Small Craft Advisory 891, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 AM AKST Tue Nov 13 2018

.ANALYSIS AND UPPER LEVELS...
Warm temperatures have been observed across the southern tier of
Alaska, and multiple locations have been in the 40s since
midnight. The active weather pattern continues across the state
this morning, which can be seen on satellite with a series of lows
from Kodiak to Kamchatka. A frontal system associated with a low
center situated over the Bristol Bay region is bringing
precipitation in the form of rain for Southcentral. This system
has brought about an inch an a half of rain to Cordova and Seward,
and an impressive 6.55 inches to Portage Glacier over the past 24
hours. A second, North Pacific low sits south of Kodiak Island
moving northward towards the Gulf. The latest ASCAT pass has
detected gale force winds near Kodiak, south of the Eastern
Aleutians and in the Eastern Bering.

Winter weather advisories over the Kuskokwim Delta and Lower
Kuskokwim Valley with mixed precipitation expected throughout the
morning. Snow has been observed so far for most areas overnight,
which has switched over to freezing rain for Bethel in its latest
observation.

&&

.MODEL DISCUSSION...
In the short term period (48 hours), the ensemble members of the
GFS are tightly clustered with the lows near Kodiak and south of
of the Chain and mimics the Fujiwhara effect as the lows are
projected through time. By Wednesday, the clustering is actually
retained with the low near Kamchatka and the new north Pacific
low. The operational models are also handling the synoptic
features well. However, the thermodynamics are slightly off. There
are some concerns with the precipitation type even in the first
period (a.k.a. this morning) and concerns over potential fog
development.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Patchy fog is
possible around the Anchorage Bowl this morning. The current
thinking is that there is enough influence from the local winds
to keep the fog at bay at PANC. If that changes, an amendment will
be issued.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Thursday)...
A strong short-wave tracking northwestward across Southcentral
early this morning will quickly exit to Interior Alaska, bringing
an end to steady rain across the region. An unstable air mass
moving up from the Gulf combined with a weak upper level wave
will maintain some showers through at least midday, especially in
the Prince William Sound region. As the merry-go-round of lows
down to the south consolidates into a single low over the
southern Gulf tonight, low level flow across Southcentral will
shift to northerly and drier conditions will settle in. This will
allow temperatures to begin to fall from the very lofty values
currently being observed. The cooling trend will persist through
Thursday as upper level flow then shifts around toward the north
advecting some marginally colder air from the Interior.

There is some uncertainty in the forecast for the Copper River
Basin tonight through Wednesday as an upper low tries to back in
from the Yukon. For now have confined a chance of precipitation
to the Wrangell Mountains, but this bears watching.

Kodiak Island will be on the northern edge of the departing low
complex through Wednesday morning, keeping showers in the
forecast. A highly unstable airmass over the southern Gulf may
also produce a few thunderstorms today.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
The parting shot of snow and freezing rain will clear through the
northern portion of the area this morning before calmer weather
ensues. This last shot is due to a very strong low driving through
Bristol Bay. First, the Winter Weather Advisories remain in place
this morning for the YK- Delta and the Lower Kuskokwim Valley.
The band of precipitation that is currently moving through that
area has produced on and off bouts of just about all types of
precipitation, to include snow and freezing rain. The Bethel 12z
sounding does still show a nearly 2000' warm layer above the cold
pool at the surface. As this wave slides towards the coast this
morning, it should allow cooler air to filter in. The net effect
will be that any remaining precipitation will turn over to all
snow (as many locations in the Kuskokwim Valley are starting to
do). This cold air could also lead to a few pockets of snow
showers and blowing snow along the Northern YK-Delta Coast. Winds
will continue to remain very gusty from the north to east.
However, the warm air and lack of snow at the surface will
severely limit any reduction to visibility.

As the low pivots and dives south back across the Alaska
Peninsula, conditions will generally start to improve for all the area
this afternoon. Winds will slowly abate as a drying trend sets up
through the middle of the week. In fact, winds do look light
enough by early Wed morning that fog and/or low stratus could be
significant across the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue and
Wed)...
The Bering will see 3 synoptic features impact the weather for the
next couple of days. In chronological order, a strong low will
spin through Bristol Bay early today. This will bring a slug of
moisture and warmer air to the region. It will turn and push back
into the North Pacific by this afternoon. However, it will provide
a ripe environment for some showers and gusty winds across the
Alaska Peninsula and through the Central Aleutians. The 2nd piece
is cold air over the Central Bering. The aforementioned low will
pull this cold air further south. This will exacerbate the showers
and will create some gusty northerly winds, especially through
north- south oriented terrain. It will take some time, but
eventually the cold air will start to mix some snow into the rain
showers. Then finally, part #3 is a new warm occluded front that
will slide towards the Western Aleutians late Wed night into Tue.
This will cause the winds to shift out of the southeast and
increase back up to gale-force. It will also push yet another
round of warm air into the area bringing steady rain with it.

&&

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

The long-term forecast begins Wednesday night with a continuation
of an active pattern across Alaska. The main features are a broad
upper-level trough axis positioned from the Bering to well south
of the AKPEN with a pair of flattened mid-level ridges bookending
the trough. There is also a (sub) tropical connection to the
trough as it draws up plenty of warmer air and moisture
originating north of Hawaii. This is reflected at the surface by a
low moving north through the Pacific on Thursday and centering
near Kodiak Island for Friday with its warm front stretching along
the southern coast. This feature and the upper-level trough then
both become negatively tilted as the system occludes and the ridge
downstream reamplifies across the Alaska Panhandle. By Saturday,
the low begins to weaken as it and its corresponding upper-level
shortwave eject northeast over the northern Alaska Panhandle in
response to a second, deeper trough sliding east across the
Aleutians and reinforcing the longwave trough centered over the
eastern Bering/AKPEN.

As is the case in the long term, there are plenty of subtle
differences concerning the exact timing of the shortwaves moving
through the Gulf and across the Aleutians. The GFS has been the
most bullish (most progressive) of the suite of extended range
guidance, but it has also been performing the best of late
(although the EC and Canadian also agree on the long wave setup).
Therefore, the extended forecast is weighted more toward the
GFS/GEFS solution. This keeps a weak trough over the eastern
Bering through the end of the week as the northward moving
Pacific low spreads clouds and precipitation over the AKPEN by
Thursday and over southcentral by Friday. Farther west, a
weakening occluded front over the western Aleutians on Thursday
will spin up a new triple point low over the central Bering by
Friday. This is the feature that will intensify quickly as it
moves into the eastern Bering. Precipitation type may be a
concern again later in the week as the weather pattern remains
similar to that (areas of colder air at the surface with much
warmer air over-running aloft) which brought the mixed bag of
precipitation to much of southern Alaska this past weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisories 152 155.
MARINE...Gale Warnings 150 160 179 180 181 185 414.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS/ALH
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...TM