Small Craft Advisory 173, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
335 PM AKST Sat Feb 25 2017

.ANALYSIS AND UPPER LEVELS...
A broad upper ridge extends from the northeast Pacific to over
Southcentral Alaska. Stable conditions associated with this
feature strengthened inversions and led to stratus and fog across
much of Cook Inlet, western Kenai Pen, Anchorage Bowl, and Mat-Su
Valleys this morning. Conditions have since improved across the
Kenai Pen and Mat Valley. However, areas of low stratus and dense
fog continued into the afternoon hours across portions of the
Anchorage Bowl. Generally fair skies were observed elsewhere
across Southcentral with local outflow winds across western
Prince William Sound and through Thompson Pass.

Broad southwesterly flow aloft and an associated surface frontal
trough dominated the weather across much of the eastern Bering
Sea, Southwest Alaska, and the AK Pen. Rain and snow were observed
across the Delta with snow encroaching on the remainder of the SW
Mainland. Strong southwesterly winds were evident across much of
the Bering Sea, central/eastern Aleutians, to the Delta/Western
Capes of Southwest Alaska.

&&

.MODEL DISCUSSION (through Monday afternoon)...
Models overall are in good agreement with a few minor differences.
Generally used the NAM/GFS in the west with the GFS and hand edits
in the east.

&&

.AVIATION...
PANC...Stratus and fog will likely be slow to clear this afternoon
and may allow IFR conditions to linger into the evening hours.
The inversion responsible for these conditions weakens some
tonight with increasing upper level clouds. The weakening
inversion and a little more mixing tonight may help to improve
visibilities in fog. However, there is still the strong
probability of MVFR/IFR ceilings to continue into Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The ridge aloft building into Southcentral Alaska today will
flatten tonight through Sunday allowing the flow aloft to turn
more westerly. While the flat ridge will be less favorable for
the formation of fog tonight, there is not any mechanism in place
to scour out the lower levels yet, so expecting to see patchy and
less dense fog to redevelop in similar locations to last night.
However, southwesterly winds pushing into Kachemak Bay and
southern Cook inlet should keep fog out of those areas.

A cold upper level trough sweeping in from the west Monday afternoon
and evening will bring a substantial increase in northwesterly
winds and drop in temperatures as well as quickly scouring out the
moist lower layers of the atmosphere. Precipitation associated
with this trough will be brief and focused mainly along west
facing slopes where snow showers will be orographically enhanced.
Strong westerly gap winds through Portage Valley, Whittier and
Passage Canal as well as northerly gaps winds through Broad Pass,
Thompson Pass and Resurrection Bay will develop Monday night and
increase through Tuesday. Additionally, strong northwesterly flow
through Barrens and out of gaps and passes along Shelikof Strait
and southern Cook Inlet will build as strong cold advection pushes
in behind the upper level trough.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A warm and moist front pushing through the Kuskokwim Delta and
into the Bristol Bay area this evening will bring a mixture of
rain and snow to the area, as well as some gusty onshore winds.
Precipitation in the delta will remain as snow with a few pockets
of rain mixed in during the early evening hours. Even though
warmer air is advecting into the region from the Eastern Bering,
it will pass over the ice along the Southwest coast. This has been
causing the air pushing into the Southwest coast to come in
slightly cooler than expected, and keeping conditions cold enough
to support snow. The rest of Southwest AK will remain cool enough
to continue to support snow as the front continues to stretch
further inland and slowly move southeast. As the front moves
southeast, temperatures across the area will start to cool tonight
and allow for more snow to fall. Some warming limited to the
Bristol Bay zone will cause snow to mix with rain Sunday, but by
Sunday night and into Monday morning, cold air filters back into
the region from the Northwest. However by that time, only
diminishing snow showers are expected to be lingering over the
area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The front extending across the central and eastern portions of
the Aleutians and Bering will slowly weaken and push southward
through the remainder of the weekend. This will keep a mix of rain
and snow along with small craft to gale force gusts over these
areas through Sunday night. Colder northerly flow wrapping around
the high pressure building over the Western and Central Bering
Sunday night into Monday. This will begin to push the above
mentioned front into the Northern pacific and cause temperatures
over the area to drop, making snow the more dominant precipitation
type into early next week.

&&

.LONG TERM FORECAST (Days 3 through 7)...
After cold air filters through the southern mainland early next
week it remains in place through Friday with the upper level
trough situated over the mainland and ridging in place over the
Bering. Offshore northerly flow will be gusty at times for
channeled terrain, especially along the coasts, through the end
of the week as the upper low provides a continuous tap from the
Arctic. All operational models were in good agreement through the
long range forecast, although the 12Z GFS started to break the
Bering ridge down toward Friday which was an outlier on this
solution. Only minor changes were needed today with a weighted
forecast toward the ECMWF and it's ensembles.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 120 131 185.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...KH