Small Craft Advisory 972, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Sun Dec 9 2018

.ANALYSIS AND UPPER LEVELS...

A negatively tilted upper level trough is located over the western
half of the mainland and extending south well into the Pacific
Ocean. A subtle shortwave ridge is building in behind the trough
and a strong zonal jet is located just south the of Aleutians
supporting a broad complex of surface lows in the Southern Bering.
The jet is helping to finally displace the persistent upper
trough to the east, allowing the short wave ridge to briefly push
over Southcentral. At the surface, a decaying low is located over
the Southcentral interior rapidly filling and moving north. A
deformation zone on the backside of the low is leading to a band
of moderate to sometimes heavy snow over the Mat-Su Valleys and
east up the Glenn Highway corridor, with reports of up to 6
inches of snow on the highway near Chickaloon. A narrow window of
clear skies can be seen over the southwest mainland, though low
stratus and fog has formed in many places this morning. A frontal
system can be seen pushing across the Eastern Aleutians and AKPEN,
with the cloud shield approaching the Bristol Bay coast. Cold air
wrapping south and east behind the front is helping to enhance a
broad swath of gale force winds along the Central Aleutians and
Bering.

&&

.MODEL DISCUSSION...

Models are overall in good agreement through the end of tonight as
the next storm system pushes east from the Bering into the Gulf,
with only minor timing and positional differences. By Monday,
models begin to struggle as the upper trough repositions itself
over mainland. Numerous waves and surface lows are being depicted
with lots of inconsistency between the numerous model runs. There
is at least some agreement that the storm track should take a
more southern route, allowing colder air to return to the southern
mainland.

&&

.AVIATION...
PANC...VFR conditions should prevail through the day as the low
pushes north. There may be a brief period of MVFR conditions this
morning as a weak upper trough pushes up the Cook Inlet. Skies
should clear up some this afternoon as a weak ridge builds in
ahead of the next system. Good up inlet flow with post frontal
pressure rises will continue to cause gusty southerly winds
through the early afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Monday night)...
An upper level short-wave trough will lift northeast across
Southcentral this morning and combine with low level cold air
advection to produce some areas of snow or snow showers. Favored
areas for some light accumulation are the Susitna Valley and
eastern Prince William Sound. While Valdez and Cordova are
reporting rain early this morning, do expect a changeover to
snow over the next few hours.

Very progressive upper level flow means active weather will
persist as we head through the next couple days and beyond.
An occluded front along the Alaska Peninsula will cross Kodiak
island today, then move onshore along the north Gulf coast
tonight. The air with the occlusion will only be marginally
warmer. Expect snow changing to rain for most of Kodiak Island.
As precipitation spreads onshore along the north Gulf coast
expect a similar trend along the immediate coastline, while
inland areas remain safely all snow. Strong east to southeast
low level flow will produce downslope drying for much of the
western Kenai as well as Anchorage and the Matanuska Valley
tonight into Monday.

The forecast becomes a whole lot more complicated behind this
front, as a series of fast moving upper waves and weak surface
lows transit the Gulf and Southcentral tonight through Monday
night. Forecast confidence in the track of individual features
is low. Overall, expect periods of rain and snow from Kodiak to
Prince William Sound, with snow as you head inland from the coast.
Interior Southcentral will also have multiple brief shots of
mostly light snow.

&&

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

A front moving across the southern Bering is expected to move into
Southwest Alaska today. Forecast trends continue to slow down the
front's arrival, which now looks to be late this morning through
this afternoon. As it moves inland, the front is expected to
quickly weaken into tonight as the associated low pressure makes a
turn towards the south, removing much of the forcing driving the
front inland. Despite a somewhat slower movement, trends continue
to weaken the front, and as such, most areas are likely to stay
under 2 inches of new snow accumulation. A new low is expected to
form along the Alaska Peninsula tonight and track northeastward
into Bristol Bay on Monday morning. This feature will bring more
widespread snow shower activity primarily to Bristol Bay areas
through the day Monday, with snow primarily building northward
along the Kuskokwim Mountains in the western Lower Kuskokwim
Valley. As a result, after this morning's break in the weather
ends, most areas look to have a prolonged period of scattered snow
shower activity, that may briefly become both heavy and more
numerous at times with any more organized small scale features
moving across the area. Temperatures are expected to remain steady
in the 10s and 20s the next couple days.

Looking ahead, a reinforcing Arctic blast of cold air is expected
to move across Southwest Alaska Monday night behind the low. The
cold air looks to persist for much of the rest of the workweek.
Many areas may actually see BELOW normal temperatures...gasp!...
for the second half of the week with sub-zero lows becoming
increasingly likely for much of non-coastal Southwest Alaska as
soon as Tuesday night.

