Small Craft Advisory 288, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
340 PM AKDT Sat Apr 13 2019

.ANALYSIS AND UPPER LEVELS...

The core of the Pacific jet stream is south of the Aleutian Chain
this afternoon with a weak blocking high over the center of the
Bering Sea. A 989 mb low is tracking east along the Central
Aleutian Chain, but it is weakening as it does with a broad
confluent flow regime aloft. Another surface low, also rather
weak, is crossing the Southern Gulf of Alaska. Over the mainland,
a shortwave trough is digging south across Western Alaska into
Southwest Alaska. A closed low is centered over the North Gulf
Coast, but this circulation is only a remnant circulation,
although it does look impressive on visible satellite as a tight
mesoscale circulation wrapping convective showers around it. With
all these details, however, weather is rather quiescent region
wide, with weak pressure gradients and largely dry weather. The
main concern is low stratus and fog across parts of the Kuskokwim
Delta as well as over the Bering Sea.

&&

.MODEL DISCUSSION...

The numerical weather models are in very good agreement with the
general pattern through Tuesday. They are in lesser agreement with
the complex interactions between merging arctic shortwave troughs
with a low in the Gulf of Alaska Monday and Tuesday. The impacts
are not large, but there will be changes to the forecast by Monday
night, especially across Southwest Alaska where there will be
higher chances for precipitation as an arctic trough digs south.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

Light winds and limited precipitation will be the story for
Southcentral Alaska the next couple of days. The low over the
northern Gulf will slowly drift south through Monday which will
allow for some light, isolated showers along the north Gulf
coast. Cloud cover over the Cook Inlet area will diminish into
Monday leaving mostly clear skies. Meanwhile, cloud cover over
the Copper River Basin will increase through Sunday before
clearing out again Monday.

&&

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

A shortwave centered over the western Alaska interior will drop
south across the Kuskokwim Valley this evening. As it does, its
associated surface trough will move across the region, getting
hung up over the Alaska Range tonight. These features will combine
to produce areas of rain and snow showers from Sleetmute and the
Nushagak Hills east to the Alaska Range through late Sunday as the
shortwave continues its southward movement over the AKPEN. Behind
it, a cool, northerly low-level flow is yielding low stratus and
patchy fog from Norton Sound south across the Y-K Delta. This is
expected to continue (although not as widespread as today) through
the overnight and early morning hours Sunday as the upper-level
ridge axis moves overhead. As the ridge expands, an offshore flow
will set in over the mainland bringing drier conditions for the
remainder of Sunday into Monday. The next shortwave then drops in
form the north late Monday into Tuesday bringing another shot of
mixed rain and snow from interior Bristol Bay to the Kuskokwim
Valley and a return to more seasonable temperatures across the
Y-K Delta.

&&

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

The main weather feature is a 988 mb vertically stacked low south
of Atka and its occluded front draped south of the central and
eastern Aleutians. This system will continue to weaken and fill
tonight as a new triple-point low forms well south of the AKPEN
and quickly advances east. Farther west, a deepening shortwave
east of Kamchatka will spin up a new surface low over the western
Aleutians by Sunday morning. An amplifying ridge/trough pattern
upstream will yield the development of strong cold air advection,
enhancing northerly wind gusts on the back side of the surface low
through early next week as the low drifts south and east.
Widespread rain and snow showers in cold air advection are likely
along the backside of the low, while a moist southerly flow ahead
of the low will maintain warm conditions and rain across the
eastern Aleutians and southern AKPEN. Over the Bering, widespread
low stratus will persist through the remainder of the weekend as
an upper-level ridge dominates.

&&

.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.

Toward 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...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...MSO