Small Craft Advisory 964, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKST Sun Dec 15 2019

.ANALYSIS AND UPPER LEVELS...

A large, mature low pressure system can be seen on satellite
moving slowly northwest in the western Gulf of ALaska. Its
occluded front is pushing north through Kodiak and towards the
southcentral coast. This low is a closed low both aloft at 500MB
and at the surface. High pressure still hangs on inland north of
the low. The pressure gradient between the two is creating some
gusty winds in the Susitna Valley south to the Kenai Peninsula.

Out in the Bering, a weak low spins over the eastern Aleutians,
causing low clouds and rain there. There is a shortwave ridge
currently over the central Aleutians, providing a brief break in
the weather there. This ridge is sandwiched between the low to the
east and the next big system that is just approaching the western
Aleutians this morning. This low is another large and occluded low
moving through the north Pacific. A triple point low can be seen
via satellite beginning to form as it moves into our forecast area.

&&

.MODEL DISCUSSION...

Models continue to be in good agreement. Some disagreement exists
with the low center position and movement of the low that is
currently in the Gulf of Alaska. However, the disagreement is not
enough to make much of difference in the forecast. Therefore, we
are confident in the forecast through the next few days.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Southerly
winds just above the airfield should diminish enough early this
morning to eliminate low level wind shear.

&&

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

A strong low south of Kodiak Island will merge with the low near
Dutch Harbor this afternoon bringing the new low center over the
Alaska Peninsula. The front associated with the Kodiak low is what
will have the greatest impact on Southcentral Alaska and the Gulf
region today through tonight.

There is a bit of warm air associated with this front which will
keep precipitation all rain at lower elevations along the gulf
coast. However, the low center pulling back to the southwest will
keep this warmer air from penetrating too far inland. This will
also allow cooler air from inland to change precipitation to snow
over the Kenai and Chugach Mountains. One of the bigger challenges
for this forecast package was to determine the elevation at which
the precipitation should go from rain to snow and if that will be
low enough to bring the snow to the Sterling Highway in Turnagain
Pass. At this time it looks like it should be mostly rain into
this evening with any snow melting on contact with the ground.
However, precipitation may quickly turn to snow overnight through
Turnagain Pass.

By Monday, the merged low over the Alaska Peninsula from this
afternoon will have weakened as it moves into the Gulf of Alaska.
The front will also be much weaker, though it will remain along
the north gulf coast. It looks unlikely for any significant
precipitation to make it across the coastal mountains into inland
areas over the next two days.

&&

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

Active weather continues over Southwest Alaska today as a front
associated with a complex low moves in from the southeast. Gusty
southeasterly winds and rain are expected across the Alaska
Peninsula today. Farther north over the Kuskokwim Delta,
temperatures will be cold enough for snow. Current thinking is
that as the front passes through later this afternoon, trace
amounts of snowfall are possible for the Kuskokwim Delta Coast. As
the low drifts south Sunday night into Monday, winds will weaken
and precipitation will taper off. Some scattered showers of mixed
precipitation are possible Monday night into Tuesday over the
interior areas of Bristol Bay, otherwise general offshore flow is
expected to persist through Tuesday ahead of another front that
moves in from the west by Tuesday evening.

&&

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

A 970 mb low is still centered just north of Unalaska Island. It
is starting to drift southward, and will eventually phase with a
second low (located south of Kodiak Island and moving northward)
late this morning. The biggest forecast update made overnight was
to keep the boundary of strong westerly winds just west of Dutch
Harbor. It has been a challenge to pinpoint exactly where the low
center will track as models have struggled to resolve this.

This newly phased low will then move over the AKPEN and
eventually into the Gulf, but not before generating some gusty
northerly flow over the central and eastern Aleutians this
afternoon. Temperatures will generally be warm enough for rain,
though a brief change to snow cannot be ruled out over the central
Aleutians later today as colder air gets pulled in from the
north. The next storm in the Bering arrives over the western
Aleutians this afternoon, spreading in another round of southerly
gales over the central Aleutians Mon night into Tue morning. As
the low center tracks eastward, reaching the central Aleutians by
Tue morning, colder northerly flow on the backside will bring
stronger gusts to the western Aleutians for Tuesday.

&&

.MARINE (Days 3 through 5)...
The storm track during this period will be from the Aleutian
Islands eastward to the Gulf of Alaska. High end small craft
advisories to gale warnings are expected as this low pressure
system moves through. Additionally, expect light to heavy freezing
spray from Hooper Bay to Nunivak Island.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The global models are in decent agreement with the longwave
pattern for north Pacific and Bering through Friday December
20th. At the onset of this forecast period there are two lows,
one near Kodiak and a Kamchatka low. There is a ridge axis
between the two systems. There are some minor differences with
the timing of two different surface lows but the forecast
intensity is only a few millibars off and the forecast track is
very similar. Overall, the run to run continuity with these two
lows has been decent. Expect these systems to move from west to
east with the first storm tracking into the Gulf of Alaska and
later towards SE AK. All the while, the second storm will head
into the Bering with the frontal boundary pushing across the
Aleutian chain. Again, for the Anchorage metro area the chances of
snow in this forecast period range from zero to slight chance.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...STORM: 119 120 130 131
         GALE:  125 132 136-139 141 155 160 165 170 172-178
                351 352 411 413
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...PJS