Small Craft Advisory 198, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 AM AKST Mon Mar 6 2017

.ANALYSIS AND UPPER LEVELS...

A highly amplified upper level pattern remains intact across
Alaska and the Bering Sea once again this morning. A strong upper
level ridge remains entrenched across the Bering Sea while an
arctic trough remains over the Alaska mainland and continues the
feed of cold air from the arctic. Once again, temperatures were
well below normal for pretty much the entire state of Alaska
except for the Aleutian and Pribilof Islands as a weakening warm
front remains draped across the Bering Sea. Once again,
temperatures have fluctuated greatly across Southcentral Alaska
with outflow winds blowing strongly through the typical gaps. As
the winds have decoupled this evening, locations have dropped into
the low single digits to below zero temperatures. Elsewhere where
winds remain strong, temperatures have only dropped into lower
teens.

The strongest winds overnight have been observed through Thompson
Pass where winds gusts on occasion have reached or exceeded
hurricane force. Winds in the town of Valdez have been a bit
weaker with wind gusts upwards to 60 to 65 mph. Diurnal gusty
winds are expected across much of Southcentral Alaska again today
once the nocturnal inversion erodes and stronger winds aloft are
able to mix down. Abundant sunshine under clear skies will help
these inversions erode quickly through the morning hours.

&&

.MODEL DISCUSSION...

Models are once again in excellent agreement. This persistent and
steady-state weather pattern has really allowed numerical guidance
to get a firm grasp on the synoptic conditions. There are always
going to be a few challenges when it comes to the details
especially the local effect winds across Southcentral Alaska.
There is high confidence that the winds over Thompson Pass and
Valdez are peaking or are just past peaking and should gradually
trend downwards throughout the day. The models have struggled the
most with nighttime temperatures as large thermal gradients have
been observed as some locations decouple and other do not. This
has led models to be off 10 to 15 degrees at times which can
create quite the headache. Luckily these winds and temperature
concerns should become less of an issue in the coming days. The
high resolution NAM was once again used for the forecast package
for it's increased resolution.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (through late Tue night)...

The upper blocking pattern remains in place and will in general
continue the dry, cold, and locally windy pattern through Tuesday
night. Winds should peak in the Valdez to Thompson Pass areas this
early morning and safely fall below high wind potential by mid
morning. Other gap flows across Southcentral will weaken throughout
the afternoon hours, but favored areas will continue with brisk
winds through tonight.

On Tuesday, gap flows will increase during the latter part of the
day as a northerly jet on the forward side of the ridge
intensifies and surface pressure gradients increase.

&&

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

There is no change in the sensible weather pattern as dry north-
northwest flow aloft dominates the regional weather pattern.

A weak remnant shortwave trough will cross the N Bering Sea and
move into southwest Alaska Tuesday night through Wednesday night,
bringing in some higher cloud cover and slightly warming
temperatures by Thursday as the Bering ridge shifts east.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A stalled front over the Central Bering and Aleutians will remain
in place today then weaken tonight as secondary shortwave trough
becomes the dominant upper level feature over the extreme Western
Bering Sea. This occluded low will move toward eastern Siberia
Tuesday before another gale force low and associated warm front
move across the Western bering Sea Tuesday night and Wednesday.
High pressure will continue to dominate the Eastern Bering
through the period

&&

.LONG TERM FORECAST (Days 3 through 7)...
Tuesday night and Wednesday, the large scale upper level ridge
centered over the eastern Aleutians and Bering will tilt east
into coastal Southwest Alaska. With significant warm advection
aloft accompanying the ridge and strengthening night time
inversions, expecting to see fog and low stratus impacting coastal
portions of the Kuskokwim Delta Tuesday night, spreading to
include the Bristol Bay area Wednesday night and then pushing
further inland to impact most of the lowlands of Southwest Alaska
by Thursday night. To the west of the large ridge, southerly flow
and an active storm track will keep the western Aleutians in a
wet and windy pattern through the week.

For Southcentral Alaska, the offshore pressure gradient and
resulting gap winds will increase again Tuesday night through
Wednesday morning as the tilting upper level ridge pushes the deep
trough east and puts Southcentral Alaska underneath the stronger
northerly flow aloft in between the two features. The entire
pattern continues to shift east Thursday and Friday as an upper
trough out of Russia, rides up and over the ridge and flattening
it somewhat. This shift will bring a gradual moderation in the
offshore winds as well as ushering warmer air aloft into
Southcentral Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HIGH WIND WARNING...131
MARINE...Heavy Freezing Spray...121 126-131 138 139 141 185
                          Gale...127 127 177 178 185

&&

SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHCENTRAL ALASKA...RMD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...JR