Small Craft Advisory 259, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKDT Tue Apr 2 2019

.ANALYSIS AND UPPER LEVELS...
A large scale, northwest to southeast running upper level ridge
stretches from eastern Russia, across mainland Alaska and into the
Gulf. A small closed upper low centered just to the east of Norton
Sound is embedded within the ridge and becoming in phase with a
cold upper level trough digging south from the Arctic Ocean
across the northeastern corner of Alaska and the northern Yukon
Territory. Further to the west, an elongated upper low stretches
from west to east south of the Aleutians. One surface low
vertically stacked with the upper low is located south of the
western Aleutians while a second developing surface low is lifting
to the northwest towards the central Aleutians. A gale force
front stretches across the southern Bering and over the eastern
Aleutians and western Alaska Peninsula.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement through the short-term
forecast period. The smaller scale details regarding how they
handle the upper level trough digging south through eastern Alaska
Tuesday and Wednesday and its interaction with and incorporation
of the upper low currently over western Alaska have come into
better agreement as well. Model consistency begins decreasing
Wednesday night and Thursday regarding the position of an eastward
tracking low south of the western Gulf, however impacts from this
system will likely be limited to marine areas.

&&

.AVIATION...
PANC...VFR conditions with light and variable winds will persist
through Tuesday night. Northerly winds begin increasing through
the day on Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Wednesday night)...
The prolonged period of dry and warm weather is about to come to
an end. The upper level low east of Norton Sound will open into a
trough and drop southward across Southcentral tonight. Right on
its heals, the arctic trough will drop southward across the
eastern Alaska mainland Tuesday night, then settle right over top
of Southcentral on Wednesday. This will usher in a much colder
airmass. 850mb temperatures will fall from -2 to +2 this morning
down to -6 to -14 degrees Wednesday night, with the coldest air
settling over the Copper River Basin. The cooling trend will begin
Wednesday, but will likely be most noticeable on Thursday, with
daytime highs 10 to 15 degrees cooler than what has been observed
in recent days.

Aside from the cooler weather, the passage of the troughs combined
with the low level cold advection will produce gusty winds across
the region. As is typical, the strongest winds will be through
mountain passes as well as bays along the coast. However, most
everywhere will see a little bit of wind as the cold air advects
southward on Wednesday. With colder air coming in via the Copper
River Basin, this is a favorable set-up for winds in the Matanuska
Valley. Some of this wind will bend southward and affect north to
west Anchorage. Since many areas are now snow-free and green-up
has yet to occur, there could be quite a bit of blowing dust in
the Matanuska Valley. As far as these "bora" type of events go,
this will be a relatively weak one. The upper trough stalls
overhead and doesn't push south into the Gulf. Thus, the advection
of cold air into the Gulf is weak and coastal gap winds will
remain well below warning thresholds.

Lastly, expect some light snow in the mountains surrounding the
Copper River Basin as the upper troughs move through Tuesday night
into Wednesday. Think the low levels will remain too dry for snow
in Glennallen and vicinity, but there could be a few flurries. The
one roadway that could see a dusting of snow is the Glenn Highway
west of Glennallen, toward Eureka and Tahneta Pass. The remainder
of Southcentral will remain dry, though there will be some passing
clouds.

&&

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

The upper ridge will slowly weaken as the upper level flow becomes
northerly and a series of shortwave troughs move south. This will
increase northerly flow aloft and offshore winds tonight through
Tuesday night. Winds will become rather gusty by Wednesday,
especially across the Kusko Delta. This pattern will keep the
region dry through Thursday.

&&

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

A mostly stationary front will remain over the Bering today and
tonight, supporting moderate gale force winds over most of the
Aleutian Chain. Stronger gusts to near 50 knots will be possible
today, especially in areas with local gap winds. Rain will
accompany this front, but that will diminish as the front weakens
and lifts northwest on Wednesday. A COL will form over the Bering
thereafter, keeping winds below gale force, and precipitation more
showery and intermittent through Thu.

&&

.MARINE (Days 3 through 5/Thursday through Saturday)...

Gulf of Alaska...Confidence remains high for sub gale force winds
throughout the Gulf of Alaska through Saturday.

Bering Sea/Aleutians...A swath of near Gale force wind is still
possible across the northern Bering Sea Thursday, although models
have backed off on the strength of the wind. Gale force wind is
possible in advance of a front approaching the Western Aleutians
late Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The long-term period begins Friday with an upper low dropping
south over the Copper River Basin accompanied by cold air that
will drop temperatures back to more normal temperatures. The
influx of moisture with this upper trough and cold air will keep
the threat of snow over the eastern Copper River Basin through
Saturday. Warmer air then returns Sunday, ending the threat for
snow. Moving through the weekend a surface low moves into the
Gulf of Alaska and the upper level flow becomes southerly. This
supports increased clouds and the possibility of precipitation
along with slightly warmer temperatures across Southcentral
Alaska. There remains uncertainty with regard to whether the
upper low tracks further to the west as progged yesterday, thus
the inconsistency in model solutions is leading to less confidence
in any one forecast as we move into early next week.

Out over the Bering Sea another low and front is expected in the
western Aleutians Saturday. As this low tracks to the northeast
into the central Bering Sea, gale force winds and a mix of rain
and snow are likely across the central Aleutians. However, warm
air rapidly moves in post frontally.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 155 165 170 171 172 173 174 175 178 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...SA