Heavy Freezing Spray Warning 103, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKST Wed Dec 21 2016

.ANALYSIS AND UPPER LEVELS...

An upper level trough extends south across the AK mainland this
morning, with a building upper ridge over much of the Bering. At
the base of the mainland AK trough is the beginning of a closed
off low that will move over the Gulf later today. Currently the
upper low is over southwest AK and producing areas of snow over the
Bristol Bay region. Snow showers are also moving into portions of
Southcentral ahead of the Bristol Bay feature, on southwest flow
aloft. At the surface a low is consolidating over the northern
Gulf of AK, while the Bering is covered with weak low pressure.

&&

.MODEL DISCUSSION...

Models are in good agreement, with the low continuing to
consolidate over the northern Gulf through tonight before drifting
southeast through Thu night. A north Pacific ridge will build into
the void left by the departing low, reaching into the Bering Thu
and southwest AK Thu night into Fri. A weak low developing in the
northern Bering later today will remain north of the forecast
area. To the west of the ridge the next north Pacific low will
begin to affect the western and central Aleutians beginning on
Thu.

&&

.AVIATION...
PANC...An upper level low consolidating over Southwest AK this
morning is sending a trough northeast in the direction of
Anchorage. This is spreading snow showers to the west of the
airport, but also providing enough mixing to begin clearing fog
from the area. This should continue through the early morning. VFR
conditions are expected by midday and should persist through Thu
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Broad southerly flow aloft and surface lows moving north into the
northern Gulf will continue the threat of Snow and snow showers
today, heaviest along the North Gulf Coast, and to a lesser extent
inland. That said, the NAM is trying to develop a deformation
band which extends north into the Copper River Basin this
afternoon. This may be a little overdone, but bears watching.
Areas of fog across the Anchorage Bowl, Knik Arm, and into
portions of the MAT Valley should dissipate by afternoon and not
make a reappearance tonight. The ridge builds in behind the upper
level trough late tonight/early Thu bringing clearing and cooler
conditions. Pressure gradients will also increase tonight and Thu
signaling the onset of brisk outflow conditions, strongest across
the eastern Kenai Pen, western Gulf, and North Gulf Coast. These
conditions should continue into Thu night. The biggest concern
with this change is for Thompson Pass and the stronger winds
blowing around any loose snow beginning Thu.

&&

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

A cold upper trough continues to remain over the region, with a
rather impressive embedded surface low situated over the far
eastern portion of Bristol Bay. Low level convergence along this
boundary is sufficiently strong to support an area of accumulating
snowfall. With northerly cold advection winds behind this front,
expect some areas of local blowing snow from Dillingham east today
before winds diminish. High pressure builds in tonight and will
support the coldest temperatures in a while before another weak
shortwave trough spreads in clouds and snow chances Thursday night
and Friday.

&&

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

Quiet weather after the very active storm cycle of the last week
will continue as weak upper ridging moves across the Bering Sea
ahead of a weak upper level trough. Broad westerly flow behind
this feature will support rather widespread rain and snow showers
over the Bering Sea as it moves east through Thursday. Thereafter,
a deepening North Pacific low will move into the Western and
Central Bering Sea Thursday night, bringing with it gale force
east-southeast winds.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Outflow winds will be the big issue Thursday night into the early
weekend as high pressure settles over inland areas from the west
and low pressure in the southeastern Gulf continues moving
southeast and away from the region. Attention then turns to a
series of systems that will move northeast into the Bering toward
the mainland, but will encounter resistance from downstream
ridging and eventually result in a west-east longwave trough and
surface-front orientation from the Bering to the Gulf by early
next week. While there is good synoptic agreement in this pattern
occurring, the devil will be in the details of the individual
systems that develop along this frontal boundary.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 130- 132 138 150.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...CC