Gale Warning 518, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKDT Fri Sep 28 2018

.ANALYSIS AND UPPER LEVELS...

The state is being dominated by a variety of synoptic patterns. A
vertically stacked upper level low in the western Bering Sea is
slowly moving eastward extending a long-wave trough over the
western Aleutians. This low sits in favorable jet stream position
which is causing the low to slightly intensify and create a band
of increased vorticity along the western Aleutians on the leading
edge of the trough. The trough is bumping into a building ridge
over the Gulf of Alaska which is encountering southerly flow on
the western edge. This southerly flow is advecting a moist,
subtropical airmass over the AKPEN and Southcentral. At the
surface, a warm front on the leading edge of the ridge is aiding
in the rain's progression northward. In addition, a nearly
stationary front sits between the trough and ridge over the
eastern Aleutians which is increasing rainfall throughout the
eastern Bering and supporting the more typical fall like weather
throughout the area.

&&

.MODEL DISCUSSION...
The models remain in agreement with the trend to keep the
moisture and rain farther west and with a slower progression of
the rainfall over the southern mainland. After this trough clears
to the north and west, high pressure is expected to build from the
Gulf of Alaska into the southern mainland this weekend. The
models are doing a good job and are consistent with a surge of
warm air moving over south central Alaska and southwest Alaska by
Saturday. This will result in more late summer time temperatures
as opposed to late September temperatures across these regions
this weekend.

Out west the models remain in very good agreement essentially
pin-wheeling the two lows in the Bering Sea and one south of the
eastern Aleutians. The low now south of the eastern Aleutians is
forecast to dissipate to a trough over the Pribilof Islands this
evening as the low now over the Pribilof Islands is handled
similarly by the models moving to the northwest Bering Sea this
evening. The third low in the mix is the one now in the western
Bering Sea. the models have it moving east then northeast across
the Bering Sea as it dissipates to a trough over the central
Bering Sea Saturday evening.

&&

.AVIATION...
PANC...Rain will taper off this morning and clouds will gradually
clear through the day with VFR conditions and light and variable
winds through the forecast period.

&&

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

The front is lifting from south to north along the North Gulf
coast through Cook Inlet early this morning. It will weaken and
then dissipate as it heads northward today. Behind this front
there is a large ridge building into the region out of the Gulf of
Alaska. This warm ridge will become the dominant weather feature
this weekend into early next week. The biggest uncertainty with
the forecast is whether there will be widespread fog that develops
overnight Friday night and how long it may persist into Saturday.
The subsidence and warm temperatures associated with the ridge
all support fog development. However, there is also drying
northerly flow developing Friday night as well. While this
northerly flow is not terribly strong, it may be enough to
mitigate the fog.

After any fog that does develop dissipates Saturday morning, there
will be sunny weather and warm afternoon temperatures for the
region over the weekend. Some daily record high temperatures may
also be reached during this upcoming warm and dry period.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Sunday)
Today expect more wet weather across Southwest Alaska. As of 4 AM
local time or 12Z, the radar at Bethel continues to ping into a
precipitation shield from Kipnuk to Bethel to Sleetmute. The
NESDIS Percent Of Normal TPW has detected a vast area of +200
percent of normal for atmospheric moisture. This band of
precipitation will steadily lift to the north today. Anticipate a
warming and a drying trend for Saturday and Sunday. Temperatures
will be well above normal for most (if not all) of Southwest
Alaska. Expecting King Salmon to get into the mid 60s this
weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Sunday)
A wet and unsettled period will continue through the weekend, as
the remnants of two lows wobble around the Bering with one south
of the chain. The latest ASCAT/advanced scatterometer pass has a
low northeast of Shemya and a weak low south of Atka with a faint
frontal boundary connecting them. There is an additional
circulation near the Pribilof Islands. This batch of unsettled
weather will result in periods of showers and seas will be
enhanced. These lows will interact with each other showcasing the
"Fujiwhara Effect". Looking to the west Pacific, there are a
couple of interesting lows near Japan that will eventually impact
the Aleutians. The first one is expected early Sunday and this
storm should bring another round of gales to the region.

&&

.LONG TERM FORECAST (Days 4 through 7)...
Beginning the extended period forecast Sunday night, there will
still be a very impressive 575 dm 500 hpa blocking high (4 sigma)
over the state of Alaska with a large negative tilt trough over
the Bering Sea. A secondary low will be in place south of the
Alaska Peninsula. It does now seem that the blocking high will
remain strong over Alaska through Monday night and may even hold
through Tuesday, keeping most of Southern Alaska dry into
Wednesday before precipitation begins to move north along the Gulf
Coast. This trend means precipitation threats will be a day or so
later than previously thought.

Attention quickly turns to the western Bering Sea where the
remnants of Super Typhoon Trami will begin to exert an influence
on our waters. Unfortunately, much uncertainty remains at this
juncture, even though the low is expected to move into western
Bering Monday night and Tuesday (less than 5 days out). The global
numerical models range from a weak and southerly track barely
clipping the ALeutians (12z ECMWF) to a powerful 948 hpa storm
force low over the entirety of the western Bering Sea and Western
and Central Aleutians (18z GFS). The 12z Canadian NH splits the
difference between the outlier models in both track and strength,
so that solution will be favored for now which will likely hint at
potential gales for the far SW Aleutians. It should be noted, for
marine purposes, that this storm has the potential to be very
strong, but much uncertainty remains at this juncture so a more
conservative approach will be taken, although not as conservative
as the weak ECMWF solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 165 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...JA