Small Craft Advisory 991, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
708 AM AKST Fri Dec 14 2018

.ANALYSIS AND UPPER LEVELS...

The long-wave trough that has held firm over the Bering is in the
process of shearing into a series of disorganized shortwaves and
is becoming more meridional in orientation as the axis shifts
toward the western mainland. One shortwave is currently moving
northeast along the Kenai Peninsula and is associated with a
fast-moving 990mb surface over Prince William Sound and a weak
trough extending along the Cook Inlet. Snow falling from near
Kenai north to Palmer and east to Cordova intensified overnight
due in part to the shortwave passing over the region coupled with
an increase in RH and omega (upward vertical motion) in the mid-
levels. This system is moving quickly to the north and east so
accumulations over southcentral are expected to remain light with
snow coming to an end this morning. A meso-low north of King
Salmon is producing snow for locations around Lake Iliamna. The
bigger story for the southwest mainland, however, is the arctic
air holding tight over the region. Along the coast, overnight
temperatures are ranging from 0 to 10 above, but inland values
have dropped to as low as 10 to 20 below zero.

Farther west, another shortwave and surface low near the Pribilofs
are bringing clouds, snow showers, and a reinforcing shot of
arctic air with northerly winds over the Bering. A Kamchatka low
is sweeping a front across the western Aleutians where surface
winds have now dropped below gales. The jet stream remains south
of the chain with the left exit region positioned over a deepening
surface low well south of Dutch Harbor. The latest scatterometer
pass denotes a healthy swath of gales wrapping around the center
of circulation and IR imagery shows cooling cloud tops indicative
of strengthening. This system is moving east toward the western
Gulf, sweeping a front, currently south of the AKPEN, ahead of it.

&&

.MODEL DISCUSSION...

Overall, models remain in good agreement through the period
regarding the track of the North Pacific low as it moves toward
the Gulf through the weekend. Prior to that, there is some
discrepancy between models concerning precipitation amounts along
the lee of the Chugach and Kenai Mountains and across the
Anchorage Bowl as an arctic boundary sets up over the
southcentral coast Friday. Most guidance (GFS, EC, Canadian) show
this feature (quasi-warm front) lifting quickly across the region
from southeast to northwest by Friday evening with precipitation
ending Friday night as downsloping takes over. The NAM, and to a
lesser degree, the high-res models, are much slower with the
movement of this feature, keeping precipitation going about 6
hours longer. This solution would allow for more overrunning of
warmer air over the colder air entrenched along the higher terrain
and yield more snow along the lee of the mountains. Given that
the NAM and GFS are similar regarding the evolution of the larger,
synoptic features, blend of the GFS and NAM was used as a middle-
of-the- road approach for timing and precipitation (snowfall)
amounts across Southcentral. Farther west, guidance continues to
perform well with respect to the development of another North
Pacific low and a weaker Kamchatka low both tracking east through
the weekend.

&&

.AVIATION...

PANC...MVFR conditions with light northerly winds will persist
through the morning with light snow ending early this morning.
Snow will return Friday afternoon with better chances of IFR
ceilings and visibility late Friday afternoon through early
Saturday morning.

&&

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

An upper level disturbance that brought some light snow to the
Cook Inlet region overnight is moving over north toward the Alaska
Range and will be out of the area by late morning.

Most of the effort on the forecast revolves around the strong
front and low that will be moving into the Gulf of Alaska today
into Saturday. As this system approaches the area, it will advect
moist air over the Arctic air that is in place over Southcentral
Alaska. This will also push the arctic front back toward the
northwest and this is where the forecast gets a bit tricky for the
Cook Inlet region including the Mat-Su valleys. The snow will
begin over the Kenai and Chugach mountains today and then head
toward the Cook Inlet area by late afternoon. The question is how
fast that arctic front retreats northwestward since that will
determine when the snowfall shuts off for these areas. As the
arctic boundary is in place it will prevent the downsloping on the
leeward side of the mountains and snow will occur. As soon as it
gets west of the mountains in the Cook Inlet /Mat-Su region a
sharp back edge will develop and shut off the snowfall. There is
some significant spread in the models in how fast this arctic
boundary moves and that is the difference in whether the Anchorage
area sees the snow end by mid-evening or six-to-eight hours
later. This is the difference in 2 to 4 inches of snowfall or 6 to
12 inches. Most reasonable situation at this time is for snow to
end sometime around midnight with 4 to 6 inches total
accumulation. This is subject to change as the system develops and
850 mb temperatures from the 12Z sounding in Kodiak and Anchorage
can be compared to the 12Z model runs and therefore help resolve
the issue.

The north Gulf Coast will see the snow showers this morning
become steady snow by afternoon. Precipitation should remain all
snow with this system along the north Gulf coast.

Kodiak Island will see the snow mix with rain at lower elevations
this afternoon and then turn to all rain at lower elevations this
evening. Higher elevations of Kodiak Island will see some periods
of heavy snowfall overnight.

&&

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

Not much change in the forecast for the next couple of days over
the southwest mainland as an upper level low slightly shifts
north, allowing high pressure to build in. As the high builds in,
skies will continue to clear as northerly flow pulls cold arctic
air south over the area. Most locations are set to see their
coldest temperatures of the winter so far Saturday and Sunday
morning, with many inland locations seeing temperatures fall below
-20F. Brisk offshore flow will drop wind chills to near -40F
 Saturday and especially Sunday morning near the Kuskokwim Delta
 Coast. On Saturday, a deformation zone will develop over the
 Kuskokwim River Valley and AKPEN allowing for a narrow band of
 snow to form. Snow totals by Saturday night look to stay below
 6", though amounts will be higher in the upper elevations. Models
 have been struggling some with the exact location of where this
 snow band sets up, so snow amounts could vary greatly over a
 small distance.

&&

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

A strong 950 mb low will continue trekking east today, skirting
just south of the Aleutians. Most precipitation from this system
should stay just to the south of the island chain, though some
light snow and rain showers over the Eastern Aleutians are not
out of the question. As the low pushes east, strong northerly
flow, aided by cold air spilling out of the Arctic, will develop
leading to gale force winds on the south side of the Eastern
Aleutians. Winds could gust up to 50 mph through channeled terrain
overnight. Ridging will build in behind the departing low leading
to broad cold northerly flow and scattered snow showers across
most of the region. Another strong Pacific storm approaches the
Western Aleutians tomorrow, but looks to take a similar southerly
track as its predecessor, with just increased winds along the
Aleutians while the bulk of precipitation will stay just off
shore. Northerly flow will continue to dominate through the end
of the period.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with the longwave trough extending
from Southwest Alaska to the Gulf Of Alaska. Meanwhile upstream
there is an upper level ridge that spans from Hawaii to the
Eastern Aleutians. As the forecast period evolves, another vortex
of brutally cold air will descend from Siberia and the longwave
pattern will become more highly amplified. The deterministic
models have depicted a series of lows impacting the Aleutians,
the Gulf of Alaska and the Prince William Sound. However, the
precise tracks of these lows remains uncertain. The models have
been struggling this winter locking onto a solution past 72 hrs
and that trend will continue for this forecast package.

PJS
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning...351 352.
	     Gale Warning...119 120 130 131 132 136 138 139 150 155 172.
         Heavy Freezing Spray Warning...160 180 181 185.

&&

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