Gale Warning 584, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
543 AM AKDT Sat Oct 31 2020

.ANALYSIS AND UPPER LEVELS...

At the upper levels, a well defined shortwave trough is moving
southeastward into northwestern portions of Alaska this morning.
Further south, a decaying upper low is found just south of the
Aleutian Chain, heading towards the North Pacific. A fairly zonal
jet is set up across the Gulf of Alaska with a weak embedded
disturbance heading towards the Alaska Panhandle.

At the surface, northerly flow has become well entrenched across
the AKPEN through Southcentral, with nighttime microphysics
imagery indicating significantly less low clouds/stratus this
morning versus the past couple of days due to the offshore flow
in place.

A front extending from the low south of the Aleutians is
weakening and, moving south/east this morning, with the wind/rain
impacts in these areas subsiding and clearing skies taking over.

&&

.MODEL DISCUSSION...

Models are in great agreement with regards to the progression of
the shortwave trough currently entering Northwest Alaska. There is
good agreement on this shortwave approaching the Northern Gulf
Coast by later this weekend, then cutting off and remaining
centered around Prince William Sound into Monday.

Confidence remains high on the expected wind event through this
weekend as the shortwave approaches and passes the area, with a
very cold airmass in place across the interior. The main forecast
uncertainty revolves around how quickly this low cuts off which
could influence the overall peak gap winds across the Northern
Gulf Coast.

&&

.AVIATION...

PANC...VFR conditions will persist. Northerly winds will
increase, potentially gusting up to 20 kts by this afternoon.

&&

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

The forecast is not too spooky to start off this Halloween
Weekend. In general, it will feature some bone-chilling
temperatures in some locations with gusty winds in many of the
typical gaps. First, some low clouds and even some patchy fog have
crept in this morning over the Copper River Basin and near
Portage. These will both be very short-lived though as dry air
continues to push into the area with gusty conditions developing
during the daylight hours. Winds will start to increase more
significantly late tonight into Sunday as the upper level support
pushes south from the AK Range over Southcentral. The exact timing
of these winds will make for a challenging low temperature
forecast for Sunday morning. Where winds develop and persist
overnight, temperatures will not fall as far. However, the
opposite will be true bringing very cold conditions to locations
where winds are calm.

Winds should peak late Sunday night into early Monday through many
of the typical gaps. This includes the Matanuska Valley, Thompson
Pass, Seward, Whittier, and the Copper River Delta. A reinforcing
piece of energy will drive down from the AK Range at that time and
help to mix down the strongest winds. The exact location and
timing of the upper level low remain somewhat uncertain. Model
trends overnight showed a subtle weakening and slowing of this
feature, so much of the forecast remains consistent. However, it
still appears that warning level winds (75 mph) will be possible
for Thompson Pass, so anyone with plans to travel through that
area should stay tuned to the forecast. There will also be
potential for strong gusts in Valdez as the upper level low passes
over and mixes down some of the mountain-top winds from time to
time. With this package, we have also ramped up the winds over
West Anchorage and for the Matanuska Valley. This looks to be a
fairly long duration event continuing through the beginning of the
week.

&&

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

The models have anomalies related to temperature over SW AK from
today through day five. On the latest satellite image, there is an
area of fog over Bethel and a swath of clear skies from Bristol
Bay to the Western Alaska Range. Radiational cooling is
anticipated across much of SW AK. There is an arctic low north of
the Brooks Range that will track southward today. There is not a
lot of moisture with this arctic system but flurries and light
snow showers are possible this evening and overnight. The biggest
impact will be the intrusion of arctic air with some locations
falling below zero degree Fahrenheit and the chances of afternoon
temperatures getting above freezing is slim. Light freezing spray
is possible for portions of the Kuskokwim Bay and Bristol Bay.

&&

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

There is a dome of high pressure over the northern and central Bering
which will trap a layer of marine stratus over much of the Bering
Sea. There is an elongated occlusion located across across the
central and eastern Aleutians which will continue to bring
precipitation (rain/rain showers) and gusty winds to the region.
The surface low associated with the occlusion will continue to
weaken and track southeast today. This will re-orient the winds
at the surface. A second low southwest of Attu Island, continues
to deepen and move closer to the Western Aleutians. This new
system has a lot of jet support and a tight pressure gradient at
the surface, however this low will also track south of the Chain.
Unsettled weather is expected this weekend but the high seas will
experience the brunt of the storm. One of the interesting features
of this storm is that when we change channels on the satellite
the moisture plume associated with this low is actually connected
to the moisture plume of Super Typhoon Goni which is near the
Philippines.

&&

.MARINE (Days 3 through 5) Monday through Wednesday...

A North Pacific low and front tracks southward through Wed.
Models are mixed, but confidence is good for easterly small craft
winds and waves with local gale force winds over the Aleutians and
Alaska Peninsula diminishing Tue. Upper level troughs bring
northerly small craft with local gale force winds over the Eastern
Bering and Bristol Bay Tue and Wed. A strong front approaches the
Western Aleutians and Bering with widespread high end gale force
winds for Wed.

For the Gulf, a weakening low over the northern Gulf lingers
through Wed. Track and development vary between models, but two
areas of westerly gale force winds and waves occur Mon, the first
from the Barren Island into the Central Gulf, the second from the
Alaska Peninsula into the Western Gulf, diminishing Tue. These two
areas become northwesterly gale force winds that fan out into the
western Gulf for Wed.

&&

.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. The initial thinking for the extended forecast was that
with the cold airmass, that daytime and overnight temperatures
would bottom out. Despite the cold, the strong winds will actually
help to keep temperatures up and we've made those changes in the
forecast.

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 moving 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 Warning: 120 121 125 127 128 131 172 176-178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BG
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...SS