Gale Warning 578, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
523 PM AKDT Wed Oct 28 2020

.ANALYSIS AND UPPER LEVELS...

A weak surface low southeast of Cordova continues to produce
showers across the northern Gulf and is extending clouds over
Prince William Sound as well as portions of the Copper River Basin
this afternoon. Northwesterly flow is also extending cloud cover
from Kamishak Gap across the Southern Cook Inlet and Barren
Islands. The main weather feature for Southcentral, however, is
the persistent fog / low stratus for the the southern Susitna
Valley, Matanuska Valley, Anchorage Bowl, and the northwestern
portion of the Kenai Peninsula. This area of fog and low stratus
is expected to remain in place throughout the evening and into
Thursday morning.

Elsewhere, low stratus clouds remain over Southwest Alaska, with
only a few breaks across interior Bristol Bay and the Kuskokwim
Valley. Farther west, a nearly stationary low over the western
Bering is extending a surface front with areas of light rain from
the Pribilofs to the eastern Aleutians.

&&

.MODEL DISCUSSION...

Model agreement remains good through the next couple of days.
Guidance is keying in on a general increase in the outflow winds
across coastal Southcentral for Thursday as a shortwave trough
moves over the region from the northwest. This feature slides out
of the region by Friday with a near zonal flow. The more zonal
flow will allow a second wave and developing surface low over the
Gulf to track east-northeast and remain east of Prince William
Sound with moisture mainly confined to the eastern Copper River
Basin. Out west, models are also in good agreement with the
current storm-force low slowly weakening as it slides south into
the North Pacific while a second low move closer to Attu by late
Saturday.

&&

.AVIATION...

PANC...Fog and low stratus are expected to remain over the
terminal through the afternoon and evening hours. Visibility may
improve slightly for a time this afternoon and evening as the fog
thins and lifts. While conditions may briefly improve, the overall
synoptic pattern supports its persistence. Thus, the potential
remains for dense fog to return later this evening and continue
through Thursday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Friday)...

The primary forecast concerns this afternoon include persistent
freezing fog around Cook Inlet, the polar air mass moving into
Southcentral, and where chances of snow remain in the forecast.

Starting with the fog, it has persisted for much of the day,
particularly in Anchorage, the Matanuska Valley, along Turnagain
Arm, and interior portions of the Kenai Peninsula. Since the sun
is no longer strong enough to burn the fog off entirely, and with
no change in the overall weather pattern tonight, it appears
highly likely that the fog will return to dense intensity. This is
most likely of course where the fog is presently. So for
Anchorage, the Matanuska Valley, and areas along the mountains on
the Kenai Peninsula, expect the freezing fog to persist overnight,
becoming dense at times, and contributing to hazardous travel
conditions. A piece of good news however is that the fog is
already leaving behind its telltale signature, beautiful rime ice
on all the trees. With even colder air working in over the coming
days, that ice is unlikely to go anywhere anytime soon, adding to
the picturesque Alaska landscape.

As mentioned above, a polar air mass is already beginning to
encroach on Southcentral from the north this afternoon. This polar
air has full upper level support with a digging trough moving in
from the northwest. Thus, the mountains will not do much to block
the cold air from moving into the area. Temperatures will
gradually cool into the weekend. The fog may hold overnight lows
up, but have also kept Anchorage and the Mat-Su from getting above
freezing during the day. Thus, see no reason anywhere away from
the immediate coast will get above freezing anytime in the
foreseeable future. Highs will generally hover in the 20s each
day, with lows in the teens (except for the foggy areas tonight),
with single digit lows expected in the normally cold and clear
spots.

