Small Craft Advisory 204, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
409 PM AKST Tue Mar 7 2017

.ANALYSIS AND UPPER LEVELS...
A low over the far western Bering Sea is driving northward into
the top portion of the quasi-stationary upper level ridge, which
is pushing the ridge eastward toward the Alaska mainland. This is
leading to a strengthening northerly jet streak over Alaska. With
a short-wave trough dropping southward into Southcentral today
(and additional weak impulses trailing to the north), this sets
the stage for another wind event across Southcentral. Meanwhile,
to the west of the upper ridge, the latest in a series of lows is
tracking from the northwest pacific to the western Bering Sea,
pushing a gale force front toward the Western Aleutians.

&&

.MODEL DISCUSSION...
Models remain in excellent agreement with large scale features and
forecast confidence is high. The primary forecast challenge will
be determining how much wind mixes down to the surface over
Southcentral and portions of Southwest Alaska tonight because weak
surface based inversions may set in prior to arrival of the
strongest winds. The extent of wind will greatly affect the low
temperature forecast for tonight/Wednesday morning.

&&

.AVIATION...
PANC...Another upper level trough dropping through Southcentral
will lead to strengthening low level winds this afternoon through
tonight. The challenge is determining how much of this wind will
mix down to the surface. Abundant sunshine should mix out inversion
from this morning, so expect boundary layer to remain well enough
mixed to produce some gusts beginning sometime this evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Wed and
Thu)...The most noticeable change in the weather for the next few
days will be an increase in the winds for most areas. The ridge
over the Bering will push slightly further east and take on a
positive tilt. This will lead to more upper-level energy spilling
into Southcentral and help mix down some of the gustier winds.
These winds should also keep the atmosphere better mixed through
the overnight hours, leading to warmer low temperatures (versus
the records that we saw this morning). The other notable change to
the weather will be the introduction of some high clouds spilling
over the area by late Thu-early Fri.

Models have come into better agreement on a stronger shortwave
dropping south through the Fort Yukon area late Wed. As the
overall flow in the vertical column becomes more northeasterly,
this feature could be just enough to bring down some warning level
winds through Thompson Pass. A high wind watch has been issued to
address this concern. Even if winds don't quite reach the 75 mph
range, they will still be very strong through the typical north to
south gaps (Thompson Pass, Copper River Delta, Seward, etc.).
These winds will persist through early Thu. By Thu afternoon-
evening, the ridge will shift far enough east that the gradient
will relax and winds should diminish markedly. As the offshore
gradient wanes some, there will be increased concern for fog
development starting Fri morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Generally sunny skies and offshore flow will prevail into late
this week as an omega block continues over the Bering. The
strongest winds will likely occur over Bristol Bay this evening
into Wednesday morning as high pressure buildings in from the
north. This high pressure will then center over the area through
Thursday, allowing for winds throughout the region to diminish
under sunny skies and continued cool temperatures.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A persistent omega block over the Central-Eastern Bering will
provide little change in the current weather to the Bering Sea
region through late this week. Most of the activity will be
centered around the Western Aleutians/Bering where southerly gale
force winds and rain will continue into Thursday. On Friday, the
omega block will then rotate and spread into most of the Bering,
allowing for diminishing winds and clearing skies throughout the
area.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning late Thursday, expect continued dry conditions, lighter
winds, and cold, but relatively warmer temperatures to current
ones. The upper level pattern looks much the same to what we're
experiencing currently. The biggest difference in the pattern is
ridging over the Bering Sea will develop into a closed blocking
high, while troughing remains over the eastern Gulf and Panhandle.
While the flow remains northerly in an outflow pattern similar to
the current pattern, the Bering ridge nosing in from the west will
considerably moderate temperatures in the low-level. Radiational
cooling will still dominate, so morning lows will be chilly
(single digits or less), but the warmer airmass will make for
daytime highs in the 20's.

Into the weekend, the blocking high is expected to retrograde
westward. The main effect will be to keep the northerly flow, and
increase the chances of getting more arctic air back over the
area. Sensible weather looks much the same over this time period.
High confidence in dry weather, winds will be generally light
northerly, except for along the north Gulf coast which will
feature gusty winds. Temperatures will be below normal, with the
exact values dependent on the amount of wind present.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 131.
MARINE...Gale 126 127 177 178 185.
		 Heavy Freezing Spray 121 126 127 129 130 139 141.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...MTL

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1211 AM AKST Wed Mar 8 2017

AKZ185-081815-
Eastern Aleutians-
Including the cities of Unalaska and Nikolski
1211 AM AKST Wed Mar 8 2017

...Trace of Ash Expected at Unalaska Through the Morning...

Bogoslof Volcano erupted late tonight. Winds are favorable to
bring trace amounts of ash from the volcano to Unalaska Island.
Trace amounts of ash are expected over Unalaska Island including
Dutch Harbor for the next several hours.