Small Craft Advisory 180, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
428 AM AKST Tue Feb 28 2017

.ANALYSIS AND UPPER LEVELS...

The upper air pattern features a stout shortwave trough amplifying
over the Gulf of Alaska in northwest flow with a building ridge
axis over the Western Bering Sea. A polar upper trough is centered
over the Beaufort Sea, and this is the "feed" for the cold air
pouring through southern Alaska this morning. With a surface low
strengthening in the Gulf and northerly cold air advection aloft,
offshore pressure gradients are on the rise this morning. All
favored gap winds along the coast are also on the rise, and gap
winds across Southern are expected to continue to rise today as
cold air advection continues.

Conditions across the Bering Sea are much more benign with a gale
force stationary surface front west of Shemya. This is enhancing
north to east low level flow across the remainder of the Bering
Sea and Aleutians.

&&

.MODEL DISCUSSION...

The numerical models are in excellent synoptic agreement into the
weekend. There remains some discrepancy in the numerical models
with respect to embedded shortwave trough features propagating
through the flow, especially on Thursday. The GFS and ECMWF were
favored at that point with the NAM being a fast outlier.
Otherwise, high resolution models were favored to capture local
gap winds in the short term. Forecast confidence is very high.

&&

.AVIATION...
PANC...CIGS should rise this morning as cold air dries out the mid
level stratus deck. Northerly winds will increase by midday and
will continue to remain gusty into tomorrow.

&&

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

The cold air will continue to move into the region as gusty winds
persist over the area. Kodiak Island will see some of the
strongest winds as the upper level support is greatest there.
The temperature difference between the Copper River Basin and Gulf
coast is enough to bring very gusty winds to Thompson Pass. There
is enough transportable snow in the pass to cause limited
visibilities today in the blowing sow caused by the strong winds.
The biggest uncertainty is how long before there is not enough
transportable snow left to reduce visibilities. Currently a
blowing snow advisory is out for Thompson Pass until 6 PM today,
but there is a decent chance the blowing snow will be done before
that in which case we will cancel the advisory.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cold air will continue moving into the Southwest Mainland today as
northerly flow develops. This will also allow for cloud cover and
snow showers currently moving over the area to diminish by this
evening, which should allow for temperatures to fall even colder
tonight. These cold temperatures along with northerly winds will
create wind chills to as low as 40 below in the Kuskokwim Valley
overnight tonight, it does not appear at this point that these
conditions will be widespread or long lived enough to require an
advisory. The potential for advisory level wind chills will
continue to need to be monitored over the coming days as the
region continues to gradually cool with persistent northerly
flow. This will be the primary weather concern into the end of
this week as sunny, dry, and cool conditions spread throughout the
area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The broad ridge of high pressure has setup over the Central
Bering will become nearly stationary omega block that will persist
through mid-week. This will keep much of the active weather
confined to the Western Bering where nearly persistent gale force
winds and rain/snow will continue as a frontal system becomes
stationary over the area. Over the Eastern Bering broad northerly
flow and cold air advection will bring snow showers to the Bering
Sea coast of the AK Pen into Wednesday, however even these will
begin to diminish as the upper level trough descends into the Gulf
and allow ridging to spread into the Eastern Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
By the beginning of the extended forecast period Thursday much of
the region will have already undergone a transition in the
overall pattern, with a broad upper level trough over much of the
Mainland and Gulf, and strong ridging out west over the Bering Sea
and Aleutians. This general setup will persist through early next
week, which will consist of colder air filtering down from the
arctic and strong outflow winds over parts of the Southern
Mainland. Models remain in good agreement with the overall pattern
into the weekend. As expected though, there is more uncertainty
with regards to the finer details, especially with respect to low
amplitude shortwave troughs diving south toward the Gulf of Alaska
coast Friday through Monday.

All three major global deterministic models, (the ECMWF, Canadian
GDPS, and GFS) along with their respective ensembles, are also
depicting a rather significant bora polar air outbreak across much
of Mainland Alaska. There is potential of seeing 850 hpa
temperatures reaching as low as -20C to -30C across the Southern
Mainland with cold offshore flow along the coast. The March sun
should moderate some of the cold air at the surface, but the
reality of this situation is it will be quite cold and well below
average for the next 7-9 days, dominated by dry offshore flow and
cold interior temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Weather Weather Advisory For Snow 131.
MARINE...Gale 121 127 130 131 132 136-139 150 176-178 180
Heavy Freezing Spray 121 126 127 130 136-139 141 150 160 165 180
185.

&&

SYNOPSIS AND MODEL DISCUSSION...JRA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...PEPE