High Wind Watch 4, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 PM AKST Sun Feb 12 2023

.ANALYSIS AND UPPER LEVELS...

An upper level long-wave trough continues to dominate most of
western Alaska and the Bering Sea, anchored by a broad upper
level low in the vicinity of St. Lawrence Island. Several short-
waves are rotating around the broad upper low, with one primary
short-wave rotating across Southwest Alaska.

At the surface, a low pressure system is slowly moving east across
the northern Gulf, pushing moisture into Southcentral and Prince
William Sound. A tight pressure gradient has set up across Kodiak
Island to Homer, creating an area of strong westerly winds.

Areas of generally light snow were occurring from the central to
eastern Aleutians, with pockets of light snow occurring in
Southwest AK. Southcentral received light snow with pockets of
freezing drizzle this morning.

&&

.MODEL DISCUSSION...

Deterministic models remain in good agreement with the synoptic
features through Wednesday. The biggest challenge in the near-term
continues to be in Southcentral where small fluctuations in
location and strength of any developing low pressure circulation
along the front can have a significant impact on localized
conditions. At this time, models show a surface low developing
near Kodiak Island by 12z Tuesday. The GFS is showing a much more
defined circulation, which then dissipates with another
circulation developing farther east. The NAM shows a more cohesive
progression of the developing low.

Deterministic models all show a very similar handling of the
strong low pressure system moving across the North Pacific at
the start of the work week. The biggest question there is how
far north will the front make it into the Bering by Monday night.

&&

.AVIATION...

PANC...Precip has changed over to all snow after a period of
mixed precip this morning. Cold air advection along with a deep
layer with the temperature in the dendritic snow growth zone will
lead to periods of moderate snow late this afternoon and evening.
While MVFR conditions may prevail, do expect periods of IFR or
even briefly LIFR in heavier snow. Ceilings will then gradually
lift overnight as the heavier snow shifts eastward toward the
mountains, with VFR conditions likely by morning. Occasional gusty
southwest winds are possible through the evening as cold air
continues to advect into the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Wednesday)...

An active pattern will gradually quiet down during this forecast
period. Snow will continue across Southcentral Alaska this evening
and tonight. The snow may be heavy at times due to very high snow
to liquid equivalent ratios and a deep dendritic growth layer.
A Winter Weather Advisory has been issued for Anchorage through
5 AM AKST Monday morning due to 4-8" of new snow expected with
locally higher amounts along the hillside, East Anchorage and
Eagle River.

The other big story will be the strong westerly winds across the
waters that encompass the western Gulf of Alaska. Strong cold air
advection associated with these strong winds will result in heavy
to extreme freezing spray. Gusts will reach hurricane force in
spots prone to localized funneling.

The pattern will gradually quiet down from Monday into Tuesday.
Temperatures will become much colder, though no significant
weather is to be expected.

&&

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

The biggest story for the start of the week will center around the
blast of bitterly cold, Arctic air streaming southeast around a
persistent upper level low drifting near the Norton Sound. A bit
of light snow will continue for a few more hours over the Lower
Kuskokwim Valley as an shortwave trough lifts past the region
towards the Interior, but otherwise low level clouds and light
snow will continue to break up through the overnight tonight as
the low levels dry out under a stout shortwave ridge moving in
from the eastern Bering. Diminishing winds and clearing skies will
make for ideal conditions for frigidly cold overnight lows to
materialize, with widespread temperatures bottoming out in the
-10s to -20s through early tomorrow morning. Most of the day on
 Monday will continue to be dominated by mostly clear and cold
 conditions, with highs struggling to break the zero (Fahrenheit)
 mark.

