Small Craft Advisory 946, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 PM AKST Thu Nov 29 2018

.ANALYSIS AND UPPER LEVELS...
The upper level longwave pattern has a trough extending out of
Russia over the Bering Strait and then curving southeastward to
Southcentral Alaska and into the Gulf of Alaska. The 250 mb jet
stream is running west-to-east just below the Aleutian Islands
and then splits with the main push heading southward under a
trough in the Pacific and then continuing eastward. The section
that splits northeastward is bringing a jet streak over the
Eastern Aleutians and Alaska Peninsula today which will amplify
some as it heads over southwest mainland Alaska Friday afternoon
and then weakens Saturday over Southcentral Alaska. This is a big
source for the warmer air moving into the area, but the big push
of warm air will remain south of Alaska.

At the surface there is an elongated low in the Bering Sea with
centers in the northwest Bering Sea. The front associated with
this low is moving into Southwest Mainland Alaska and then curves
over the Alaska Peninsula. A low is beginning to develop along
this front southwest of the Alaska Peninsula which will rapidly
deepen tonight as it moves northeastward. The impacts of this will
be talked about later in the discussion.

&&

.MODEL DISCUSSION...
Models continue to show a similar synoptic pattern but change
run-to-run on details such as how far northward the warm sector of
air reaches Friday and Saturday. Overall, the NAM and GFS look to
be giving good enough information to be used, but there are still
some challenges especially in forecasting precipitation type
Saturday.

&&

.AVIATION...
PANC...With some partial clearing after the snowfall from the
past day, it looks increasingly likely that there will be stratus
and/or fog that will produce reduced ceilings and/or visibilities
tonight through tomorrow morning. The current state of the
atmosphere tilts a little more toward the stratus and less fog,
but reduced visibility is likely.

&&

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

An interesting weather day is in store for the region as we head
into the Friday evening through Saturday morning timeframe. More
on that in a minute. Earlier we hoisted a dense fog advisory for
the Glenn Hwy from Chickaloon to near Glennallen, as area web cams
showed visibilities to near zero at times. With a transient ridge
building overhead, this will likely continue through early
afternoon tomorrow. As a side note, the afternoon sounding from
Anchorage showed multiple low-level inversions already developing,
but this can be seen even better on the King Salmon sounding.

Then the real fun part of the forecast begins Friday evening, with
the fun being will the models lock onto a solution in terms of the
finer details, such as how fast will surface temperatures warm and
thickness levels increase. As has been the case for the past
couple of days, the NAM appears to be too aggressive with the
warm advection at 850 mb. Meanwhile, the Canadian Regional, ARW,
and HRRR models are slower with the warming at this level, and
more so at the surface. This then leads to concerns for a period
of freezing rain developing, as 850 temps warm above zero while
the surface temps remain below freezing. The latter of the three
models all differ on how long of a period this potential is.

So in short, look for patchy fog overnight and during your morning
commute, then for snow to gradually change from snow, to freezing
rain, to rain for portions of Southcentral in the Friday evening
through Saturday morning timeframe, with all locations changing
back to snow Saturday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Thursday
night through Sunday)...

The first of two fronts is currently on the doorstep of the
Southwest coast with snow beginning to spread inland over the YK
Delta. Ahead of the front, southeasterly winds are increasing and
are expected to peak overnight as the front pushes inland.
Snowfall totals will be on the lighter side with this system due
to the quick movement of the front; however, blowing snow will be
a concern overnight from Dillingham to Bethel as the southerly
wind gusts in combination with the falling snow will reduce
visibilities to around one mile at times. Although warmer air is
advecting in aloft, temperatures are expected to stay cold enough
for precipitation north and east of Dillingham to fall primarily
as snow. Enough warmer air is expected to move in for areas around
Bristol Bay where a changeover to rain will take place by early
tomorrow morning.

A North Pacific low will then begin to track northward, lifting a
warm front across the region beginning midday Friday. With the
center of the low tracking to the west of the mainland and a
strong southerly flow ahead of it, this system will advect much
warmer air over the southwest mainland. Rain will continue along
the AKPEN and around Bristol Bay with snow quickly transitioning
to rain farther inland as the front lifts north and the moves into
the eastern Bering. Snowfall amounts will once again be light and
mainly confined to far interior location like Aniak and Sleetmute
as well as into portions of the Kuskokwim Delta where the colder
air will linger the longest. As the warmer air works north, there
will also be a narrow band of mixed precipitation as snow
transitions to rain. Most of it will be a rain/snow mix, though
there is a chance for a brief period freezing rain north of
Bethel toward Hooper Bay. By early morning Saturday, colder air
will begin to wrap around the northward tracking low changing any
lingering rain showers back to snow. This weather pattern then
repeats itself for late Saturday into Sunday as yet another
system moves toward the southwest mainland from the north
Pacific with another snow to rain precipitation event.

&&

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

Behind this front, zonal flow will develop over the western Bering
for the next couple of days as vertically stacked low east of
Kamchatka remains stationary. Intermittent small craft winds and
periods of rain/snow will develop within this zonal flow as
shortwaves eject from the parent low and transit over the
southern Bering. Conditions remain quite active over the eastern
Aleutians and Bering, however, as the first of two North Pacific
lows lift north into the eastern Bering. The first is a double-
barrel low and slides into the eastern Bering tomorrow. As this
system intensifies, widespread gales and areas of storm-force wind
will develop from the eastern Aleutians north to Cape Newenham.
The second low then lifts north Saturday with another round of
widespread rain and gales for the eastern half of the Bering and
Aleutians.

&&

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

The forecast period commences with a longwave trough across the
Central Bering and portions of the North Pacific with a ridge
axis over the Yukon Territory and the Gulf Of Alaska. Between
these longwave features southwest flow aloft will dominate much of
the mainland. There are numerous shortwaves embedded in the upper
level trough. At the onset of the forecast period, the
Canadian/GFS and ECMWF have a surface low near Dutch Harbor,
another surface low west of St. Matthew Island and a dome of high
pressure east of SE AK. Even at the onset of the forecast period
the global models struggle with the placement of the surface low
although they have these features. As the forecast period
elapses, the model divergence becomes more amplified. Generally
speaking, expecting a progressive and unsettled pattern for the
Bering, the Aleutians and SW AK. However, due to the lack
consistency, this forecast package reflects a blend of models.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 155 165 172. Gales 132 138 150 160 170 173-176 180
181 414.

&&

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