Small Craft Advisory 355, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 AM AKDT Thu Apr 23 2020

.ANALYSIS AND UPPER LEVELS...

Low pressure still continues to spin in the North Pacific south of
the Aleutians. A spin-off low is currently under development to
the northeast of this system, which can be observed on satellite.
This system is tracking north towards the eastern Aleutians as it
develops and sending cloud cover into southwest AK. Also out
west, the leading edge of the front anticipated to arrive in the
western Bering is showing up on satellite. An ASCAT pass shows
continued northeasterly winds in the eastern Bering and easterly
winds in the western Bering. This verifies our forecast and means
its on track.

A shortwave ridge has moved into the Gulf of Alaska and is nosing
into the southcentral part of the state. This ridge is trapping
moisture under it, so low clouds and fog continue for many areas
in mainland AK. So, not a whole lot of change in the pattern.

&&

.MODEL DISCUSSION...

Models continue their generally good agreement in the synoptic
pattern. Disagreement in the placement of low centers both in the
upper levels and the surface begin to develop Friday, indicating
a possible decrease in model performance going into the weekend.
Namely, models have placement difficulties for a low moving
through the southern Gulf of Alaska and are having trouble
tracking a low that moves along the western and central Aleutians
Friday into Saturday. Mainland Alaska does not appear to be
affected by these developing disagreements, so confidence is
still high for most of our forecast area.

&&

.AVIATION...

PANC...MVFR ceilings are expected early this morning over the
airfield this morning with the continuance of low-level cloud
cover. Clouds should lift late morning to VFR conditions with some
day-time heating. However, MVFR ceilings will redevelop tonight.
Visibility is not expected to be an issue with fog not in the
forecast. Winds will remain light.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A moist air mass will remain over the area through Friday for
continued showers and considerable cloudiness. A weather front
south of Kodiak Island will work its way up to the Kenai Peninsula
on Friday and then stall. On Friday night high pressure will build
into Southcentral from the north. This will weaken the front and
push it back southward across the Gulf. This will also begin to
push the moist airmass southward, for the beginning of a drying
trend.

&&

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

A North Pacific low that is currently south of the eastern
Aleutians will gradually curve northwestward to over the AKPEN. A
warm front associated with this low will move over Southwest
Alaska today and tonight and allow for temperatures to slowly
rise as warm air advection will dominate across the region. This
front will bring rain to Bristol Bay today. Precipitation will
begin as showers in the morning with a more steady rain developing
in the afternoon. Due to the strong warm air advection from the
southeasterly flow off the Pacific Ocean, temperatures from the
surface through 850 mb remain above freezing and thus
precipitation type will be in the form of rain. As the front
continues northwestward on Friday, rain will gradually taper off
in the morning. Temperatures will remain mild as southeasterly
flow will prevail as the low center will remain west of the
region.

&&

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

An area of low pressure is currently developing east of the Kurill
Islands and southwest of the Aleutians from being located in the
left-exit region of the subtropical jet. The left-exit region is
an area along a jetstreak that is conducive to storm development.
This low is expected to move northeastward toward the Aleutians
today. This low will bring Small Craft southeasterly winds to the
western Aleutians early this afternoon. Winds will gradually
strengthen across the western Aleutians Thursday night into Friday
morning as the low center moves closer to the western Aleutians.
Due to the increased pressure gradient between high pressure over
eastern Siberia and the center of this low, low-end Gale force
easterly winds will develop north of the western Aleutian chain
Friday morning. Winds will then gradually ease Friday afternoon as
the low begins to move southeastward into the North Pacific.

An area of low pressure over the eastern Aleutians and western
AKPEN will bring precipitation to the Pribilof Islands late Friday
morning through Friday evening. As 850 mb temperatures remain
steady or slowly rise, precipitation type will be in the form of
rain. There is some uncertainty if precipitation will start off as
a rain/snow mix as 850 mb and 925 mb temperatures will remain
slightly below freezing during the late morning and early
afternoon hours on Friday at the onset of the precipitation.
Though, there is much higher confidence that any rain/snow mix at
the onset will change to all rain by mid-afternoon.

