Small Craft Advisory 943, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 AM AKST Thu Nov 29 2018

.ANALYSIS AND UPPER LEVELS...

An active pattern is still holding over much of the state this
morning. A broad upper level trough remains in place anchored by a
996 mb surface low just south of Kodiak Island. Embedded within
the broader parent trough is a shortwave trough located over the
Kenai Peninsula. This shortwave is responsible for the continued
snowfall over Southcentral, extending from the Anchorage Bowl
northward to the Mat-Su Valleys, through the overnight hours. Cold
air advection continues to fill in behind this surface low,
bringing mixed precipitation to the north Gulf coast. Surface
observations indicate what initially began as rain has gradually
turned to snow or remained a rain/snow mix along the coastal
areas of Prince William Sound.

Surface observations also indicate precipitation over Southwest
in response to the front associated with the low south of Kodiak
Island. Southeasterly surface flow combined with northerly flow
aloft has allowed a decent band of precipitation to set up over
Bristol Bay during the overnight hours. Scatterometer data
(satellite-derived surface winds) indicate a patch of gale-force
winds over the waters just south of Sand Point.

Looking west, an upper level ridge is positioned over the central
Aleutians while an upper level Kamchatka low has begun advancing
into the western Bering. The leading edge of the front associated
with the low is draping gale-force winds over the central Bering
as it pushes eastward.

&&

.MODEL DISCUSSION...

Models are in good agreement through Thursday night and have
locked onto the timing and track of a strong gale force front
ripping east across the Bering today. Major differences pop up by
Friday morning as a sharp upper level trough with a pair of
surface lows with tropical connections moves north from the
Pacific into the Bering. The GFS/NAM are in decent agreement with
timing, strength, and track while the Euro seems to be the outlier
with a quicker and weaker solution. Have gone towards the GFS/NAM
solution due to consistency, which brings strong gale to storm
force winds to the Eastern Bering/Aleutians/Bristol Bay area
Friday night into Saturday.

&&

.AVIATION...

A weak surface wave tracking west across the area overnight has
mostly pushed west of the terminal this morning, with the bulk of
radar returns showing up on the other side of the inlet. Some
lingering moisture this morning will lead to temporary marginal
conditions in snow before completely clearing the terminal by mid
morning. Skies should scatter out some this afternoon as ridging
builds in from the west. This of course means increased chances of
fog developing this evening. The challenge is figuring out how
quickly the next system moves into the area tonight and if
fog/stratus is able to develop before mid level clouds push in.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Light showers will linger around Southcentral Alaska this morning
but diminish as the upper level low over the area weakens as it
moves to the northeast. Ridging will then build in over the area
allowing for temperatures drop back down closer to normal levels
for this time of year. However, colder temps will be short lived
as another system will bring a strong push of warm air across
Southcentral Alaska Friday.

As the front from this system moves through, most locations will
start as snow with many low lying areas near the coast
transitioning to rain. One of the challenges will be determining
if freezing rain will occur during the transition. For areas
around Cook Inlet this looks to be a possibility for locations
that can hold on to cold air at the surface. Another challenge is
with the models struggling with the timing and location of this
system. These differences will play a big factor in timing of
precipitation and amounts of precipitation throughout Southcentral
Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Thursday
through Saturday)... Two rounds of weather are expected the next
couple of days. The first round will push through southwest Alaska
later today into tonight and is the landfall of the front
currently pushing through Bering Sea. Temperatures are expected to
stay cold enough for any precipitation to fall as snow. Blowing
snow was a concern going into the tonight, but temperatures look
too warm for snow to really get blowing around.

The second round of weather should move into southwest AK tomorrow
night. This round of weather will move in from the southwest,
bringing warm air with it. Snow will quickly transition to rain as
this system comes ashore, so this will be mainly a rain event.
There will be a narrow band of mixed precipitation as snow
transitions to rain. Most of it will be a rain/snow mix, though
there is a chance for freezing rain around Bethel.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3: Thursday through Saturday)... Low pressure continues to push
east through the Bering Sea today. Gale Force winds accompany the
front as it pushes towards the southwest coast. The front is
expected to weaken as it goes, largely loosing its Gale Force
winds as it moves through the eastern Bering Sea. Behind this
front, the western Aleutians will see zonal flow the next couple
of days, with winds intermittently strong enough for small craft
advisories. Meanwhile, the eastern Aleutians will see a brief
break in the weather, than another wave of energy will cross the
eastern Aleutians from the south tomorrow night, increasing wind
speeds again as it heads towards the southwest coast. Weather then
quiets Saturday with weak low spinning just north of the Pribilof
Islands.

&&

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

The upper level pattern for the extended forecast will feature a
longwave trough centered over the northwest Bering Sea, and a
ridge over the central/eastern Gulf of Alaska. Between the two
features, an active jet stream with several embedded disturbances.
Guidance is keying in on a potent disturbance coming off east
Asia, which will interact with baroclinic zone draped over the
northwest Pacific. The result will be a reshaping of the long-
wave as a stronger surface system develops south of the Western
Aleutians over the weekend. That system will become the new
center of the long-wave and shape the pattern into the middle of
next week.

Saturday begins under the southwesterly jet aloft, an embedded
trough, and plume of moisture from the Pacific. Unfortunately for
snow lovers, a bit warmer air mass will also be accompanying. As
that system exits a frontal boundary will stall from the western
Gulf through the southwest mainland. The pressure gradient around
this time will favor southeasterly winds through Turnagain Arm and
possible into Anchorage itself. As the previously mentioned
stronger system spreads into the Bering Sea we will return into a
deeper layer southeasterly flow pattern, meaning drier and warmer
with above average temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 155 165 170 172-176 179-181 185 412 413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP/KO
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...ML

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
957 AM AKST THU NOV 29 2018

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

0443 PM     SNOW             RABBIT CREEK RD         61.08N 149.81W
11/28/2018  M5.5 INCH        ANCHORAGE          AK   PUBLIC

0413 PM     SNOW             LAKE OTIS / HUFFMAN     61.10N 149.82W
11/28/2018  M4.8 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0405 PM     SNOW             RABBIT CREEK PARK       61.10N 149.83W
11/28/2018  E4.5 INCH        ANCHORAGE          AK   PUBLIC

0321 PM     SNOW             HUFFMAN                 61.11N 149.85W
11/28/2018  M4.8 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0222 PM     SNOW             TURNAGAIN               61.19N 149.95W
11/28/2018  M1.5 INCH        ANCHORAGE          AK   NWS EMPLOYEE

1256 PM     SNOW             OCEAN VIEW              61.11N 149.88W
11/28/2018  E1.8 INCH        ANCHORAGE          AK   PUBLIC

1255 PM     SNOW             BEAR VALLEY             61.07N 149.73W
11/28/2018  M3.0 INCH        ANCHORAGE          AK   PUBLIC

            STILL SNOWING AT THE TIME OF THE
            OBSERVATION.

1251 PM     SNOW             UPPER HUFFMAN RD        61.11N 149.78W
11/28/2018  M4.0 INCH        ANCHORAGE          AK   PUBLIC

            ACCUMULATION OVER 5 HRS.

1250 PM     SNOW             INDEPENDANCE PARK       61.13N 149.85W
11/28/2018  M2.8 INCH        ANCHORAGE          AK   NWS EMPLOYEE

&&