Gale Warning 631, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 PM AKST Tue Nov 3 2020

.ANALYSIS AND UPPER LEVELS...

An upper-level low remains nearly stationary just south of Prince
William Sound this afternoon. Multiple lobes of vorticity are
spinning around this low. These vort maxima, coupled with the
strong, northerly wind flow along the Southcentral coast, are
spinning up multiple meso-scale surface lows across the northern
Gulf. The deepest of the three continues to spin southeast of
Cordova. The southeasterly flow out ahead of the low is tracking
moisture in the form of snow showers across the Copper River Delta
and Cordova, where some of these showers have been quite intense,
reducing visibility to less than one half mile at times this
morning. The snow showers have also brought up to 2 inches of
snow to the Cordova area. Light snow has also fallen across the
southern half of the Copper River Basin, where up to an inch of
snow fell this morning. Farther west, skies remain mostly clear
with northerly gaps winds continuing, albeit much weaker than
yesterday, from the Matanuska Valley south to Kenai.

For Southwest Alaska, a vigorous shortwave and associated surface
front is moving southeast toward the Kuskokwim Delta today.
Southerly winds with some light snow is occurring in Bethel, but
winds have shifted to the northwest and are increasing for Toksook
Bay and and Hooper Bay. A cold, northerly flow across the AKPEN
is producing a few snow showers for Cold Bay and Sand Point.

Elsewhere, a large ridge of high pressure is keeping conditions
rather tranquil across the rest of the Bering and Aleutians.
Southerly flow is beginning to develop, however, for Attu and
Shemya ahead of the next storm-force front approaching from the
west.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement in the short term, but with
some caveats. First, guidance is struggling to pick up on the
strength and exact placement of the meso-lows over the northern
Gulf. This has introduced uncertainty regarding snow and snow
amounts for places like Cordova through this evening, as none of
the models have keyed in on precipitation along the immediate
Copper River Delta coast. However, with guidance still lifting the
surface and upper-level lows to the northeast tonight, snow
showers are expected to diminish in both intensity and coverage.

Guidance is also in good agreement with a shortwave transiting
Southcentral Wednesday night into Thursday. Hi-res models are
focusing in on a second high wind event for Thompson Pass during
this time, especially given the general uniform northerly wind
flow through the troposphere. Models do also diverge slightly
with regard to the front moving through Southwest Alaska Thursday
into Friday. The GFS is a bit slower, with the NAM and Canadian
more in line with each other. Sensible weather differences look
minimal, however, with most of Southcentral seeing light snow by
Thursday night.

High model uncertainty creeps in by the weekend with a North
Pacific low moving toward the AKPEN. The EC drives the brunt of
this system north to the Y-K Delta while the GFS and Canadian
push it northeast to the northern Gulf. These solutions would
yield significantly different wind fields and precipitation types
across the Southern Mainland. Thus, forecast confidence drops
precipitously (pun intended) by the end of the week.

&&

.AVIATION...

PANC...VFR conditions and light northerly winds will persist.

&&

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

Continued quiet weather is expected across much of Southcentral
for tonight through the day Wednesday. Any wind that remains
across the area is expected to become light to calm during the
evening and overnight hours. As a result, decent radiational
cooling is expected for much of Southcentral tonight and Wednesday
night as insolation gained during the daylight hours will quickly
be lost to space after the sun goes down. This will make way for
the coldest mornings thus far for this winter season.

A weak surface low in the northern Gulf will finally start moving
off to the east overnight as the upper level pattern finally
begins to change. Cordova had snow all morning due to this system
and has since tapered off. Cordova is expected to remain dry
through the overnight and for the day Wednesday with just the cold
temperatures remaining. Middleton Island has been in the brunt of
dealing with the impacts of this low in the Gulf with snow
falling all day. This is expected to continue into the overnight
hours before finally tapering off.

Next, the High Wind Watch previously in effect has now been
upgraded to a High Wind Warning for Thompson Pass. The warning is
for the time period from 9pm Wednesday evening to 9am Thursday
morning for wind gusts as high as 75 mph. In addition to the wind,
due to the very cold air mass in place and temperatures in the
single digits to near zero, wind chills may approach as low as 35
below zero. An increasing pressure gradient with support from the
upper levels (i.e. increasing jet support and upper level trough)
is expected to be the main driver for this wind event. There are
some things working against the high winds but there was enough
confidence to go ahead with the warning.

Thirdly, a strong upper level wave is expected to move southward
through the AKPEN and into the North Pacific overnight into
Wednesday morning. As a result, gusty winds are expected for
Shelikof Strait for Wednesday with gusts upwards of 50 kts. The
city of Kodiak is expected to remain dry but this system will
bring a period of light snow to the southern part of Kodiak Island
overnight into the early morning hours Wednesday.