&&

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

An area of low pressure over the Western Aleutians is in the
process of elongating along a warm front that stretches into the
eastern Bering this morning. Multiple centers of low pressure are
expected to develop along this front during the day, though no one
low is expected to be particularly strong. This is occurring as a
result of the front stalling out any northward progress in the
face of strong northerly winds streaming south out of Russia and
the Bering Strait. As this cold air reaches the front, some areas
will have more success pushing south, resulting in the development
of localized circulations (areas of low pressure) that are
entirely the result of the large temperature gradient between the
two air masses. This pattern will continue to favor snow showers
nearly constantly streaming south in the cold air mass, and
enhancing the frontal precipitation across the Bering. The
Pribilofs are right along this front, and have seen reduced
visibilities in snow at temperatures just below freezing.

A primary low is expected to establish itself somewhere near
Bristol Bay late tonight into Monday morning, which will help the
cold northerly airflow over the central Bering to resume making
its southward progress. The low will eventually cross over the
Alaska Peninsula into the western Gulf, probably by Monday
evening, though the details of that are still very uncertain.
Regardless, the snow shower activity with the cold air will take
up an increasingly larger chunk of the Bering through Monday
night, with snow showers likely across the eastern Bering even
through the Aleutians and Alaska Peninsula by Monday night. A
small area of high pressure will build into the western Bering
Monday afternoon allowing for a relaxation of the winds there.

&&

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

The long term forecast begins Tuesday night with an active
weather pattern remaining across most of the forecast area. A
long-wave trough remains anchored over the Bering as a ridge
downstream moves east over the eastern Gulf while a second, more
elongated ridge sits over the northern Pacific. This pattern
favors a zonal flow across the Aleutians with a series of quick-
moving surface lows across the region and a persistent west to
southwest flow across the Gulf of Alaska, helping to steer the
surface lows from the western Gulf northeast through the northern
Gulf and up toward Prince William Sound.

The first of these systems is a weakening compact low situated
near Prince William Sound late Tuesday through Wednesday. This low
will continue to bring mixed rain and snow along the coast with
areas of light snow farther inland as an upper-level trough
swings across the southern mainland. Snow may linger across
interior southcentral moving into Thursday, however, with both
the GFS and EC suggesting that the trough stalls, intensifies,
and becomes negatively tilted before lifting north. A stronger
shortwave and surface low undercutting this trough now looks to
track farther south across the Gulf late Wednesday into Thursday,
impacting areas east of Cordova and along the Alaska Peninsula.

The next system, the strongest of the bunch, is expected to track
along the Pacific side of the Aleutians Thursday into Friday and
enter the western Gulf near Kodiak Island late Friday into
Saturday. There is good agreement between the various guidance
solutions that this system will rapidly intensify into a 950mb
low with widespread precipitation and storm-force winds as it
moves south of the AKPEN and into the western Gulf. The low will
then stall near Kodiak Island and weaken slightly, but a strong
southeasterly flow will push plenty of moisture north along the
southern coast, with the GFS more bullish on the northern extent
of the precipitation compared to the EC. Colder air in place
should allow the onset of precipitation to be all snow. Warm air
advection in the mid and lower levels, however, should once again
make for a transition to rain for locations along the coast with
downslope winds limiting precipitation for the Anchorage Bowl.

Farther west, a trough extending north from this low will develop
into a secondary low pressure system near Bristol Bay. This
surface low will help draw the warmer air in the mid-levels across
all of southwest Alaska, bringing a snow to rain precipitation
event late Friday into Saturday. Across the Bering, a strong
northerly flow along the back side of the double-barrel low will
keep the Aleutians colder than normal with periods of snow
showers.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray 185
Gales 119 120 125 130 131 132 138 139 150 155 165 170 172 173 174
176 177 178 180.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...TM

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE ANCHORAGE AK
1221 PM AKST SUN DEC 9 2018

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

0456 AM     SNOW             8 MILES SSE EAGLE RIVER 61.24N 149.44W
12/09/2018  M5.4 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0800 AM     SNOW             EAGLE RIVER             61.32N 149.54W
12/09/2018  M2.5 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0950 AM     SNOW             16 S ANCHORAGE          60.94N 149.11W
12/09/2018  M3.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0958 AM     SNOW             UPPER O'MALLEY TRAILHEA 61.12N 149.72W
12/09/2018  M2.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

1000 AM     SNOW             4 WNW PALMER            61.63N 149.21W
12/09/2018  M4.0 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

1038 AM     SNOW             SUTTON                  61.72N 148.88W
12/09/2018  M7.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1150 AM     SNOW             1 ENE WASILLA           61.58N 149.44W
12/09/2018  M3.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1204 PM     SNOW             23 E CHICKALOON         61.80N 147.78W
12/09/2018  E6.0 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

1214 PM     SNOW             10 ESE KNIK             61.39N 149.48W
12/09/2018  M1.1 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            SNOW AND RAIN. PRECIPITATION EQUIVALENT OF
            0.18 INCHES.

