Gale Warning 664, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 PM AKST Wed Dec 4 2019

.ANALYSIS AND UPPER LEVELS...
An upper level short-wave ridge is sitting overhead of Southcentral
today helping to stabilize the airmass. As would be expected, this
has led to development of areas of low clouds and fog. A weak
surface low sitting over the northern Gulf is combining with an
upper level short-wave approaching from the west to produce a few
snow showers over the southern Kenai Peninsula. The short-wave is
moving into a dry stable airmass further north, so don't expect
any impacts from this trough as it crosses Cook Inlet, Anchorage,
and the Mat-Su during the afternoon and evening hours. Offshore
gap winds continue to blow along Prince William Sound and the Gulf
coast thanks to persistently tight pressure gradients between the
low over the Gulf and high pressure inland.

Out west, a surface frontal system is tracking eastward across the
Bering Sea and toward the Southwest Alaska coast. Winds ahead of
the warm front and occluded front are strengthening over the
northern to eastern Bering Sea as the fronts push into and up
against the Arctic airmass in place over mainland Alaska.
Precipitation out ahead of the fronts is in the form of snow,
with rain back behind them. Meanwhile, a trailing cold front is
progressing eastward across the central Aleutians, advecting in
marginally colder air. However, a short-wave trough digging over
the northwest Pacific will amplify the pattern, with warm air
heading back northward before too long.

&&

.MODEL DISCUSSION...
Models are in good agreement with handling of large scale features
through about Thursday night. Solutions then radically diverge
Friday along the central to eastern Aleutians. The NAM/GFS both
show rapid cyclogenesis with a 960 mb storm force low in place by
Friday afternoon. Despite a similar jet structure and the presence
of an upper level short-wave, the ECMWF and Canadian models do not
depict cyclogenesis. Model differences continue to grow farther
out in time, fundamentally affecting the forecast from the eastern
Bering Sea to the southern mainland of Alaska. Ensembles seem to
favor the presence of a low, perhaps not as deep as the operational
GFS and NAM. Sine the current forecast indicates a gale force low
in the same general area as the NAM/GFS low, will leave forecast
as is and just ensure the general trends of the pattern shift are
reflected in the forecast.

&&

.AVIATION...
PANC...With dry mid to upper levels and a low level inversion in
place, conditions are favorable for fog/stratus. Localized areas
of fog have already formed north of Anchorage on up to the
Matanuska Valley. Low level flow is quite light, but with a light
north winds at and near the surface, some of this fog could drift
south to the terminal. Have introduced fog into the TAF, but
confidence in timing is low.

Light overrunning snow will develop late Thursday, leading to
lowering ceilings and visibilities. There is some uncertainty in
timing, but would expect MVFR conditions to develop late Thursday
afternoon or Thursday evening.

&&

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

An elongated surface low over the northern Gulf will continue to
push slowly eastward tonight. There is enough moisture with this
system to produce an area of light snow across the southern half
of the Kenai Peninsula and across the Copper River Delta,
especially as a weak upper-level shortwave moves east in tandem
across the norther Gulf this evening. The light snow will come to
an end quickly tonight for the Kenai Peninsula while snow showers
continue into Thursday across the southern half of the Copper
River Basin as weak southerly flow draws moisture over the area.

Fog is also a concern tonight through early Thursday morning for
portions of the Copper River Basin and the northern half of Cook
Inlet, including Knik and Turnagain Arms. Fog has persisted east
of Gulkana and over Palmer this afternoon, and as the ridge over
the interior continues to build, it is likely the fog will expand
in coverage, possibly moving over Anchorage overnight. Thus, lows
overnight will be tricky in these locations as any fog
development will moderate the downward trend in temperatures.

On Thursday night, a front lifting north from Kodiak Island and
the western Gulf will move toward coastal Southcentral. This will
bring an increase in southeasterly gap winds as a southerly flow
develops. Even with this southerly push, it should remain cold
enough both at the surface and aloft for the precipitation to
stay as snow, with light accumulations likely from the western
Kenai Peninsula north across the Susitna Valley through Friday.