As for chances of snow, only two areas will have that concern
through Friday. First up will be the Copper River Basin. A well
defined circulation is over the northern Gulf, just off Cape
Suckling. Associated moisture is drifting north over the Basin.
With the low not moving much over the next few days, the mountains
and far eastern Copper Basin will be under a constant threat of
light snowfall, but outside of the mountains are not expected to
amount to more than a few inches over several days. A point of
uncertainty lies with how far west the snow can get, with
Glennallen on that western fringe. If the snow moves in,
particularly with an upper level disturbance digging in during the
day Thursday into Thursday night, there could be enough moisture
and lift to result in a few inches of snow from Glennallen east.
For now, the forecast is for most of that snowfall to remain east
of Glennallen proper. On Friday afternoon into Friday night, a
front approaching from the south could bring chances of snow to
Kodiak. However, the question of whether there is enough cold air
to overcome the warm sea surface temperatures provides the most
uncertainty. Either way the chance of snow is only at the start,
as warmer air sooner or later will change the precipitation over
to rain, which will be the predominant precipitation type in
Kodiak for most of the event.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Saturday)...

Low stratus and overcast sky coverage will continue into at least
Thursday morning before clouds begin to clear. An upper level
ridge over the Bering will ease cloud developments over Southwest
Alaska in the coming days as the ridge moves closer to the coastal
area Thursday morning. Looking ahead, two separate shortwaves
from north of Yukon Delta, combined with cold air advection aloft
and the onset of the upper level ridge from the Bering will help
reduce air temperatures in the teens to mid 20s between Thursday
night and Friday night. A chance of snow is feasible over lower
Kuskokwim Valley and Kuskokwim Delta tonight and Thursday, with a
chance of rain over AKPEN tonight and Thursday before conditions
start to elevate there beginning Thursday afternoon. A frontal
boundary from the Aleutians will extend to the south of AKPEN on
Friday morning, which could bring more rain and a chance of snow
through the end of the week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Saturday)...

A Northwest Pacific low will bring gales over much of central
Bering and the Aleutians on Thursday and Thursday night, while
storm force winds will concentrate heavily over Adak Island on
Thursday. A lot of rain and a chance of snow will associate with
this expected system, but overall, the storm low is still expected
to stay south of the Aleutians into the weekend. Winds will
gradually weaken on Friday as the low treks back over the North
Pacific through Friday night.

&&

.MARINE (Days 3 through 5)...

Northwesterly gales are likely across the bays and passes of the
eastern Aleutians and southern coastal AKPEN late Saturday into
Sunday. Winds and waves over this area will generally diminish
through the day Sunday; however, some gales are still possible
southwest of Kodiak Island into Monday. A gale-force front skirts
the western Aleutians late Saturday and the central Aleutians
SUnday into Monday before before moving south.

For the Gulf, northwesterly gales are likely Sunday morning
into Monday across the northern Gulf, especially through bays and
passes and out over the Barren Islands.

&&

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

The main weather story moving through the weekend and into next
week will be the intrusion of arctic air across the Southern
Mainland in the wake of a vigorous surface low moving from the
western Gulf to the Alaska Panhandle. The core of the coldest air
looks to arrive on Monday as the polar upper-level low settles
over Southcentral. 850 mb temperatures (~4500ft AGL) look to
plummet to -18 to -20C across interior Southwest and over the
Anchorage Bowl and Kenai Peninsula. The Copper River Basin could
see 850 mb temperatures fall to -22 to -25C. This would translate
to highs struggling to climb into the teens to lower 20s for most
locations not immediately along the northern Gulf coast by early
next week. Daytime highs for parts of the Copper River Basin
during this time could remain near zero. Needless to say, low
temperatures would likely be in the single digits for most areas
and well below zero for the Copper River Basin and normally colder
locations like sheltered valleys.

Gusty outflow winds will also remain a potential along the
immediate coast through next week due to the build up of colder
air inland and a lingering gradient between an interior ridge and
trough along the coast. Models, however, are indicating that one
or more compact, meso-lows could spin up at the surface over the
northern Gulf due to the interaction of the upper-level dynamics
and the increased surface winds and thermal gradient. If this does
happen, there could be additional cloud cover that could help
moderate temperatures nearer the coast and also bring about an
increased chance for snow showers through early next week.

Farther to the west, a ridge of high pressure will build over the
western Aleutians and expand east by next week. Multiple
shortwaves diving south between the ridge and the aforementioned
polar low to the east will allow for clouds, snow showers, and
gusty northerly winds to persist from the central Bering to the
Southwest Alaska coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 165 170 172-176 178 179 180
		 Storm 177
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...TM