Small Craft Advisory 42, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 AM AKST Sun Jan 14 2018

.ANALYSIS AND UPPER LEVELS...

The overall weather pattern features a deep trough over the Bering
Sea and a highly amplified ridge over western North America that
extends all the way up to the Alaska-Canada border in the Arctic.
This has once again set mainland Alaska up under a warm and wet
southerly flow pattern. A strong low crossed over the AKPEN from
the Gulf to the Bering overnight, which is the storm center
associated with the strong weather front that pushed through
Southcentral Alaska yesterday. Behind the front, temperatures at
most locations have warmed into the upper 30s to lower 40s, with
gusty southeast winds and showers that are mostly getting hung up
along the coastal mountains. The exception is across the Copper
River Basin, where cold air remains entrenched and heavy snow is
falling from Glennallen eastward. A Winter Weather Advisory
remains in effect for this area.

Meanwhile, Southwest Alaska is more under the influence of the low
center itself (rather than the weather front) with easterly winds
gradually becoming more southerly. Temperatures have spiked above
the freezing mark across most of the Bristol Bay zone as the
winds shifted directions, and this same thing will occur later
this morning across the Kuskowim Delta as the storm center slowly
tracks northward about 50 miles west of the coastline.

West of the low, most of the Bering/Aleutian region is seeing
fairly benign conditions in the heart of the longwave trough, with
cold air from the Russian Far East being pulled offshore and
producing gusty winds and snow showers.

&&

.MODEL DISCUSSION...

The models are in pretty good synoptic agreement for the next few
days, but continue to struggle with the placement of the low
moving up toward Prince William Sound on Monday that will bring
snowfall for somewhere from Anchorage to the western Copper River
Basin. A very sharp cutoff between no snow and several inches of
snow is expected on the western end of this storm, and so the
exact position will be critical. Unfortunately, the models
continue to vary significantly, with the previously snowy (for
Anchorage) NAM backing off to zero snow, and the previously dry
ECMWF now suggesting 4 of more inches of snow possible in
Anchorage. The other models are somewhere in the middle. Based on
forecast persistence and also conceptual models (highly amplified
jet well to the east of the Anchorage area), the current thinking
remains on the drier end of the forecast envelope. Stay tuned, as
this will be the main forecast challenge examined during the day
today.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist as lingering
southeast winds gradually return to a more northerly direction
today.

&&

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

Warm advection continues over southcentral Alaska today in
southeast flow aloft. The Prince William Sound region will see
rainfall with the cooler Copper River Basin receiving mostly
snow. A Winter Weather Advisory is in effect for the Copper River
Basin through tonight for snowfall with heaviest amounts south and
east of Glennallen. There is also a good chance there will be
some areas of freezing rain over the basin as well due to the warm
air pushing 850 mb temperatures to 0 C or higher, and surface
temperatures remaining below freezing. The most likely time for
the freezing rain will be this evening and overnight.

This system will edge westward tonight and bring in a chance of
snow to Anchorage late tonight into tomorrow. At this time it
looks like most or all of the snow will be east of Anchorage so
just went with a chance. The Palmer area has a better chance of
receiving snowfall being farther east but the majority of the snow
will be in the Copper River Basin.

Mild temperatures are expected to persist for the first half of
the week across southcentral Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

A strong gale force low has now moved into Bristol Bay with the
associated occluded front quickly moving north into the interior.
With the exception of the lower Kuskokwim Valley, strong east to
southeast winds are mixing down to the surface allowing for a
rather dramatic warm-up over the region. The surface low will
eventually lift north toward the Bering Strait through Sunday
night, keeping a showery regime over Southwest Alaska with those
showers favoring coastal locations.

Winds diminish and precipitation wanes on Monday and Monday night
before another low moving west from the Gulf of Alaska spreads
clouds and moisture over Southwest. Depending on how the upper
level deformation zone forms, there exists an increasing chance
for snow for parts of Southwest as the surface low stalls near
Kodiak Island. There is uncertainty with the exact track of this
low, so stay tuned to future updates.

&&

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

The gale force low over Bristol Bay will slowly weaken and lift
north toward the Bering Strait through Monday. The wrap-around
precipitation will continue to support snow for the Pribilof
Islands along with rain showers over the Eastern Aleutians.
Northerly flow and rather cold air originating from Eastern
Siberia will move over the Bering, keeping a showery regime in
place through Tuesday. A low end gale force low will then move
into the western Bering Sea Tue night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
This forecast period commences with a low in the Gulf of Alaska
and a vortex of cold air spanning from NE Russia and into the
Central Bering. At the surface, there is a ridge axis that
extends from the north Pacific into the Western Aleutians. The GFS
and the ECMWF are not exactly in sync with the low in the Gulf Of
Alaska but they are clearly converging on a solution that brings
a deep fetch of moisture to Prince William Sound and the Kenai
Peninsula which merited bumping up pops and QPF this forecast
package. By Wednesday, the models bring new lows into the north
Pacific one which will move towards Southeast Alaska and the
other which will track towards the Western Aleutians. The models
continue to bring cooler temperatures aloft into Southwest Alaska
by midweek with the vortex slowly shifting eastward.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales: 130 132 138 150 160.

&&

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