Gale Warning 684, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
602 AM AKST Tue Nov 24 2020

.ANALYSIS AND UPPER LEVELS...

Satellite analysis shows a broad, complex region of low pressure
over the western Bering with a strong front extending from Atka
to Saint Matthew Island. The primary low can be found along the
front to the west of Saint Paul Island with additional low centers
north of Shemya and another in the northern Bering. ASCAT has
measured widespread gales over the western Bering in the region of
strong cold air advection. An occluded front extending off of the
low in the northern Bering can be seen on the radar over
Southwest Alaska, where precipitation has transitioned from snow
to rain over the coastal areas. To the northeast of this front,
temperatures have been increasing steadily this morning as the
warm air moves in. For Southcentral, a ridge of high pressure
continues to move in from the west, which has lead to clear skies
and cooling temperatures over Southcentral.

The primary front over the Bering brought strong winds over the
central Aleutians late last night and this morning. Stations in
Adak and Atka measured gusts from 72 to 82 mph around midnight.
Winds are also increasing over the Pribilof Islands with gusts
consistently around 65 mph this morning.

&&

.MODEL DISCUSSION...

Overall, model solutions remain in good agreement. A few minor
discrepancies exist in the strength and speed of propagation of a
triple point low that forms in the southern Cook Inlet Wednesday
morning. Models have trended towards a more progressive system
with an additional low over Southwest Alaska tracking northeast.
The timing and progression of these two features will impact the
strength of winds over the Anchorage Bowl.

&&

.AVIATION...

Upper level clouds will increase through the day today but
ceilings will remain VFR. Wind shear is likely to develop
Wednesday morning as a front enters the western Gulf.

&&

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

A brief respite in the active pattern today will quickly give way
to another storm arriving in the Western Gulf late tonight. First
things first, today will primarily feature diminishing
outflow/offshore winds and generally increasing clouds through the
day. Then attention shifts to a very dynamic storm system set to
drive right up Cook Inlet early Wed. This system has multiple
subtle facets that are sure to impact the forecast. The good news
is that models are generally in agreement with one another in how
they resolve the evolution of the storm.

This system is tracking much farther north than the system from
Sun-Mon. This means it will be able to usher in much warmer air in
the lower levels. This will create more of a rain/snow mix for
inland portions such as Portage Valley, whereas coastal sites will
be warm enough for mostly just rain. In addition to the warm air,
the track of this low also favors strong downsloping winds across
the Kenai and Chugach Mountains. As the air comes down the
mountains, it will warm adiabatically and cause surface
temperatures to rise a couple more degrees. This could lead to a
wintry mix initially for parts of the Kenai Peninsula and the
Anchorage Bowl.

Winds with this system will also be impressive. As the low moves
up Cook Inlet, it deepens into the low 970 mb range. It will also
have abundant upper level support with a strong shortwave at
500mb and a potent jet of 100 kts at 300mb. High Wind Watches are
in effect from Portage Valley, down the Turnagain Arm, and into
the higher elevations of the Anchorage Hillside. The track of the
low and some other subtle features over SW AK will be the key
components regarding how strong the winds get and exactly where
they land.

Good news for snow lovers is that the system will usher in cold
air as it exits the area. So even though there will likely be a
brief warm up and some wintry mix, the event should finish late
Wed evening with at least a couple inches of snow. For those at
higher elevations such as Hatcher, Turnagain, and Thompson Pass,
expect snow accumulations between 6-12".

&&

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

A front associated with a strengthening storm system over the
Bering sea will continue to impact Southwest Alaska today through
Wednesday. The front will impact the region in two waves. The
first wave is pushing through the region currently with light snow
occurring across the Kuskokwim Valley and across northern Bristol
Bay. Coastal regions of Bristol Bay started as light snow
overnight before transitioning over to plain rain now as
temperatures have warmed above freezing. Any precipitation across
the above areas east of the Kuskokwim Mountains are expected to
taper off later this morning with a lull in the action until later
this evening through the overnight and into the day on Wednesday.
The YK-Delta also started as snow earlier in the night and has
since transitioned over to plain rain south and west of Bethel.
Bethel as of this writing was still holding onto light snow even
though temperatures have warmed above freezing. With a stronger
push of warm air expected a bit later this morning, expect this to
change over to plain rain as the warm air eventually wins out.
The YK-Delta will stay in rain for much of the day especially west
of Bethel as the front continues pushing through.

