Small Craft Advisory 124, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKST Thu Feb 8 2018

.ANALYSIS AND UPPER LEVELS...
The main weather feature is a broad trough driving the active
weather pattern across the Bering with several strong short waves
within its flow evident on Water Vapor satellite imagery. A
closed low center spinning within the trough is over Shemya early
this morning. Abundant moisture is streaming up through the
central and eastern Bering between this low and a ridge over
mainland Alaska. At the surface, ridging remains dominant over
the southern mainland returning colder than normal temperatures
along with weak offshore flow. Gusty southerly flow is predominant
across much of the Bering with a surface low crossing to the
Bering over the central Aleutians. This system is re-enforcing a
warm front across the eastern half of the Bering.

&&

.MODEL DISCUSSION...
Models continue in relatively good agreement with the synoptic
pattern through the end of the work week. Changes were made to
increase winds across the Bering as confidence increased
for storm force marine winds as the pressure gradient tightens
between the Bering low and ridge to its east. Blowing snow was
also added along the Kuskokwim Delta coast as the frontal
boundary shifts closer Thursday night into Friday.

&&

.AVIATION...
PANC...IFR to LIFR visibilities may briefly occur this morning as
a steep inversion just of the surface is trapping low level
moisture along the northern inlet. Patchy fog may redevelop
tonight as low level moisture increases in a continued stable
environment. Light winds persist through the TAF package with a
dominant surface ridge over the central mainland.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The Suomi (polar orbiting satellite) VIIRS RGB Nighttime
Microphysics imagery clearly shows some localized areas of
fog/stratus along the Knik River Valley into Palmer, along Knik
Arm, along the east end of Turnagain Arm (and nearby valleys
including Girdwood), and portions of the Copper River Basin along
the south to west side of the Wrangell Mountains. Otherwise, clear
skies dominate Southcentral this morning under an offshore flow
regime. An upper level short- wave which dropped southward across
the eastern Copper Basin overnight led to an upswing in gap winds
in Valdez/Thompson Pass as well as along the Copper River Delta
into the northern Gulf. These winds will gradually diminish today
as pressure and thermal gradients slacken.

Aside from a continued threat for localized areas of fog and
stratus, dry clear and seasonably cool conditions will continue
into Friday. A strong frontal system taking shape out west will
then approach Kodiak Island on Friday with rain and gale force
winds spreading northward across the western Gulf. Precip will
reach the southern tip of the Kenai Peninsula Friday night, with
clouds spreading northward across the rest of Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
Warm southerly flow will continue over southwest Alaska with
ridging to the east and a longwave trough to the west. Late
tonight a front will move in and will at first bring precip to
areas along the coast and then will spread inland on Friday.
However, the majority of the heaviest precip will stay along the
coast. Precip looks to start as snow for most locations and
transition to rain, with the exception of farther inland locations
that will stay as snow. Winds will also increase throughout
southwest Alaska as the front pushes in. Along the Kuskokwim Delta
coast the initial onset of snow combined with winds will bring
blowing snow conditions to the coastline Friday. Friday night both
precip and winds will start to diminish and Saturday will
continue to see improving conditions.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
With the longwave trough extending over the western Bering,
multiple systems look to track north into the central and eastern
Bering through Saturday. These systems will bring warm air, rain
and winds to the eastern Aleutians and Bering as they move
through. Marine areas around eastern portions of the Alaska
Peninsula look to see storm force winds Friday. Further west,
temperatures will stay colder without the influence of the North
Pacific systems bringing in warmer air from the south. This will
result in light snow showers for locations around the Western
Aleutians into the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Saturday will be a day the pattern is shifting to a warmer one
over Southcentral and Southwest Alaska due to an amplifying upper
level ridge along the West coast of North America. The low in the
Bering Sea will push into this ridge over the weekend and start a
somewhat progressive upper level pattern over the state for next
week. Models are struggling quite a bit with the progressive
pattern, but in spite of the noise, it looks like we can get a
reasonable idea as to where the longwaves will set up and those
will be the basis for everything else. In general, the highly
amplified ridge/trough pattern has been dampened a bit on the long
term guidance which is to keep the forecast from fluctuating too
far as some of the details become more clear. This will also keep
a lot of 'chance of' POPs to remain in the long term forecast.

Overall, next week will be much more mild with many locations
experiencing temperatures above freezing. Precipitation type will be
an issue with this situation, and the details will not come into
focus until we get closer to these systems moving through the
area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 150 160 180.
         Gale Warning 130-132 136-138 155 165 171-173 179 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...EZ