&&

PRELIMINARY LOCAL STORM REPORT...SUMMARY...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
1235 PM AKST SUN DEC 9 2018

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

0456 AM     SNOW             8 MILES SSE EAGLE RIVER 61.24N 149.44W
12/09/2018  M5.4 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0800 AM     SNOW             EAGLE RIVER             61.32N 149.54W
12/09/2018  M2.5 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0950 AM     SNOW             16 S ANCHORAGE          60.94N 149.11W
12/09/2018  M3.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0958 AM     SNOW             UPPER O'MALLEY TRAILHEA 61.12N 149.72W
12/09/2018  M2.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

1000 AM     SNOW             4 WNW PALMER            61.63N 149.21W
12/09/2018  M4.0 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

1038 AM     SNOW             SUTTON                  61.72N 148.88W
12/09/2018  M7.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1150 AM     SNOW             1 ENE WASILLA           61.58N 149.44W
12/09/2018  M3.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1204 PM     SNOW             23 E CHICKALOON         61.80N 147.78W
12/09/2018  E6.0 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

1214 PM     SNOW             10 ESE KNIK             61.39N 149.48W
12/09/2018  M1.1 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            SNOW AND RAIN. PRECIPITATION EQUIVALENT OF
            0.18 INCHES.

1234 PM     SNOW             NWS ANCHORAGE           61.16N 149.98W
12/09/2018  M0.7 INCH        ANCHORAGE          AK   NWS EMPLOYEE

&&

PRELIMINARY LOCAL STORM REPORT...SUMMARY...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
116 PM AKST SUN DEC 9 2018

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

0456 AM     SNOW             8 MILES SSE EAGLE RIVER 61.24N 149.44W
12/09/2018  M5.4 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0800 AM     SNOW             EAGLE RIVER             61.32N 149.54W
12/09/2018  M2.5 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0950 AM     SNOW             16 S ANCHORAGE          60.94N 149.11W
12/09/2018  M3.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0958 AM     SNOW             UPPER O'MALLEY TRAILHEA 61.12N 149.72W
12/09/2018  M2.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

1000 AM     SNOW             4 WNW PALMER            61.63N 149.21W
12/09/2018  M4.0 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

1038 AM     SNOW             SUTTON                  61.72N 148.88W
12/09/2018  M7.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1150 AM     SNOW             1 ENE WASILLA           61.58N 149.44W
12/09/2018  M3.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1204 PM     SNOW             23 E CHICKALOON         61.80N 147.78W
12/09/2018  E6.0 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

1214 PM     SNOW             10 ESE KNIK             61.39N 149.48W
12/09/2018  M1.1 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            SNOW AND RAIN. PRECIPITATION EQUIVALENT OF
            0.18 INCHES.

1234 PM     SNOW             NWS ANCHORAGE           61.16N 149.98W
12/09/2018  M0.7 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0100 PM     SNOW             8 NNW PALMER            61.69N 149.23W
12/09/2018  M9.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

&&

PRELIMINARY LOCAL STORM REPORT...SUMMARY...CORRECTED
NATIONAL WEATHER SERVICE ANCHORAGE AK
241 PM AKST SUN DEC 9 2018

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

0456 AM     SNOW             8 MILES SSE EAGLE RIVER 61.24N 149.44W
12/09/2018  M5.4 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0800 AM     SNOW             EAGLE RIVER             61.32N 149.54W
12/09/2018  M2.5 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0950 AM     SNOW             16 S ANCHORAGE          60.94N 149.11W
12/09/2018  M3.0 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0958 AM     SNOW             UPPER O'MALLEY TRAILHEA 61.12N 149.72W
12/09/2018  M2.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

1000 AM     SNOW             4 WNW PALMER            61.63N 149.21W
12/09/2018  M4.0 INCH        MATANUSKA-SUSITNA  AK   NWS EMPLOYEE

1038 AM     SNOW             SUTTON                  61.72N 148.88W
12/09/2018  M7.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1150 AM     SNOW             1 ENE WASILLA           61.58N 149.44W
12/09/2018  M3.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

1204 PM     SNOW             23 E CHICKALOON         61.80N 147.78W
12/09/2018  E6.0 INCH        MATANUSKA-SUSITNA  AK   PUBLIC

1214 PM     SNOW             10 ESE KNIK             61.39N 149.48W
12/09/2018  M1.1 INCH        ANCHORAGE          AK   TRAINED SPOTTER

            SNOW AND RAIN. PRECIPITATION EQUIVALENT OF
            0.18 INCHES.

1234 PM     SNOW             NWS ANCHORAGE           61.16N 149.98W
12/09/2018  M0.7 INCH        ANCHORAGE          AK   NWS EMPLOYEE

0100 PM     SNOW             8 NNW PALMER            61.69N 149.23W
12/09/2018  M9.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

0159 PM     SNOW             5 ESE WASILLA           61.56N 149.32W
12/09/2018  E3.0 INCH        MATANUSKA-SUSITNA  AK   TRAINED SPOTTER

0234 PM     SNOW             12 NNW HOPE             61.06N 149.76W
12/09/2018  M2.0 INCH        ANCHORAGE          AK   TRAINED SPOTTER

&&