Lastly, a frontal system moving eastward from Southwest Alaska
will be the first in a series of storm systems that will impact
Southcentral. Clouds are expected to increase during the day
Thursday followed by light snow Thursday evening into Friday
for Anchorage, Kenai Peninsula, and the Mat-Su valleys. A few
inches of snow is likely from this area-wide.

&&

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

A shortwave trough is moving across Southwest Alaska this evening
and it is bringing snow to the area. Observations across the area
shows areas of reduced visibility in moderate and heavy snowbands.
Winds and gusts have also increased along the coast of the
Kuskokwim Delta. Moderate to heavy Ocean-Effect snow bands are
expected tonight as strong straightlined northwesterly flow will
persist in the wake of this shortwave trough. Forecast soundings
showed a steep lapse rate with a 13-15 degree Celsius difference
between the surface and 850 mb. The straightlined flow will allow
for these snowbands to form and not be sheared. There is also a
very high thermal cap which will make for a deeply mixed boundary
layer to promote convection. A Blizzard Warning remains in effect
for locations along the coast for the Kuskokwim Delta for winds of
at least 35 mph and visibility of a quarter mile or less. A front
will move through Southwest Alaska Wednesday night through
Thursday evening. Precipitation will start as snow across the
Kuskokwim Delta and quickly change to rain early Thursday morning
as strong southwesterly flow will quickly warm temperatures above
freezing. Precipitation will remain as snow farther inland across
the Kuskokwim Valley as well as in the higher elevations of the
Ahklun and Kilbuck mountains as temperatures will remain below
freezing in those locations. There is no elevated warm layer in
the forecast soundings, thus there is little to no chance of
freezing rain.

Coastal flooding is another concern from the strong southwesterly
flow, especially in areas where the coastline is concave
(coastline bends inland). A Coastal Flood Watch has been issued
for the Kuskokwim Delta from late Wednesday night through
through Friday afternoon from Toksook Bay to Hooper Bay for the
potential of storm surge reaching 3-5 feet above highest
astronomical tide. The watch is in effect from Thursday morning
through Thursday afternoon for Toksook Bay through the Kuskokwim
Bay for the potential of storm surge reaching up to 3 feet above
highest astronomical tide.

&&

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

An active pattern is expected for the Bering/Aleutians over the
next 3 days as a strong frontal system moves across the area. The
front will enter the western Bering/Aleutians Tuesday evening. The
front will then move eastward into Southwest Alaska late Thursday
morning into Thursday afternoon. Widespread storm force winds with
hurricane force gusts are expected in the western and central
Bering/Aleutians Wednesday night through Thursday night. The front
will then weaken in the central Bering/Aleutians and produce gale
force winds with storm force gusts early Thursday morning before
moving into the eastern Bering/Aleutians late Thursday morning.
Large 25-35ft long period seas associated with this front will
enter the western Bering/Aleutians Wednesday morning. These seas
will gradually abate across the western Bering/Aleutians Wednesday
evening. Seas will increase to 25-30ft in the central Bering
Wednesday evening into Wednesday night and then gradually abate
Thursday afternoon onward. These seas will also be long in period.

&&

.MARINE (Days 3 through 5)...

Saturday will be the most active weather day of this period as a
developing low will move into the western Gulf of Alaska with
some storm force winds likely with it. There is a bit of
uncertainty as to the track of this low as it could end up farther
west in Bristol Bay, but that would still bring Gales over most
of the Gulf region.

A Gale-force low should move through the Bering sea from the
southwest Saturday through Sunday. However, there is a bit of
uncertainty with this low and it is uncertain whether it will be
quickly followed by a stronger low on Monday, or end up being a
little slower in moving into the Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long term pattern is a bit uncertain through the weekend with
a low moving from south of the Aleutians to the Western Gulf of
Alaska on Sunday. There is a chance this low will take a more
westerly track into Bristol Bay which would mean warmer weather
for Southcentral Alaska, but the most likely solution looks to be
for the development of a triple point low which becomes the main
low by the time it approaches Kodiak on Sunday. This more likely
solution will bring an increased chance of snow for Southcentral
Alaska.

The Bering Sea will have a low move from the southwest corner
of the Bering to the northern part of it Sunday through Tuesday.
However, there is a high degree of uncertainty with this storm.
This is due to a secondary low that will trail the main low and
the interaction of these lows on Monday and Tuesday will dictate
how this system will work out. One of the the biggest impacts will
be the temperatures in Southwest mainland Alaska as the track
will bring a good push of southwest flow over the region and warm
up the temperatures there. This could bring in a mix of
precipitation types with snow, rain and freezing rain all possible
across the Kuskokwim Delta.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131 (Thompson Pass).
         Blizzard Warning 155 (Kusko Delta coast).
 Coastal Flood Watch 155 (Kusko Bay to Hooper Bay).
MARINE...Storm Warning 150 173 174 175 176 177 178 179 185.
         Gale Warning 127 128 130 131 132 138 155 160 165
                      170 172 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...MV
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...EZ