Small Craft Advisory 189, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 AM AKST Thu Mar 7 2019

.ANALYSIS AND UPPER LEVELS...

An negatively tilted upper level ridge is currently in place over
the eastern and central mainland. Associated high level cloud
cover is indicated well on current IR imagery. Mixed
precipitation is occurring over Bristol Bay and the western
mainland along a stalling occluded front. The associated 960mb low
continues to move slowly across the western Bering. Gales are
currently being reported in areas south of the low, namely Adak.
Given the zonal flow pattern, winds are generally out of the west
over the Aleutians and as such, any higher gusts typically
associated with cold advection are not an issue at this time.

&&

.MODEL DISCUSSION...

Models remain in good agreement. The aforementioned 960mb low
will gradually weaken as it becomes vertically stacked. A second
strengthening low will merge with the current low, which will also
strengthen the pressure gradient out ahead of it. As such,
confidence is moderate for storm force winds over the Barren
Islands on Saturday and is reflected in the latest forecast. The
front is expected to be fast-moving as it will be aided by a
nearly 170 knot upper level jet.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. A front
approaching the area will lead to wind shear conditions developing
this afternoon as stronger southeasterly winds cross the
mountains. The wind shear will persist throughout the night into
Friday morning. Mid-level moisture crossing the mountains may
cause a shower or two to develop in the vicinity of the airport.
The Turnagain Arm wind, while active, is expected to remain south
of the airport through the TAF period.

&&

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

A front is approaching Southcentral Alaska from the
southwest and it is on track to bring precipitation to areas
throughout the region. The heaviest amounts will be along the
north Gulf coast and Kodiak Island with the eastern Kenai
Peninsula seeing the most. Precipitation type will be a challenge
with the front, with locations around the north Gulf coast
hovering around the rain/snow line. Conditions look to be cold
enough for precipitation to start as snow then transition to rain
by the afternoon for low lying areas and then back to snow
overnight. However, there is less confidence with the exact timing
of the transitions and how far inland the rain could make it.

Precipitation will last into Friday when it will be reinforced by
a trough approaching from the south. Similarly to Thursday,
precipitation will be heaviest along the north Gulf coast,
however, Kodiak Island will be less impacted by this second round.
Inland locations will still be mostly protected by the coastal
ranges and only see minimal amounts of precipitation. This setup
will also bring strong winds to some of the favored east/west
oriented gaps such as Turnagain Arm. Winds look to pick up this
evening and last into Friday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tonight
through Friday afternoon)...

An occluded front in the eastern Bering is quickly approaching
Southwest Alaska. The front will move inland overnight and weaken
as it continues to travel away from its parent low. However, for
a brief period tonight, gusty southeast winds will kick in along
the coastal areas of Southwest Alaska and will last through the
early morning hours on Thursday. This may cause snow on the ground
to be lofted, creating brief periods of blowing snow along the
Kuskokwim Delta coast through the overnight hours. Precipitation
will initially fall as snow, though a change to a rain/snow mix
and eventually rain will occur along the AKPEN for Thursday
afternoon. Upper level support over this surface front dissipates
quickly, leaving a mostly disorganized and weaker front stalling
over Southwest Alaska through Friday. As colder air wraps around
the dissipating parent low, any lingering precipitation will
likely change back to snow Friday morning, especially over the
Kuskokwim Valley. Showers are expected to last through Friday
afternoon, ahead of another front that arrives over Southwest
Alaska for the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tonight
through Friday afternoon)...

A storm force low centered east of Attu Island continues to slowly
move into the Bering. Its leading front moves into the eastern
Bering this evening with gale force winds diminishing from west to
east as the occluded front weakens and nears landfall over
Southwest Alaska. Meanwhile, cold air wrapping around the
backside of the low will produce an area of westerly storm force
winds south of the Rat Islands before diminishing to gales
Thursday morning. Precipitation associated with the front will
start as snow, however as warm air is advected northward, a change
to a rain/snow mix and eventually rain is expected. Behind the
front, a showery regime will fill in across the Bering for
Thursday. By Thursday afternoon, the low is centered west of the
Pribilof Islands and weakens rapidly as it becomes vertically
stacked. On the south side of the low, gale force westerlies are
still expected to persist across the central Aleutians, however
they will quickly diminish early Friday morning. This persistently
active pattern will allow for another storm force low to arrive
over the western Aleutians Friday afternoon.

&&

.MARINE (Days 3 through 5)...
(Saturday through Monday)

...Bering Sea and coastal waters of the Aleutians/AKPEN...

More of the same looks to be in store for the region, as the
overall pattern remains relatively unchanged through the period.
In general, look for another strong (~955 mb) low to pass roughly
250 nm south of Shemya Friday evening, reaching the Bristol Bay
area late Sunday morning while weakening somewhat.

The associated front will track north across the Aleutians late
Friday evening. Given the frontal orientation and track of the
low, winds will increase to sustained gales along and ahead of the
front for most of the Bering. South of the chain, gales will
develop following frontal passage by early Saturday morning, with
seas building in excess of 30 feet.

A brief respite will then follow for Sunday across western
portions of the area, as ridging builds across the region, with
decreasing winds and seas. However, another system (albeit
significantly weaker) will then enter the western Bering/Aleutians
for Monday.

...Gulf of Alaska...
(Sunday and Monday)

Following Saturday's frontal passage and strong winds, look for
diminishing wind speeds and wave heights across the Gulf for
Sunday and Monday. Southwesterly to westerly winds in the cyclonic
flow will allow good fetch length to be maintained. However, a
gradual decrease in the pressure gradient will result in coastal
areas still seeing small craft conditions on Sunday, before
improving for Monday.

Forecast Confidence: High

&&

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

The models remain in pretty good agreement through Monday, before
more disagreement develops. Overall, the pattern will feature a
fairly strong area of low pressure located near the Kuskokwim
coast of Southwest Alaska on Saturday. The associated surface
front will bisect the Gulf from northwest to southeast, with
short-wave ridging developing across the western Bering and
Aleutians. Another low (albeit weaker than its predecessor) will
quickly move into the area behind the ridge, with the surface low
tracking generally along the chain for Sunday into Monday, before
turning more northeast towards Southwest Alaska Tuesday morning.

It's after this point where a possible pattern change develops,
making the forecast more uncertain. Typically during these
transitional phases, the models struggle to agree and this
situation is now different. The overall theme is for an area of
low pressure to lift northward into the Gulf from the Pacific
sometime Wednesday. Here, the GFS portrays a 960 mb low
approaching the Gulf, with its European counterpart (the ECMWF)
almost 20 mb weaker, and the Canadian Global just showing a trough
entering the region. Looking at the ensemble means would lend
more support towards the European solution. The GFS was the only
model showing a series of strong lows developing for the Bering a
week or two ago, with the other models playing catch-up as the
time approached.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 170 172-178 185 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...PD/TM