Small Craft Advisory 227, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKDT Sat Mar 17 2018

.ANALYSIS AND UPPER LEVELS...

 There are two main weather features on the map today. The first
is a weather front draped across the eastern Bering associated
with a low back near Russia. Meanwhile, a shortwave ridge of high
pressure is evident over southcentral Alaska from this morning,
which caused widespread high clouds stretching from Kodiak to the
Copper River Basin. Underneath this ridge, widespread dense fog
developed around upper Cook Inlet. Surface winds are switching
around to the south across coastal sections of Southwest Alaska
ahead of the approaching front. Snow and rain showers are
occurring across the area, and will be on the increase this
evening as the front approaches the area. Way out west,
stratocumulus clouds over the western 2/3 of the Bering are
evidence of a decent shot of cold air working in behind the front.
More evidence: it's snowing at Shemya (western Aleutians), with
gusty southwest winds approaching 50 mph.

&&

.MODEL DISCUSSION...

The models remain in good overall agreement over the next few
days, and all depict the front approaching the southwest getting
reinvigorated on Sunday and surging eastward into the Cook Inlet
area as a cold front. The GEM-Global is hanging onto it's slightly
faster track with this storm, while the ECMWF and GFS are both
hanging onto their slower track. The NAM had previously trended
toward the slower solution, but is now back to the faster
solution. We still prefer the run-to-run consistency of the GFS/EC
solution, and will be making only minor tweaks to the forecast
today. If the faster solution were to verify, it could lead to a
longer snow event in the Susitna Valley Sunday/Monday, with higher
snow totals. At this point that seems unlikely, but stay tuned to
future forecast updates.

&&

.AVIATION...
PANC...A slow improvement trend should continue through early
this evening at which time low-level winds reverse and start to
come out of the south. It is possible that the leftovers of these
low clouds will come back in tonight. There is some reason to
think visibilities may stay up a bit tonight as inversions may not
be quite as strong due to the presence of mid level clouds.
However, fog can't be totally ruled out through Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An area of high pressure that was centered over Southcentral
during the day today will continue to move off to the east. A weak
shortwave skirts through the region tonight bringing increased
cloud cover and a few snow showers, which should be mostly
confined to the Northern Susitna Valley. Due to cloud cover and
unfavorable dynamics, widespread dense fog is not expected to
develop across the Cook Inlet region tonight and tomorrow morning.

The next system approaches the area tomorrow afternoon as a
longwave trough digs south over the Eastern Bering sending a front
across SW Alaska and the Alaska Range. There has been some
uncertainty regarding timing of this system but recent model runs
have come into better agreement, thus we are more confident in a
late Sunday night/early Monday morning arrival. Strong southwest
flow aloft will again favor the Susitna Valley/Hatcher Pass area
for precipitation. A push of warm air ahead of the system will
mean rain will mix with snow up to the Southern Susitna Valley,
though precipitation should remain mostly snow for Talkeetna
north, with 4-6" of wet snow expected along the Parks Highway,
with up to a foot of snow expected in the Hatcher Pass area
through Monday afternoon. For the Anchorage and Matanuska Valley,
temperatures are expected to keep precipitation mixed, though may
briefly change over to all snow around the morning rush hour time
frame as temperatures fall associated with a cold front passage.

As the front pushes east across the Copper River Basin Monday
afternoon, high pressure will begin to build across the western
part of the state bringing dry and cold offshore northerly flow.
The resultant pressure gradient increase will lead to the
development of a strong offshore wind event along the coastal
areas and into the Gulf of Alaska Monday evening through the
middle of the week.

&&

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

A front associated with a low near the Kamchatka peninsula will
move onshore on Sunday, bringing stratiform precip to most of
Southwest Alaska. This front has a relatively warm profile aloft,
so precip type has been the forecast challenge for Southwest AK
today. Current thinking is that the warm air will mix to the
surface and bring rain in the Bristol Bay region, but precip will remain
a wintry mix for the Kuskokwim Delta and Kuskokwim Valley.
Although the air mass moving through is a bit more moist than
models initially indicated, precip amounts are not expected to be
significant and the mixed precip type will limit snowfall
amounts. After this front moves through, the parent low will
transition eastward and this will allow colder air to move
overhead, bringing more seasonable temperatures to the region on
Monday. Skies should clear overnight on Sunday, but with unstable
air aloft and moderate surface winds, extensive areas of fog are
not expected.

&&

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

As the low near the Kamchatka peninsula shifts northward and then
eastward over the next couple of days, west to southwesterly flow
will move across the Aleutians and Bering Sea. Gale force winds
are currently blowing through the Western Aleutians, and these
are expected to move northward and diminish with the weakening of
the parent low. The front currently crossing the Central Aleutians
will move eastward and onshore Sunday night, bringing more showery
conditions to the entire region. Beginning Monday, a pattern shift
is in play for the Aleutian Chain with high pressure moving in.
Some patchy fog is expected under the main core of the high, but
this should mostly remain to the south, with minimal gale force
westerly winds dominating the Bering Sea.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The mid/long term forecast begins Tuesday with a strong upper
level ridge that is building across the Bering. East of that is an
upper level trough that is digging south of the panhandle. These
two features will be dominating the weather pattern, and become
the source of model contention as we head into the middle of the
week.

As the ridge builds to the west, strong northerly winds will
build across most of Alaska, keeping temperatures down and any
precipitation at bay. As we move closer to the weekend, guidance
really begins to diverge from any real consensus at the lower
levels. Both models wrap shortwave energy around a parent low in
the northern Bering Sea. The GFS is stronger than the EC and with
the energy and attempts to break down the ridge by Friday. The EC
on the other hand, is strong with the energy but does not cut
through the ridge, rather it dives south. In reality, this is
status quo for the GFS and the long term forecast will be shying
away from that output. We will trend closer to the ensemble output
from the EC. This trend weakens the ridge by this weekend as a
low undercuts the ridge and moves into the Gulf.

The primary takeaway of the extended forecast is that we will
undergo two pattern changes through next weekend. The first change
is as we move into the early week, and the shift to colder
temperatures with northerly winds. The second is as we move
towards the end of next week and into the weekend. The pattern
looks to shift to more of an active storm track, bringing
temperatures and chances for precipitation back up.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 174 176 178 185.

&&

SYNOPSIS AND MODEL DISCUSSION...AMD
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LF
LONG TERM...SS