Small Craft Advisory 112, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
412 AM AKST Wed Feb 6 2019

.ANALYSIS AND UPPER LEVELS...
Water Vapor Satellite imagery shows a broad ridge spanning across
the mainland and Gulf of Alaska early this morning with a closed
low embedded just north of the ridge center. This formation
started yesterday as a strong shortwave developed along the
western periphery of the ridge, and then organized enough to
maintain a small mesocyclone which is slowly tracking eastward.
Not much impacts with this interesting feature; however, there is
a fluctuating pattern of patchy fog and low stratus along the
southern mainland.

To the west...low pressure dominates the Bering with a closed
center moving northward along the western Aleutians. Gusty
southerly winds are noted across much of the Bering will rain
developing along a frontal boundary extending from the Alaska
Peninsula and north to the west coast.

&&

.MODEL DISCUSSION...

Models are good agreement with the overall synoptic pattern
through early Friday, then begin to diverge on solutions. The
overall pattern is shifting to an amplified ridge over the
mainland with a trough over the Bering. The difficultly for the
models, heading into the second half of the week, arises in
handling the energy of multiple lows through the upper level flow.
The first system crosses to the Bering today and becomes an open
wave on its northward track. This is quickly followed by another
significant wave early Thursday which is expected to follow a
similar pattern, closing a center over the western Bering waters
before lifting north into an open wave.

&&

.AVIATION...

PANC...Low ceilings and patchy fog this morning are expected to
improve during the afternoon, however conditions will dimish again
tonight a ridge traps low level moisture near the surface.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Some precipitation remains with a system over the eastern portion
of Southcentral, from about Valdez eastward. This will slide east
of the area this morning. An upper level ridge will build over the
area today and remain in place through Thursday night.
Considerable stratus remains over the western portion from Homer
north through Anchorage and the Matanuska and Susitna valleys.
Some fog and flurries are also expected this morning. This will
be slow to clear today. More clouds will move over this western
portion of the area tonight through Thursday night as the ridge
axis sets up just to the east, at around 145 degrees west
longitude.

&&

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

The forecast remains on track with great agreement seen in the
numerical model forecasts, leading to an above average forecast.
The main challenge will be determining how quickly inland
locations warm, especially east of the Kilbuck Mountains and
north of the Nushagak Hills. These valleys may experience more of
a mesoscale cold air damming effect (when the warmer air aloft
flows over the mountains versus fully mixing down to the valleys,
due to the steepness of the terrain and the diurnal cycle of the
valley/ridge breeze). This leads to some mixed precipitation
concerns, but areal and temporal considerations looks like it will
have a minimal affect for the populated locations.

In general, we can't rule out some light precipitation for the
coastal locations early today, but it looks like the main brunt
of the precipitation will move into the region during the late
afternoon to evening time frame. As the front spreads from
southwest to northeast across the region, a fairly stout low-
level jet in excess of 50 knots and 700 mb RH progs (courtesy of a
very weak tropical connection) will help to ensure there's plenty
of moisture available. Now the question is, what will ignite this
precipitation to develop...the answer looks to be a
weakening/stalling surface occlusion front. Model progs
(projections) indicate enough surface convergence to initiate
parcel lift, with strong lobes of vorticity in the mid levels
providing ample ascent (lift) aloft for decent cloud growth
resulting in widespread and efficient precipitation development.
With multiple weak surface waves aided by strong warm/moist
advection aloft, and strong disturbances in the mid and upper
levels of the atmosphere, a rather wet period looks to continue
through the end of the work week.

&&

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

A weakening 970 mb low currently south of Shemya will continue
moving towards the Pribilof Islands while weakening by tomorrow.
Its associated surface front will stall and weaken near the coast
of the Kuskokwim Delta and extend down towards the tip of the
AKPEN. Multiple disturbances in the flow at both the surface will
keep an active period over the eastern half of the Bering and
Aleutians, and into the AKPEN. Winds will be decreasing through
the day to small craft criteria for the offshore waters. However,
the coastal waters taper off slower through Thursday, before
lowering for Friday.

Further west, multiple lows will play tag with the western border
of the forecast area, with periods of showers. The real story will
be a strong surface low deepening to sub 970 mb, and its
associated surface front, which will move into the western Bering
Thursday afternoon and sweep eastward through Friday. widespread
precipitation will develop, with sustained gales likely ahead of
the front and near the low itself.

&&

.MARINE (Days 3 through 5)...

Beginning Friday, there is generally good confidence that a ~970
mb low will track just to the west of Attu bringing a wide swath
of southwesterly gale force winds to the Bering/Aleutians west of
the Pribilofs through Friday night. There is also the chance for
some areas of storm force winds to develop over the western
Aleutians/Bering by Friday evening, though these are not expected
to be long lived. Winds and waves are expected to subside some as
the low pushes north by Saturday morning, with generally high
confidence that winds further diminish Saturday night into Sunday
as ridging temporarily builds in over the western Bering.
Confidence is increasing regarding a strong ~960 mb low
approaching the western Aleutians from the southwest by Sunday
afternoon. The initial front associated with this low is likely to
produce gale force winds from the southeast across most of the
Bering. There is also increasing probability that a swath of storm
force winds will push through the western Bering/Aleutians by
Sunday night.

Over the Gulf of Alaska, high pressure will generally lead to
tranquil conditions through Saturday. By Sunday, a low is expected
to approach the western Gulf, though there is still a fair amount
of disagreement regarding this system. Generally expect winds and
waves to increase over the western Gulf through the day on Sunday.

&&

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

Beginning Friday, an upper level ridge will dominate the long wave
pattern over most of the Gulf of Alaska and the eastern mainland.
An upper trough will be located over the western Bering leading to
an active storm track across the western half of the area. Strong
southerly flow in between the ridge and trough will pump warm and
moist air north into the southern mainland, with precipitation,
mostly as rain, expected over the southwest mainland Friday and
Saturday. The forecast challenge will be determining how far east
the ridge axis pushes and weakens over the weekend. The GFS keeps
the ridge a bit further east and flattens it, allowing the storm
track from the Bering to push further east into Southcentral,
while the EC amplifies the ridge, leading to more of a blocking
pattern. Regardless, both solutions show a system pushing a front
into the western Gulf by Sunday, bringing increasing chances of
precipitation to Southcentral, especially along the coast.

By Sunday, a strong cold front looks to push east across the
Bering and mainland nudging the upper ridge to the east. This
will bring precipitation across most the southern mainland with
colder air pushing in behind the front through Monday morning. An
even stronger low will push east across the Bering on Monday
bringing a wide swath of precipitation across the mainland through
Tuesday with a surge of warmer air ahead of the system, thus
precipitation types will be challenging.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 155 165 172 178 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION/AVIATION...KH
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...KVP