Small Craft Advisory 86, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
406 AM AKST Sat Jan 27 2018

.ANALYSIS AND UPPER LEVELS...
Currently the axis of the large, high amplitude blocking ridge
extending from roughly 20N to 80N stretches from the central
Aleutians, north across the Bering to the Gulf of Anadyr and
Bering Strait. To the east of the ridge, north to northeast flow
aloft with a long arctic fetch, continues to pump cold, dry air
into Mainland Alaska. The upper level low in the far southwestern
Gulf continues to track south while troughs associated with it
rotate from east to west across the Gulf. To the west of the
large ridge, a north to south running front has stalled over the
western Aleutians and western Bering.

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.MODEL DISCUSSION...
Models remain in good agreement regarding the large scale
synoptic features, however continue to have timing and track
differences with the smaller scale troughs rotating through the
Gulf. Additionally, model differences regarding a northern stream
upper level shortwave tracking across southcentral around Sunday
evening will impact the magnitude and aerial coverage of a
potentially strong mountain wave/northerly outflow wind event
Sunday night or Monday.

&&

.AVIATION...
PANC...VFR conditions and light north to northeast winds will
persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The northern portion of the upper level high centered over the
Bering Sea will tilt into northern Alaska today, and then
southward into southern Alaska by Sunday. The upper trough, and
any chances of precipitation, will remain over the northern Gulf.
This will bring increasing northeasterly winds aloft to the area
Sunday and Sunday night. The surface offshore gradient will also
increase over Southcentral, with increasing winds. The strongest
winds will be through gaps and passes, including the Matanuska
Valley, the Copper River delta, and the Thompson Pass/Valdez area.
An upper level short-wave trough will drop south into the area
later Sunday and Sunday night. This will focus increasing winds
through Thompson Pass and Valdez. The Nam shows 850 mb winds
peaking around 70 kts Sunday evening. The dynamics due to the
short-wave are favorable for winds to mix downward and winds could
approach warning levels in the pass and even in town. Cold
advection is minimal, so the wind forecast for now is kept below
75 mph, but this is something worth watching.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A dry offshore flow regime will prevail across the southwest
mainland into early next week in predominantly northeast flow on
the periphery of the Bering Sea ridge. Temperatures will continue
to gradually moderate through Monday with the help of slowly
rising heights over the region. These warmer temperatures should
be enough to offset the continued breezy conditions and mitigate
any significant wind chill concerns compared to recent days,
however seasonably cold temperatures and wind chill values dipping
well below zero should still be expected through Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure will dominate the Bering through Monday as the ridge
closes off and drifts north towards the Gulf of Anadyr. This will
promote very benign conditions across much of the Bering, with
the most notable sensible weather impacts being found along the
southern periphery of the upper high. With that in mind, a few
snow showers will continue to linger along the Akpen and the
Eastern Aleutians through Sunday while slowly diminishing in
intensity as flow pivots to the northeast and cold advection
wanes. Out west, a nearly stationary frontal boundary will
continue to bring rain and southeasterly gales to Shemya and
vicinity through Sunday before finally drifting off to the
southwest and allowing quieter conditions to settle in for the
beginning of next week.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The Omega Block feature will be on-going into the extended period
with the ridge axis over the Eastern Bering and Eastern Aleutians.
This configuration will keep Southwest Alaska in North-
Northeasterly flow through at least Tuesday. This pattern,
combined with tight low-level surface pressure gradients, will
keep a risk of strong wind flow through channeled terrain
especially along the Alaska Peninsula, Thompson Pass and the
Copper River Delta.

Tuesday, a cold front drops out of the Arctic, but bypasses
Southcentral thanks to tall Alaska Range mountains. The deep cold
spills into Southwest and is supported by a newly formed upper-
level low that undercuts the blocking High over the Chukchi Sea.

The remainder of the week will feature a gradual return to
southerly flow over the Eastern Aleutians and Southcentral as the
new upper-level low drifts westward through the Southern Bering
Sea. Timing between the Euro and GFS shows up with a difference on
the scale of days at this point. It will be worth watching the
Pacific Jet to monitor any subtropical connections and potential
atmospheric river effects into Northern Gulf Coast communities.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 127 128 165 172 178 352 411.
         Heavy Freezing Spray Warning 127 160 180.

&&

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...DS/DEK