Small Craft Advisory 126, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
553 AM AKST Fri Feb 7 2020

.ANALYSIS AND UPPER LEVELS...

There is an upper-level low situated over Bristol Bay this
morning. At the surface, a low near Port Heiden is swinging an
occluded front over the AKPEN and across the Barren Islands. Areas
of light snow have developed along the front as it lifts north
toward Cook Inlet. Ahead of this front, an upper-level shortwave
is kicking off widespread snow showers across Prince William
Sound. A secondary surface low near Dillingham is interacting with a
stalled arctic front draped across the Kuskokwim Delta. An area of
diffluent flow resulting from this interaction is providing enough
lift for widespread light snow across Southwest Alaska. This frontal
boundary marks the delineation between a warmer, southwesterly flow
across Bristol Bay where temperatures this morning range from the
mid 20s to lower 30s and an arctic airmass and gusty northeasterly
winds lowering temperatures well below 0F across the Kuskokwim
Delta.

Over the Bering, an upper-level shortwave is producing some snow
showers across the Pribilofs and is also enhancing the gusty
northeasterly winds. A 1000 mb low over the AKPEN is pushing a
trough and some scattered showers toward Kodiak Island this
morning. A storm-force 964 mb low is spinning southwest of Attu.
This system is draping a front across the western Aleutians with
a triple-point low developing south of Atka. This low has
deepened quite a bit overnight as it lifts north toward the
central and eastern Aleutians.

&&

.MODEL DISCUSSION...

Guidance continues to remain in decent agreement on the synoptic
pattern into the weekend. The biggest difference continues to be
with the evolution of the hurricane-force low crossing the eastern
Aleutians late Friday then weakening as it moves over the Y-K
Delta by late Saturday. The Canadian GEM remains the slowest
solution by 6 hours or more while the EC is still the fastest. The
recent 06z GFS run has come more in line with the 00z and 06z NAM,
thus a blend of the two was used for the morning forecast package.
The biggest challenge with this system is precipitation type
across the northern coast of Bristol Bay and strength of the winds
from the eastern half of the Aleutian Chain to Southwest Alaska.
But given the minimal impacts any model discrepancies have on the
sensible weather, average forecast confidence remains.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through late
morning before a frontal system brings snow and lower ceilings
over the airport complex around 21z. MVFR conditions are expected
through this afternoon with the potential for IFR conditions in
steadier snow. Light southerly winds will develop with VFR
conditions returning later this afternoon as the front departs
and the snow tapers off. LLWS is then expected to develop after
midnight as a low-level jet and Turnagain Arm wind develop ahead
of the next frontal system. Gusty southerly winds are then
expected to move over the terminal by early Saturday morning.

&&

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

An occluded front spotted on radar this morning crossing over the
Kenai Peninsula will continue its track eastward throughout the day.
As an upper level trough moves over southcentral this afternoon, the
Anchorage bowl and MAT-SU Valleys will see a short period of snow
with southwesterly flow at all levels, which could produce an inch
or two of new snow. The snow showers will taper off for these areas
overnight with brief upper level ridging building in.

Southcentral will see a more active weather pattern on Saturday as
a warm front moves into the region from the southwest. Moist
overrunning from this front will bring abundant snowfall starting
Saturday morning into late Saturday night, which will produce
6-12 inches of snow for the Susitna Valley. Therefore, a winter
weather advisory has been issued for this area. Kodiak will stay in
the rain as the front moves over them, while temperatures are
slightly above freezing for the Kenai Peninsula and Mat- Valley
could see a mix of rain and snow.

In addition, expect stronger winds and a slight warm up in
temperatures. The low over southwest Alaska will help keep an east-
to-west pressure gradient set up early Saturday morning into the
afternoon hours. This will lead to a period of higher winds through
Turnagain Arm, with winds peaking near 65 mph. As the front moves
north gusty southerly winds will spread across the Anchorage bowl in
the afternoon. Temperatures in town will rise during these stronger
chinook winds, causing high temperatures near 40 degrees. Winds
across the Copper River Basin will also be stronger starting
Saturday afternoon.

