Gale Warning 632, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
549 AM AKST Mon Nov 6 2023

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Key messages:

* Areas of light snow today through Tuesday morning.

* A major winter storm will bring widespread impacts from wind and
  precipitation to Southcentral and the Gulf Tuesday night through
  Wednesday night.

The upper level low which brought accumulating snow to Anchorage
and vicinity yesterday is now tracking into the eastern Gulf. A
trough axis extends from the low northwestward across the Kenai
Peninsula and Cook Inlet. A weak surface low has formed over the
northern Gulf to the north of the upper center. A vorticity-
maximum rounding the upper low is tracking northwestward toward
Prince William Sound, with precipitation spreading out ahead of
it from eastern Prince William Sound (Cordova/Valdez areas)
across the southern Copper River Basin and Prince William Sound
toward the northern Kenai Peninsula, Anchorage, and the Mat-Su.
Expect light snow to overspread these areas as we head through
the day before gradually tapering off from south to north
tonight. Snowfall intensity will be quite light, so generally
expect an inch or two accumulation. Meanwhile, the vorticity
maxima will help pull the upper low and trough northward,
ultimately lifting into the Copper River Basin tonight then
exiting northward on Tuesday. This will bring widespread
accumulating snow to the Copper Basin this afternoon through
Tuesday morning.

All attention will then shift to what will be a major winter
storm for Southcentral and the Gulf Tuesday through Wednesday
night. A short-wave trough digging across the Aleutians into the
north Pacific will lead to cyclogenesis south of the Alaska
Peninsula and Kodiak Island on Tuesday. In response to this, a
high amplitude ridge will form downstream, forming a strong
southerly jet streak and atmospheric river pointed at Southcentral
Alaska. Gale to storm force winds will accompany the leading
front northward across the Gulf on Tuesday with strong winds and
heavy precipitation spreading onshore along the north Gulf coast
and Prince William Sound Tuesday night. Cold air in place will
lead to snow or a rain/snow mix as precipitation first arrives,
but temperatures will quickly warm with snow levels rising to 2000
feet or higher for the coastal zone by Wednesday afternoon.

The orientation of the leading front along with the low position
south of Kodiak means that downsloping across the Kenai and
western Chugach Mountains will be fairly weak. This will allow
precipitation to make it over the mountains to the western Kenai
Peninsula, Anchorage, and portions of the Mat-Su Tuesday night
and Wednesday. Similar to the coast, precipitation will start as
snow, but warmer air will advect in aloft leading to a transition
to mixed precipitation or rain. Strong pressure and thermal
gradients ahead of the front approaching from the Gulf will lead
to gusty northeast winds in the Matanuska Valley, with northerly
winds continuing down Knik Arm to north to west Anchorage and down
Cook Inlet. These winds will maintain drier air, which could
limit the ability of precipitation to reach the ground, especially
in the Matanuska Valley. Still, with strong dynamical forcing and
the presence of an atmospheric river, ultimately expect some
precipitation to fall in all areas.

The primary area of uncertainty with this storm system is on
Wednesday with the track and strength of upper level short-waves
and surface frontal waves from the Gulf up to Southcentral. A
stronger surface frontal wave crossing the northern Gulf would
strengthen what will already be a strong barrier jet along the
north Gulf coast and into Prince William Sound. The GFS is the
strongest solution, brining hurricane force winds from Cape
Suckling to Prince William Sound. Meanwhile, the upper level
waves will intensify precipitation, especially along the coastal
mountains. The constant barrage of short-waves moving up from the
Gulf will help drive the front onshore Wednesday night. With
southerly low level flow behind the front, surface temperatures
will likely rise well above freezing for much of the region.

The weather impacts will be many with this system, leading to
widespread hazardous travel on water and on land. As we get closer
Advisories and Warnings will likely need to be hoisted for much of
the area. Here are some of the potential impacts:

1. Storm total rainfall (or rain and melted snow) of 2 to 4" with
localized amounts up to 6" possible for the eastern Kenai
Peninsula/Prince William Sound areas (highest over the eastern
Kenai Peninsula).

2. Multiple feet of snow in the coastal mountains, with hurricane
force wind gusts possible for the east Turnagain Arm mountains.

3. Blizzard conditions possible through Thompson Pass.

4. Snow to rain transition for most of Southcentral.

5. Snow and blowing snow possible for the Mat Valley, Anchorage,
and western Kenai Peninsula Tuesday night through Wednesday
morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...