&&

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

An active weather pattern begins tonight and continues through
Saturday as several storm systems develop across the Bering and
North Pacific and push onshore. The main impacts are expected
with the first system moving inland this evening with blowing
snow reducing visibility and accumulating snow.

For this evening, a low over the northern Bering sends a front
across the Alaska Peninsula to the Southwest coast. Gusty offshore
flow increases along the boundary as it slowly moves inland
overnight. Snow develops along the front which spreads eastward
through the night. The frontal boundary weakens as it gets closer
to the western Alaska and Aleutian ranges for the afternoon,
therefor conditions are expected to improve. There will be a few
locations that may see blowing snow during the afternoon; however,
durations should be brief after the main frontal passage.

The next front quickly follows pushing across the Alaska Peninsula
Thursday night and then to the Bristol Bay coast early Friday
morning. There is still a bit of uncertainty on the timing and
strength of this system. At this time, the second front is
expected to bring a much warmer air mass across the region with
precipitation changing from snow to rain as it moves toward the
interior.

&&

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

Several low pressure systems will bring unsettled weather across
the Bering through Saturday. The main challenges through this time
will be capturing the timing and strength of these quickly
evolving systems from a largely data-void area over the North
Pacific.

The first system currently tracking across the eastern Bering
slides inland across the west coast tonight with the next low
rapidly organizing as it lifts north to the Central Aleutians
tonight. Gale force easterly winds are expected across the
western Bering/Aleutian Chain with this second system, which
takes a northerly track through Thursday. As the low lifts north
winds switch out of the south to westerly direction across the
southern waters in the afternoon. Rain will accompany this front
changing over to snow showers in the wake of this system as cold
air wraps around the core of the low.

The next low is also a quickly developing system which lifts north
to the eastern half of the Aleutian Chain early Friday. There is
high uncertainty with the track and strength of this system. This
low has the potential to bring storm force winds across marine
areas along the eastern Aleutians and Alaska Peninsula. A colder
air mass will advect from the western Bering toward the Southwest
coast for the start of the weekend.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

The Gulf of Alaska remains quiet until early next week, when
southerly flow and low pressure is expected to bring in gale force
winds on Monday. Confidence is moderate in this occurring. With
southerly flow we will definitely have elevated winds and waves,
but to what degree is a bit of a question. Gale force looks to be
a reasonable solution for now.

In the Bering, low pressure will be present in multiple areas. Two
lows will be present over the weekend, one in the western Bering
and the other over the Central Aleutians. Both are forecast as
gale force for now. Models are quite erratic and consistency is
poor both over time and between different models. We are confident
in low pressure producing elevated winds and waves, but exactly
where it will be windy and how strong those winds(some models
think storm force) could be is a big question. We are not
confident in the specifics. So we will need to hone this in the
next couple of days.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through Wednesday)...

Long term analysis still points to southerly flow and considerable
warming over the weekend for Southcentral AK. These conditions
look to persist into early next week. The high amplitude ridge
responsible for this looks to form rapidly Saturday along the
ALCAN border and push all the way into the arctic. We can also
expect low pressure embedded in this flow to bring precipitation
to coastal areas and elevate winds. However, model guidance is
continuing to be inconsistent over southcentral, so details will
have to come in time. However, we are confident in a big warm up
- threatening our snow pack - for Southcentral this weekend.

Out west, broad low pressure is expected to be present over the
entire area. An entrenched long wave trough will have several low
pressure systems of varying intensity spin through for the
duration of the forecast period. Right now, the forecast starts
with two low pressure systems (called a double barrel low) with
one low in the western Bering and the other over the Central
Aleutians. Models are quite erratic in the long term so the
solution for low pressure will evolve, but there is enough
agreement in models to go with this for now over the weekend. Low
placement will likely change as guidance does and it should be
noted that by early next week multiple lows will be rotating in
the long wave trough and right now it's not clear where they will
be...So stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155 161.
MARINE...Gale 174 178 180-185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...BB