Small Craft Advisory 290, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 AM AKDT Sun Apr 14 2019

.ANALYSIS AND UPPER LEVELS...

Synoptically, the pattern throughout southern Alaska is fairly
quiet. The jet stream continues to flow in a zonal pattern well
south the Aleutian Islands today at 30,000 ft. A weak vertically
stacked low is positioned near Adak this morning. Cold air is
wrapping around this low with generally north-northwest flow on
the backside. A second weak stacked low sits over the central Gulf
of Alaska. This low is at the base of a trough encompassing much
of Southcentral and the Interior. Between these two upper- level
lows, a negatively tilted ridge has built over much of the Bering.

Weather remains fairly benign. Satellite imagery shows that the
main area of cloud cover exists over the western Bering and along
mountain ranges throughout the southern mainland. Rain showers
this afternoon provided enough moisture, when combined with
dropping temperatures overnight, to cause fog to form throughout
the Copper River Basin and northern Gulf coast.

&&

.MODEL DISCUSSION...

Through Tuesday, models are in good agreement.  Monday night,
a low stemming from the Arctic is moving into the southwestern
mainland. This low is well handled by the numerical models at this
point. The first aspect in which models are struggling is a
Pacific low moving into the Gulf of Alaska on Tuesday. The GFS,
NAM and Canadian all depict a surface low moving into the eastern
Gulf Tue morning while the ECMWF has yet to incorporate this
feature.

&&

.AVIATION...
PANC... Light and variable winds will persist through Monday
morning. There will be periods of scattered showers throughout
Sunday morning, however, they will be flurries at most.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level low over the northern Gulf will drift very slowly
south and east through Monday night. In general dry conditions are
expected over Southcentral. However, some showers are possible
over the area during the next couple of days as the flow aloft
remains weakly cyclonic. Most of these showers will be over the
western Kenai Peninsula through tonight, and the Copper River
Basin through Monday. These will be somewhat diurnally enhanced,
and more prevalent over the mountains.

&&

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

Weak low pressure persists across the Gulf and along the central
Aleutians into the beginning of the week. This keeps the Southwest
coastal areas under an offshore regime with light and variable
winds inland. An upper level disturbance over the western interior
dives across the Kuskokwim Valley to Bristol Bay through this
afternoon. A few showers are expected to develop mainly along the
western Alaska Range and Aleutian Range...however there is some
uncertainty on the surface convergence which might be further west
than currently expected. A quick look a King Salmon's sounding
this morning supports brief small hail and downburst activity
(gusty winds where storms develop). Monday should be drier as a
surface ridge limits precipitation through the afternoon, then
expect increased chances for snow and rain through Wednesday as an
arctic cold front slides southward along the west coast becoming
nearly stationary over the Kuskokwim region on Wednesday.

&&

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

A broad area of low pressure across the central Aleutians remains
nearly stationary through late Monday before taking a southward
turn Monday night. Increased winds across the Bering on Monday
into Tuesday as the surface low re-organizes with a significant
short wave diving south over the western Bering before wrapping
around the core of the low. Cold air advects across the Bering on
Tuesday as an Arctic low tracks south along the western coast. A
few snow showers are possible across the eastern Bering as
shortwaves propagate away from the parent low.

&&

.MARINE (Days 3 through 5...Tue through Thu)...
Over the Bering Sea...confidence is high in small craft northerly
winds to start the period, primarily in the Eastern Bering. As
high pressure builds in from the west, winds should slowly
decrease leaving very few concerns for the area.

Gulf of AK...the period will start out relatively quiet with high
confidence in sub- small craft cyclonic (counter-clockwise) flow.
As we move towards Thu, models diverge drastically with a strong
low moving quickly from south to north into the area. The GFS is
the strongest solution advertising a mid 970 mb low with gale-
force winds. However, the GFS is an outlier and the consensus is
to take this low further to the east and keep it slightly weaker.
Thus the forecast will continue to advertise small-craft winds or
less for winds. Because of these model differences, forecast
confidence remains low to moderate.

&&

.LONG TERM FORECAST (Days 3 through 7...Tue through Sat)...
As is often the case in this timeframe, the extended forecast is
starting to become slightly more certain as it draws closer to the
short term. Early in the period, confidence has improved markedly in
a weak deformation band setting up over the interior portions of
SW Alaska on Tue. All models are advertising a potent upper level
shortwave dropping in from the Bering Strait area. With weak
cyclonic flow swinging moisture in from the Gulf of AK, this will
set the stage for some potential rain and/or snow Tue- Wed. There
is still some slight uncertainty regarding the exact timing of
the upper level wave. So for this forecast package, we have only
trended PoPs up rather than putting in a widespread area of
likelys. The other important note with this pattern would be a
return to much cooler (more seasonable) temperatures for the
middle portion of the week, primarily from Aniak west.

Towards the Gulf of AK, things look to remain fairly quiet early
in the period. There will be a weak low creating cyclonic
circulation. Where exactly that low sets up and exactly how strong
that flow is remains in flux. Models are trending slightly further
east and weaker, so this forecast moves ever-so-slightly in that
direction. This could also lead to another day of mostly sunny
skies for Southcentral.

As we move into the latter half of the week, model disparity jumps
up significantly. Over the Gulf of AK, all the models agree a
strong low moves from south to north by early Thu. However, the
GFS is an outlier with this system as it is by far the strongest
and further west. Thus, we are trending the forecast slightly
further to the east for the low track using some of the ensemble
model guidance. Either way, it looks like the potential for more
winds and rain for coastal areas. Over the Bering, a closed upper
level high builds in during this time. This will leave the Western
Bering relatively quiet, but will serve to dig out a longwave
trough over SW AK. That type of pattern supports unsettled weather
with cooler temperatures over SW with cloudy and showery
conditions extending over to Southcentral AK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...MSO