Gale Warning 633, NWS Anchorage, AK

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

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

Key messages:

* Areas of light snow tonight 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.
Today's light snow will gradually taper off from south to north
tonight. 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 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 shortwave 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.

SB/ER

&&

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

A 984 mb parent upper low sits over the northern Bering this
evening. Rotating around the southern periphery of the low is a
secondary low/trough, currently passing through the Central
Aleutians. This secondary low is forecast to deepen and become the
dominant low as it passes across the Eastern Aleutians into the
Pacific side of the Alaska Peninsula by tomorrow morning.
Satellite imagery shows the pair of lows, almost mirror images of
each other at the moment. Widespread cloud cover is blanketing
the Bering with the areas of clearing filled with scattered
convective showers.

What this means for Southwest Alaska is diminishing precipitation
and lighter winds through Tuesday evening and beyond. What the
weather pattern means for the Bering and Aleutians is that
unsettled conditions are unlikely to change anytime in the near
future. ASCAT passes over the Bering show westerly gales
associated with the Aleutian low moving into Amchitka currently.
These winds and storm force gusts will spread eastward with the
low tonight, briefly clipping Adak and Atka over the next 6 hours
or less. For tomorrow, northerly small craft to gale force winds
will spread across the central Bering on the backside of the
broader Aleutian low. At the same time, westerly gales are
forecast to overspread the Eastern Aleutians and Alaska Peninsula
as a front lifts toward the Western Alaska range over the latter
half of the day. Westerly winds are to transition to a northerly
direction over the Eastern Aleutians and Alaska Peninsula by early
Wednesday with gales spilling out of the gaps and passes.

The next upper low and front sweeps across the Aleutians late
Wednesday into Thursday with renewed westerly gales and potential
storm force winds, especially on the Pacific side of the island
chain. An extended period of showers may accompany the front
through Thursday as it becomes somewhat stationary over the
southern Bering.

BL

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

A series of well developed low centers stretching from Siberia
across the Bering into the Gulf of Alaska gets interrupted by a
short wave ridge that moves over the Aleutians and Alaska
Peninsula through the weekend. This ridge also introduces
uncertainty in the forecast models with the changes through the
weekend. The longwave upper level trough reorients from the
Western Chukchi Sea along the Asian Coast, while the Gulf low
gains support from a developing Beaufort Sea low. This shortwave
trough continues Southward along the Canadian Coast. The shortwave
ridge flattens out over Southwest Alaska by Sunday. Widespread
precipitation and windy conditions continue across the Bering,
while stronger winds build along the Aleutians as the ridge moves
East. Northwesterly gales will dominate with isolated areas
approaching storm force Friday.

Gulf of Alaska weather will remain unsettled as the low center
loops nearly stationary around the Gulf. Heaviest and most
continuous rainfall will occur along the Southcentral Coast.
Further inland, conditions will be a mix of rain and snow, with
snow at the higher elevations. Gale force winds extend into the
North Pacific from the Alaska Peninsula and around Kodiak Island.
Return flow sets up a gale force barrier jet along the North Gulf
Coast, diminishing late Saturday.

KZ

&&

.AVIATION...

PANC...MVFR and occasional IFR conditions will continue through
the evening and into the overnight hours. Light snow will
gradually taper off this evening, with increasing visibility as a
result. The ceiling forecast is a bit more uncertainty with plenty
of moisture through the low-levels. CIGs are likely to prevail in
the MVFR height range through the night, though some instances of
IFR CIGs are certainly possible. Conditions will improve late
tomorrow morning as drier air mixes into the layer, raising clouds
well into VFR range.

Brown

&&

SPECIAL WEATHER STATEMENT...UPDATED
National Weather Service ANCHORAGE AK
337 AM AKST Tue Nov 7 2023

AKZ102-103-112-121-071345-
Anchorage-East Turnagain Arm-Matanuska Valley-
Western Kenai Peninsula-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Girdwood, Portage, Palmer, Wasilla, Sutton, Chickaloon, Kenai,
Soldotna, Homer, and Cooper Landing
337 AM AKST Tue Nov 7 2023

...SNOW, BLOWING SNOW AND RAIN LIKELY LATE THIS EVENING THROUGH
WEDNESDAY EVENING...

An area of low pressure will bring strong winds and widespread
precipitation to much of Southcentral Alaska from late today through
late Wednesday. Strong winds are expected from the Matanuska
Valley southward through the Cook Inlet. Winds gusting to 30 mph
will result in reduced visibility to one half mile at times.
Stronger wind gusts out of the Matanuska Valley may reduce
visibility even further in locations such as Palmer and Wasilla.

Snow is expected to begin across the Kenai Peninsula and
Anchorage late today and then the Mat-Su Valley early Wednesday
morning. A couple inches of snowfall are expected for much of the
area before transitioning to rain on Wednesday as temperatures
warm above freezing. Rain on snow may reduce blowing snow
potential.