Small Craft Advisory 693, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
455 AM AKDT Mon Sep 10 2018

.ANALYSIS AND UPPER LEVELS...

A persistent omega blocking pattern continues over the region
with a 1025 mb surface high anchored over the Eastern Bering and
Western mainland leading to dry northerly offshore flow. A very
weak inverted upper level shortwave is swinging south across the
mainland on the east side of the high, though the only sign of
this feature is an area of upper level clouds currently moving
from the southern mainland into the Gulf of Alaska and Northern
Pacific. An offshore pressure gradient across Southcentral is
still leading to gusty northerly winds through the typical
channeled terrain along the Gulf Coast. The offshore winds are
warming adiabatically as they descend from the higher terrain to
sea level. This has led to very warm temperatures for coastal
locations, with Valdez (72) and Seward (72) both breaking their
daily record high temperatures yesterday by nearly 10 degrees.
Additionally, Valdez tied its all time high temp for the month of
September.

Out west, strong subsidence underneath the ridge axis is leading
to widespread low stratus and areas of fog over most of the Bering
Sea, which is beginning to move ashore over the Kuskokwim Delta
Coast early this morning.

&&

.MODEL DISCUSSION...
Models are in great agreement through the short term.

&&

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

&&

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

A large area of high pressure remains centered over the Bering Sea
with ridging persisting over Southcentral Alaska today. The upper
level flow remains northerly over Southcentral Alaska and the
Gulf of Alaska. A short wave embedded in this upper level flow
brought a period of mid-to-upper level clouds to the region
yesterday evening that were a bit thicker than anticipated. The
high clouds are still visible in the IR satellite from the Kenai
Peninsula to Kodiak Island early this morning, but should continue
to move south and out of the area today. Other than a few high
clouds with other weak short waves, the region will remain under
mostly sunny skies with near-record high temperatures for the next
few days. The only exception looks to be Tuesday night into
Wednesday for the Wrangell Mountains and far eastern portion of
the Alaska Range where a stronger short wave will bring some
clouds and some showers.

&&

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

A band of mid- and upper-level clouds continue to move across
Bristol Bay and the AKPEN this morning as an upper-level
shortwave slides south toward the north Pacific. As this system
moves south, high pressure will build back in along the west coast
and slowly drift over the Southwest mainland through early
Wednesday. With high pressure dominating the overall weather
pattern, expect continued dry and mild weather through the first
half of the week. A few clouds are possible around the Bethel area early
Tuesday, however, as another weak shortwave rides up and over the
top of the ridge. Calm winds and clear skies during the overnight
hours will allow for areas of fog and low stratus to form,
especially within low-lying valleys and along the coast. A front
will track east across the Aleutians on Wednesday with clouds and
rain developing over the AKPEN by Wednesday afternoon and along
the southwest coast by Wednesday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A strong, vertically-stacked area of high pressure will remain
centered over the Bering for the next 36 hours before beginning to
slide east toward the Alaskan coast as an area of low pressure
moves up from the north Pacific. Widespread areas of stratus and
fog will persist across a vast majority of the Bering and Aleutian
chain while under the influence of this ridge. An occluding front
associated with the low will begin a push north Monday evening,
reaching the central Aleutians by Tuesday afternoon. Near gale
force easterlies are possible briefly just south of the central
Aleutians prior to the front moving through. A deep, southerly
fetch will advect plenty of subtropical moisture northward as
showers continue behind the front. These areas of scattered
showers will become more numerous early Wednesday morning and
again Wednesday evening as a second and third trough move north
in advance of the low as it sits stationary southwest of Adak.

&&

.LONG TERM FORECAST (Days 4 through 7)...
(Thursday through Sunday)

Much like a scene from a movie you may know, todays numerical
solutions have the same issues as has been the case for a few days
now. The GFS (American) continues to be faster to break down the
persistent and anomalously strong ridge over the central
Bering.  The ECMWF (European) remains consistent through Friday,
with the Canadian global model similar to its European
counterpart, albeit a touch stronger in amplitude. These two
models are in better agreement in breaking down the ridge as
multiple shortwaves bump into it while riding its periphery. The
issue remains with the speed at which the ridge breaks down and
its overall positioning. Given the better agreement in this, it
does look like rainfall chances may begin to increase for
southwest and southcentral Alaska, along with increasing cloud
cover and decreasing afternoon temperatures.

For the weekend and beyond, the models really start to run-a-muck,
as none of the three models are in even modest agreement. The
previously consistent EC model had deflected arctic shortwave
energy to our northeast as it rounded the top of the ridge. Now,
it allows a strong disturbance to enter the Bering from the west,
then tracks it across the AKPEN into the Gulf. Meanwhile, the GFS
allows the low to enter the Bering and then portrays the low
stalling near the Pribilof Islands as it meanders in that region.
Both models allow this to occur as a strong area of high pressure
develops somewhere along the North Slope or across the
northeastern interior/Yukon border. Finally, the Canadian global
has a middle of the road approach with the positioning and timing
of these features.

Given the uncertainty due to poor model agreement, we decided to
show a breakdown of the ridge in the forecast, just at a much
slower pace, which is more in line to the European and Canadian
models with a slight nudge towards the GFS. This is also aligns with
WPC guidance during this time period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 176.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...PD