Small Craft Advisory 711, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKDT Wed Sep 19 2018

.ANALYSIS AND UPPER LEVELS...
A very slow moving upper level trough over the Seward Peninsula
has brought widespread cloud cover to the North and West Mainland
and is beginning to spread rainfall to the Kuskokwim Delta as the
front makes landfall. This system is expected to very slowly make
a eastward track through the interior, bringing rainfall primary
to the northern portions of the AOR. Ahead of this low, weak mid-
level ridging once again brought stratus and fog to much of
Southcentral this morning. This cloud cover began to clear late
this morning, leaving some areas with a partly sunny afternoon
before the cloud cover returns from the west. Further to the west
over the Bering Sea, the same low that brought gale force winds
the Western Aleutians yesterday as spread in the Central
Aleutians. This low will slowly track through the Bering today,
and be the focus for a large scale pattern change to much of the
Mainland beginning Friday and Saturday.

&&

.MODEL DISCUSSION...
The models remain very consistent with both the track of the
Arctic trough that will skirt just north of the area as well as
the Bering low and its associated front into this weekend. As a
result confidence continues to increase that rainfall from the
Arctic trough will primarily bring rain from Bristol Bay
northward, while the incoming Bering low will produce rainfall
over most of the AOR by this weekend.

&&

.AVIATION...
PANC...MVFR stratus clouds will continue burning off this
afternoon allowing for VFR conditions for the evening hours. CIG's
are then expected to fall to MVFR or possibly even IFR early
Thursday morning as the low level stability increases again.
CIG's are not expected to be as widespread or low as what occurred
this morning however as cloud cover will also be moving in from
the west.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

There are two main weather concerns for Southcentral through the
start of the weekend. The first is the potential for fog again
overnight tonight through tomorrow morning for areas along Cook
Inlet north into the Susitna Valley. The synoptic set up is
similar to last night with weak surface winds, light up-inlet
flow, and lingering low-level moisture. The limiting factor
tonight, however, will be a lack of additional moisture with fewer
pop-up showers across the region and an increase in cloud cover
from an advancing shortwave trough helping squelch any significant
radiational cooling. Nonetheless, patchy fog is possible from
Homer to Kenai northward to Anchorage and Wasilla through mid-
morning Thursday. The aforementioned trough will move through
tomorrow, keeping clouds and scattered high-terrain showers in the
picture over Southcentral Alaska.

The second item of focus will be a deepening low moving into the
central Bering Thursday night. Its associated warm front will
swing across the AKPEN and into Southcentral Alaska by Friday
afternoon, increasing the southwesterly flow at the surface and
aloft and bringing with it widespread rainfall. For Anchorage,
this system will bring the first measurable rainfall to the city
(well, airport) since August 26th. A secondary triple-point low
will begin to develop near the Kenai Peninsula Friday night. As
this happens, the flow will turn to the south and southeast,
keeping rainfall going along the coast and upslope sides of the
mountains through Saturday. For areas along the lee of the
mountains, like the Anchorage Bowl, chances for rain will diminish
as downsloping takes hold. In addition to the rain, the development
of the low in the Gulf will induce the formation of a coastal
ridge. This will work in tandem with the main Bering low to
increase in pressure gradient as it approaches the west coast
Friday afternoon. The tightening gradient will help drive an
uptick in the southerly gap winds and southeasterly Turnagain Arm
winds late Friday through Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (through Saturday)...
A more active flow pattern is beginning to take shape over the
mainland. Tonight, a weak front and trough will bring clouds and
rain to the Kuskokwim Delta, spreading eastward and weakening
overnight as it moves eastward. Some sprinkles and showers will
linger on Thursday as the system moves out of the area. A front
and low pressure system from the Bering moves in on Friday. While
it currently has a good moisture tap, the front will occlude as it
reaches the Alaska Peninsula. However, the front will still bring
a shot of steady widespread rain on Friday, tapering off to showers
on Friday night into Saturday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A gale-force low and front will move into the central Bering Sea
overnight. Those winds will slowly taper as the front moves
toward the Alaska Peninsula late Thursday. Once the front passes,
the Bering stays within the large scale trough, generally
characterized by weaker flow and spotty rain.

&&

.LONG TERM FORECAST (Days 4 through 7/Saturday night through
Wednesday night)...

At the start of the extended period (Saturday night) a mature low
will be sitting along the west coast of Alaska, with widespread
rain across Southwest Alaska and areas of rain lingering over
Southcentral. Expect slow improvement Sunday through Sunday night
as this system weakens and lifts northward.

Beyond that (for the first half of next week - and beyond) the
latest model guidance (including ensembles) is all over the place
with details of specific storm systems. Thus, forecast confidence
is about as low as it gets in the ability to say what might
happen on any specific day. However, there is quite a bit of
agreement in the overall large scale pattern, with a long-wave
trough over the Bering Sea/Aleutians occasionally extending right
across mainland Alaska and a ridge centered over the northeast
Pacific/western North America. Key differences are in the
interface between the trough and ridge (in vicinity of the Gulf
of Alaska on up to Southcentral Alaska). The overall amplified
pattern supports development of deep surface lows with a general
storm track from the North Pacific northward toward mainland
Alaska, with additional storms moving through the trough out
in the Bering Sea/Aleutians. In this type of pattern we would
generally expect wet conditions across most of Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 165 170 172-177 179.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL
LONG TERM...SEB