Heavy Freezing Spray Warning 79, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
626 PM AKST Thu Dec 8 2016

.ANALYSIS AND UPPER LEVELS...

A broad inverted trough extending north from an upper level low
centered west of Washington and Oregon covers the Gulf of Alaska.
An easterly wave rotating around the northern end of the trough is
progressing from the northern Gulf into Prince William Sound. In
addition to bringing rain and snow to the northern Gulf coast,
this wave is further tightening the offshore pressure gradient
and bringing rather windy conditions to Thompson Pass and the
Copper River Delta today. With the very dry air mass in place, it
has been taking a while, from the arrival of initial moisture
band via the radar, for snow to start reaching the surface. While
Valdez and Thompson Pass are on the northern periphery of the
precipitation the combination of snow and strong winds are
bringing increasingly poor visibility conditions to Thompson Pass
with scattered showers possibly creeping over into portions of the
Copper River Basin. Over the Susitna Valley, northerly flow is
keeping the area cold and dry.

Over the western Gulf, a wrap around frontal band is lifting
northwest and will reach Kodiak Island this afternoon. Further
west a frontal system stretched across the Bering and Central
Aleutians has stalled and is in the process of shearing apart.
Over Southwest Alaska, mid level easterly flow is bringing some
snow showers across the Alaska/Aleutian Range which are primarily
affecting portions of the Greater Bristol Bay Area. At the
surface, cold north to northeasterly flow continues.

&&

.MODEL DISCUSSION...

There remains significant model uncertainty regarding the timing
of the frontal wave crossing the northern Gulf and spreading into
Prince William Sound today. The 12Z Canadian model is the fastest
bringing in the precipitation and the 12Z NAM the slowest. Radar
imagery supports a somewhat faster solution though not as fast as
the Canadian. The 18Z NAM is coming into better alignment with
both this timing and the snow band falling apart fairly rapidly as
it crosses the Chugach Mountains this evening, which given the
cold dry air mass in place and the lack of supporting upper level
dynamics seems a reasonable solution.

&&

.AVIATION...
PANC...Dry northerly flow will continue with VFR conditions
persisting through the day today. A frontal band rotating in from
the east will bring some snow showers across the mountains this
evening through the overnight hours. However, with the dry air
mass in place it is expected that accumulations will be light with
conditions remaining predominantly VFR and only briefly dropping
to MVFR as showers move through. Snow showers will taper off
quickly Friday morning with dry weather and VFR conditions
prevailing thereafter.

&&

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

A front moving westward across the Gulf is the focus of lift for
the next round of winter weather to impact Southcentral tonight.
Here are the details as of now:

Except along the immediate coast, plenty of cold air is entrenched
across Southcentral to make precipitation type an area of high
confidence: Snow. Mid-level moisture associated with the front
has already enveloped much of Prince William Sound this afternoon.
As the front pushes westward, any feed of moisture from the Gulf
will be lost. In the lower levels, moisture is currently lacking.
The source of lift for snowfall generation is the front itself.
The front moving through the upper-levels will be decoupling from
the near-surface front, thus the farther north and west one goes,
the less likely snow will become. Another source of lift is the
warm air advection associated with the advancing moisture through
the mid-levels. Instability will be confined to the portion of
the atmosphere it especially needs to be: the snow growth layer
around 700 mb. The cold dome of air wedged along the mountains
and light southeasterly cross- barrier flow should prevent the
usual downsloping from being much of a factor with this incoming
snow event.

Thus, the points of greatest uncertainty are how quickly will the
snow generated in the mid-levels reach the lower elevations and how
long will all the ingredients for snow remain in place. With the
front expected to steadily advance through the entire region,
the window for snow will only be open for a brief time. Between
that and the need for the atmosphere to moisten the lower levels
enough that all the snow doesn't sublimate before reaching the
ground, the forecast of anywhere from a few snow showers to
around an inch looks on track for Anchorage and the Kenai
Peninsula, with less further north towards the Matanuska Valley.

In addition, building surface high pressure behind the front will
continue the occasionally strong outflow winds through the usual
passes and waters downwind of those passes. Temperatures should
remain a bit below seasonable levels for the next few days, as
rising heights aloft indicative of warming air are offset by a
gradual settling-in of cold high pressure near the ground,
resulting in increasing atmospheric stratification.
Building high pressure should also help make for a very quiet
period weatherwise after tonight's front goes through.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The Southwest Alaska region continues with offshore flow through
Friday at the surface, with upper lows developing over the
mainland and dropping southwest into the eastern/southeastern
Bering through Saturday. The main forecast challenge with respect
to cloud cover is in sheltered areas where there is enough low-
level moisture for low stratus to develop. Chances of light snow
are possible mainly toward the Bristol Bay region on the periphery
of easterly waves moving from the Gulf of Alaska to the southern
Alaska Peninsula coastal waters.

&&

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

Upper flow over the Bering region will continue to be relatively
weak with upper ridging across the central Bering. Easterly waves
will continue to bring light snow chances to the Alaska Peninsula
region, while northerly flow over the near-shore ice will continue
to generate snow showers over the eastern Bering to the Eastern
Aleutians, and possibly to the Pribilofs. To the west, the
weakening front over the Central Aleutians and west-central
Bering will continue to dissipate over the next 24 hours.

A weather front looks poised to enter the Western Aleutians
Friday night and likely reach the Central Aleutians Saturday, possibly
bringing winds of gale force to the Western Aleutians and small-
craft-advisory level to the Central Aleutians.

&&

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

Beginning Saturday evening, the big story will be the continued
development of a high latitude blocking high centered over
mainland Alaska. This will result in warming temperatures aloft
and strengthening low level inversions which will keep
temperatures in low elevations cold. Fog and low stratus will
become more of a threat with weak pressure gradients inland.
Meanwhile, the main storm track will remain over the Western and
Central Bering, as a series of increasingly strong lows and
associated fronts move north through the area. This will continue
to strengthen the ridge axis across the mainland heading into the
late next week. Temperatures will begin to moderate closer to
climatology as strong cross-polar flow drops polar air into North
America. This pattern will keep dry conditions over much of
Southcentral Alaska, with only weakening fronts impacting
Southwest mainland coast. Even with guidance continuing to differ
with the exact details through the weekend and much of next week,
forecast confidence remains high in the sensible weather outcomes
as most model solutions continue to show ridging rebuilding
across the eastern Bering Sea and western Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning Thompson Pass.
MARINE...Gale warning 127 128 130 139 178. Heavy Freezing Spray
160 180 181 185 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC
LONG TERM...JA/TP