Small Craft Advisory 247, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKDT Fri Mar 23 2018

.ANALYSIS AND UPPER LEVELS...
The large upper level ridge remains over Southcentral Alaska and
eastward into the Yukon Territory with the main upper level low
center to the northwest of the Gulf of Anadyr in eastern Russia. A
weakening front is slowly moving through Southwest Mainland
Alaska today but will mostly dissipate as it crosses the Alaska
and Aleutian Ranges. Colder air is evident in satellite imagery
over the central to western Bering Sea from the presence of the
cumulus clouds and showers in that area. Most of this colder air
will remain well west of mainland Alaska as a low in the western
Aleutians moves to the northeast and cuts off the colder air.

&&

.MODEL DISCUSSION...
Models are in good agreement through the first day-and-a-half but
start to significantly diverge after this in the eastern Bering
Sea. The ECMWF has been used for the afternoon package its its
track and speed make the most sense. The GFS continues a trend of
being too strong and too far north and the GEM is significantly
farther south. Model differences over the Gulf and Southcentral
Alaska are much less than in the eastern Bering and Southwest
mainland Alaska.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. The though of
stratus overnight was considered but with the dry windy past few
days and the subsidence inversion weakening, it is more likely
that there will not be any fog or stratus at PANC overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sat and
Sun)...
A weak front is stretching out over Kodiak Island as it makes a
slow trek towards the southern Kenai. There is weak upper level
elongated energy associated with this feature. This will bring
some mid and high level clouds to Southcentral in the overnight
hours. There is some uncertainty regarding the potential of some
low cloud development across the Knik Arm/Northern Cook Inlet
early Sat AM. The afore mentioned energy and high clouds will
likely keep much stratus from developing, but can't rule it out
completely as some moisture backs up along the Chugach Mountains
for a brief period in the morning.

As a surface low develops along the decaying front overnight, this
will bring another period of offshore flow to the usual places
(Whittier, Thompson Pass, Seward). The resulting winds will not be
anywhere near as strong as earlier this week. The gradient will
favor more of a northwest (instead of north) direction, so we
expect the strongest winds through the Barren Islands. These winds
will be short lived in the morning as the low pulls away quickly
and a weak upper-level ridge slides in.

A parent low over the Eastern Bering will sling yet another front
towards Kodiak Island early Sun. This feature is quite a bit
stronger than the afore mentioned front and will have much more
moisture to work with. Thus, have kept PoPs quite high from Kodiak
up through the Gulf Coast/Prince William Sound. The front slows as
it slides across the Gulf. Just as it seems about ready to stall
out, another piece of energy comes along from the west and
reinvigorates it. This leads to a triple- point low forming along
the front over the northwestern Gulf late Sun. As this low tracks
through the Northern Gulf, it will produce an increase in easterly
flow across into Prince William Sound and through the Turnagain
Arm. If this tracks holds true, it should allow enough cold air to
be pulled from N to S across Southern AK. This should result in
most of the precipitation for the Gulf area falling as snow. Not
expecting anything extraordinary at this point, but this looks to
produce a few inches of wet snow in Seward and Whittier. For
inland areas (Anchorage, Palmer), it does not look favorable for
much (if any) precipitation through the weekend with robust
cross-barrier flow.

&&

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

The weather front that came onshore yesterday is falling apart
over the lower Kuskokwim Valley, with fairly light precip falling
around Sleetmute but coming to an end in the next few hours.
Meanwhile, a break in the weather is moving in from the Bering
with clearing skies over the Delta and Bristol Bay zones. This
will likely lead to some fog development again tonight across the
area, with thickest/most widespread fog likely up in the lower
Kuskokwim Valley where the snow occurred most recently. Saturday
will be a nice day overall across the area, before east/southeast
winds start to pick up Saturday night with clouds and rain/snow
showers approaching the area by Sunday morning. This next storm
doesn't look like a big impact for the southwest Mainland as the
low center itself stays out to see and drifts into the northern
Bering by the end of the weekend. This should result in cloudy and
showers conditions for the start of the week with temperatures
near the freezing mark across much of the area.

&&

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

Clouds are already building into the area in advance of the next
storm that is currently approaching the western Aleutians. Rain
and gusty (Gale-force) winds will impact the Central Aleutians
tonight as the low moves into the Bering. This low will largely
stay at the same strength as it moves to near the Pribilof Islands
Saturday night and then starts to drift northward and weaken for
the start of next week. This storm isn't going to have a very
strong or organized front associated with it, so the heaviest
precipitation and strongest (gale-force) winds will be confined to
the immediate vicinity of the low pressure center.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Extended forecast for Sunday and Monday remains on track, with a
low in the Bering Sea trailing a front through Southwest Alaska
and then developing a triple point low south of the Aleutian chain
and moving it onshore in the Gulf Coast. This system still looks
like it will weaken rapidly as it moves northward, dumping most of
the precipitation along the Gulf Coast. After this system moves
through, uncertainty in the model solutions remains high, as the
run-to-run consistency has been poor. Yesterday's ridge on
Wednesday is now a strengthening low, with varying positions and
speeds in the Gulf of Alaska, depending on which model solution
you choose. General track currently pushes this next low to the
northeast by Thursday, but where the low moves onshore will have
much effect on how much precipitation falls along the Gulf Coast
and when. Currently, EC is the fastest and furthest west
solution, with the Canadian Regional taking the Red Lantern award
as it pushes the low into the Panhandle, and the GFS solution
lying somewhere in the middle. Meanwhile, all three models show a
ridge building in the Bering Sea, which is consistent with
yesterday's forecast. For Wed through Fri: high confidence in
improving weather for Southwest, the AK Pen and Aleutians, with
moderate confidence that a low will affect the eastern Kenai and
Prince William Sound. Amounts and locations of precipitation east
of the Kenai Peninsula are still highly uncertain for the end of
the week.&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 130 131 177 413.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AD
LONG TERM...LF