion of colder air from out of Russia is the first issue that
needs to be resolved, as that will likely spell the end of the
westward movement of the low center somewhere around Dutch Harbor
by Sunday morning. At the same time a poorly resolved influx of
energy in the form of a triple point low is likely to be picked up
by the low and drawn northward into it by Sunday. This will in
turn induce a brief Fujiwhara effect where the two lows rotate
around each other. Some of the models, namely the NAM and
Canadian, eventually make this second low the stronger, while the
GFS/EC keep the initial low the stronger. Depending on which side
wins out, this will have ramifications as to how much warm air is
drawn northward into mainland Southern Alaska and where the
strongest winds will be. As far as impacts go, the EC/GFS solution
would be the preferred, as then the strongest winds will be out
over the Pacific and not barreling into Southwest Alaska.

In addition to the treatment of the low, another aspect of the
forecast inducing huge uncertainty initially will be how much warm
air is transported into Southcentral and Southwest Alaska, and how
quickly the rather small amount of cold air currently over the
area gets mixed out. This will ultimately decide the impacts as
far as: 1) freezing rain persists over the Copper River Basin 2)
how quickly the snow expected to cause blizzard conditions over
Thompson Pass becomes a heavy, wet snow that is much more
difficult to blow around, and 3) for nearly everyone in
Southcentral, when the changeover occurs from snow to rain.
Another feature of importance deals with how quickly strong
cross-barrier winds across the Chugach ramp up, which in turn
determines how much precipitation impacts Anchorage, the Mat-Su
Valleys, and the Kenai Peninsula get from this system. A similar
problem is over Southwest Alaska, as cross-barrier flow may have
impacts on precipitation type, which in turn will impact the
degree that blowing snow can occur over the Kuskokwim Delta and
how long any freezing rain can persist in the Lower Kuskokwim
Valley. Thus, numerous details have yet to be determined with any
certainty, and as such, forecast confidence is unusually low given
the imminent arrival of the precipitation and wind to
Southcentral and Southwest Alaska.

&&

.AVIATION...
PANC...An area of fog and low ceilings are currently impacting the
area. As northerly winds increase later this morning, that will
help move the fog away. Tonight, a front moving into the area will
cause scattered snow showers early, which will become rain
showers later in the night as temperatures rise. While not yet
explicitly mentioned in the TAF, as the magnitudes are marginal,
there could be wind shear concerns as the northerly winds persist
at the surface while east to southeast winds increase aloft from
06Z Sunday on.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Ridging remains over Southcentral Alaska but a new system is on
its way from the south. This new system is pulling up a long
stream of warm air and moisture from the Pacific that looks to
raise temps throughout Southcentral Alaska as well as bring an
extended period of precip to areas along the north Gulf Coast.
The front from this system will move through Kodiak Island today
and make it to the north Gulf coast this evening. Precip for most
locations should start as snow but will transition over to rain
due to the influx of warm air that will be pulled up from the
south. Areas at higher elevations and places further inland will
hold on to snow longer before they transition allowing for higher
snow accumulations for places in Thompson Pass and southern
portions of the Copper River Basin.

There is also a chance for the warm air to overrun colder air at
the surface resulting in periods of freezing rain. The most likely
area for this to happen is southeastern portions of the Copper
River Basin where a Winter Weather Advisory is out for snow and
freezing rain tonight into Sunday morning.

This system will also cause gap winds for locations along the
north Gulf coast and Turnagain Arm. One of those locations will be
Thompson Pass and the winds combined with the snow tonight will
result in blizzard conditions through the Pass.

The pattern of warm air and rain over Southcentral Alaska will
continue through Monday with several more shortwaves reinforcing
the pattern as they approach from the south. The majority of the
precip through the weekend and into Monday will be along the north
Gulf coast with only occasional lighter precip making it very far
past the coastal mountains for much of Southcentral Alaska.
Precip looks to diminish Monday night as the pattern shifts.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
A very challenging forecast is the store for the area through the
weekend. 3 separate areas of low pressure will merge into one
broad low complex later today just south of the Alaska Peninsula.
As they do so, they will launch several rounds of moisture from
south to north over SW AK. The first will hold just offshore
through the day today, so most places can expect to stay dry
through daylight hours. However, as the low center moves north by
this afternoon, there will be a marked increase in gusty offshore
winds. This should help to pull some more cold air into the area
from the interior portions of the state.

