Small Craft Advisory 144, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 AM AKST Tue Feb 13 2018

.ANALYSIS AND UPPER LEVELS...

The upper air flow pattern is currently in transition toward a
more progressive and less amplified regime as a deamplifying
shortwave trough, which brought widespread snow to Southcentral,
is moving east toward the Yukon. Over the western Bering Sea there
is a large low with an associated gale force front moving east
over the central Bering Sea. In between the two troughs is a
shortwave ridge axis which is moving east over Southwest Alaska.

At the surface, general westerly flow has encompassed southern
Alaska as the surface low continues on a track along the North
Gulf Coast toward the Yukon. Offshore westerly flow is increasing
this morning from Kodiak Island to the eastern Kenai Peninsula.
Inland flow over Southcentral is rather weak, so there is
widespread inland fog and stratus which is being blocked from
moving east by the Talkeetna and Chugach Mountains.

&&

.MODEL DISCUSSION...
The models are in excellent agreement with the large scale pattern
but poor agreement with the details as Alaska switches regimes
into a more progressive one. The first system to cause headaches
is one moving from the eastern Bering Sea into Southcentral
Wednesday. Models have not handled this shortwave trough well, and
are now much faster than yesterday. However, with meagre moisture
available and an unfavorable track north of the Alaska Range, any
snowfall from this system will not be widespread or heavy. Higher
uncertainty than normal continues into the late week as another
low moves into the Bering Sea then tracks into the Gulf of Alaska
and Southcentral. The atmospheric dynamics of this pattern are
subtle and difficult for the models to capture, especially since
there will be a low level flow originating from the near
subtropics of the North Pacific. For this forecast, conservative
changes were made favoring a blend to all the models to capture
the uncertainty with the wave pattern aloft.

&&

.AVIATION...
PANC...Local fog is going to be a challenge this morning with some
breaks in the fog developing based on local observations. However,
with plentiful low level moisture and weak winds and clearing
skies, it is expected that fog will be around until at least the
late morning before diurnal heating mixes the atmosphere out by
the afternoon. Thereafter, weak winds and VFR conditions are
expected to prevail through the remainder of the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The trough which brought the mixed bag of weather to Southcentral
is moving east and should clear the Copper River Basin late this
afternoon. Expect snow showers is this system heads east today
across the Copper River Basin. Abundant moisture dropped
yesterday (Monday) from the MAT-SU Valleys, across the Anchorage
Bowl, to the Western Kenai Peninsula combined with clearing higher
level clouds has given lead to low stratus and fog formation.
These conditions will likely hold around through this morning with
improvement by afternoon. Strong outflow conditions are
developing over the North Gulf Coast and eastern Kenai Peninsula
this morning with Gales to Storms over the western Gulf. Winds
should peak in the aforementioned areas during the morning hours
then slowly diminish during the course of the afternoon and
evening. A weak trough moving across western Alaska brings
increasing chances at snow/snow showers to Southcentral
Wednesday.

&&

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

The longwave pattern features a ridge axis moving across Western
Alaska (including the AKPEN) with a low vortex east of Kamchatka.
The ridge will rapidly push eastward this morning and then the
region will be impacted by the frontal boundary that is currently
in the Bering. The models continue to suggest that the
precipitation shield will impact Nunivak Island first and
gradually move inland towards Bethel and other parts of Southwest.
Forecast thermal profiles from the NAM/European/GFS all suggest
that the precipitation from this portion of the front will start
out as a rain/snow mix and then transition over entirely to snow.
By Wednesday it is safe to say that the boundary is undergoing
frontolysis or weakening. A few lingering snow showers are
anticipated on Wednesday, but significant accumulations are not
expected.

&&

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

There is a surface low east of Kamchatka with a mature frontal
boundary bisecting the Bering and the Aleutians. Based on the
recent surface observations, most of the Chain is still in the
warm sector. The latest ASCAT/advanced scatterometer has detected
gale force winds over the Central Bering and portions of the
Aleutians. Looking at the thickness values at 1000-850 MB, after
the frontal passage there will be sufficient cold air to support
snow showers later today and over night. Late Tuesday/early
Wednesday, the pressure gradient slackens with the boundary and
the potential for gales decreases. Wednesday, a new low enters the
domain from the North Pacific. Both the GFS and the NAM bring the
surface low towards Adak. This low will really deepen once its
jumps north of the Aleutians over the Bering, but for this
forecast period is more of a area of interest.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A weak trough will cross Southcentral Alaska Wednesday night. A
ridge will build behind this trough, over southwest Alaska on
Wednesday night and then into Southcentral on Thursday. An upper
low over the central Aleutians Wednesday night will move quickly
into the central Bering by Thursday afternoon. This low will then
push into southwest Alaska on Friday, weakening and then racing
across the Gulf. A large ridge will then build over mainland
Alaska late Friday, keeping the area protected from any weather
systems through Saturday. Another Bering system will push into
southwest Alaska on Sunday, and then weaken and/or dissipate as it
moves across Southcentral Sunday night into the fairly strong
ridge. By this time the models differ on the timing and details of
this system and various systems over the Bering and Aleutians.
They do, however, agree fairly well on the over-all synoptic
situation for early next week with a ridge over the northeast
Pacific and most of the mainland, and a deep trough over the
Bering Sea and Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 121 131 132 165 170 172-174 176-181 185 351 352
412-414.
Storm 131 .

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...BL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
239 PM AKST TUE FEB 13 2018

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

0200 PM     SNOW             POTTER VALLEY           61.05N 149.78W
02/13/2018  E3.1 INCH        ANCHORAGE          AK   PUBLIC

            REPORT FROM THE PUBLIC OF 3.1" OF SNOWFALL
            IN THE PAST 24 HOURS.

&&