Small Craft Advisory 914, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
511 PM AKST Sat Nov 30 2019

.ANALYSIS AND UPPER LEVELS...

In the upper levels a large low is centered over the northwest
Bering Sea with a trough extending southeast across southwest
Alaska into the Gulf of Alaska. Strong southwest flow follows in
on the back side of this trough into southwest Alaska. A surface
low is south of the central Aleutians, caught in the strong
southwest flow, and is rapidly moving northeast toward the Alaska
Peninsula. A moderately strong low is in the southwest Gulf of
Alaska with a weak trough extending north over south central
Alaska. This is pushing moisture into Prince William Sound and
south central Alaska, resulting in rain and some rain and snow
across the region. This is in the decaying phase and the
precipitation associated with it is diminishing.

&&

.MODEL DISCUSSION...

Differences in the models while not great, result in significantly
different possibilities in the weather for Bristol Bay tonight
and across Southcentral Alaska through Monday afternoon. This is
leading to less confidence in the forecast, especially for the
Kenai Peninsula, the Anchorage area, and the Matanuska and Susitna
Valleys. The main issues are the track of the low as it moves
east. If it takes the more southerly track advertised by the NAM,
snow will be much less across the Bristol Bay zone than the GFS
more northerly solution. We are leaning to the NAM for the Bristol
Bay forecast and are backing off on the snow and wind previously
anticipated, with the heaviest snow mainly in the Aleutian
Mountains and Lake Clark Pass. As this storm moves east, the
models are similar in bringing it into Prince William Sound on Sunday
night and Monday. Before the low moves into the Sound, weak over-running
will occur Sunday. The challenge is whether it will be cold
enough for this initial shot of precipitation to be snow. It looks
to be marginally cold enough, and most likely snow will remain
mixed with rain for most of Sunday. Sunday night and Monday, the
Nam has a slightly more east and south solution than the GFS, and
is more aggressively bringing the colder air into the Anchorage
area than the GFS. This can result in significantly different
snowfall amounts for the south central region that will bear
watching.

&&

.AVIATION...

PANC...The south wind is expected to drop off at or after 00Z, at
which time lower CIGS are likely. MVFR CIGS should persist
overnight, followed by IFR conditions toward the morning hours,
around 15-18Z. The low conditions should persist through the day
Sunday.

&&

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

Snow mixed with rain will continue this evening over the northern
Kenai Peninsula, Anchorage, and southern Susitna Valley as an
upper level trough departs the area to the northeast. Farther
north in the northern Susitna Valley, temperatures are cold enough
for all snow, leading to a Winter Weather Advisory through early
tomorrow morning.

For the most part, midlevel winds will remain out of the
southeast through Sunday, drawing in warm air over Southcentral.
This translates into precipitation remaining a rain/snow mix
through Sunday. However by Sunday evening, a strong low moves into
Prince William Sound bringing easterly gales and steady rain to
the northern Gulf coast. Meanwhile the presence of this low will
disrupt the warmer midlevel southeasterly flow, giving the air
column a chance to cool. Additionally, a large amount of cold air
that has dammed up along the west side of the Alaska Range will
eventually spill eastward. At this time, we are thinking any mixed
precipitation falling on Sunday will have the ability to change
to snow late Sunday night into Monday morning when the cold air
falls into place. Accumulations are expected for the western Kenai
Peninsula, Anchorage Bowl, and Mat- Su Valleys, though amounts
vary greatly on the timing of the cold air arrival. We encourage
you to follow the forecast closely for updates.

&&

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

Clear and dry conditions this afternoon will transition to a
wetter pattern overnight as an arctic front lifts across the
Alaska Peninsula north through the Bristol Bay region by the
Sunday morning hours. The biggest forecast challenge is in the
amount of snowfall that will occur with this fast moving system.

Current thinking is that the low responsible for potential snow
will stay farther south (NAM solution) with little snow
accumulations across interior Bristol Bay. However, there is still
uncertainty in this forecast due to model disagreement. The GFS
solution would keep the low center farther north, increasing the
amount of snow that would fall across the area, easily reaching
advisory criteria. For this reason, we have decided to keep the
Winter Storm Advisory out for this region even though the amount
of snow accumulations are reduced from this morning's forecast.

Areas stretching from King Salmon to Port Alsworth could see new
snow totals of 2-4" from this system if the NAM wins out, with
higher snowfall totals of 5-7" along the Alaska and Aleutian
Ranges through Monday morning. Stronger winds could cause blowing
snow or resuspending snow that is already on the ground, causing
a reduction in visibility. The Kuskokwim Delta should remain dry
through Monday morning when a chance of snow approaches coastal
areas. Conditions across the remainder of Southwest Alaska improve
on Monday into Tuesday as the low exits the area.

&&

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

Gale force winds south of the central Aleutians will continue to
progress eastward through Sunday afternoon as the low moves inland.
The associated low will also cause an area of small craft winds
to move from the eastern Bering this evening into Bristol Bay
overnight, diminishing by Sunday afternoon. Precipitation will
stay primarily rain with this system, with light snow
accumulations possible across the Pribilofs. Small craft winds
will move into the western Bering and Aleutians by Sunday
afternoon spreading across the Bering through Monday associated
with a new area of low pressure.

&&

.MARINE (Days 3 through 5)...

A progressive pattern is expected this forecast period. On
Monday, low pressure will move into Prince William Sound with a
surface pressure of 974 mb to 977 mb. The Gulf Of Alaska can
expect gale force winds Monday and Tuesday. The low will slowly
decay and the pressure gradient forces which are responsible for
the wind intensity will decrease late Tuesday and Wednesday.

A mature front will push across the Central Aleutians and towards
the AKPEN which will produce small crafts and gales. In the wake
of this frontal passage another robust Kamchatka low will bring a
batch of unsettled weather to the Western Aleutians late Tuesday
and Wednesday. With this system expect another round of gale force
winds.

&&

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

The models are struggling in the long term forecast period but
there are a few key themes with the the global models. A. Its
highly progressive. B. There are 3 discernible fronts. By
Tuesday, the trough reaches the northern Gulf and expect this
feature to slowly begin to fill. To the west, there is another
front near the Aleutians supported by cold air over the Bering and
a strong jet core. A surface low near the central Aleutians will
continue to push eastward. A cold upper level trough swinging off
of the Kamchatka Peninsula Tuesday night and over the western
Aleutians Wednesday/Thursday will bring another batch of unsettled
weather to the region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145 161.
MARINE...Gale 119 120 131 132 150 155 172 174.
		 Storm 130.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...SA