Gale Warning 785, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
553 PM AKST Sat Dec 8 2018

.ANALYSIS AND UPPER LEVELS...
The upper level pattern continues to build colder air into the
region from the west under a trough which extends over southwest
Alaska into the Gulf of Alaska. This trough axis is the track the
storms are training along through the Gulf and into Prince William
Sound. The strong low from yesterday moved across Prince William
Sound this morning and will continue to weaken as it moves into
the interior of the state. Another low is right on its heels and
has moved into the northern Gulf of Alaska this afternoon. This
low should take a similar but slightly more eastward track
overnight.

Out west, the 250 mb jet stream is moving from west-to-east and is just
south of the Aleutians with a core around 160 kt in that area.
This is feeding a surface low that is in the western Aleutians and
elongating across the Aleutian Chain.

&&

.MODEL DISCUSSION...
Models are in good agreement in the overall synoptic pattern well
into next week and keep colder air aloft over mainland Alaska and
the storm track through the southern Bering Sea and into the Gulf
of Alaska with lows moving onshore along the north Gulf coast. That
is the good news. The problem is the details that drive important
forecast elements such as the timing of wind and precipitation
are fluctuating wildly with each model run. There is not a favored
model. Therefore forecasts are focusing more on the favored
weather in the general pattern so adjustments can be made as the
picture (hopefully) becomes more clear.

&&

.AVIATION...
PANC...With snow occurring much of the night, ceilings are likely
to be mostly MVFR with visibilities occasionally dropping to MVFR
as well. By Sunday morning ceilings and visibilities should
improve, but there is always the chance for some fog to develop
after the precipitation.

&&

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

The North Pacific parade of lows continues this evening as a once
strong low has moved inland while a second low is quickly moving
north toward eastern Prince William Sound. Rain and snow is
switching over to snow as colder air filters north up Cook Inlet.
Heavier snow will overspread higher elevation coastal areas like
Thompson Pass and Turnagain Pass as the second low moves inland
this evening. A deformation zone will form and enhance the already
ongoing snow showers, with ranges from 2-3" or so for Anchorage,
with up to 4-5" in the Matanuska Valley and Susitna Valley and
western Copper Basin, by Sunday morning.

A cold upper trough will move through Southcentral tonight with
much colder air moving into the region as 850 hpa temps drop to -8
to -10 C. Flurries and snow showers will still be possible
tomorrow given the cyclonic flow aloft and the lingering moisture.
The next gale force front will quickly move into the Gulf of
Alaska Sunday afternoon, then move onshore by Sunday night
bringing mostly snow to the coast although some locations will
switch to rain/snow. The orientation of the front with strong
easterly flow and upper level shortwave trough elongating and
shearing will generally keep interior locations heavily
downsloped.

Another low in the parade of lows moves into the Gulf of Alaska
Monday, but uncertainty grows a lot with this system, both with
the strength of winds and the chances for inland precipitation.
With that said, it does look like inland snow chances will again
ramp up late Monday evening into Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Sunday
through Tuesday)...
A weak surface ridge is still anticipated to work through
southwest Alaska tonight. This will calm the atmosphere enough to
allow the possibility of some patchy fog to develop in low lying
areas along the coast.

Tomorrow, the outer reaches of the broad low moving through the
Bering Sea will arrive at the coast. The front moving inland from
this low will quickly fall apart as it pushes over land. However,
it will bring enough moisture and energy with it to drop 1 to 3
inches of snow, mainly from Dillingham and areas inland from
there. The possibility exists of an inch or two more of snow on
the eastern slopes of the Kuskokwim Mountains due to upslope flow
there. Snowfall totals for this storm are not expected to be close
to advisory level. It should also be noted that though this
system is bringing some relatively warmer air with it, temps will
stay cool enough for the precipitation type to remain snow.

