Small Craft Advisory 903, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
550 AM AKST Wed Nov 25 2020

.ANALYSIS AND UPPER LEVELS...

A high amplitude ridge exists over the central Alaska mainland
with a negatively tilted trough over the Bering Sea. A large front
is present on the leading edge of the trough extending from the
Kenai Peninsula to Kotzebue Sound. Aloft, a 80 kt jet core exists
over the front. On the backside of the front, cold air advection
and westerly winds can be seen on satellite in the form of closed
cell cumulus clouds and scattered snow showers.

Near the surface radar indicates the front continuing to progress
across Southwest. Surface observations along the west coast
indicate precipitation has transitioned back to snow as westerly
cold air advection begins to take hold of the region. Over the
interior, rain, warm temperatures, and lee side warming west of
the Alaska Range continues.

Both the Middleton Island and Kenai Radars are beginning to fill
in as the front moves into the Southcentral. Down inlet flow can
be seen at Kenai with northerly winds as the lee side trough
develops in the western Gulf. Winds are also increasing in Portage
as the front moves over the north Gulf coast.

&&

.MODEL DISCUSSION...

Model agreement in the short term remains good. Models have held
true to their own solutions in regards to the low moving up the
Cook Inlet on Wednesday. Model agreement continues to be poor past
36 hours. The main feature that the models are struggling with is
a surge of cold air moving across the north Pacific this morning
which can be seen on satellite as a developing baroclinic leaf
near 175W. The models all develop 3 upper level and surface lows
that are expected to move into the Gulf on Friday. The latest
model runs show a large spread in the location of these features
which result in a lower forecast confidence for precipitation over
Southcentral. Over the western Bering there is some model
discrepancy over the timing of two strong lows merging into one
on Friday, however the wind field is similar between the models so
forecast confidence for this system is high.

&&

.AVIATION...

PANC...A front has moved over the terminal this morning producing
southeasterly winds and low-level wind shear. Precipitation is
expected to remain away from the airport through the afternoon
when the Turnagain Arm wind may briefly enter the terminal
bringing a rain and snow mix. The arrival of the Turnagain Wind
around 23z will bring an end to any wind shear. The onset of
precipitation will bring down ceilings to MVFR. Snow is likely
after sunset, which should lower visibility to IFR conditions
through the overnight hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Wed and
Thanksgiving)...A storm system developing near Kodiak Island is
right on track to bring active weather across Southcentral through
the day today. Winds have already started to increase across many
of the marine areas and will continue to due so, especially from
Prince William Sound over to Passage Canal. There were no
significant changes to the thinking from the previous forecast, so
overall changes were minimal into early Thanksgiving. The low
will track up Cook Inlet later today. High winds are still
expected along Turnagain and the higher elevations of the
Anchorage Hillside. There will also be strong winds through
Portage Valley. These winds will generally be across the terrain.
This will cause localized warming thanks to some downsloping.
Temperatures will generally push into the mid 30s.

Precipitation is set to push from Kenai late this morning to the
Anchorage Bowl late this afternoon. Due to those locally warmer
temperatures, the onset is likely to be a mix of rain and snow or
potentially just all rain. However, it will not be very much
precipitation during that warm period. As the upper level energy
arrives from the west, it will be coincidental with much colder
air. This should allow for a couple inches of snow accumulation
from Anchorage up to the Matanuska Valley, with the highest
amounts favoring west-facing terrain. As the front lifts through
late this evening, winds will rapidly diminish across all areas.

But the active pattern is not over yet. Forecast confidence is
increasing in the potential for another, weaker low to more
along the northern Gulf coast Thu afternoon into Fri. There is
still a fair amount of model uncertainty with this system, but one
thing is quite certain, it will be much colder. This should allow
for the precipitation type to remain safely all snow across the
area. This is a much more compact system, so pinpointing the exact
location of best QPF will take some more time to refine. That
said, we will be keeping a close eye on it as it has the potential
to produce several more inches of snow across many of the
population centers heading into the Holiday Weekend.

&&

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

A frontal system moving through Southwest Alaska this morning will
continue moving from southwest to northeast. This front is bringing
mostly rain to many areas due to a decent amount of warm air with
this system. Rain has changed back to light snow across the
westernmost portions of the YK-Delta this morning with
temperatures dropping back down to around freezing as the cold air
moves back in. The cold air will continue moving across Southwest
today in a southwest to northeast direction with areas across the
northern portions of the Middle Kuskokwim Valley seeing the cold
air return last. Windy conditions are also moving through
Southwest in association with the front with Iliamna reporting a
gust upwards of 66 mph in the last hour. Winds will remain the
strongest through northern Bristol Bay and into the Middle
Kuskokwim Valley through later this morning before gradually
diminishing in the afternoon.

Quieter conditions are expected across Southwest for later this
evening into Thursday, though snow showers are expected
especially across the coastal regions of Bristol Bay and the YK-
Delta. There is the possibility of some these snow showers making
it far inland with even areas like Bethel, Sleetmute, and
Sparrevohn possibly getting in on the action.

One thing that is being watched is a system moving northward near
Kodiak Island that may send a band of snow into the Iliamna area
for Thursday afternoon. However, this part of the forecast remains
uncertain at this time even with it being 24 hours out. Right now,
thinking that if this band forms it will stay over the mountains
or on the other side closer to Kodiak Island north through Cook
Inlet.