&&

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

A challenging forecast is expected through Saturday as a storm
system deepens on a northward track tonight. The impacts for the
next two days include cold wind chills across the Kuskokwim Delta
and snow and blowing snow across the southwest mainland.

Wind chills across the Kuskokwim Delta coast are expected to
continue through late tonight as gusty northerly flow persists.
Temperatures are expected to moderate warmer late tonight which
should improve wind chills. The front associated to the much
anticipated storm approaches Bristol Bay late this afternoon as the
surface low crosses over the central Aleutians at 964 mb. The front
is expected to spread snow quickly across Bristol Bay to the
Kuskokwim Delta overnight as a southerly low level jet elongates
northward from the Alaska Peninsula. Winter weather advisories are
now out for all of the Southwest locations as widespread snow along
with gusty winds reducing visibilities impact most of the region
tonight through Saturday.

Warm air intrusion is expected to change snow over to rain across
eastern areas of Bristol Bay, east of King Salmon during this
event. However, there is a bit of uncertainty on how quickly the
atmosphere will warm near King Salmon to Dillingham and north to
Koliganek. These locations are expected to at least briefly mix
with rain during the early morning hours on Saturday. The 850 mb
temperatures begins to cool through the column again Saturday
afternoon with cold air advection changing precipitation back over
to all snow Saturday night. Saturday night looks like the peak time
frame for snow amounts as the storm system lifts north toward the
interior and weakens. Products may need to be extended into Sunday
if this storm moves slower than current forecast trends.

&&

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

A powerful storm system currently developing south of the Central
Aleutians will bring strong gusty winds, rain and snow across the
Bering through Saturday. Gusty easterly flow will increase across
the Bering through this afternoon as the associated surface low
deepens on a northward track. Rain and snow will spread across the
Aleutian chain and Alaska Peninsula this morning changing over to
all rain by this afternoon for most of these locations. Kiska west
should stay cold enough to support snow. Pre-frontal snow
development is possible across the Pribilof Islands this afternoon
with diminishing weather expected this evening as blowing snow
brings blizzard conditions through tonight. High wind warnings were
issued with the morning package as models are in good agreement on a
large swath of winds developing late tonight. There is high
uncertainty on how quick the low progresses to the northeast, so
timing will likely need to be adjusted.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

A gale force storm will move through the central and western
Aleutians into the western Bering from late Sunday into Tuesday.
Storm force winds are possible along the frontal boundaries. There
is still considerable uncertainty for the timing and strength of
this system.

For the Gulf, gale force winds over the northwest Gulf and
Shelikof Strait Sunday morning will quickly diminish during the day.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday through Thursday)...

We start the long term period Sunday with a weakening low near
the Seward Peninsula, high pressure south of the Gulf of Alaska
and another storm on tap moving toward the Aleutians and Bering
Sea. Models are in generally good agreement with the location of
the low moving into the Aleutians Sunday. However, they differ
significantly with the intensity of this storm. The models have
similar solutions with the timing of the progression of the front
from this storm across the Aleutians and Bering Sea through Monday
but continue to diverge significantly with the storm center
through Wednesday, thus leading to little confidence in the
current model solutions. The approach is to use an ensemble and
blended approach beyond Monday.

This pattern and scenario support the Arctic air pushing back
across the southern mainland Monday and persisting through
Thursday, thus returning the mainland to the colder air, similar
to the past few weeks. However, here the models differ
significantly, and temperature forecasts Tuesday through Thursday
differ by nearly 20 degrees, again leading to very low confidence
in any one solution and low confidence in the extended forecast
period. The Bering Sea and Aleutians will be in the highly active
weather pattern, with frequent periods of strong winds and
precipitation through the extended period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155.
 	     Winter Weather Advisory 145 152 155 161.
 	     Blizzard Warning 195.
	     High Wind Warning 181 185.
MARINE...Hurricane Force Wind Warning 172 174.
         Storm Warning 155 160 165 170 171 179 180 412 413 414.
         Gale Warning 119 120 125 130 131 132 138 139 140
         150 173 173 175 176 177 178 181 185.
		 Heavy Freezing Spray Warning 160 179 185 411 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...AH/BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...BL/SA