High Wind in Alaska, November 26, 2017

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKST Sun Nov 26 2017

.ANALYSIS AND UPPER LEVELS...
The most significant feature over the forecast area is a
deepening low pressure system spreading across much of the Bering
Sea and Aleutian chain. This low has intensified significantly and
is causing northerly hurricane force winds throughout the Central
Bering and Aleutians. Hurricane force winds are already being
observed over Adak at 89 mph and continue to increase. Current
scatterometer data has also been observed, showing 70 mph winds
throughout the Western Bering and shifting eastward while
intensifying. The winds are being supported by an upper level jet
streak with wind speeds near 150 mph with the low sitting in the
upper-left exit region. Showery precipitation has also been
reported on the backside of the low in the Western/Central
Aleutians and is mostly falling as rain. The associated front is
moving toward the Kuskokwim Delta Coast and Bristol Bay bringing
snowfall and increasing winds to the coastline.

While not as strong as the storm in the Bering, there is a
weakening low in the Northern Gulf which brought 4 inches of snow
to Cordova yesterday. The weak wave on the northern periphery of
the system that produced the above mentioned snowfall, continues
to track westward through Prince William Sound while weakening.

&&

.MODEL DISCUSSION...
The biggest model question for the past few days is the behavior
of the intense Bering Sea low after it moves into Bristol Bay
tonight into Monday. The ECMWF and Canadian models began to shift
the low farther south and across the Alaska Peninsula with
another strengthening low in the vicinity of Adak on Monday.
Whereas the American models continue to trend toward the
EC/Canadian solution with each passing model run. This agreement
continues to raise confidence that there will be little, if any,
coastal flooding in the Kuskokwim Delta and Bristol Bay as the
track of the low and subsequent weakening of it are not conducive
for flooding.

The north Gulf coast also has a bit of a change that the models
are currently working out. The weak low tracking westward over the
northern Gulf from the east is moving faster and progressing
farther west than the models have been showing. This system is
expected to bringing some snow over the eastern Kenai this today,
with a very small chance of bringing some snow flurries as far
west as Anchorage.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist
today.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The low over the northern Gulf (and associated snow showers) will
continue a slow westward track today. The low level flow is in
opposition to the upper level flow, so have slowed down the
arrival of snow showers in western Prince William Sound. With
weaker advection, it looks less and less likely that any snow will
make it over the Chugach and Kenai Mountains. Cold and dry air in
place would likely lead to evaporation of snow before it reaches
the ground anyway. Contrary to this, have upped the likelihood of
snow over the Susitna Valley this afternoon and evening as an
upper low over Fairbanks area tracks westward, with potential for
some weak lift on the south side of the low to interact with mid
level moisture moving in from the northern Gulf. In any case, any
snow that does fall in the Susitna Valley will be very light.

The focus for upcoming days is on the deep Bering Sea low, which
is setting up a change in pattern from the cold dry and windy
weather we've seen of late to warmer and "wetter" weather. The
first sign of this change will be a trough and surface front
headed to Kodiak Island today. This is on track to bring light
snow accumulation to the island before transitioning to rain this
afternoon and evening. The upper trough becomes fairly diffuse as
it continues northward toward Southcentral, so expect the bulk of
precip to be confined to the coastal zone. More specifically, low
level southeasterly flow will enhance snow amounts over western
Prince William Sound and the west side of Cook Inlet on Monday.
Precipitation type will remain mostly snow, with the possibility
of mixed precip along the immediate coast by Monday afternoon.
Meanwhile, inland areas will stay mostly dry, but cloud cover and
weak warm air advection will lead to moderation of temperatures.

&&

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

Snow continues to push inland early this morning, accompanied by
gusty winds. Some observations have approached blizzard criteria
at times, but this threat looks to be limited both temporally and
spatially enough to preclude any upgrade to the current
headlines. Otherwise, as a strong ~941 mb low continues to slow
its progression as it nears the Pribilof Islands, and the
associated surface front occludes west of a Bethel to King Salmon
line. Areas of light to moderate snow are developing along and
immediately behind the front.

