Small Craft Advisory 1005, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 AM AKST Sat Dec 30 2017

.ANALYSIS AND UPPER LEVELS...

The upper level pattern is changing for Southern Alaska. A trough
over the Bering and vertically-stacked low over the Aleutians is
digging southward into the Pacific Jet which is largely south of
45 north. This is causing the Jet's eastward extension to bend
northward towards the Gulf of Alaska and Southcentral. Building
upper level ridging along the west coast of North America in
response will enhance the developing southerly upper level flow.

Benign weather conditions continue for much of Southcentral with
seasonably cold temperatures. Snow showers are impacting the north
Gulf Coast around Seward, the western side of Cook Inlet and the
western Susitna Valley, with lighter flurries over the western
Kenai. Cold air is entrenched across the Copper River Basin with
temperatures colder than 20 below. The winds are picking up
through portions of Southwest Alaska, keeping temperatures warmer
through that area with increasing cloud cover ahead of the next
system. Rain continues across the Aleutians and the Alaska
Peninsula in association with the low over the central Aleutians
with snow being reported in the Pribilofs with temperatures
hovering around the freezing mark.

&&

.MODEL DISCUSSION...

The models are all in good agreement through the next couple days
on the broad synoptic scale pattern. As usual, the timing and
strength of fronts and smaller features remains the point where
there is more disagreement. However, those details can have large
implications on the forecast. For this model cycle, there are some
position differences with a wave tracking northwestward into the
Alaska Peninsula tonight, then progressing along the coast during
the day Sunday. The models are in close enough agreement tracking
the next strong low that moves into Bristol Bay by Monday evening
that a simple ensemble approach was used for that system. Forecast
confidence is average.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Low level wind
shear will pick up by midday as a front turns the winds out of the
southeast aloft, while surface winds remain out of the north. The
wind shear increases further late tonight through Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong low moving north toward the Alaska Peninsula today
combined with a leading weather front heading toward the Gulf will
signal a change to a more active weather pattern. Gales to storm
force winds will spread north across the Gulf marine areas
through Sunday. Inland areas will see increasing gap flows this
afternoon in favored locals (Turnagain Arm, Portage Valley, Mat
Valley). Winds in these areas will continue to strengthen through
today before peaking Sunday morning. The strongest of these gap
winds will be along Turnagain Arm and Portage Valley where winds
will exceed 60 mph tonight through early Sunday afternoon. Snow
will intensify across the eastern Kenai Peninsula tonight into
Sunday afternoon. Combined with strong winds it appears that
blizzard conditions are possible from Portage Valley through
Turnagain Pass. The challenge for this area on Sunday, does the
air warm enough for the snow to change over to rain or a mix in
Portage Valley. Either way, higher elevations of the Seward
Highway should remain mostly snow through Sunday.

Elsewhere, snow will likely mix with rain along the Coasts with
downsloping limiting snow west of the Chugach Range. The
exception would be upslope enhanced snow along the Alaska/Aleutian
Ranges over western Cook Inlet/Susitna Valley. Kodiak will see
moderate rain at times today and tonight until the front passes
to the north Sunday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An approaching front will bring a mixed bag of precipitation to
the mainland today. Winds will be the constant for most of the
area, strongest through Greater Bristol Bay. These easterly winds
will also bring temperatures into the 30s and even some low 40s.
Most areas along the Alaska/Aleutian ranges will be downsloped and
dry while areas along the Kuskokwim Mountains will see upslope
precipitation, with snow expected this morning turning to rain in
the afternoon. Expect wind gusts into the 40 to 50 mph range. The
front passes early Sunday morning followed by subsiding winds.

The forecast is a little trickier for the Kuskokwim Delta. The
mountains will block most of the warm air push, though areas in
the lee of the mountains will approach the freezing mark. The
front will bring accumulating snow to areas west of Bethel along
with 20 to 30 mph winds which will reduce visibility in blowing
snow, mainly this afternoon and evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The main frontal boundary is moving from the Alaska Peninsula
toward the mainland today with a portion arcing toward the
Pribilof Islands who will have a little accumulating snow. The
majority of the western and central Bering Sea will see a long
period of instability driven snow showers for the next couple
days. Another front and low pressure system approaches the Alaska
Peninsula on late Sunday night/early Monday morning.

&&

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

The long term forecast period begins with New Year's Eve, which
will see a low over the Aleutians and a front draped from
Southwest AK across the Kenai Peninsula and into the Gulf.
Although this isn't an unusually strong system, it does bring warm
air to the state, which will start our transition to a more
active pattern. The warming temperatures aloft should bring mixed
precip to Southwest AK, changing over to rain in the lee of the
mountains. For Southcentral, this system looks like it will be
primarily snow and possibly some quite heavy snow over the
mountains on the Kenai Peninsula. Precipitation will remain mostly
in the mountains, so not looking for a snow event for most
communities along the road system.

The next system approaches on a very similar track beginning
Tuesday, bringing stronger winds and warmer air. Precipitation
will most likely fall as rain for most of the forecast area,
including Southwest AK. In Southcentral, the current track and
strength of this system has the possibility to bring a Chinook
wind event, which would enhance our already warm temperatures for
an early "January thaw". The first chance for a return to cooler
temperatures begins on Wednesday, when the low in the Gulf slides
northward, bringing cooler temperatures on the west side
(backside) of the low. However, with recent model performance,
confidence in the 7-day forecast is low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 125.
Winter Weather Advisory 155.
MARINE...Heavy Freezing Spray: 160 180 181 185.
Storm: 120 130 131 132 138 351.
Gale: 119 125 136 137 139 140 141 150 160 165 177 179 180 181 185
352 411 412 413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ML
LONG TERM...LF