Gale Warning 777, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 AM AKST Thu Dec 6 2018

.ANALYSIS AND UPPER LEVELS...
A deep upper level trough covers most of the eastern Bering Sea
and western mainland, while an upper ridge is centered right over
the southeast Panhandle. The upper trough extends as far south as
30N as a strong upper jet taps into subtropical moisture, sending
it north into the Gulf of Alaska and Southcentral. General
southerly surface flow associated with this feature is keeping
warm moist air moving into the southern mainland with temperatures
well above normal in most locations. Radar imagery shows rain
showers moving north across the coast from Valdez to Cordova as
well as across the southern Kenai Peninsula. Further west, a very
weak and subtle surface trough is producing a narrow band of
precipitation over the western half of the Kuskokwim River Delta.
ASOS stations in these areas have been reporting mixed
precipitation, with some areas of freezing rain.

Over the Central Bering and Aleutians, an upper level ridge is
building in from the west leading to a brief period of tranquil
weather with light winds and scattered clouds. A front associated
with a low over Siberia is pushing east across the Western Bering
and Aleutians, shearing apart as it runs into the upstream ridge.
Strong post-frontal winds are ramping up over the Western
Aleutians as a 150 kt westerly jet streak pushes in south of the
island chain. Recent scatterometer passes show high end gales to
even low end storm force winds just to the south of Attu.

&&

.MODEL DISCUSSION...

Models have been struggling to sort out a series of lows in the
Gulf of Alaska today and tomorrow, differing with placement and
timing as the lows interact with each other. Overnight model runs
have begun to zero in on a solution so forecast confidence has
increased through Friday night. Heading into the weekend, models
are struggling with the details as another surface low swings up
into the Gulf from the North Pacific. Regardless, there is
increasing agreement in the synoptic level features through the
end of the weekend.

&&

.AVIATION...

PANC...Light winds and VFR conditions will prevail through the
day. Conditions should begin to deteriorate some by tomorrow night
as the next system moves into the gulf.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Friday night)...
This forecast is about as challenging as it gets, as a complex
series of surface lows and upper level waves move up from the
North Pacific. There is a large spread in numerical guidance,
along with poor run-to-run continuity and the environment over
much of Southcentral is right on the edge of rain and snow.
Thus forecast confidence is low.

Most model guidance is now picking up on a frontal wave tracking
northward across the Gulf to Prince William Sound early this
afternoon, then quickly inland late this afternoon. Precipitation
will spread westward out ahead of this low from eastern Prince
William Sound across the Copper River Basin and backing into the
Matanuska Valley and portions of Anchorage. The latest Anchorage
sounding shows a prominent above freezing warm nose in place,
indicative of rain or freezing rain. However, if you saturate the
sounding it would be right on the threshold between rain and
snow. Precipitation is most likely north of the Anchorage bowl up
to the Matanuska Valley where the airmass is likely a bit cooler
(away from the warming influence of Turnagain Arm winds). Thus,
expect whatever falls to be primarily in the form of snow. No
matter what, it does look like a quick shot of precipitation.
Elsewhere, precipitation will be rain along the coast and mostly
snow in the Copper River Basin.

Meanwhile, a weak vertically stacked low will lift northward
toward Kodiak Island, with rain continuing along a stationary
front out ahead of it, namely for Kodiak Island. More significantly,
a strong trough further south will become negatively tilted and
lift northward toward the Gulf tonight through Friday. This will
bring a reinforcing deeper low to Kodiak Island and spread rain
and strong winds across the Gulf to Prince William Sound. The
leading upper waves will likely bring another shot of precipitation
to interior portions of Southcentral this evening. Temperatures
across much of the Kenai Peninsula, Anchorage, and the Matanuska
valley will be at or above freezing by this point in time, so
freezing rain should not be a big problem (though there may be
some localized areas).

For Friday night and beyond there is very large uncertainty in
whether a frontal wave will develop into a full blown low or
whether another system will move up from the Pacific - both
scenarios being advertised by different models. Stay tuned as
we continue to work out the details of this very active weather
pattern.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Thu and Fri)...
The area will be split by two weather patterns to start the period
before cold air spills over the whole region by the weekend.
First, a couple of weak shortwaves are helping to produce some
light precipitation this morning. One is stretching through the
eastern YK-Delta from Quinhagak through Aniak. Temperatures in
this corridor are hovering right near freezing, so a mix of rain
and snow will continue there through the early morning. The other
is a weak deformation axis along the eastern side of the Aleutian
Range. This shield of precipitation is sliding slowly east. It is
a bit more rain near the coast and a bit more snow inland.
Surrounding both of these features is widespread fog with abundant
low level moisture. This fog could linger through the day with
little wind or strong forcing to clear it out.