An occluded low producing gales near St Lawrence Island is moving
east across the northern Bering Sea this morning with an
associated front pushing inland across Southwestern Alaska. Snow
is being reported along the occluded front from Bethel to
Dillingham. Snow is expected to continue through late this
morning. On satellite, a large area of showers follow behind this
front and expands across much of the Bering Sea under zonal flow
south of the low center. The triple point of the occlusion is
located just south of the Alaska Peninsula and its cold front is
crossing the Alaska Peninsula this morning, bringing rain and snow
showers to Pilot Point, Port Heiden, and Nelson Lagoon. Showers
are not expected to diminish for Southwest and the Alaska
Peninsula until tonight into Tuesday.

The Aleutians, especially the Western and Central portions of the
chain, is situated under a stronger upper level jet that will
support the development of a low from west to east today.
Accompanied with the low will be gale force winds with gusts to
around 50 to 60 mph near from Amchitka today to Adak tonight. This
low will continue to travel east along the Aleutians today and
tomorrow. Northerly winds with cold air advection behind these
lows will gradually spread east and the interaction of these lows
will maintain northerly winds across the Alaska Peninsula Tuesday
night into Wednesday morning. As the low center nears the Alaska
Peninsula and Kodiak Island, easterly gale force winds will
develop north and south along the Alaska Peninsula coast Tuesday
afternoon, followed by the northerly enhancement through terrain
gaps near Nelson Lagoon and Cold Bay Tuesday night into Wednesday.
Higher winds and seas will also be likely through Dutch Harbor
into Wednesday morning. As the low moves into the Gulf, a band of
precipitation will lift towards the Bristol Bay Interior and
Middle Kuskokwim Valley, but there is low confidence on exactly
how far north this band will lift Wednesday afternoon.

Another front reaches the Western Aleutians around midnight
Tuesday through Wednesday morning. The parent low will trail
nearby through Wednesday and will also bring about similar
magnitude winds to Aleutians as recent systems. Then on Thursday
morning, the winds look to substantially increase to storm force
near Adak. While it seems likely at this time, the forecast is
still a few days out and can change. Indicated on integrated vapor
transport, more moisture will be associated with this system,
compared to the previous.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through Sunday)...

A well developed upper level longwave trough extends from the
Russian Far East across the Bering to a closed low in the Gulf of
Alaska. The trough continues into the Northern Pacific through
Sunday. This longwave trough guides surface lows across the region
through the weekend. Timing of the lows and waves will be the
biggest issue. Locally and periodic heavy rains and windy
conditions expected across the Bering and Southwest Alaska. High-
end Gale force winds over the Aleutians and Alaska Peninsula could
peak higher with additional cold air advection.

In the East, perturbations around the lows bring locally heavy
rainfall and breezy conditions to much of the Southcentral Coast.
Gustier gap winds may become an issue around Kodiak Island and the
Kenai Peninsula. Precipitation becomes more showery inland, and
with additional cold air wrapped into many areas with snow or
mixed rain and snow and lighter winds.

KZ &&

.AVIATION...

PANC...
LIFR to IFR cigs expected through approximately 17-19Z this
morning. Expecting low to mid level stratus to linger through much
of the day, but brief break possible this afternoon between
20-00z with cigs lifting into MVFR/low VFR. IFR cigs with MVFR
vis expected once again by 00-02z as -SN builds in from the E
under variable winds.

&&

Preliminary Local Storm Report...Summary
National Weather Service Anchorage AK
135 PM AKST Mon Nov 6 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0819 AM     Snow             SE Anchorage            61.07N 149.77W
11/06/2023  M1.0 inch        Anchorage          AK   Public

            1 inch of snow overnight (Sunday into
            Monday).

1227 PM     Snow             SE Anchorage            61.07N 149.77W
11/06/2023  M1.8 inch        Anchorage          AK   Public

            1 and 3/4 inches of snow overnight and
            through noon Monday.

&&

Preliminary Local Storm Report
National Weather Service Anchorage AK
314 PM AKST Mon Nov 6 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0314 PM     Snow             null                    61.06N 149.76W
11/06/2023  E3.5 inch        Anchorage          AK   Trained Spotter

            24-hour snow total of 3.5 inches. Liquid
            water equivalent of 0.26 inches.

&&

CL