Small Craft Advisory 887, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKST Sun Nov 19 2017

.ANALYSIS AND UPPER LEVELS...
A strong short-wave trough continues to dig south of Kodiak Island,
supported by a 160 kt upper level jet streak diving southward
across the eastern Bering Sea on the front side of a high amplitude
ridge. Cold air advection across the eastern Bering and Alaska
Peninsula is leading to widespread strong gusty winds, with near
warning level winds (75 mph) being observed through channeled
areas along the southern Alaska Peninsula and Eastern Aleutians.
Even colder "Arctic" air over northern Alaska and the northern
Bering Sea is poised to advect southward through this same corridor
as an upper level low over interior Southwest weakens and gives
way to a strengthening low over the Gulf.

Out ahead of the trough light snow has been falling across nearly
all of Southcentral Alaska today. However, snow is beginning to
lighten up and taper off as energy is transferred down to the
Gulf and low level offshore flow strengthens. Gusty northerly
winds along Cook Inlet are also leading to blowing snow and
significantly reduced visibilities across portions of the western
Kenai Peninsula. Conditions will improve here as the snow lightens.

Kodiak Island is in a precarious position right along the upper
trough axis and just west of the developing surface low over the
Gulf. Temperatures have warmed into the mid to upper 30s, but cold
air advecting in from the west will cool the shallow above
freezing layer near the surface and change precipitation to snow.
Heavier bands of snow will approach from the Gulf as the low
deepens, likely affecting the northeast end of the island,
including Kodiak City. However, the relatively warm surface
temperatures will limit blowing snow.

&&

.MODEL DISCUSSION...
Overall, model guidance is in very good agreement with large scale
features. The primary forecast challenge in the short-term is
determining how much precip from the Gulf low will make it to
Kodiak Island and how quickly surface temperatures will fall
once snow develops. All guidance indicates some wobbles in the
low as it develops and tracks southward, so will need to keep an
eye on satellite imagery to track heavier bands of snow.

&&

.AVIATION...
PANC...Ceilings and visibility will steadily improve this
evening as snow ends and northerly low level flow strengthens.
The winds should keep any fog or stratus from forming, which often
occurs in the wake of fresh snow. Cold air advection will
increase on Monday and though not strong, it looks like just
enough to help mix down stronger winds from aloft to the surface.
Thus, have trended up with the wind forecast.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
The front across Southcentral slides to the Gulf this evening
bringing snow to an end across inland locations. A triple point
surface low which developed along the western Gulf this afternoon
deepens tonight as a strong upper level wave rounds the base of
the mid level trough. This strong wave closes off a new center
over the southern Gulf waters tonight keeping mixed precipitation
across the Gulf with a few showers moving into coastal locations
through early Monday morning. Skies will clear on Monday from west
to east across the mainland as subsidence occurs along a ridge
shifting in from the Bering. The main forecast challenge on Monday
surrounds the strength and timing of wind speeds across channeled
terrain as offshore flow develops in the wake of the low. Only
minor changes were made to increase speeds across the Matanuska
Valley and keep slightly higher gusts through midnight for
Thompson Pass as a jet streak moves overhead while an upper level
wave moves east from the Chugach Mountains. Tuesday will be
relatively quiet across the region as the low in the Gulf moves
further southeast.

Kodiak and the Gulf waters... Updated the forecast to take out
the Winter Weather Advisory as snow amounts are not expected to be
as high as previously expected. Snow remains in the forecast
through Monday as a developing shortwave moves from the Southwest
to the western Gulf, however snow accumulations were lowered.
Snow will taper off Monday night as the low pressure system slides
to the south.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The low pressure system that brought snow, freezing rain, and
strong winds to the area will continue to move off to the
southeast into the Gulf of Alaska tonight. A few remaining snow
showers over the Kuskokwim Delta and higher terrain to the east will
taper off this evening. Cold air and strong winds will filter
down from the north on the backside of the low, which could lead
to some areas of blowing snow where light powdery snow had
previously accumulated, especially inland areas to the north.
Along the Western Capes, strong northerly winds with gusts to
40-50 mph will continue as a low level jet just to the west over
the Bering Straight moves through the area.

By tomorrow, a transitory upper level ridge moves in bringing
clearing skies and diminishing winds, though temperatures will
drop to their lowest levels of the season at some locations.
Cloud cover will increase again by Monday night as the next system
approaches from the northwest bringing another round of
precipitation and windy conditions on Tuesday. This system looks
to be a bit weaker than its predecessor, though snow should fall
across most areas with rain still likely along the Western Capes.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Strong winds will continue tonight along the Eastern Bering and
Aleutians as a northerly low level jet descends through the Bering
Strait. Aided by a 160 kt jet streak at 300 mb and mid level cold
air advection from the Arctic, winds will mix down and accelerate
through channeled areas and terrain gaps leading to gusts up to
75 mph from Dutch Harbor east. Winds will quickly diminish tonight
and tomorrow morning as an upper level ridge moves over the
Eastern Bering. Therefore, no changes were made to the current
high wind warnings and are on track to expire at midnight for
Dutch Harbor and 6 am for Cold Bay

Further west, another system approaches as a strong warm front
attached to a low anchored over Eastern Russia sweeps northeast across
the Western Bering/Aleutians bringing rain and gale force winds to
the area tonight through tomorrow night. The pattern continues as
high pressure builds in rapidly behind the departing system
bringing a quick relief in the active weather before another
strong low pushes in from the southwest Tuesday night. This storm
is progged to rapidly deepen as it moves across the Bering on
Wednesday and looks to be the makings of a possible strong winter
storm late this week for the Eastern Bering and Southwest mainland.

&&

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

On Tuesday, a low well north of the Arctic Circle will bring a
front through the southern portion of the mainland, with a brief
shot of snow for the entire region and some short lived outflow
winds for the Southcentral area. Tuesday night and Wednesday will
be the literal calm before the storm for most of the state, as a
low pressure system enters the western Bering late Tuesday night
and strengthens rapidly as it crosses the Bering.

Models have been relatively consistent with the track of this
storm varying a little to the south over the past day. Current
forecast is for the low to cross the northern Bering and move
onshore near Nunivak Island late Wednesday night, before moving
southeastward over the Kuskowim Delta and Bristol Bay before
heading into the Gulf on Thursday night. It is still expected to
be a storm force low and significant winds and waves are currently
in the forecast for the Bering Sea...with some concern for high
winds and seas in the Bristol Bay region on Wednesday. Behind this
storm another northwesterly punch will bring high winds for the
Pribilofs and the AK Pen, not unlike the pattern from this
weekend. As the storm moves into the Gulf, northwesterly outflow
winds will again be the concern for Southcentral on Friday. On
Saturday, the pattern begins again with another low approaching
from the southwest, although this system appears to take a more
northerly track along the Kamchatka Peninsula, hopefully lessening
the wind and wave threat in Alaska for next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 181 185.
MARINE...Gale 127 130 138 139 160 173 174 178 179 181 185.
		 Storm 119 120 131-137 150 155 165-172 180.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...LF