Small Craft Advisory 175, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKST Sun Feb 25 2018

.ANALYSIS AND UPPER LEVELS...
A large vertically stacked low is tracking northeastward across
the central Bering Sea with a frontal system way out ahead of it
approaching Southwest Alaska. A strong low level jet on the south
side of the low in an area of cold air advection and instability
is leading to storm force winds along the western Aleutians and
on up into the southern Bering Sea. Shemya reported consistent
gusts to around 60 kt this morning, so there may be a few
hurricane force gusts across the aforementioned marine areas. A
strong upper level jet streak on the front side of the upper
trough is what is helping to drive the low level frontal system
eastward. Snow is quickly spreading onshore in Southwest Alaska,
with severely reduced visibilities along the Kuskokwim Delta coast
where winds ahead of the front are strongest. A Winter Weather
Advisory for blowing snow remains in effect for this area until
7pm Sunday evening.

Further east, Southcentral Alaska is under the influence of high
pressure today. Areas of fog and stratus expanded this morning
over northern Cook Inlet, Anchorage, and portions of the Susitna
Valley as a shallow surface inversion steepened. The sun is
helping to erode this as the day wears on. Weak cold advection
and tight pressure gradients behind the last system continue to
produce localed gap winds along the north Gulf coast, though they
are trending downward.

&&

.MODEL DISCUSSION...
Models remain in good agreement and overall forecast confidence
remains high. There will be some forecast challenges Monday night
through Tuesday with determining timing and impact from a series
of upper level short-waves which drop southward across western
Alaska then move across Southcentral. This has potential to
prolong snowfall, particularly on the west slopes of mountain
ranges (with persistent strong upslope flow), and result in
locally higher snowfall amounts (most notably for the west side
of the Chugach and Talkeetna Mountains including the Anchorage
Hillside). One other forecast challenge will be determining how
strong to go with winds across Southwest Alaska and into the
western Gulf as polar air advects southward on Monday. The
environment will be favorable for strong erratic gusts to mix down
to the surface, so winds could be quite variable from one
location to the next.

&&

.AVIATION...
PANC...Fog has moved out of the terminal, but is still sitting
over northern Cook Inlet. As southerly low level flow moves up
the Inlet early this evening the fog could briefly move back in
before the strengthening winds and arrival of clouds bring an end
to the fog altogether. Will monitor latest trends closely and
adjust TAF accordingly.

For the the next system, deep southwest flow will lead to another
round of snow. While MVFR conditions are expected to prevail
during the period of snow, there could be a short duration of IFR
during the heaviest snow, most likely around 12z. After the first
upper level short-wave moves through early Monday, the mean low
to mid level flow will become a bit more westerly. From here on
out would expect the heaviest snow and lowest ceiling/vis to be
over east Anchorage and the front range of the Chugach. The
terminal will be on the western edge of snow (and MVFR
conditions), with VFR conditions likely prevailing over northern
Cook Inlet just to the west of the terminal. This will remain the
case for a good portion of Monday and Monday night. It's
challenging to determine what the prevailing conditions will be,
but would expect a mix of MVFR snow and some periods of low end
VFR conditions in between upper waves.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Brief high pressure and outflow winds will quickly give way to
increasing clouds and eventually snow as the next low moves into
the North Gulf of Alaska tonight. This will bring a widespread but
quick hitting shot of snow tonight and early Monday morning. With
deep layer southwest flow and a dendritic zone solidly in that
southwest flow regime, it is expected that snow ratios should be
rather high with fluffy, rapidly accumulating snow likely.
Thereafter, by Monday afternoon, the snow event will shift to an
orographic lift "event" with snow favoring the west facing slopes
of the Chugach, Talkeetna, and Wrangell Mountains. The heaviest
snow accumulations will be across east Anchorage and the Hillside,
Eagle River, and up into the Matanuska Valley where 5-8+ inches
of snow is possible. Snow now looks likely to linger through
Monday night into Tuesday morning across those areas, otherwise,
there is little change in forecasting thinking from yesterday.