The first major shift in the pattern will be a sweeping cold front
plowing through the YK-Delta later today into tonight. This will
produce some steady snow with a couple of inches accumulating west
of the Kuskokwim Mountains. The bigger story with this weather
system will be the cold air. 850 mb (5000') temperatures start at
-4C this morning. They will plummet down to -13C by this time
 tomorrow morning.

The other weather pattern will hold to the eastern half of SW AK.
This will be more or less continuous precipitation right along the
Alaskan and Aleutian Ranges, highlighting the Lower Kuskokwim
Valley. As a stronger, wetter low develops in the Gulf of AK, it
spins more moisture in from the east Fri morning. This will allow
the steady precipitation to intensify. As the aforementioned cold
air moves in from the west, it will drive under some of this
moisture helping to create even more lift. This could result in
some fairly significant snow accumulations across the Kuskokwim
Valley. Stay tuned for exact amounts as models come into better
agreement and the event draws closer in time.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Thu and
Fri)...
An elongating col will create adverse conditions over the Western
Aleutians while a compact low delivers cold, northerly winds to
the Eastern Bering. First, the col is being stretched out by a
potent upper level shortwave. This is create a quick switch from
southeast winds to westerly winds. As the westerly flow pushes in,
it will bring in widespread gales and seas greater than 20 feet.
Precipitation associated with this feature should be mostly snow
with some cold air advecting in.

The Eastern Bering will see a developing low dictate the weather
for the next couple of days. It is currently just south of the
Seward Peninsula. As it taps into some colder air, it will start
to gather strength and slowly track to the east. This will allow
for some much colder air to dive south across the Eastern Bering
into the Alaska Peninsula. The net effect will be gusty (gale-
force) winds and snow showers. These showers will be especially
pronounced along the north side of the Alaska Peninsula as we head
into the weekend. Mariners should also be on the lookout for some
gusty NW winds to develop out of bays and passes on the south side
of the Peninsula late Fri.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern will continue for the region through the
upcoming week, with southwest/southcentral Alaska becoming the
"battle zone". It'll definitely be interesting to see if the
models, which continue to come into better agreement, push some
Arctic air into southcentral, or keep it bottled up west of the
Alaska Range. Conversely, how much tropical-laden moisture gets
thrown into a developing deformation band near the Alaska Range
and into southwestern parts of the state.

Now for the details. Model continuity remains good regarding a
strong mid-level low dropping south from the Chukchi Sea. There
is a new wrinkle though in that they now stall the system near
Norton Sound on Friday and hold it there through the weekend. As
the surface reflection develops, much colder air will plunge into
the eastern two-thirds of the Bering and southwest Alaska, through
the eastern half of the Aleutians, and into the north Pacific.
The further north positioning (compared to near Nunivak Island
yesterday) then in turn allows a pair of strong storms to move
north to northwest into the north-central Gulf. With the further
west track, warmer air will push across most of Southcentral. This
means rain will likely be the predominant precipitation type,
with snow across southwest Alaska and the eastern half of the
Aleutians, and a mix for Kodiak Island.

These Gulf lows will weaken while moving northeast, as a new
system enters the picture to start the week. This storm will
undergo significant deepening, as it drops from 980 mb (per the
GFS) south of Adak late Sunday morning to 958 mb south of Kodiak
Island late Monday morning. This will put the AKPEN and the
southern part of southwest Alaska in a potential for some fairly
decent snowfall accumulations if the current forecast holds. It'll
then take up residence in the southern Prince William Sound region
for Tuesday and Wednesday, as the storm slowly fills and weakens.
This is perhaps the "best" potential to see some snowfall for
Southcentral, but even then, seems like the dynamics and moisture
will be on the downward trend before enough cold air makes in into
the region. Still, with it being several days out things can
change. Stay tuned!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 119 120 125 130 131 138 139 150 177 178 185
351 352.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...PD