Small Craft Advisory 221, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 AM AKDT Thu Mar 15 2018

.ANALYSIS AND UPPER LEVELS...

A weak trough and associated precipitation that brought several
inches of snow yesterday to Palmer and the mountains east of Eagle
River is now drifting eastward across the Copper River Basin.
Associated localized reductions in visibility and ceiling heights
in that region have occurred.

All eyes then turn to the next storm system currently lifting
northeastward across the eastern Aleutians and the Alaska
Peninsula. The center of the storm remains south of Nikolski, but
the associated precipitation extends well out ahead of it. This
storm will be the next weather maker for nearly all of Southern
Alaska. More on that below.

In the upper levels, a jet representing the delineation between
troughing to the north and ridging to the south is right over
southern mainland Alaska. This flow is helping spread the
precipitation with the Aleutian storm towards the area more
quickly. The upper low associated with the Aleutian storm is also
lifting northeastward towards southwest Alaska.

&&

.MODEL DISCUSSION...

Short-term agreement between the GFS, EC, and GEM remains good,
while the NAM has a somewhat different take on the evolution of
the upcoming storm over the Aleutians. The NAM is by far the
wettest solution when it comes to precipitation across the Susitna
Valley with the system Thursday night, while it is the driest
further east across Anchorage and the Matanuska Valley. Meanwhile,
the NAM keeps the center of the low further intact as it
approaches the Western Capes, while all of the other models
degrade the low more quickly and keep the center further west
into the Bering. By the end of day 2, the GEM deviates from the
other models significantly and was thrown out. Thus, a GFS-
centric solution was the preferred model for this forecast
package.

&&

.AVIATION...

PANC...Isolated pockets of fog have developed in the vicinity of
the airport this morning and will remain a threat until the sun
burns it off later this morning. The next round of snow moves in
this evening, though developing downsloping on southeast winds
should limit accumulations and confine the worst snowfall
conditions to the evening.

&&

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

The next system will approach the area from the southwest today.
Precipitation will reach Kodiak Island this morning, then spread
across the rest of Southcentral through the afternoon and evening.
For the western Kenai Peninsula through Anchorage and the
Matanuska valley, strong low to mid level southeasterly flow
will limit precipitation due to downsloping. As a result, fairly
strong winds are expected through Turnagain Arm. For the Susitna
Valley, upslope conditions will likely produce a decent amount of
snowfall tonight through Friday. This system is also warmer than
the last few systems, and it is possible that some rain could mix
in over coastal locations and even some inland areas. Conditions
will dry out Friday night as an upper level ridge builds over the
area.

&&

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

There is very little change in thinking with this forecast package
as a slow moving low is still expected to bring widespread
precipitation to Southwest Alaska (predominantly in the form of
snow with a mix of rain along the Alaska Peninsula and on the lee
slopes of the Alaska Range). The best swath of snow is still
expected to be from the Kilbuck Mountains eastward to Dillingham
and north to Sleetmute. These areas may see 4-7" with locally
higher amounts close to 10-11", and the winter weather advisories
will remain in effect unchanged. Moving west from Bethel, snow
amounts will dramatically drop off, but strong northerly winds
will help to blow snow around and reduce visibilities, so the
winter weather advisory for blowing snow remains in effect. The
low lifts north and weakens Friday, with weak ridging building in
and diminishing chances for precipitation.

&&

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

A warm front associated with a North Pacific low is currently
crossing over the Eastern Aleutians into the AKPEN. Any leftover
snow will quickly change to all rain as the warm air moves
overhead. The front will stretch out and weaken through Friday,
but not before a swath of gale force winds impacts most of the
area from the Pribilof Islands eastward. There will be a sharp
delineation to the precipitation edge over the Pribilof Islands
such that Saint George may see a rather long period of light snow
while Saint Paul remains dry.

Over the western Bering/Aleutians, a weakening gale force front
will be replaced by a more substantial near storm force front
Friday and Friday night.

&&

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

The long range forecast begins Saturday with a broad upper level
trough that is slowly pushing east through the Bering. With this
feature there will be a few rounds of precipitation that'll hit
the west coast and spread inland. The first is well handled by all
guidance and will involve a surface low with an attendant front
lifting through the Bering Strait. A second, and stronger
shortwave will round the base of the trough and shoot towards SW
Alaska Sunday afternoon. It is here that models begin to fall
apart. National Centers prefer more of a focus on the operational
EC than the GFS due to run to run continuity. On Sunday, a second
low will repeat the motions of the first low, but push farther
inland this time. This may help to bring a chance for snow early
in the week for southcentral Alaska.

Around this time, a strong ridge will begin to build across the
Bering Sea. It is expected to strengthen through mid week but
stall over the AKPEN by Wednesday. This ridge will get pinched off
by a strong Pacific jet that will dive south of 40N. Most of the
area should see northerly winds through this time. For the
Anchorage Bowl, temperatures will be near normal. Sunny skies will
help to make it feel warmer and melt some of the lingering road
ice through mid week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...

PUBLIC...Winter Weather Advisory: 152 155 161.
MARINE...Gale: 155 160 170 172 173 177-181 411 414.
Heavy Freezing Spray: 160 180.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...SS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
1245 PM AKDT THU MAR 15 2018

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

1115 PM     SNOW             CHICKALOON              61.80N 148.48W
03/14/2018  E8.0 INCH                           AK   PUBLIC

            ABOUT 8 INCHES STARTING AT 7AM.

1115 PM     SNOW             10 NW MOOSE CREEK STATE 61.77N 149.31W
03/14/2018  E7.0 INCH                           AK   PUBLIC

            HATCHER PASS RECEIVED ABOUT 5-7 INCHES BETWEEN 2AM AND
            8PM.

&&

AHILL