High Wind in Alaska, October 5, 2017

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT Thu Oct 5 2017

.ANALYSIS AND UPPER LEVELS...

The large scale pattern mainly consists of a large trough over
much of the Bering and western Alaska, an upper low centered over
the northern Bering, and a stout 190 kt upper-level jet stretching
across the southern Bering and Aleutian chain. The surface low
stacked beneath the upper low over the northern Bering will start
to slowly wane in intensity this morning, as the upper-level
support and baroclinicity decreases today. Recent observations
from the Pribilof Islands continue to show winds steadily
increasing, as the core of stronger winds aloft begin to mix down
and push over the area this morning. A strong upper level
disturbance along the base of the above mentioned trough is in the
process of breaking off from the main system, and progressing
into the Gulf this morning. At the surface just ahead of the
upper-level wave, a triple point low is beginning to take shape
just north of Kodiak Island and moving eastward.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement throughout the short
term, with some very minor differences in the placement and
intensity of the Bering low as it weakens and moves onshore near
Hooper Bay today. Models have come into slightly better agreement
with the development and track of a triple point low this morning
south of the Kenai Peninsula, and tracking into Prince William
Sound by this afternoon. The EC stuck to its previous solution and
is now a more southerly outlier, keeping the low over the
northern Gulf rather than bringing it into Prince William Sound.
These minor placement variations have some impacts on the timing
of localized winds associated with the low. The GFS was the
preferred model solution to best capture the localized winds and
synoptic features.

&&

.AVIATION...

VFR conditions will begin to transition to around MVFR conditions
this morning as a front pushes in from the west, and rain begins
to pick up over the Anchorage bowl. As the front tracks further
east, VFR conditions are expected back by early this afternoon,
as conditions become more showery and ceilings lift slightly.

&&.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The frontal system currently in the southwestern Gulf will lift
northeast today with a compact surface low deepening through the
morning as it tracks northeast just offshore of the Kenai
Peninsula and then swings across the entrance to Prince William
Sound this afternoon. Associated rain will be heavy at times along
the Gulf Coast but will taper off quickly once the low moves
past. A secondary low will cross from Southwest Alaska tonight,
moving across Cook Inlet and the Kenai Peninsula then continuing
and into the northern Gulf Friday morning. Troughing along the
northern Gulf Coast will persist through Saturday with breezy
offshore Northwesterly gap winds developing. The next frontal
system will push across the Kodiak Island Saturday evening and
into the central Gulf and southcentral Alaska Saturday night
through Sunday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Thu and Fri)...
The warm occlusion associated with the massive storm over the
Bering has made it through the area. But there is still plenty of
very active weather coming behind it. Showers will continue
through today, some of which could be heavy at times as some
colder air starts to move in aloft. Can't entirely rule out an
isolated thunderstorm along the coast this afternoon either. The
bigger story will be the long duration of the strong westerly
winds through the period. Sustained winds will increase through
the day and should peak this evening. The strongest gusts will be
along the Western Capes and the Alaska Peninsula. These winds
will usher in higher seas, especially from Kipnuk south through
Bristol Bay. A coastal flood advisory has been issued to address
this. The low itself will move right through the YK-Delta as it
tracks quickly into the Cook Inlet region by Friday morning. This
will leave the area under showery conditions, but winds should
start to diminish through the day Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Thu and
Fri)...
The well-advertised Bering Sea storm is dominating the weather
scene. Wave heights of 30 feet have already been reported with
winds hitting 65 mph so far. The high wind warning remains in
effect for the Pribilof Islands until 1 pm Thursday. This massive
system will leave broad westerly flow in its wake into Friday. The
westerly flow will pull a little bit of cooler air down from
Siberia and make for widespread showers across the region. The
next low will start to take shape south of the Central Aleutians
late Fri. However, this system will not be nearly as strong or as
large as the current storm.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The extended forecast period begins Friday night which will
consist of a fast upper level westerly flow pattern with one
departing trough moving into Canada and another developing trough
over the Bering Sea and North Pacific. The main upper jet will be
over the Pacific, so weaker features will be in place inland with
largely dry conditions and precipitation favoring mountain upslope
regions. On Saturday night into Sunday, a low will move up from
the base of the developing trough into the Gulf of Alaska. This
feature has been giving the global numerical models huge errors
for the past three days, especially the GFS and Canadian GDPS.
Now, it seems, that the lower amplitude and weaker ECMWF solution
will likely verify as all the models have trended toward this
direction. What this means is the low will barely clip the Gulf
coast and may even completely miss it, keeping the bulk of rain
and stronger winds farther south. For this forecast, the new
blended WPC forecast approach will result in a slightly farther
south track and lower chances for inland precipitation.

Thereafter, the global models are all now in good agreement
shifting the upper trough east over Southern Alaska and bringing
in a much cooler air mass Monday night through early Wednesday as
the storm track shifts to the Bering Sea. It does look like that
as early as late Monday that a series of strong lows will move
through the Bering Sea, so mariners should keep a keen interest
on this as it may become quite stormy again.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 195.
         Coastal Flood Advisory 155 161.
MARINE...Storm 179 180.
         Gale 119 120 125 130-132 136-138 150 155 160 165 170-173
              175 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...TP
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JA