By Monday evening, a notable shift in the pattern will begin as a
strong Bering low pushes its front northeast along the Alaska
Peninsula and into Bristol Bay by Monday night. Snowfall will
begin across much of Bristol Bay from north to south, persisting
into Tuesday as the then weakening low skirts south of Kodiak
Island. As has been the case with our recent frontal systems, the
highest totals will be focused between Dillingham and Koliganek,
where about 4 to 7 inches of new accumulation will be possible
between late Monday and Tuesday afternoon. Snowfall will taper
off between Tuesday evening and Wednesday as the weakening system
shifts out into the Gulf and as another shortwave ridge moves
into place from the Bering.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 to 3: Tonight
through Wednesday)...

Satellite and scatterometer data are showing an impressively
strong low approaching Hurricane force barreling towards the
central Aleutians from the south this afternoon, with very strong
easterly winds with gusts peaking near 80 mph expected to begin
shortly for Adak and Atka. An Arctic air mass out ahead of the
low's approaching front will allow for the somewhat unusual
combination of high winds, heavy snowfall and blowing snow as
precipitation begins later this evening. A Winter Storm Warning
has been issued for the central Aleutians through early Monday
for the expected combination of high winds, intense snowfall and
blowing snow reducing visibilities to less than half a mile.
Temperatures will warm overnight as the front lifts north,
resulting in a quick changeover to rain as winds also drop off
behind the frontal passage. Hurricane force sustained winds are
meanwhile expected overnight to the west towards Amchitka, but
expect Shemya to stay out of the worst conditions for this system,
with winds peaking in gale force range.

Going into Monday, the low will continue east just off to the
south of the Aleutian Chain as the front lifts north at gale to
storm force towards the Pribilofs. Blizzard conditions will begin
on Monday morning as snowfall and strong east to northeast winds
pick up, with snow and whiteout conditions then persisting for
many hours through Tuesday morning. Gusty north winds in gale
force range will push along the Aleutian Chain into the southern
Alaska Peninsula as the low exits towards the Gulf. Meanwhile, the
next system in this onslaught of intense lows will only just be
getting started farther west.

On Tuesday, another extremely powerful low will rapidly deepen
south of the Kamchatka Peninsula, kicking another storm to perhaps
Hurricane force front into the western Aleutians by Tuesday
afternoon. Temperatures ahead of this front may again be cold
enough for a short window for snow to fall for a few hours before
warmer air transitions falling precipitation to rain, with
accumulating snow also possible as the front progresses towards
the central/eastern Aleutians and Pribilofs heading into Wednesday.
Very strong winds will be possible as the front first enters the
Bering on Tuesday afternoon across the western Aleutians as well,
with southeast gusts pushing 80 mph possible near Shemya early on
Tuesday afternoon as the front moves past. If it still needs to be
said, there is really no end in sight to the active pattern at
this point.

&&

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

At the onset of this forecast period, expect a highly amplified
pattern with the trough axis near the western Aleutians. Expect a
series of strong lows forecast to track across the region. There
is model divergence that increases with time. But there are some
trends. The Gulf of Alaska continues to look active. Out west, a
low near Kamchatka, potentially as deep as 952 mb, pushes a front
into the central and eastern Aleutians on Thursday. The ECMWF and
the Canadian bring the remnants of this front into Southwest AK
on Friday. A new surface low forms south of Adak on Friday. The
GFS is the outlier. With a potentially strong low pressure at the
surface portions of Alaska will receive gusty winds and light to
moderate precipitation. This appears most likely for the Aleutians
but it is becoming more likely for the southern coast as well,
dependent on the eventual storm track. Additionally, expect a
series of shortwaves embedded in the long wave pattern that will
influence the sensible weather.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning 187.
         Blizzard Warning 195.
         Winter Weather Advisory 101.
MARINE...Heavy Freezing Spray Warning 121 129 130 131 132 136 137 138
                                      139 141 150 160 179 180 185.
         Hurricane Force Wind Warning 177.
         Storm Warning 120 130 131 175 176 178.
         Gale Warning 119 121 132 136 137 138 139 141 150 155 160 165
                      170 171 172 173 174 179 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...NR
SOUTHCENTRAL ALASKA...ED
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
LONG TERM...PS