&&

Marine Discussion (Days 3 - 5: Saturday through Monday)

Bering Sea/Aleutians

A large area of low pressure over the Gulf of Alaska coupled with
high pressure building into the western Bering Sea will result in
persistent northerly flow through Monday for most of the Bering
Sea. A low and front do move into the extreme western Aleutians
and extreme western Bering Sea late Sunday into Monday that has
the potential to bring southerly gale force wind to those regions
Monday. Confidence is high with the overall pattern, and average
with respect to gale force wind Monday.

Gulf of Alaska

The large area of weak low pressure will persist over the Gulf of
Alaska Saturday through Monday. We do not anticipate any gale
force winds in the Gulf or coastal locations with this weather
pattern. There is high confidence in the forecast as the models
have been consistent and in good agreement.

&&

Long Term Discussion (Days 4 - 8: Sunday through Thursday)

The pattern will be slow to change across the Southern Mainland,
while the Western Aleutians see much more active weather during
this period. First, for the Southern Mainland, a broad area of low
pressure will slowly transit across the Gulf of AK from Sun
through Mon. It will then set-up residence over the Eastern Gulf
through early next week. As the low shifts across the Gulf, this
will be key for the amount of moisture that can make it into
Southern AK. It looks like coastal locations will remain
vulnerable to showers through early Mon. After that though, the
low moves too far east while it actually deepens some. The net
effect for Southern AK will actually be a slight increase in the
offshore gradient (flow). This should lead to continued drying and
maybe even some breaks in the cloud cover as we head into next
week. Temperatures will remain seasonal, especially if the sun can
come out. The coldest air in the state will remain trapped north
of the AK Range.

The Aleutians will be quite a different story.  Sunday will
represent a transition across much of the Aleutians. Some weak
cold air advection across the Eastern Aleutians and Alaska
Peninsula will help to make some breezy N-NW winds and could
produce some spotty showers (especially on the north side of
terrain). These winds will get an extra 'push' from a shortwave
ridge of high pressure building over the Central Aleutians late
Sun-Mon. This ridge should lead to a drying trend across that
area, but could also bring enough stability to bring foggy
conditions. The first in a series of fronts will move into the
Western Aleutians Mon. This will bring near gale-force winds and
rain. This system dissipates fairly quickly as the parent lift low
drifts north towards the Kamchatka Peninsula. A much stronger
system will approach the Western-Central Aleutians Tue-Wed. Models
are still trying to pin down some of the differences with this
feature. The GFS is currently advertising it down into the mid 960
mb range by late Wed. The EC is not quite as bullish. Regardless,
this will be a key feature to watch as the primary weather maker
across much of the area by mid-late next week.

A large upper level trough will be in place over the entirety of
the Mainland next weekend. There is fairly good model agreement
regarding a persistent area of low pressure over the Gulf of
Alaska this weekend and into early next week. This will likely
bring easterly flow along the northern Gulf coast with overcast
skies and intermittent precipitation as several upper level waves
rotate in over Southcentral. Meanwhile, an upper level ridge over
Siberia gradually attempts to build eastward, producing a large
area of northerly flow over the Bering. The progression of this
ridge eastward will help to create a more amplified pattern,
ultimately breaking up the largely zonal jet currently in place
south of the Aleutian Chain. By Sunday, a North Pacific ridge
builds over the western Aleutians. Previous model runs have
suggested this ridge would be strong enough to displace the trough
over the Mainland as it continues eastward, however at this time
it looks to remain stationary. By Monday, models suggest the
presence of a large ridge in place over the Bering while low
pressure remains over the Gulf. By the middle of next week, models
suggest a strengthening low approaching the Bering, however there
is little consistency with this feature, and thus very low
confidence in the forecast beyond the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...SA/MSO