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000
FXAK68 PAFC 171429 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
629 AM AKDT Tue Oct 17 2017

.ANALYSIS AND UPPER LEVELS...
The impressive subtropical jet with a core of greater than 180
knots at 300 mb continues to steer a mature low across the
Aleutians. Gale force and storm force winds associated with the
low were observed in the vicinity of the western Aleutians Islands
with the latest ASCAT pass. While the north Pacific and Gulf of
Alaska have been very unsettled, southcentral including the
Anchorage Metro Area can expect another brisk morning and more
blue skies.

&&

.MODEL DISCUSSION...
More progressive flow will be on tap this period. At 500 MB, the
models appear to be on lock step depicting the synoptic pattern
through Friday. At the surface, there appears to be reasonable
consensus about the occluded low that is currently over the
Aleutians and its progression eastward. However, the NAM/GFS and
ECMWF have slightly different placements of the low and the
intensity by 12z Wednesday. The NAM is slightly deeper and more
bullish with the track and the GFS/ECMWF not as deep at 12z
Wednesday with similar triple point low positions with each other.
And then 24 hrs later, the same models bring the surface low into
SE Alaska near Dall Island. The solutions ping into very similar
pressure gradient fields which will result in a Gale force front
affecting the Aleutians, Kodiak Island and the Gulf Of Alaska. In
the wake of the frontal passage, a ridge axis will build over the
western Aleutians. By Friday, a new cyclone will enter the west
domain and another triple point low will develop over the
Aleutians.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak upper level trough is moving into the Susitna Valley today
and will move slowly eastward into the Copper River Basin
tomorrow. This feature, combined with a dissipating low along the
north Gulf coast will be the cause of some clouds and a few snow
showers east of the trough as the surface low pumps in just enough
moisture. There is a sharp delineation in the clouds as seen on
the satellite early this morning. Clouds are over Prince William
Sound, the Palmer and Wasilla areas, and northward to the
Talkeetna Mountains. West of this line (including Anchorage,
Talkeetna, and Seward) skies are mostly clear, but east of this
line the clouds should persist throughout most of the day. Some
snow and rain showers are expected to develop in the Prince
William sound area which may make it into the southernmost part of
the Copper River Basin as well. No significant snow accumulations
are expected.

The largest synoptic weather feature will be the low and
associated front that will traverse the Gulf of Alaska the next
few days bringing Gales to the Gulf marine areas and rain to
Kodiak Island. This low will remain just south of the Gulf and
cause gusty northerly winds to develop along the coast tomorrow
into Thursday.

&&

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

Light to moderate rainfall and gusty winds will continue across
the Alaska Peninsula as a front associated with an area of low
pressure northeast of Unalaska moves through the area. The front
is expected to move south of the area as the low pressure system
tracks into the southern Gulf of Alaska later this morning and
afternoon. Elsewhere across Southwest Alaska, cloud cover will
continue to envelop the area as moisture rotates around the
aforementioned low pressure system. A few stray showers may be
observed but overall a fairly dry pattern looks in store for much
of the region. Temperatures will return to near normal values by
the second half of the week as an arctic air mass moves overhead.

&&

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

Scatterometer data was able to do a wonderful cross sections of
the low currently situated just to the northeast of Dutch Harbor.
It looks like widespread 40 to 55 kt winds are being observed on
the back side of the low pressure system with the strongest winds
staying just outside of Alaskan water. Nevertheless, storm force
winds will be observed south of Dutch Harbor today before the 180+
kt jet quickly shunts the low pressure system into the southern
Gulf of Alaska. A period of showery weather will persist across
the Bering Sea through Thursday as warm SST`s couple with colder
air aloft acting to steepen low-level lapse rates. Another front
will enter the western Bering Sea Wednesday afternoon but will be
weaker than this last system as the low pressure system weakens as
it moves into the Bering Sea. This low will take a similar track
as the current low over the Bering Sea thanks in large part to the
orientation of the upper level longwave pattern.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The mid and long term forecast beginning Wednesday night has cold
outflow winds over the southern mainland and a low tracking east
along the Aleutians. There is good agreement amongst the models
with this low until it moves into the Gulf Friday night. At this
point there are differences in how it approaches the southern
mainland. Some of the solutions bring the majority of the
precipitation to the north Gulf coast with downsloping over the
northern Cook Inlet area. However, there are some solutions that
would bring snow to the Anchorage area. At this point there
remains too much uncertainty for a high confidence forecast one
way or the other.

By early next week there is some agreement that the remnants of
Typhoon Lan could track towards the western Bering, but there
remains huge differences on its strength and position.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 150 155 170 172 173 174 175 177 351 352 132.
         Storm 176.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...DWK