Small Craft Advisory 72, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
433 PM AKST Tue Jan 22 2019

.ANALYSIS AND UPPER LEVELS...
Ridging remains entrenched over eastern portions of the state
extending from southeast Alaska through the northwestern part of
the state. There is also a low near the central Aleutians slowly
tracking to the northwest with its front stretches over southwest
Alaska and Kodiak Island on its way to the Kenai Peninsula. This
setup has resulted in southerly flow for most locations that is
pulling up an abundance of warm air resulting in many locations
switching from snow to rain.

&&

.MODEL DISCUSSION...
The models are in decent synoptic agreement until Wednesday
afternoon when they struggle to track a new system approaching the
Aleutians from the south. They are many differences in how
multiple lows will develop along the front with this system.
However, there is still some agreement with timing of the front.
On Thursday models are having even greater difficulty with
position and movement of multiply lows in the region. The worst is
with a low approaching the Alaska Peninsula Thursday afternoon.
There is a general consensus of it approaching the peninsula, but
timing, intensity and position are all off.

&&

.AVIATION...
PANC...VFR conditions and light surface winds will persist.
Turnagain Arm winds look to increase this evening but should not
move in over the city at the surface. However, some of the
Turnagain Arm winds will move in overhead resulting in windshear
conditions.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main forecast challenges over the next 48 hours will be
precipitation type coverage as temperatures warm, and increasing
wind speeds for the Turnagain Arm and Upper Hillside. We've
already received a report of icy road conditions this afternoon at
Anchor Point as the initial precipitation ahead of an approaching
front moved into the area. This front will reach the Kenai
Peninsula tonight before and then lift into the Mat-Su
Valleys/Anchorage Bowl by early afternoon. Although the front will
be weakening somewhat as frontolysis takes hold (the weakening of
a front), the pressure gradient will still be sufficient to cause
an increase in winds for Turnagain Arm into the Upper Hillside.
Here, winds could reach the 50 to 70 mph range. With strong
downsloping in the typical rain shadow areas, look for mainly
spotty flurries/drizzle with some brief increase in coverage as
frontal passage occurs.

Our attention then turns to a much stronger front that will cross
the region for the Wednesday evening through Thursday morning
time frame, as a strong low lifts into the Bering Sea. The front
will be in the occlusion process with very strong upper-level
support as a lobe of vorticity (this brings increasing lift with a
front helping to enhance precipitation potential, while also
having strong downward mixing potential) crosses the region.
Strong dry advection behind frontal passage will likely cause some
gusty wind conditions with increasing temperatures as well.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A complicated weather pattern exists for southwest Alaska. Low
pressure in the Bering Sea is slowly moving northward, with
several waves of energy circulating around it. This is making for
an active weather pattern and timing out these pulses of energy
will be the forecast challenge for the next few days. Models are
not handling the timing of these waves either, making the forecast
that much more challenging. Past Wednesday, the forecast
confidence is low due to poor model quality and the complexity of
the situation.

The next pulse of energy will move through tonight bringing rain
and snow with it. Rain and mixed precipitation will be present on
the lee side of the mountains where temperatures have warmed
enough. Snow will be elsewhere as low level northeasterly flow
will keep temperatures cool enough for snow. Winds will quickly
shift southeast tomorrow, bringing in warmer temperatures and
transitioning some areas to a rain or rain/snow mix in the
afternoon on the tail end of this next wave. The following wave is
expected to arrive early Thursday, but will need some more detail
on that system with model data performing poorly. Overall, a
complicated forecast the next few days so stay tuned for updates.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Low pressure continues to slowly move northward through the
Central Bering Sea. This low has several waves of weather
circulating around it, making for an active weather pattern over
the entire area. Timing out these waves of energy will be the
forecast challenge for the next few days. Models are not handling
this pattern well, with timing issues with each of these waves
coming through which adds to the complexity and challenge of
figuring out what is going on. This also means forecast confidence
is low past Wednesday.

Where we do have some confidence is in the first 48hrs. The
initial wave passed through yesterday with the second passing
earlier today. A third wave is approaching the eastern Aleutians
now and will move through tonight. It will bring rain showers and
gusty winds with it. Models did agree on some stronger winds in
the Bristol Bay area, so upped winds to gale force level there.
The following round of energy can be seen on satellite in the
North Pacific and is expected to pass through the Eastern Aleutians
tomorrow night and will be slightly weaker. Yet another round of
energy is expected Thursday for the eastern Bering, but this is
where confidence gets low so will wait to detail this to when it
gets closer in time.

Meanwhile in the Western Bering and Western Aleutians northerly
flow will be present on the backside of the low. This means cooler
temperatures and snow showers. This cooler air is also getting
entrained into the low and wrapping around it, making for a messy
precipitation type profile around the low, with a mix of rain and
snow around the low in the Central Bering. All in all, a very
complicated forecast for the next few days, so stay tuned.

&&

.MARINE (Days 3 through 5)...
An active and challenging pattern will continue through the
weekend. Thursday and Friday will see a strong low lift across the
AKPEN bringing high end gales, with the possibility of storm force
winds. Model guidance is still uncertain and is keeping confidence
levels moderate on the track and timing, but high on intensity. On
the heels of that system is one that will move into the Bering
this weekend. The current track slides a deepening low through
Adak and towards the Pribs. High end gales are expected. Storm
force winds, and possibly higher on Sunday through the central
Aleutians are not out of the question.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The longwave pattern isn't changing much through the weekend.
Ridging across the Gulf of Alaska will slide east and weaken as a
deepening trough moves into the western Bering. That signal
spells continued active weather for much of the extended forecast.
The first system will slam into the AKPEN Friday and into this
weekend. A second system will track into the Bering and both will
bring strong winds, warm air and unseasonal conditions. Above
freezing temperatures are expected for south central and coastal
locations, while some inland areas may cool below freezing
overnight. The return to winter (temperatures) is looking to occur
early into the next work week, though a shunt to warm may be on
it's heels as well.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 120 13>132 136 137 150 155 160 178 185.
 Heavy Freezing Spray 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...SS