Small Craft Advisory 207, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
423 PM AKST Sun Mar 1 2020

.ANALYSIS AND UPPER LEVELS...

A 996 mb low over the western Aleutians is slowly drifting
southeastward. An occluded front extends across the central
Bering Sea. The trough-axis associated with this low extends
southwestward from the western Aleutians into the North Pacific.
A deep 980 mb low is centered over the Ahklun Mountains and is
moving eastward. Teleconnections support a strong jet stream
across the eastern Pacific as both the Arctic Oscillation (AO)
and Eastern Pacific Oscillation (EPO) are in the positive phase.
The jet stream currently extends northeastward from the subtropical
Pacific into the southern Gulf before curving eastward into
southeastern Alaska. An 1001 mb low is currently developing in
the left exit of this jet where strong difluent flow in the
upper- levels becomes influenced by the low over the western
Bering. Strong high pressure continues to remain in place over the
East Pacific south of 50N.

At the surface, radar imagery shows snow moving across the
northern AKPEN towards Cook Inlet. Another band of snow is moving
southeastward across the Kuskokwim Delta towards the Kilbuck
Mountains. Snow showers are also moving northeastward across Cook
Inlet and across the northern Gulf.

&&

.MODEL DISCUSSION...

Models remain in good agreement with the overall synoptic pattern
through the middle of the week as an Omega Block develops. Despite
some differences in the amplitude and orientation of the 300 mb
jet, all models show this pattern setting up. Models also show
good agreement at the surface with respect to the placement and
movement of highs and lows. Some of the subtle differences that
remain uncertain will be exactly where the best snow accumulations
will fall and the potential for blizzard conditions coming across
Cook Inlet to the higher elevators near Homer. High resolution
models were favored for local effects due to their ability to
resolve mesoscale features.

&&

.AVIATION...
PANC...Conditions will quickly deteriorate this afternoon/evening
as the next wave of snow moves in. Conditions should dip into the
IFR range where they will remain overnight in light snow. By
tomorrow morning, the snow should become more showery in nature
and allow for ceilings and vis to slowly improve.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Sunday
night through Wednesday)...

A broad area of low pressure continues to impact all of Southcentral
AK. The first round of snow with this system moved through
overnight last night into this morning. Snow panned out as
expected, meaning the forecast is on track. The second round of
snow is currently moving up Cook Inlet, with snow now falling in
Anchorage Bowl. With the forecast being on track we have not
adjusted snowfall totals overnight tonight. We have also kept in
place advisories and warnings that are currently issued. The
Copper River Basin will also see about 6 inches of snow overnight,
with some areas on the south side of the Alaska Range seeing a
bit more with upslope flow. We held off on an advisory for now
though. We did push back the start time of the blizzard warning
for areas near Homer as the cold air and cross inlet flow that
will cause the Blizzard are not expected to arrive until this
evening. We are still expecting the Blizzard to occur though.

Another additional change we made was to snowfall tomorrow
morning in the Anchorage Bowl. We are now expecting light snow to
linger through the morning hours, finally tapering around midday.
Additional accumulation from this snow should be around 1.5
inches. This means the morning commute for those in Anchorage and
surrounding areas will be impacted by snow and degraded driving
conditions. An SPS was issued today to notify the public.

After this storm exits tomorrow, cold air advection will barrel
in behind it. So while stormy conditions will calm, there will
continue to be significant changes in the weather with a return to
a colder pattern for the rest of the week. Cold air will cause
very gusty winds through gaps and passes in Southcentral AK. Areas
we are currently keying on that will see strong winds are Passage
Canal including Whittier, Resurrection Bay, the Barren Islands,
and Prince William Sound. Blowing snow in Whittier is especially
of concern, with blizzard conditions possible there later this
week. We will continue to analyze this developing situation. For
the rest of Southcentral AK, it looks like winter will hang on a
little while longer.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Things are generally winding down across Southwest this afternoon.
A weakening low that brought blizzard conditions yesterday through
this morning will exit to the east this evening. Lingering snow
showers will last through the evening before tapering off
overnight. A clearing trend is expected behind the low as cold
Arctic air moves in from the north. Gusty northerly winds will
persist, especially along the Kuskokwim Delta and Bristol Bay
coasts through the evening. Overall, generally clear and cold
conditions can be expected across southwest Alaska through
Wednesday as a ridge over the Bering dominates the weather pattern
for the early part of the week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3)...

An area of low pressure centered northwest of Adak will produce a
swath of gale force winds over the southwestern Bering through
Monday morning. Scattered snow showers are also likely along the
western Aleutians with this feature. This low will then gradually
weaken and move south of the Chain Monday night. Overall,
conditions will remain rather quiet across the Bering as cold
northerly flow returns for the beginning of the week. A ridge of
high pressure builds in for Wednesday. By Wednesday night, another
front approaches the western Aleutians.

&&

.MARINE (Days 3 through 5...Wednesday through Friday)...

A low in the northeastern Gulf gradually weakens and moves
southward. A building ridge of high pressure in place over the
Bering will create a strong pressure gradient, producing gales
over the eastern Bering and Pacific side of the Alaska Peninsula.
Channeled gaps and passes could see storm force winds at times
during the day on Wednesday. These strong northerly winds could
persist through Thursday as the ridge moves eastward. By Wednesday
afternoon a strong area of low pressure approaches the western
Aleutians bringing another round of gale force winds to the
Bering. Though there is still uncertainty, the current track of
the low is to the southeast, centered south of Sand Point by Fri
morning.

&&

.LONG TERM FORECAST (Days 4 through 7...Thursday through Sunday)...

An upper level low over the western Aleutians moves southeastward
into the western Gulf, giving way to a broad upper level ridge
moving into place over the Bering by Friday morning. Upstream of
this ridge, another deep upper level low moves in, forcing the
ridge eastward toward mainland Alaska. Model solutions are
trending toward strengthening this low as the upper level trough
digs sharply to the south over the weekend. As it does so, model
solutions indicate another strong surface low could impact the
Southwest coast, and eventually the rest of the southern mainland,
Saturday into Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 121.
         Winter Weather Advisory 145 152.
MARINE...Storm Warning 132
   Gale Warning 119 120 125 130 131 137 138 139 150 155 160 177.
   Heavy Freezing Spray Warning 130 138 139 150 165 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...KO