The one part of the forecast that is still somewhat uncertain is
across the Kuskokwim Valley. It is here that there is some
question as to just how warm they will get. Temperatures have been
steadily coming up overnight and most guidance has much of the
region reaching the mid to upper 30s later today. Since nothing
appears to be in place to hold the cold air in place, suspect the
warm air will come in and change snow to at least a mix of rain
and snow for Sleetmute to Sparrevohn this afternoon through
Wednesday morning. As a result of the warm air expected to come
in, snowfall accumulations were lowered this update to a general
1-3 inches with the lowest south and highest amounts north where
the cold holds on longer. Some areas north of Crooked Creek may
see several inches of accumulation. One other thing that will
accompany the front will be windy conditions with the strongest
winds expected through the Kamishak Gap area where gusts may reach
as high as 45-55 mph tonight through Wednesday morning.

Wednesday, the cold air comes back in from west to east
throughout the day bringing temperatures back below freezing for
highs. Snow showers are possible especially across the Kuskokwim
Delta Coast and on the Kuskokwim Mountains and the western Alaska
Range Wednesday through early Thursday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tuesday
through early Thursday)...

A strengthening storm system continues moving northward today and
exits the Bering to the northwest this afternoon. Its front
continues to move across the eastern Bering and eastern Aleutians
this morning with storm wind force winds and hurricane force gusts
especially around the Pribilofs to the coastal waters north and
south of the eastern Aleutian Chain. Periods of rain also
accompany the front as it moves through bringing a wind swept
rain. Wave heights will also be in the 25-35 ft range both in the
marine zones north and south of the Aleutians.

Cold air advection can already be seen on satellite (nighttime
microphysics) this morning due the popcorn nature look to the
clouds indicating an unstable atmosphere. As the cold air
continues to move across the Bering behind the aforementioned
front, widespread snow showers are expected across much of the
Bering Sea and Aleutian Chain especially Wednesday. Lowered
visibilities and ceilings are expected within the snow showers.
Small crafts as well as gale force winds are expected especially
across the western portions of the Chain and Bering as the
pressure gradient remains tight over the coming days.

&&

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

Gulf of Alaska:

Fair confidence with overall pattern but much uncertainty remains
in the details during the 3-5 day timeframe. Thursday night and
Friday morning a Gale force may clip the southern Gulf. The low
will weaken and move into either the northeastern or eastern Gulf
Friday morning. however low confidence on track and strength.
Friday night a gale force front moves into the Gulf there is a low
chance of pockets of storm force winds but low confidence on
strength and higher confidence with track. A gale force low will
move in behind the front sometime late Saturday, however very low
confidence on timing and track.

Bering Sea:

Good confidence as a weak front with small crafts moves across
the eastern Bering Thursday morning and moves into Southwest
Alaska by Thursday afternoon. Another low enters the western
Aleutians Thursday afternoon with high end Storms mainly across
the Pacific side of Aleutians. This low will move near the
Aleutians through the south Bering on Friday and move east into
the Gulf sometime on Saturday. Fair confidence on track, but
lower confidence on strength of low.

&&

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

Progressive flow will continue to dominate the long term with a
series of fronts and lows moving across the Aleutians and Bering
into Southwest and Southcentral Alaska. These systems may bring
chances of accumulating snow. Good confidence on the overall
pattern, but lower confidence on details of track and strength of
systems.

A weak low is expected to move across the Gulf Friday with light
rain and snow mainly near the coast and coastal mountains, but
minimal impacts. But uncertainty still remains with the strength
and track of this low.

A strong low will move across the southern Bering near the
Aleutians on Friday and with an accompanying front moving south
of the Aleutians. This will produce high end storm force winds
mainly on the Pacific side of the Aleutians. Late Friday into
Saturday Southwest Alaska may receive some accumulating snow on
north side of this low in the deformation zone. Fair confidence
with this system through early Saturday, however lower confidence
as we head into Saturday as models vary a bit with track of low.

This low and frontal system will track into the Gulf late Friday
and Saturday with at least Gale force winds along front and
possibly pockets of storms but low confidence at this point. There
is a chance of heavier rain and snow from Kodiak Island towards
the Kenai and Prince William Sound. Snow may move into parts of
Southcentral late Saturday into Sunday. However, a lot of
uncertainty Saturday and Sunday with the evolution of this system
progressing across the Gulf.

Yet another strong low tracks near the Aleutians Sunday and
Monday still a bit of spread among the models, but relatively good
agreement considering its 6 days out. Most models show storm
force winds, however the EC is a bit weaker at this point. The
front associated with this low will track into the Gulf on Monday
bringing winds, rain, and snow.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 185, 195.
		 High Wind Watch 101, 125
MARINE...Storm 119, 125, 155, 165, 170, 172-174, 179-181, 414
		 Gale 120, 130-132, 137-139, 150, 160, 175-178, 185, 411-413
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...RC