Small Craft Advisory 128, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 AM AKST Fri Feb 9 2018

.ANALYSIS AND UPPER LEVELS...
An active pattern is underway across the Bering with a broad
trough centered over the central waters. The next closed low
within the flow is on a northward track just south of the Central
Aleutians early this morning. Abundant moisture streaming from
the north Pacific spans across the Alaska Peninsula to north of
the Bering Strait. The synoptic pattern remains highly amplified
with a ridge centered across the mainland, however its center has
shifted position to the south over the Kenai and northern Gulf of
Alaska.

At the surface, a relatively quiet regime continues for Southcentral
as the surface ridge shifts across the Copper River Valley. The
Southwest is starting to see increased southeasterly flow with the
next front squeezing the pressure gradient along the eastern
Bering and coast. The surface low driving the front is stacked
under the closed upper low south of the Central Aleutians.

&&

.MODEL DISCUSSION...
Models are generally in good agreement with the synoptic set up
through the beginning of the weekend. However, there are
important differences between the models in the evolution and
track of the storm system heading to the southern Bering today.
The lack of model continuity on this system leads to lower
forecast confidence as the 00Z model runs shifted the associated
front slightly further to the east.

&&

.AVIATION...
PANC...Patchy dense fog across the northern inlet keeps LIFR/IFR
conditions through much of the morning before improving VFR this
afternoon. Fog return is not anticipated tonight as low level flow
is expected to increase across Turnagain Arm which should bring
enough mixing down to the surface to inhibit development. Patchy
fog may develop along the Knik Arm but should not bring impacts
to the vicinity. Light surface flow prevails through the TAF package
as a surface ridge settles across the southern mainland today and
slowly shifts east on Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Areas of fog and stratus in the Anchorage and Matanuska Valley
areas should largely dissipate by this afternoon as low level
winds begin to increase out ahead of a front moving into the
southwestern Gulf. This will create just enough vertical mixing
to help mix out the fog. Otherwise, Southcentral will see another
sunny and seasonably cool day before high clouds move in tonight.

The front moving into the Gulf will start out quite strong, with
high end Gales to low end Storm force winds expected across much
of the western Gulf today through tonight. The northward
progression of the front will be slow, so expect a prolonged
period of moderate to heavy precipitation along the east side of
Kodiak Island. Though precipitation may start out as a rain/snow
mix as precipitation spreads to Kodiak Island this morning,
temperatures will quickly warm leading to a change to all rain.
In fact, snow levels will rise to 200 feet or higher tonight.
Expect one to two inches of rain along the east side of Kodiak
Island by tomorrow morning.

With strong ridging holding steady from Southeast Alaska to
Northern Alaska, the front will begin to elongate and weaken as
it continues northward across the Gulf on Saturday. In the upper
levels one short-wave trough will head northwestward across
Southwest Alaska tonight while another drives eastward into the
base of the ridge, further weakening the front Saturday night.
Therefore, expect the bulk of precipitation won't make it further
north than the Kenai Peninsula. Temperatures look like they will
remain cold enough for some light snow, mainly along the immediate
coast.

There will be one final short-wave trough lifting northward
across Southcentral Saturday night through Sunday. However, it
will be in a state of weakening as heights rapidly build behind
it ahead of the next storm system moving up from the Pacific.
Have maintained a broad-brush chance of precipitation across
Southcentral. If any snowfall does materialize, it will be very
light.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The front over the eastern Bering is making its way over southwest
Alaska and as it does it will bring precip and winds to the area.
There is some disagreement on how far inland the heavier precip
of the front will reach and how much warm air will be ushered in
from the south. The most likely scenario is areas around Bristol
Bay changing to rain quickly while areas around the Kuskokwim
Delta look to stay snow longer. However, all areas of southwest
Alaska will see temperatures rise through the weekend.

Along the coast of the Kuskokwim Delta the combination of snow
and winds will bring blowing snow conditions to the area into the
evening. The blowing snow will bring visibilities down to one half
mile at times. Tonight the heavier precip and winds will diminish
somewhat reducing the blowing snow but light showers will remain
over southwest Alaska through the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
With the longwave trough to the west, the Bering and Aleutians
will see a series of lows track north over eastern portions of the
area through the weekend. This will bring warm air and rain to
these locations as the lows and their associated fronts move
through. Out west, ridging will build in from the west and track
east over the central Aleutians by Sunday. Behind the ridging a
front will move in over the western Aleutians bringing warm air
and rain to the islands Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The early part of next week is beginning to come into better
focus as models are becoming consistent enough to see a reasonable
track and timing for major features. There is still a lot of
uncertainty for the second half of next week however as the
pattern will become more progressive than it has been in a number
of weeks.

Sunday and Monday will see much warmer temperatures around
Southcentral Alaska with many areas reaching above freezing.
Precipitation is a bit more likely for Monday but downsloping may
keep the majority relegated to the coastal areas. The interesting
part of this forecast is that, while lower levels are showing
downsloping, there is enough south-to-southwest flow aloft over
Southcentral that it could end up bringing in some precipitation
to the Lee side of the mountains where it typically remains dry.
The precipitation-type will also be a challenge with the potential
for a mix of different precip types. Beyond Tuesday, temperatures
should remain mild, but not as warm as Sunday and Monday.

On Sunday, the low in the Bering Sea will have occluded and
washed a bit of the warm air out aloft over Southwest Alaska.
Monday will see another pulse of warm air in conjunction with the
next low which is projected to move through Bristol Bay and then
inland. There will be a sharp delineation between the warm and
cold sectors with this low with areas west of the low remaining
cold and east of the low warming up. This continues to cause
precipitation-type challenges for Monday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Advisory 155.
MARINE...Storms 130 150 160 180.
         Gales 120 131 132 136 137 138 139 155 165.

&&

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