Small Craft Advisory 6, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKST Tue Jan 2 2018

.ANALYSIS AND UPPER LEVELS...

A strong area of low pressure is moving into Southern Alaska this
morning from the Pacific. Many areas are reporting precipitation
in association with the low. Generally the closer to the coast
one is, the more likely one is getting rain, though as warmer air
moves inland, more and more inland locations, including Anchorage
and the Matanuska Valley, have potential of getting some rain. The
low, centered near Bristol Bay after crossing the Alaska Peninsula
earlier last night, has essentially exhausted its supply of warm
air, and temperatures aloft are already starting to cool. For
Dillingham and points west of the low, cold air has remained in
place keeping surface temperatures below freezing, and well below
freezing over the Kuskokwim Delta. Meanwhile southeast winds,
downsloping, and warm advection are seeing many areas of
Southcentral above freezing this morning. Anchorage, Palmer, and
much of the western Kenai Peninsula are already up into the 40s
this morning. These temperatures are comparable to predawn
conditions around Tampa, Florida!

The upper level disturbance that invigorated the low moving into
mainland southern Alaska has become vertically stacked with the
surface low, and the upper level feature is rapidly becoming
absorbed into the broad upper level low in place over the entire
Bering. Thus, that broad trough remains the dominant steering for
each of the lows impacting us in Southern Alaska, with the jet
just clipping the far eastern Copper River Basin.

&&

.MODEL DISCUSSION...

As expected, the models are handling the big low currently
impacting Southcentral well. Behind the low, most of the models
are getting a better handle on a much weaker low tracking
northward across the Gulf Tuesday night, with a small area of high
pressure following that through the end of the short-term period
Wednesday night. However, the next strong low out over the Bering
involves considerably more uncertainty, as in typical fashion, the
parent low occludes over the western Bering, and triple-point low
development occurs along the occluded front further east across
the Aleutians. There is very little agreement at all on where the
secondary triple point low develops. Each model has its own
solution. As such there remains high uncertainty and low forecast
confidence by that point on Wednesday. However, the models do have
general broad agreement on the speed and position of the occluded
front itself. Thus, forecast confidence is average to above
average over Southcentral, with less confidence out west over the
Bering. The higher-resolution NAM was the model of choice at
least until some better agreement can be obtained with future
model runs.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty southeast winds with the
Turnagain Arm Jet will persist through the morning, then gradually
diminish in intensity this afternoon and tonight. There's a chance
for some passing snow showers early Wednesday morning.

&&

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

A weakening low is moving north across Southwest Alaska and its
associated weather front is lifting  rapidly north across
Southcentral. Many areas across Southcentral will be windy and
unseasonably warm with temperatures climbing in the mid 30s to the
low to mid 40s. Temperatures will cool off tonight and will trend
much lower on Wednesday as winds diminish and weak cold advection
spreads in from the west. Strong pressure rises behind the front
will continue brisk to strong winds (mostly southerly) across many
areas, especially favored gaps. Winds diminish significantly this
evening as pressure gradients relax. The brunt of the
precipitation will fall along the coasts as rain with mixed or
snow at higher elevations. Precipitation types inland will be more
challenging today with increasing freezing levels and some pockets
of cold air. That said expect a heavy lean toward rain at lower
elevations and snow to mix at higher elevations, Copper River
Basin, and northern Susitna Valley. Cooler air will bring a tilt
back toward snow tonight/Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
Southwest AK is divided into two air masses this morning. From
King Salmon along the Aleutian Range up through Iliamna,
temperatures remain quite mild, especially for early January.
However to the west, from Dillingham up through the YK-Delta,
temperatures range from just below freezing to the low teens. This
means that precipitation to the east is mostly falling as rain or
a rain/snow mix. Precipitation along the west coast and for
interior areas is generally falling as snow. There remains a
chance to pick up a few additional inches of snow this morning
with the best shot near or along the eastern flank of the
Kuskokwim Mountains. The good news is (for winter lovers at least)
cold air is starting to work back in from the west. This will
continue through the day as the low pressure system (currently
overhead) moves off to the north and east.

Wednesday will bring a return to much quieter conditions with
temperatures falling closer to normal. Snow showers will taper off
from west to east, lingering along the AK Range through much of
the day. As weak ridging builds in with some lingering moisture,
we can't rule out the chance for some fog to develop. Another warm
front will fall apart as it approaches the area from the Bering on
Thu.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue and
Wed)...
The Bering is about to move into a very active "triple-point" low
pattern. But first, conditions today will remain similar to the
past few days. One warm surface low pressure center will slide
quickly to the north over the Eastern Bering today. This will
bring some wrap-around westerly low end small-craft winds. A
secondary weak circulation over the Central Bering will allow west
to northwest flow to continue to usher cold air over the Central
and Western Aleutians. This will keep some gusty gale-force winds
and moderate snow showers in the forecast.

By Wed, the first in the series of developing lows will slide into
the Central Aleutians. The leading warm front associated with this
system will pack a large swath of gale-force winds and moderate to
heavy precipitation. The cold air currently in place will allow
precipitation to start out as snow with the chance for some
freezing spray over the marine zones. However, this will be
short-lived as warm air behind the front pushes 850 mb
temperatures to near or above freezing by Wed evening. This front
will drift to the northeast as it falls apart over the Central
Bering into early Thu. A stronger low pressure system will take
its place.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning Wednesday afternoon remains
progressive, as low pressure over the southern Mainland gives way
to an amplifying ridge building in from the west on Thursday. In
the meantime, a strong upper low will develop over much of the
Bering sea Thursday into Friday, then begin to push the above
mentioned ridge over the pan-handle and western Canada by the
start of the weekend. As the ridge builds into the southern
Mainland Wednesday into Thursday, cooler and drier conditions will
build back over the area under light offshore/northerly flow.

There is still some uncertainty regarding the finer details of the
complex north Pacific low and associated front tracking through
the Aleutians and into the Bering Wednesday into Thursday. This
is also true for the potent triple point low strengthening over
the north Pacific Thursday, then tracking through the Central and
Eastern Aleutians/Bering Friday. The associated front will reach
Southwest Alaska Friday morning and push eastward over much of the
southern Mainland by Saturday. Overall, this system will usher in
another round of wet and windy conditions for the much of the
area, as well as some warming temperatures ahead of the front as
the system progresses eastward.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale: 119 120 121 125 129 130 131 132 138 139 141 150 174
175 176 177 178 351.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...TP/DEK