Small Craft Advisory 238, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 PM AKDT Tue Mar 20 2018

.ANALYSIS AND UPPER LEVELS...
There is a large upper level ridge building from the central
through southwest Alaska. The associated surface ridge is centered
near the Alaska Peninsula. There is an upper level low centered
just south of Yakutat and moving southeast. This is vertically
stacked with a surface low. Fairly strong surface winds exist
between these features. There is and upper level low over the
Kamchatka Peninsula, with a front over the western Bering and
western Aleutians that is very slowly moving eastward. Conditions
are dry and/or drying over southern Alaska. There are a few snow
showers over the lower Kuskokwim valley area, mainly against the
Alaska Range.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement for the short term
portion of the forecast (Thursday afternoon). The exception is
that there are some differences with a low in the Bering on
Thursday and its associated front. The ECMWF has more of a bent
back occlusion and tighter surface low compared to the GFS and
NAM, which have more of a strung out low/front. The ECMWF
therefore has stronger back-side winds. Forecast confidence is
above normal, except near to below normal for the Bering low on
Thursday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through
Wednesday morning. Gusty north winds are expected to develop
Wednesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Forecast challenge for today remains the gusty outflow winds
across Southcentral and Prince William Sound. The ridge of high
pressure over the Bering Sea has moved slightly eastward, bringing
a tight northwest gradient over the Southcentral area today. Winds
are currently gusting to Gale force through the Barren Islands,
but as the ridge continues to move eastward, they will become more
northerly and affect the central portion of Cook Inlet. As we move
into Wednesday morning, an upper level jet will move southward
over the Chugach mountains, providing support for mountain waves
and warning level winds through the Valdez area. Right now, it
looks as though the strongest jet support will remain just west of
Valdez, so am expecting the strongest wind gusts out of Mineral
Creek Canyon and other northerly outflow areas. Thompson Pass will
remain windy as well but am thinking this will not be as
impactful an event in that location.

Winds are also expected to be gusty through other areas of
Southcentral, although not quite as strong as in the Valdez area.
Northeasterly winds will be quite gusty through the Mat Valley
Wednesday night and early Thursday morning. The good news is
that these winds are not backed by a significant dome of cold air
in the Copper River Basin, so they should remain below warning
level, although they will still be impactful. West Anchorage will
also see gusty northerly winds, but these are expected to be much
lower, around the 20 to 30 mph range.

Otherwise, skies will be sunny over the next few days, with
temperatures being slightly moderated by the winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
High amplitude ridging will build into Southwest from the Bering
Sea through Thursday, keeping mostly dry weather over the region.
Northwest winds will diminish and eventually become onshore by
tonight, so this may bring in some low stratus and fog to the
coastal areas. Otherwise, weather will be quiet for this time of
year as interior high pressure remains strong.

&&

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

A deep low over the Kamchatka is supporting widespread warm air
advection aloft over the Bering Sea. The associated gale force
front is stationary over the far Western ALeutians/Bering Sea.
However, a strong basal shortwave trough will round the corner and
amplify along the baroclinic zone, eventually bombing the low out
as it lifts north through the Bering Sea Wednesday night and
Thursday. This low will have a powerful bent-back occlusion as it
lifts toward the Bering Strait with storm force winds and near
hurricane force gusts. The associated gale force front will move
into the eastern Bering Sea on Thursday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
On Thursday night a large upper and surface low is over the
northwest Bering Sea with a ridge from the north Pacific through
western Alaska to the arctic coast with another low off Vancouver
Island. The ridge slowly breaks down and slides east while the Low
in the Bering Sea shunts to the north northwest through Sunday.
The upper trough extends from Asia south into the Bering Sea and
Aleutians Saturday. A surface low in the base of this trough will
move to the eastern Bering Sea Sunday then stall and dissipate.
Another low forms south of this low and that will move into the
Gulf of Alaska Monday. The upper level flow becomes zonal again
Tuesday and Wednesday. The models are not in good agreement after
Sunday, thus we will trend toward the blend of the EC ensemble
solution, though not entirely sell out to it. The pattern will
remain active through the weekend into mid week next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High wind warning 131.
MARINE...Gales 119-121 125-128 130-132 136 170 173-179 181 185 351
352 411-414. Heavy freezing 161.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...LF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...SA