Small Craft Advisory 216, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 AM AKST Thu Mar 5 2020

.ANALYSIS AND UPPER LEVELS...

The pattern continues to remain amplified as the jetstream remains
in a south to north orientation extending from the subtropical
Pacific through the central Bering/Aleutians into Southwestern
Alaska; before sharply extending southeastward into the North
Pacific. A 993 mb low located near the western Aleutians will
continue to move northeastward into the Bering. A ridge-axis
associated with a strong 1040 mb high over the North Pacific is
moving eastward into Bristol Bay. A strong 975 mb low is currently
stationary in the southeastern Gulf.

At the surface, a 1035 mb high is currently over the interior of
Alaska. Strong winds are observed in Thompson Pass, Valdez and the
Matanuska Valley as a strong southeast to northwest pressure
gradient is present combined with a cold dense airmass in the
Copper River Basin.

&&

.MODEL DISCUSSION...

Models are generally in good agreement with the overall synoptic
pattern across the area at both the surface and upper-levels for
the short-term. Models then begin to show significant differences
by Saturday with the placement of a low in the Bering. Mesoscale models
were favored for local effects due the higher resolution of these
models.

&&

.AVIATION...
PANC...VFR conditionss continue for this TAF package as ceilings
remain unlimited and visibility unrestricted. North winds at
15-20kts may gust 25-30kts at times. Winds begin to ease
at 21z and and become light at 00z.

&&

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

Outflow and local gap winds will be the main weather concerns
today. An upper low over the eastern Gulf weakens as the ridge to
the west builds east. A northerly upper jet between these two
features has setup over Southcentral and will move slowly east.
At the surface, cold air continues to spill into Southcentral and
through gaps in the Aleutian Range today which keeps a fairly
strong thermal gradient between the maritime and continental air
masses. This combined with strong pressure gradients and favorable
upper support will continue brisk gap winds through the evening
hours. The strongest winds will be along the North Gulf Coast
where warning level gusts could potentially continue into the
early evening hours across the Valdez zone and through the Copper
River Delta. Outflow and gap winds will likely diminish in most
areas overnight and be comparatively light on Friday. Cooling
temperatures and gusty winds will continue moderate wind chills
into the evening hours before gap winds diminish.

A weakening surface front and advancing upper trough will bring
clouds and chances of snow to Southcentral Friday night.

&&

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

Areas of fog have been observed on satellite and confirmed with
observations this morning from Bethel to the Kuskokwim coast.
Otherwise, clear and dry conditions are expected to persist
across the rest of southwest Alaska through Friday morning as an
area of high pressure remains over the region. By late Friday
morning, snow is expected to develop along the coast as a
weakening frontal system approaches from the eastern Bering. The
upper level energy associated with this feature will shear apart
and move southeast preventing any precipitation from moving very
far inland. Widespread snow, however, is expected to develop by
Saturday morning as a second shortwave propagates southeast from
the northern Bering. Snow will then remain across the southwest
through late Saturday.

&&

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

A pressure gradient between a ridge over the central Aleutians
and a trough in the Gulf remains strong. Therefore, storm force,
northwesterly wind gusts will continue through the bays and passes
on the Pacific side of the Alaska Peninsula through early this
morning. These winds will subside today as the ridge propagates
eastward and weakens as it moves inland over southwest Alaska.

Upstream of the ridge, a gale force front over the western and
central Bering will move into the eastern Bering by this
afternoon. Ahead of the front, strong southerly flow will bring
warm and moist conditions to the central and eastern Aleutians.
Expect temperatures nearing the 40s and snow transitioning to
rain. The front then looks to move across the Pribilofs tonight as
it enters the eastern Bering. Warm advection aloft will not
propagate that far north, keeping temperatures near freezing for
the islands. Expect precipitation to stay all snow, with 3 to 5
inches of accumulation possible by early Friday morning. Blowing
snow is also possible in areas due to higher gusts. The low will
then begin to weaken and shear apart as it reaches southwest
Alaska by Friday afternoon.

A front associated with the next low will move across the central
and eastern Bering and Aleutian Chain by early Saturday.

&&

.MARINE (Days 3 through 5 Sat through Mon)...

Gale force winds are not expected for this weekend into Monday.
The most active weather will be in over the Bering Sea and
Aleutians, where a low moving across the Bering will produce
winds that will stay just below Gale. Model uncertainty is high
enough that this system has the potential to produce gales.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through Wednesday)...

An more active pattern is expected to develop across Southcentral.
Confidence is growing for possible snow beginning Saturday morning as
a weak upper level trough moves over the region. A second
shortwave trough will track over Southcentral through Sunday,
likely bringing another round of snow. The main trough and
associated upper low will move near Prince William Sound Monday
into Tuesday. The GFS model stalls this low and continues
indicating precipitation though Wednesday, while the European and
Canadian models shows the low departing much more quickly. In any
case, while confidence is above average for accumulating snow in
the more active pattern, it is too early to determine exact timing
and amounts.

Across the Bering, the models begin Saturday in very good
agreement, with a low over the western Bering and the leading
front near the Pribilofs that moves into Southwest Alaska during
the day. Model agreement diminishes by Sunday evening, with the
models indecisive on whether the Bering low moves into Southwest
Alaska during the day, or if it remains out over the Bering. After
that, a short period of cold northerly flow returns by Monday. How
long the cold air lasts and when the next round of unsettled
weather impacts the area remain questions that involve significant
uncertainty. In summary, this weekend will be quite unsettled over
much of the area, with generally colder and quieter weather
returning heading into the workweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131.
         Wind Chill Advisory 145.
         High Wind Warning 131.
MARINE...Gale Warning 119 120 121 125 126 128 130 137 138 352 150
         170 172 173 174 175 176 177 178 411 412 413 414.
         Heavy Freezing Spray Warning 119 120 121 125 126 127 128
         129 130 131 132 136 137 138 139 141 150 165 180 185.
         Storm Warning 127 131 132.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...BL/AP/JW