Small Craft Advisory 149, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
353 AM AKST Thu Feb 13 2020

.ANALYSIS AND UPPER LEVELS...
The longwave pattern in amplifying across the forecast area
today. This is best seen by looking at the structure of the jet
stream. There is a trough/ridge couplet over the eastern Gulf Of
Alaska and the North Pacific/AKPEN. Which is followed by another
upper level trough over the western Aleutians and this feature is
associated with the 952 mb low west of Adak this morning. A tight
pressure gradient has developed with both lows, although the first
low is rapidly moving into Southeast leaving an inverted trough
over Prince William Sound. Gale and storm force winds were
detected overnight with the western system on ASCAT (advanced
scatterometer) data.

&&

.MODEL DISCUSSION...
The ensemble members of the GFS and the operational runs of the
GFS, ECMWF, NAM and Canadian are in lock step through 48 hrs.
This includes the tracks of the surface features and the advance
of the respective precipitation shields. This is echoed by the
NCEP RMOP (Relative Measure Of Predictability) which is projecting
high confidence scores through 48 hrs for the southern tier of
Alaska and the north and central Pacific.

&&

.AVIATION...
PANC...The Kenai radar (PAHG) had light returns over portions of
Turnagain Arm and near Fire Island early this morning. The main
event is over but there is a slight chance that a snow shower
could move over or in the vicinity of PANC early this morning.
Between 8-9z the cig increased from 2,200 ft to 4,600 feet.
Improvement is expected with the flight categories this TAF
package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Offshore flow will continue today as a ridge builds over the
area. The ridge will remain over the area tonight and weaken on
Friday. Gusty Gap winds will continue through today, weakening
tonight and Friday as the surface gradient weakens. Drier and
colder conditions will settle in across Southcentral through
Friday night. Another front will will reach Kodiak Island Thursday
night into Friday. Precipitation over the Island will likely be
all snow, but accumulations should not be heavy and winds will be
fairly light. Zonal flow aloft will drive this eastward across the
Gulf and keep Southcentral dry. The ridge will weaken and slide
east later Friday and Friday night as a weak upper trough
approaches from the southwest. With the approach of this trough,
clouds will increase and chances of snow will return to the the
Southcentral mainland.

&&

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

Clear and cold conditions persist for southwest Alaska. Wind
chills are currently flirting with the -40F mark in the Kuskokwim
Delta, so will stay on the safe side and leave the wind chill
advisory out through mid morning. Northeasterly flow with gusty
winds continues to move out of the Interior of the state
reinforcing the cold air in place. This is expected to change
tonight into tomorrow with the arrival of a front extending from
the storm force low currently entering the Bering Sea. Timing of
the front has varied slightly over the past day with each model
run. However, we have confidence that southwest AK will see a
shift in the weather late tonight. This will warm temperatures and
bring light snow to southwest AK.

&&

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

The storm force low has arrived in the Bering Sea. The low center
is currently passing over the Western Aleutian Islands, close to
Amchitka. The center is well defined on GOES satellite. Recent
ASCAT imagery shows a swath of storm force winds on the backside
of this tightly-wrapped low, so the wind forecast appears to be
on track. What is still in a bit of question is where the low
will end up, as models keep flip flopping on tracks that take it
to the western Bering, or more east close the Pribilofs. So stay
tuned for forecast updates on storm track.

With that said, this low is still expected to weaken after in
crosses into the Bering, with winds dropping to small craft
advisory strength by Saturday morning. The initial front
associated with this low is currently pushing through the
Pribilofs and eastern Aleutians, so periods of snow and blowing
snow are in the forecast for both locations today. The Aleutians
will some mixed precip as well with warmer temperatures there.
Another front will push through Friday with similar weather.

&&

.MARINE (Days 3 through 5)...

Bering/Aleutians: Look for an active pattern, with a series of
lows and fronts transiting the region. While there is some
question about where each low will track, generally expect a good
chance of gale force winds through the 3 day period. The strongest
winds look to be on Sunday, with a low along the western
Aleutians. There is chance of storm force winds near the low
center.

Gulf of Alaska: The Gulf will be on the periphery of the Bering
Sea storms, with the strongest winds across the southern Gulf.
It looks like winds will remain below gale force Saturday through
Sunday, with just a small chance of gales over the southern Gulf.
A more significant storm out west on Monday looks likely to bring
gales to much of the western Gulf on Monday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Beginning the extended period forecast on Friday evening, there
will be a stacked closed low over the Central Bering Sea with a
triple point low over the Gulf of Alaska, moving eastward. Onshore
flow across Southwest Alaska will bring in snow showers and more
cloud cover and warmer temps while arctic high pressure over
Southcentral continues to keep temps below average through the
weekend. A series of subtle low amplitude shortwave troughs will
pass through the Southern Mainland Saturday through Sunday night,
but there is low certainty at this juncture on track. For now,
clouds will likely be on the increase with precipitation chances
rising although the threat looks rather minimal, and this will
also favor coastal locations.

A big change is looking more and more likely beginning Sunday
evening and lasting through at least Tuesday. The flow pattern is
going to amplify dramatically, and will result in a potential
significant storm cycle across all of Southern Alaska with threats
ranging from heavy snow, coastal rain, strong winds, and more. At
this juncture details are much uncertain, but residents should be
aware and stay tuned as the details and storm track become more
certain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155.
MARINE...Gale 130 131 132 411 412 413 170 172 173 174 175 177 178
179 180 185. Storm 176. Heavy Freezing Spray 127 139 414 412 180
185. FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BJB
MARINE/LONG TERM...JA