Attention then turns to the next frontal system moving northward
along the AKPEN that may bring a round of snow to the coastal
villages across Southwest. Timing appears to be Friday afternoon
into Saturday. This system has the potential to bring a couple
inches of snow to places such as King Salmon and Dillingham over
to Cape Newenham. Some parts of the Kuskokwim Delta Coast may also
see some snow out of this but think at this point most of the
accumulating snow stays on the east side of the Kuskokwim
Mountains.

&&

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

Westerly flow continues this morning across the Bering Sea. This
is expected to continue through Thursday morning when the next
system begins to move into the area. From now till then expect
small craft winds across much of the Bering with some low end
gales across the westernmost portions of the Aleutian Chain and
the Bering. Widespread snow showers will also accompany the gusty
winds as cold air continues to flow across the relatively warm
water surface. This can be seen this morning on the nighttime
microphysics imagery in the form of cumulus clouds which have a
popcorn look to them.

The next system begins to move into the western Bering Sea and
western Aleutian Chain beginning late Thursday morning/early
afternoon. This system is expected to bring more unsettled weather
with periods of rain and high winds and seas to the area. Storm
force winds are expected in the marine waters to the south and
north of the central Aleutian Chain beginning Thursday afternoon
and continuing through Friday evening. Some model guidance is
showing the risk for hurricane force winds in the marine zone
north of Adak Thursday evening into Friday morning. Therefore,
this was placed into the forecast for that zone. However, the risk
is there that the hurricane force winds could be more widespread.
This will be fine tuned more today and sent with the afternoon
package. A High Wind Watch has been issued for Adak and Atka in
the central Aleutians for Thursday evening to Friday morning for
wind gusts upwards of 85 mph. Wave heights will also come up in
response to the winds with 30-40 ft seas expected north and south
of the central Aleutian Chain.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

Gulf of Alaska:

Fair confidence with overall pattern but much uncertainty remains
in the details during the 3-5 day timeframe. Friday a Gale force
low will move across the southern Gulf. The low will weaken and
move into the eastern Gulf Friday afternoon. however low
confidence on track and strength. Friday afternoon a gale force
front moves into the Gulf there is a chance of near storm force
winds southeast of Cordova with the barrier jet, but low
confidence on strength and higher confidence with track. This
front will move inland and weaken by Saturday afternoon. A gale
force low will move in behind the front by Saturday afternoon.
This low weakens as it moves into the northern Gulf Sunday
morning, fair confidence on track and intensity.

Bering Sea:

A storm force low moves just north of the Aleutians across the
southern Bering on Friday and Saturday. Most of the storm force
winds are expected on Pacific side of Aleutians. This low will
move east into the Gulf Saturday evening. Good confidence on
track, but lower confidence on strength of low. Saturday night
through Sunday another storm force low is expected track south of
the Aleutians, low confidence with track and strength of this low.

&&

.LONG TERM FORECAST (Days 4 through 7...Saturday through
Tuesday)...

Progressive flow will continue to dominate the long term with a
series of fronts and lows moving across the Aleutians and Bering
into Southwest and Southcentral Alaska. These systems may bring
chances of accumulating snow to Southwest and Southcentral. Models
are hinting at a slight pattern shift as the storm track and
upper level jet may move south keeping the storms south of much of
the Aleutians and Bering, with the exception of the Gulf which
may be influenced by a strong North American ridge which will
direct storms into the Gulf and possibly into Southcentral by the
middle of next week. Good confidence on the overall pattern, but
lower confidence on details of track and strength of systems.

Saturday morning an occluded front will move into Southwest
Alaska with rain near coast and snow farther inland. This front
will stretch across the AKPEN to near Kodiak Island and across the
Gulf bringing heavier precipitation to Kodiak Island and
northward across the Kenai and Prince William Sound. As the front
moves north models are starting to show a triple point forming
near Kodiak Island and tracking north, however the track and
strength of this low is very uncertain. Rain or snow is possible
across Southcentral late Saturday into Sunday as the front moves
inland, but details uncertain especially with precipitation type.
This front will also produce Gale force winds across Gulf and
strong winds, especially near the coast and the usual local effect
winds. The parent low to this front will move south of the
eastern Aleutians and AKPEN Saturday. This low will produce storm
force winds mainly on the Pacific side of the Aleutians and weaken
to Gales as it moves into the Gulf by Saturday evening. This low
then moves into northern Gulf late Sunday and weakens. Fair
confidence with this system, however lower confidence with the
details of local winds and precipitation type.

Yet another storm force low tracks just south of the Aleutians
Sunday and Monday still a bit of spread among the models, but
relatively good agreement considering its 5 days out. Depending on
the track of low and front Southwest Alaska may get some snow out
of this system as the EC and GFS show the possibility of snow in
the deformation zone, but too much uncertainty to say this will
happen at this point. The front associated with this low will
track into the Gulf on Monday bringing another round of winds,
rain, and snow. Tuesday the front will move inland and weaken,
while much of the Bering and Aleutians will be under quieter
weather as northerly flow sets up as models hint at the upper jet
dipping farther south keeping the storm track south of the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 101
 		 High Wind Watch 187.
MARINE...Storm Warning 119 130
 		 Gale Warning 120 125 128 129 131 132 139 141
		 155 176-178 351 352 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...RC

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
144 PM AKST WED NOV 25 2020

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

1233 PM     NON-TSTM WND GST PORTAGE                 60.79N 148.83W
11/25/2020  M84 MPH          ANCHORAGE          AK   MESONET

            PORTAGE GLACIER VISITOR CENTER WEATHER
            STATION.

&&