Small Craft Advisory 72, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 PM AKST Mon Jan 20 2020

.ANALYSIS AND UPPER LEVELS...
The pattern is dominated by a broad upper level trough centered
over the West Coast of AK. This is not an overly strong feature,
but just happens to be in the right location to take advantage of
some moisture over the North Gulf Coast up through Cook Inlet. The
moisture is associated with a weak surface low sitting just south
of the Kenai Peninsula. As this trough continues to take on a
negative tilt today, it is aiding in lifting the moisture through
the column. With cold air in place, the net result in fairly
widespread snow from the Kenai Peninsula up through the Mat-Su
Valleys and over to Valdez. We had a few reports of near 6 inches
from Kenai early this morning and it continues to snow from Kenai
north. Over the Bering, cold air advection snow showers continue
to be the dominant feature. For SW AK, there is plenty of low
stratus spreading over the area with some patchy fog mixing in,
especially along the coast.

At 300 mb (30,000') the jet is supporting the trough over the
West Coast at about 85 kts. The rest of the primary storm track is
over the North Pacific, well south of the mainland.

&&

.MODEL DISCUSSION...
In general, the models are handling the overall synoptic pattern
with good consistency. However, without a strong system to latch
onto, they are struggling to resolve the finer details. Some of
these finer details surround the bands of snow over the Northern
Kenai and the Anchorage Bowl. The GFS continues to be the driest
with this set up, but it remains the outlier. This forecast
package will lean mostly towards the hi-res models and some hand
edits, especially in handling this banded precipitation. Forecast
confidence overall remains moderate.

&&

.AVIATION...
PANC...IFR visibility will persist in snow through much of the
forecast period. Ceilings will linger in the MVFR range, but
could easily drop to IFR in heavier snow. The timing of when snow
ends is highly uncertain. It certainly looks to become lighter as
we go through the overnight hours which should allow ceilings and
vis to come up some.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3;
Tonight through Thursday)...

The surface low just south of Whittier will slowly drift north
through the evening. Light snow is expected to continue for
portions of Southcentral as a trailing upper level trough lifts
over the area. Another one to two inches of snow is possible from
north of Kenai up across the Mat-Su Valleys. A mix of rain/snow
is still possible through the overnight hours across parts of
Prince William Sound as the southerly flow ahead of the low has
ushered in enough warm air from the Gulf to push temperatures
above the freezing mark. Temperatures will hold steady in the
teens and twenties tonight and tomorrow for the Kenai Peninsula
north to the Susitna Valley. For the Copper River Basin,
temperatures will remain a few degrees on either side of 0F.

Precipitation will slowly taper off overnight across the Kenai
Peninsula and through Tuesday night elsewhere as the upper trough
moves north beyond Southcentral. This will allow the flow aloft
to turn northerly, ushering in a colder and drier air mass for the
second half of the week. The northerly flow will also reestablish
the gusty, northerly outflow winds through the passes and along
the immediate coast by Thursday as arctic air spills south across
Southcentral and temperatures plunge back to values we saw prior
to this current storm.

&&

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

Snow showers along the Kuskokwim Delta coast will move farther
inland and then taper off through the night as a front weakens
over the area. Snow accumulations should be minimal with this
system as it moves in towards Bethel for the early morning hours.
With lingering moisture across the region, expect low level
stratus and patchy fog to persist overnight and into the morning
hours for Tuesday for the Kuskokwim Delta and even into interior
Bristol Bay. Cold temperatures will remain across Southwest
Alaska through the week as an Arctic air mass moves in from the
northwest into the mainland on Tuesday. Cold temperatures will
continue to move in through Thursday, when strong cold advection
will bring northerly flow and gusty conditions to interior Bristol
Bay and the Alaska Peninsula.

&&

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

Cold, unstable air persists across the Bering this afternoon,
which will keep snow showers in the forecast through Tuesday. The
Pribilof Islands will likely see snow showers continuing through
the night, tapering off by the early morning hours.

Attention quickly shift towards the western Bering on Wednesday
morning when a strong North Pacific low enters the forecast area.
This system will cross over the Chain into the western and
Central Bering by Wednesday afternoon, with precipitation quickly
transitioning from snow to rain as warmer air moves in with this
low. Expect high end gale force winds accompanying this system,
with gust to storm force level as it moves over the Bering.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

Gulf of Alaska:

A low over the eastern Gulf Thursday will turn westward and move
to near Middleton Island by Friday morning. The low will stall out
and remain nearly stationary through Saturday morning, then
dissipate or get absorbed into a new low moving into the eastern
Gulf Saturday evening. This will keep gales ongoing continuously
across the Barren Islands and through bays and passes southwest
of Kodiak Island through Friday. The gales may weaken to small
craft winds on Saturday, depending on where the low is at that
point and where the next low moves into the eastern Gulf.
Significant forecast uncertainty persists on Saturday.

Bering Sea and Aleutians:

A strong and nearly stationary low centered south of the western
Aleutians will keep widespread gales with occasional embedded
storms ongoing across the central and western Bering through
Friday. Seas of 20 to 35 ft are expected over a large area of the
central and western Bering and Aleutians through Friday night. By
Saturday the low will move to near Kamchatka, which will result
in gradually diminishing winds over the western Bering on
Saturday. Strengthening high pressure over the eastern Bering will
cause gale force offshore winds to develop Friday night through
Saturday from the Southwest Alaska coast. The gales will extend
south through the bays and passes of the Alaska Peninsula and
eastern Aleutians by Saturday evening.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through Monday)...

The stagnant weather pattern intensifying on Thursday will persist
through the weekend, and potentially last even longer. A strong
and nearly stationary low centered south of the western Aleutians
will keep gale force winds and rain ongoing across the central and
western Aleutians through at least Friday night. On Saturday, the
low moves off to the Kamchatka Peninsula, but another North
Pacific low may move in to the same area to replace the old one
into Monday. There is significant model uncertainty about where
the new low tracks, however.

Meanwhile, across the eastern Bering and mainland Alaska, high
pressure will strengthen with time through at least Saturday.
After that there is some model disagreement as to how quickly the
high pressure moves westward into the Bering. The GFS is much
faster moving the high westward than the foreign models. Either
way, the high will support increasingly cold air flooding
southward on strong winds across Southwest Alaska, the eastern
Bering, and the gaps and passes of Southcentral from Saturday
through Monday. This will be a favorable pattern for another round
of wind chill hazards in these same areas.

Across the Gulf, the weather will remain unsettled as a low over
the eastern Gulf on Thursday moves to near Middleton Island on
Friday morning, then parks itself there and spins moisture, very
likely in the form of snow given the cold air around, into
coastal areas. Since the low will be relatively weak and hardly
moving, any snow that falls is not expected to become heavy, but
it looks to persist in some areas through Saturday. There is
considerable uncertainty on who gets what amount of snow and when,
so stay tuned for future forecast updates. By Monday, with the
high gradually drifting west over the Bering, the storm track may
begin to shift westward along with it into the Gulf and
Southcentral, which, if it occurs, would bring back more
seasonably cold temperatures, and much greater threats for snow
into Southcentral.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray 150 160 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...CK/TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...JW