As the low begins to fill (become vertically stacked and weaken)
this evening and overnight, precipitation will become lighter and
more showery in nature. This low will emerge into the Gulf of
Alaska by Monday evening, another rapidly strengthening low will
undergo bombogenesis (strengthening at least 20 mb in 24 hours),
as it moves from near Shemya at 980 mb around midnight tonight to
~941 mb once again in a similar location to the current storm. It
too will send another front inland across southwest Alaska.
However, this time around though temperatures in the lowest few
thousand feet will have warmed enough for any precipitation to be
of the liquid variety.

&&

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

The much advertised storm is well underway, with gusts having
topped 90 mph early this morning. Some gusts will likely top 100
mph, with perhaps a few gusts getting near 120mph. This is quite
a storm with waves forecast near 55 mph, and even a secondary
vort diving southeast on the backside of the low having produced a
squall lien with abundant lightning south of the chain.

A prolonged period of windy conditions will continue through
Monday morning, as several vort maxes rotate around the low and
help mix down the winds aloft given the low PV (potential
vorticity) heights and the degree of cold advection associated
with this system. Snow levels will be low enough for some
accumulating snow to occur along western and central portions of
the Aleutians. This storm will weaken overnight tonight into
Monday as it slides off to the southeast. However, another rapidly
deepening low will approach the region as described above, with
more hurricane force winds likely somewhere in the Monday night
through Tuesday time frame.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Forecast confidence in the long-term is fairly low at this point,
due to the run to run variation in the models over the past few
days. The overall change in the pattern from a clear. dry pattern
for the mainland to a warmer, stormier pattern still stands, but
the exact timing and effects of the low pressure systems seems to
change from model to model on a run to run basis.

Current thinking is that a low will rapidly develop into a storm-
force system in the Bering Sea on Tuesday, bringing high winds
once again to the Aleutians. This low is expected to develop a
secondary low south of the AK Pen, pushing a strong front into
South Central Tuesday night into Wednesday. A lot of warm air is
associated with this front, which will cause some challenges with
forecasting precipitation type, in particular in the Bristol Bay
and Kenai Peninsula regions. High winds with the front may move
through Southcentral -- although current model runs are slightly
weaker than yesterday.

After Wednesday, another front moves into the Western Bering on
Thursday and begins to cross the Bering Sea. Although this
indicates a continued stormy pattern, the timing and strength of
this system is highly variable between model solutions, so
confidence in current specifics of the forecast remains low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 185 187.
         Winter Weather Advisory 155 161.
MARINE...Hurricane Force Wind Warning 170 172-176 413.
         Storm Warning 155 160 171 177 180 181 185.
         Gale Warning 119 127 130 150 165 178 179.
         Heavy Freezing Spray Warning 160 181.

&&

SYNOPSIS AND MODEL DISCUSSION...TP/RA
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...LF

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
607 AM AKST SUN NOV 26 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0928 PM     NON-TSTM WND GST ADAK STATION            51.84N 176.64W
11/25/2017  M89.00 MPH                          AK   AWOS

            HIGHEST WIND GUST RECORDED. WINDS GREATER THAN 75 MPH
            FROM 856 PM ON SAT NOVEMBER 25, 2017 THROUGH 159 AM ON
            SUN NOVEMBER 26, 2017.

1200 AM     NON-TSTM WND GST 1 NNE ADAK STATION      51.86N 176.63W
11/26/2017  M91.00 MPH                          AK   MESONET

            WINDS GREATER THAN 75 MPH FROM 642 PM ON SAT NOVEMBER 25,
            2017 THROUGH 106 AM ON SUN NOVEMBER 26, 2017. THIS
            STATION IS A NOS STATION.

&&

RA