The low center will cross over the Alaska Peninsula this evening
into early Sun. This will allow for continued gusty offshore winds
at the surface. However, just above the surface, east to southeast
winds will be predominant. Thus, aviation interests should be
advised of the potential for some moderate to strong wind shear.
As the next round of precipitation slides into Bristol Bay,
there will be enough cold air initially for snow for most
locations. However, this system is also packing a healthy amount
of warm air, so while 850 mb (5000') temperatures are nearly -6C
on the King Salmon sounding this morning, they will jump to nearly
4-5C above freezing by Sun morning. This should allow most
locations across souther Bristol Bay (basically south of
Koliganek) to turn over to rain by late Sun. However, and this is
where the forecast becomes challenging, is how much cold air can
hang on across the northern portion of SW AK (the Lower Kuskokwim
Valley and Delta). It will be a battle of how much cold air the
northeast surface winds can reinforce versus how much the warm air
advection turns the precipitation type over to more rain. That
said, the primary concerns remain blowing snow potential along the
coast of the Delta and the possibility for long-duration freezing
rain over interior portions of the Lower Kuskokwim Valley
(Sleetmute, Sparrevohn). For now, the forecast remains largely
unchanged. We will have to closely monitor how cold temperatures
stay through the day today as the winds develop. The cold air in
place is not overly deep and we are still early in the winter, so
thinking the warm air aloft could easily displace it.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
Sun)...
The Bering will remain quite active with several small lows all
merging into one stronger low through the weekend. To start the
day, there are 2 tightly wrapped lows traversing from north to
south over the Western Bering. Each of these has a core of near
storm-force winds. They will move quickly through the Western
Aleutians through the day today bringing squall conditions with
potent snow/rain showers and gusty winds. Another weaker low is
bringing warmer air and rain to areas from Dutch Harbor eastward.

All of this energy will combine into one much stronger low as
we move towards Sun. By Sat evening, the low will move across the
Eastern Aleutians as it dips into the low 980 mb range. It will
have plenty of warm air on its eastern flank to produce mostly
rain there. But to the west of the system, there is a good amount
of cold air with 850 mb (5000') temperatures hovering near -11C.
As this system taps into this colder air it will continue to
deepen down into the mid 970 mb range as it pivots south of the
Pribilof Islands. Expect a wide swatch of storm-force north to
northeast winds to develop across the Central Bering in response
to this. The system will bring a rain and snow mix to much of the
area as well as some warmer air starts to replace the antecedent
cold air in place. The broad low complex will remain anchored near
the Eastern Aleutians/Alaska Peninsula to start next week.

&&

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

The long-term forecast period commences with an active pattern
Monday featuring a large low centered in the Eastern Aleutians
with multiple embedded shortwaves. The most prominent being an
impulse along a surface gradient in the western Gulf of Alaska.
There are several things that stand out with this pattern. One...a
deep fetch of moisture will get steered into Southcentral.
Two...this is a warm wet airmass with tropical origins, so expect
snow in the mountains and either rain or some type of mix in the
lower elevations. Three...the pressure gradient is tight, so
expect winds. The heftiest amounts of QPF are anticipated for the
Prince William Sound area for this event but the dynamics are such
that the Anchorage Bowl can also expect precipitation with
confidence. The longwave trough that is supporting this stormy
pattern lingers throughout the long-term period, resulting in
broad cyclonic flow for much of the Bering too.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131.
Winter Weather Advisory 141.
MARINE...Storm Warning 119 130 131 138 150 160 165 173 174 175 176
179 180 412 413 414.
Gale Warning 120 132 136 137 139 140 155 170 177 178 181 185 351
352 411.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...DWK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...PJS