Small Craft Advisory 23, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 AM AKST Sun Jan 7 2018

.ANALYSIS AND UPPER LEVELS...

The upper level pattern consists of a departing upper level low
over the northwestern Bering moving into Siberia, which is soon to
be replaced by a deepening upper level low moving into the central
and western Aleutians. Brief ridging is also pushing eastward
ahead of the second upper low. Meanwhile over mainland Alaska, the
upper level pattern is in very weak troughing along the coast,
with a building, but as yet weak upper level ridge moving
southward out of the Arctic. The weak and ill-defined flow through
much of southern Alaska is keeping the weather largely the same.
It is characterized by lots of cloud cover, weak surface
troughing along the Gulf occasionally producing a few showers, and
otherwise low stratus and fog issues. The low stratus and fog is
the primary forecast challenge this morning. Temperatures in this
pattern are around average for highs and a bit above average for
lows as the cloud cover largely keeps any heat from radiating out,
but the low sun angle and clouds prevent much daytime heating. The
Anchorage area is maintaining a robust atmospheric inversion set
up just above the surface, allowing temperatures in the Bowl to
drop to around 20, while at higher elevations, temperatures are
closer to 30.

&&

.MODEL DISCUSSION...

The models remain in very good agreement through the next few
days. The pattern remains one of two extremes: Either very little
is going on synoptically, such as over much of mainland Alaska,
or conversely, a powerful low moves into the Aleutians by Tuesday,
which is very easy for the models to track as it covers such a
large area of real estate. Thus, the primary forecast challenges
again lie with local effects, namely the strength of the upcoming
gap winds as a truly Arctic air mass moves into the Copper River
Basin, and also even shorter term dealings with low stratus and
fog in a very weak flow pattern. Forecast confidence is high. With
its high resolution and good agreement with the other models, the
NAM was the model of choice for this morning's forecast package.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
day. The potential for fog and low ceilings is increasing from
midnight tonight onward, which would lower conditions to IFR if it
develops.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The weakening front along the north Gulf coast will dissipate
later today as high pressure builds in from the north. The high
pressure will bring cold and dry air into the area from the north.
Some fog is possible in the Cook Inlet area with steep inversions
and fairly light winds. The surface offshore gradient will begin
to increase on Monday, becoming fairly strong Monday night. With
northeast flow fairly deep through the atmosphere, conditions are
favorable for winds to blow through the Matanuska Valley. The
Valdez area should also see some decent breezes. Colder
temperatures will also begin to work their way into the northern
portions of the area, especially the Copper River Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Tuesday)

A baggy area of low pressure stretches from the Alaska Peninsula
across the Alaska Range early this morning, and it's responsible
for shower development across much of Southwest Alaska as just
enough forcing combined with upslope flow has been efficient for
shower development south of a Dillingham to Port Alsworth line.
This looks to continue through the afternoon hours before a
transient short-wave ridge builds northward across the region for
tonight. With no real increase in 850 mb temps, we don't think fog
will be too much of an issue. The exception to this will be the
typical fog-prone river valleys and deltas, as light winds and
clearing skies will offer some potential for this. However,
confidence in areal coverage looked sufficiently low to preclude
mention of this in the zone forecasts.

With the clearing skies and a tightening pressure gradient across
the northern portions of the lower Kuskokwim Delta, colder air
will drain into the area from the north, helping to cool
temperatures for the Monday into Tuesday time frame. Our attention
then turns to an occluded front approaching the region from the
southwest during the day Tuesday, with precipitation developing
along and immediately ahead of it.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days through 3)...
(Today through Tuesday)

A strengthening area of low pressure was located a few hundred
miles south of Shemya this morning, with the latest observations
placing the strength near 960 mb. This storm will deepen another
10 mb or so as it treks towards Adak tonight. This system is
pretty similar to the storm that impacted the region during the
last few days. Overall, look for a strongly occluded front to
sweep eastward across the chain, while the parent low stalls.
Widespread precipitation, some of which will be moderate to heavy
in intensity will accompany the front, with more showery type
precipitation behind it and closer to the area of low pressure.

Winds will rapidly increase along and ahead of the front. For the
land zones, current indications are these winds will remain below
headline criteria. For the coastal and offshore waters, storm
force winds are likely for west central portions of the Aleutian
waters and into the southern Bering Sea, with winds increasing to
gales elsewhere along the front and near the low.

&&

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

The long range forecast beginning on Monday night through the end
of the work week generally keeps drier and colder conditions
across the southern mainland with an active windy and
precipitation pattern along the southern Bering and Gulf of
Alaska. On Monday night, an Arctic ridge shifts south across the
interior advecting a cold dense air mass toward the northern
Gulf. Dry offshore flow increases late Monday night through
Tuesday bringing gusty winds across typical gap locations.
Meanwhile, a powerful low pressure system slowly weakens over the
Bering.

Wednesday through the end of the week...a slow transition in the
pattern begins as the subtropical jet flattens out with strong
zonal flow setting up just south of the Aleutian Chain. The jet
sends the next North Pacific low further to the east than
previous systems, before taking a northward turn. This low works
against the ridge to break it down. There is still a bit of
uncertainty on how quickly the ridge influence will be overridden
by low pressure. A nearly equal blend of operational models
including their ensembles was used for updating the forecast mid
week, then trends toward ensemble solutions for the latter part of
the week. Temperatures should moderate slightly warmer at the end
of the week for the southern mainland with continued active
weather across area waters and coastal communities.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm: 173 174 175 176 177 178 413.
Gale: 155 165 170 171 172 179 180 181 185 411 412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KH