Small Craft Advisory 51, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKST Tue Jan 16 2018

.ANALYSIS AND UPPER LEVELS...
A mature vertically stacked low is tracking across the southern
Gulf of Alaska toward Kodiak Island with a warm occlusion out
ahead of it pushing westward across Bristol Bay and the Kuskokwim
Valley. There is a band of light snow right along the front, but
much warmer air has moved in behind the front, from interior
Bristol Bay across most of Southcentral Alaska. Snow levels have
risen above 3000 ft for coastal mountains, highest across the
Kenai and western Chugach ranges. Rain or rain showers are focused
along the Gulf coast and Kodiak Island, while strong easterly
winds are producing downslope drying across much of the rest of
Southcentral. The one notable exception is the Copper River Basin,
where an upper level short-wave is tracking overhead producing
fairly widespread precipitation across the basin. There are very
few observations in this zone, but based on web cams it appears
precipitation is falling mainly in the form of snow.

Meanwhile, out west much of the Bering Sea and Aleutians are under
a showery regime with weak low level cold advection and gusty
winds through channeled areas of the Eastern Aleutians and
southern Alaska Peninsula. Winds are trending stronger along the
Alaska Peninsula as the core of cold advection moves in from the
west and pressure gradients tighten ahead of the low in the Gulf.
Further west, a gale force low is approaching the far Western
Aleutians.

&&

.MODEL DISCUSSION...
Model guidance is in good agreement with large scale features and
forecast confidence is generally above average. The main forecast
challenges include local winds and temperatures across Southcentral
tonight through Wednesday along with precipitation-type in Bristol
Bay and the Copper River Basin.

&&

.AVIATION...
PANC...VFR conditions will persist thanks to persistently strong
easterly flow across the Kenai and Chugach Mountains. Low level
wind shear will become increasingly likely as the low over the
Gulf moves to the vicinity of Kodiak Island on Wednesday and
pressure gradients begin to build along western Turnagain Arm.
Also, as the low weakens near Kodiak Island on Wednesday pressure
gradients over Cook Inlet will weaken. The Turnagain Arm jet,
which is currently bending strongly southward as it comes out of
the arm, will then bend back northward. Pressure gradients do not
support the wind bending all the way into the airport, so think
wind will remain to the south near Fire Island.

&&

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

The stacked low pressure system along the southwestern gulf tracks
northwest across Kodiak Island tonight bringing gusty winds and
rain across the gulf and coastal areas. Gusty winds also increase
for Cook Inlet and around Turnagain Arm as the pressure gradient
along the coast tightens in response to the approaching low. Low
level easterly flow keeps inland areas on the drier side, however
chances for precipitation remain through Wednesday afternoon then
diminish as the stacked low shears apart between building ridges
along the eastern Gulf and Bering. Patchy fog will likely develop
along weak boundaries across the Susitna and Copper River Valleys
tonight as temperatures drop to near dewpoint values and should
improve shortly after daybreak on Wednesday. Winds across the gulf
and along channeled terrain taper down Wednesday afternoon as the
low weakens with the flow pattern changing to offshore heading
into Thursday. A trend towards colder and drier conditions begins
on Thursday as high pressure builds across the Bering forcing an
Arctic air mass slowly toward the Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wed and Thu)...
The low that pushed the warm front (and snow) through the area
this morning will drift towards Kodiak Island and fall apart.
This will allow some warm air and light precipitation to move over
much of the Aleutian Range and towards Bristol Bay this evening.
With most of the surface temperatures in the area still below
freezing, this will set the stage for the possibility of some very
light freezing rain through the evening into the overnight hours.
A Special Weather Statement has been issued to address this
concern. Areas to the west of the warm air (Togiak up through
Aniak and Bethel) will continue to see light snow. The snow should
linger through Wed over the YK-Delta resulting in a couple inches
of accumulation.

The pattern will not change much on Thu. Some colder air will get
pulled down through the Eastern Bering. This should allow for
temperatures to cool off enough to change most of the
precipitation back over to snow. With lingering moisture being
pulled up by the aforementioned low, the threat of snow will
remain in the forecast. However, none of the precipitation looks
overly heavy, so any snow accumulations would be quite light.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Wed and
Thu)...
Most of the Bering will see some cold air advection and snow
showers, while the far western portion of the domain will see a
new low from the North Pacific push through. First, the cold air
advection will start to increase tonight into Wed. This should
lead to impressive snow showers across the Eastern Bering and
especially over the northern side of the Alaska Peninsula. This
cold air will also seek to push through the gaps on the peninsula.
This will result in gusty winds at places like King Cove and out
into the marine zones south of the peninsula. These winds and
showers will slowly diminish on Thu.

Further west, the low will bring a gale-force front through the
Western Aleutians and into the Western Bering. In addition to
winds, it will also bring some precipitation that will start as
snow, then mix with rain before trying to turn over to all rain.
This low will stall out and start to fall apart Wed into Thu. As
it does so, it will wrap in some colder air from Siberia. This
will bring another swath of gale-force NW to SW winds back across
the Western Aleutians. This will also produce wide-spread snow
showers that extend all the way to the Central Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
As advertised in the past couple of long term forecast
discussions, a major pattern shift is expected over most of
mainland Alaska this weekend and into next week. The upper level
pattern, which has been leading to warm and wet conditions over
southcentral, will migrate east this weekend with a high amplitude
upper level ridge setting up over the central Bering and an upper
level trough developing over central/eastern Alaska. This will
lead to strong northerly flow through the Bering Straight and over
SW Alaska which will advect in bitterly cold air down from the
arctic. The source of this cold air is currently bottled up over
northeastern Russia where temperatures this week were recorded as
low as -80 F.

Models all show this cold airmass being pulled south into western
Alaska by the end of the weekend, though significantly modified
by the time it reaches the region. Long range ensemble guidance
shows marginal odds of minimum temperatures across western and
interior portions of Alaska being in the bottom 15th percentile of
climatology, or in layman's terms, temperatures are likely to be
well below normal next week, though not quite to record levels.
Temperatures could drop into the -30's to -40's along interior
sections of SW Alaska, with sub zero temperatures reaching the
Bristol Bay Coast. This will be the coldest air of the season so
far and may be a shock to residents who have been getting use to
the near record warmth this winter. Luckily for residents of
southcentral, the coldest air will be blocked by the Alaska Range,
though temperatures will fall back to normal or slightly below
normal values by early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 125 130 138 139 140 150 155 178.

&&

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