Small Craft Advisory 230, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT Thu Mar 16 2017

.ANALYSIS AND UPPER LEVELS...

A large ridge of high pressure remains over the Bering Sea
resulting in mostly benign weather in that area and cold (and
gusty) northerly flow across virtually all of mainland Alaska. An
upper low remains stationary in the Gulf of Alaska, with numerous
surface lows pinwheeling underneath the main circulation. One of
these surface lows is making a close approach to the north gulf
coastal areas as of this writing, with more snow shower activity
than we've seen in recent days across the northwest Gulf and
Prince William Sound. Whittier even got a few snow flakes
overnight, though snow shower activity is now beginning to pull
back offshore as the associated surface low starts its journey
southward and back out to sea. Meanwhile, cold air continuing to
pour off the Alaskan landmass is maintaining the gusty gap winds
that have been ongoing for many days now. A particularly
impressive band of winds occurred overnight near the southern end
of Shelikof Straight and on the south side of the Alaska
Peninsula. These winds were a result of a vigorous midlevel
shortwave dropping quickly southward through the area that helped tighten
the pressure gradient and mix down stronger winds to the surface,
with 60+ mph winds occurring in the area for several hours.

&&

.MODEL DISCUSSION...

The models remain in excellent synoptic/large-scale agreement. Not
surprisingly there is disagreement with the strength, position,
and track of the numerous relatively weak surface lows rotating
around in the Gulf. Based on satellite imagery, the GEM and ECMWF
seem to have the best handle on the next circulation currently in
the eastern Gulf that is headed toward the northern Gulf today.
While these discrepancies are small on the grand scale, they will
have a more substantial impact on the timing and strength of the
gap winds at many locations along the coast. Thus, the GEM was the
preferred model on the eastern domain overnight. West of the
Alaska range, models are in better agreement even on the smaller
features, and a weak preference was shown for the NAM based
largely on resolution and handling of winds through complex
terrain features along the Alaska Peninsula.

&&

.AVIATION...
PANC...VFR conditions will persist throughout the period. Mid and
high-level clouds will be slowly diminishing throughout the day,
coincident with a slow but steady decrease in the northerly winds
that have been affecting the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong vorticity maximum which dropped southward across the
western Gulf yesterday has amplified the upper trough and created
a new closed center over the northeast Pacific. This will gradually
pull the series of surface lows over the Gulf south and eastward
today through Friday, bringing an end to the latest wind event
for inland locations. Pressure gradients will remain tight enough
along the Gulf coast to maintain some gap flow.

A few showers on the northern periphery of the Gulf lows will
cross the Prince William Sound area today, with just some mid to
high clouds for the rest of the region. Skies will then gradually
clear tonight through Friday. With lighter winds and mostly clear
skies look for some chilly temperatures the next couple nights,
with inland valleys dropping below zero. With abundant sunshine
during the day temperatures will recover into the teens to 20s
each afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Gusty northerly winds will begin a slow decline today into Friday
from current levels. Not quite a pattern change as the mainland
will remain under northerly flow from the Interior, but a marked
decrease in wind. This afternoon into the evening hours, a weak
disturbance and front will push onto the Kuskokwim Delta and
stall. Expect light snow to persist from the evening into the
overnight hours before breaking up Friday morning. Snow will
likely stay confined to the coast, but a flake or two toward
Bethel isn't out of the question. That front and weak low will
actually linger near the coast through late Friday night keeping
the chance of snow through then. Light northerly winds will
continue, along with below average temperatures, for the
remainder of the period.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A gale-force front will reach the western Aleutians this
afternoon, bringing snow initially that will gradually turn to
rain through the evening. The front slows and stalls over the
central Aleutians on Friday before slowly weakening and moving
east. High pressure will set up shop over the eastern Bering
through the period.

&&

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

Persistent high pressure over the Bering and a low over the Gulf
will keep northerly winds across mainland AK portions of our
forecast area on Sat. A small craft and gale force front will be
over the southern Bering with rain and snow. An impulse moving
around the Gulf low will bring snow to the north Gulf coast, that
begins to move inland Sat night.

From Sun through the middle of next week...low pressure
consolidates and becomes stationary along the north Gulf coast.
The associated light snow over the coast will move inland where
it will persist for several days. High pressure over the northern
mainland will keep cold and dry conditions over southwest AK,
while the Bering front dissipates. Cold northeast flow across the
Bering will produce some snow showers, mainly along the Aleutian
chain through midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 150
         Gale Warning  120 121 130 131 132 136 137 139 150 155
                       173 174 175 176 177 178
         Heavy Freezing Spray 121 130 132 136 137 138 139 150
                              155 165

&&

SYNOPSIS AND MODEL DISCUSSION...AMD
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...DS