Meanwhile, as the low shifts east and strong northwest flow aloft
increases (along with increasing cold air advection), gap winds
will rapidly increase, especially through gaps which favor
westerly flow (the Barren Islands, Passage Canal and Whittier,
Kodiak City, and even Kachemak Bay). Storm force winds will
channel through the Barrens with gusts to 50-65 MPH through Kodiak
City. Snow showers will likely linger longer near Kachemak Bay,
and some blowing snow is possible, specifically over the Homer
Bluff. Expect some blowing snow along the Anchorage Hillside, as
well, Monday night and Tuesday as strong northerly winds increase.
Snow tapers off and ends Tuesday while strong offshore flow
continues.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A frontal system currently pushing into the southwest coast will
bring another round of snow and gusty winds to most locations,
with 2-5" accumulating by Monday afternoon. Over a foot of snow
is expected for some upslope locations on the western side of the
Kuskokwim and Alaska Ranges as strong westerly flow will help
promote good orographic lifting. Some warmer air intruding over
the AKPen will keep the overall precipitation type as a mix of
rain and snow through this evening, with snow being the
predominant precipitation type from King Salmon northward. Later
tonight colder air will filter back in from the west along the
southern periphery of the low, changing much of the mixed
precipitation back to snow.

Colder air will continue to move in behind the front causing
temperatures to fall throughout the day on Monday, with high
temps expected in the early morning. The cold air advection will
also lead to strong west/northwesterly winds Sunday night into
Monday across Bristol Bay and into the lower Kuskokwim River
Valley. Winds could gust as high as 50-60 mph along the Bristol
Bay coast with even higher gusts possible through channeled
terrain over the AKPEN. Because of the strong northwest flow, snow
showers will continue in the mountains through much of the day,
potentially continuing through much of Monday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An upper level trough over the central Bering sea has forced a
transient upper level ridge eastward over the southwest Mainland,
continuing the progressive pattern seen across the area this
week. The associated frontal system is currently pushing inland
from the eastern Bering, leaving some strong gusty winds and
showery condition in its wake. A 140 kt jet max on the bottom of
the upper level trough will support the development of this
system, bringing a swath of storm force surface winds (50 kt) and
building seas to the Bering Sea between the Aleutians and
Pribilofs this evening through Monday morning. The long fetch of
the westerly winds will build seas up to 40 ft by tonight, with
the core of the strong winds and higher seas clipping Saint George
Island overnight tonight.

Once the low moves into Southwest Alaska, conditions will calm
down somewhat, with a much weaker low moving into the western
Aleutians late tonight into Monday morning. This weak low will
slowly track eastward through the central Bering through Monday
night, but no hazardous weather is expected from it. The next
system approaches the Western Aleutians and Bering from the north
Pacific Tuesday afternoon, bringing another round of wet and windy
conditions to the area.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The GFS has maintained decent run to run continuity with the major
synoptic features. At the onset of this forecast period, the
latest iteration of the GFS ensembles has a tight clustering with
the low over the Western Bering, the high over the Yukon/NW
Territories, and the low over SE AK. There is a third low that all
of the global models are hinting at coming into the North Pacific
but there has been too much variance even with the more stable
model guidance. Any way you slice it, a progressive and amplified
pattern is on tap most of this period. The primary sensible
weather highlight to look for this package will be the storm force
and potentially hurricane force winds over the Western and
Central Bering as the low passes by Shemya and the surface
pressure possibly drops below 950 mb Wednesday. The GFS and the
ECMWF really bring that deepening trough with multiple shortwaves
across the Bering and into Western Alaska through Day 6.
Confidence is building that this unsettled pattern will
materialize.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 155.
MARINE...Storm Warning 120 130>132 138 160 165 175 179.
 Gale Warning 119 136 137 139 141 150 155 170>174 176>178 180
185.
 Heavy Freezing Spray 121 130.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PEPE
LONG TERM...PJS