As far as winds with this system go, impacts are expected to remain
along the coast. As previously mentioned, the front is expected
to quickly weaken, so its initial impact will present the only
time where winds will really be a factor. Winds will be 30 mph at
most, which may produce some blowing snow right along the coast.

Beyond Sunday, a disorganized area of precipitation develops as
the low pressure center will approach southwest Alaska, then stall
out. Precip will slowly taper off through early next week. For
now did more of a broad brush on precipitation as specific areas
are still a bit difficult to nail down.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Sunday
through Tuesday)...
The low we have been advertising the past few days has made its
arrival in the western Aleutians. The center is currently spinning
north of Attu Island. Scatterometer (satellite derived) winds have
picked up on a large swath of gale force, westerly winds entering
the south side of the Aleutians. There is also an area of storm
force winds to the west of the Aleutians. This means the forecast
for high winds and seas is on track. Satellite also depicts an
elongated frontal system working its way through the central
Aleutians, also as forecast.

This low is still expected to track east over the next couple of
days. The frontal timing is still generally the same, though did
slow the progression of precip eastward by just a tad. As the
front pushes east, it stretches along an east-west axis quite a
bit, with multiple low centers possible. Models are currently
having quite a bit of trouble resolving the progression of this
elongation. So, quite a bit of uncertainty with with placement of
precipitation features past Sunday. Wind direction is also
slightly different between models, due to varying placement and
number of smaller low centers in this system. Stuck with the NAM
for winds but leaned GFS for precip on this resolution. The
Canadian was not considered as it had the most extreme solution.
Hopefully better agreement and detail will emerge in subsequent
model runs.

&&

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

The long term forecast begins Tuesday night with an active
weather pattern remaining across most of the forecast area. A
long-wave trough remains anchored over the Bering as a ridge
downstream moves east over the eastern Gulf while a second, more
elongated ridge sits over the northern Pacific. This pattern
favors a zonal flow across the Aleutians with a series of quick-
moving surface lows across the region and a persistent west to
southwest flow across the Gulf of Alaska, helping to steer the
surface lows from the western Gulf northeast through the northern
Gulf and up toward Prince William Sound.

The first of these systems is a weakening compact low situated
near Prince William Sound late Tuesday through Wednesday. This low
will continue to bring mixed rain and snow along the coast with
areas of light snow farther inland as an upper-level trough
swings across the southern mainland. Snow may linger across
interior Southcentral moving into Thursday, however, with both
the GFS and EC suggesting that the trough stalls, intensifies,
and becomes negatively tilted before lifting north. A stronger
shortwave and surface low undercutting this trough now looks to
track farther south across the Gulf late Wednesday into Thursday,
impacting areas east of Cordova and along the Alaska Peninsula.

The next system, the strongest of the bunch, is expected to track
along the Pacific side of the Aleutians Thursday into Friday and
enter the western Gulf near Kodiak Island late Friday into
Saturday. There is good agreement between the various guidance
solutions that this system will rapidly intensify into a 950mb
low with widespread precipitation and storm-force winds as it
moves south of the AKPEN and into the western Gulf. The low will
then stall near Kodiak Island and weaken slightly, but a strong
southeasterly flow will push plenty of moisture north along the
southern coast, with the GFS more bullish on the northern extent
of the precipitation compared to the EC. Colder air in place
should allow the onset of precipitation to be all snow. Warm air
advection in the mid and lower levels, however, should once again
make for a transition to rain for locations along the coast with
downslope winds limiting precipitation for the Anchorage Bowl.

Farther west, a trough extending north from this low will develop
into a secondary low pressure system near Bristol Bay. This
surface low will help draw the warmer air in the mid-levels across
all of southwest Alaska, bringing a snow to rain precipitation
event late Friday into Saturday. Across the Bering, a strong
northerly flow along the back side of the double-barrel low will
keep the Aleutians colder than normal with periods of snow
showers.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...119 125 132 150 155 165 170 172 173 174 175 176
				 178.
		 Storms..177.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...TM