Small Craft Advisory 190, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Fri Mar 2 2018

.ANALYSIS AND UPPER LEVELS...
The progressive upper level pattern continues with the upper level
ridge moving out of Alaska into the Yukon while an upper level
trough in the Bering Sea approaches the southwest mainland Alaska
coast. A surface low south of the Alaska Peninsula is pushing
moisture into the Bristol Bay region. This low will get caught up
in the westerly flow aloft and not progress any closer to the
forecast area.

&&

.MODEL DISCUSSION...
Model situation is very similar to yesterday where the models
remain in good synoptic agreement through the weekend and there
are not concerning differences until around Tuesday of next week.
One feature in the short term that is of note is the sharp trough
that will move across the Kuskokwim Valley early tomorrow morning
to the Copper River Basin tomorrow evening. While snow chances
and amounts increase the farther north one goes, run-to-run
consistency on the models remains variable on the strength of this
trough. 18Z NAM and GFS runs are a little stronger with this
trough and indicate a little more snow from mainly Anchorage
northward that earlier forecasts, but this may be more of the
model uncertainty than a case where the models are actually
trending toward a little different solution. 00Z runs should shed
some light on whether this warrants a chance in the forecast.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
night. Some light snowfall is expected by late Saturday morning
which will likely bring conditions to MVFR from late morning
through the afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sat and
Sun)...
Pretty interesting set-up tonight into the weekend. A fairly
impressive low is already starting to occluded southwest of Kodiak
Island. The southern end of the island is already starting to
experience some very light rain and northeast winds. As the low
drifts to the east overnight, winds and precipitation will
increase, especially over the southern end of the island. A few
inches of snow are expected. Flow off the water should limit
accumulations for Kodiak City proper, but expect higher snow
amounts just above in the hillsides. That warmer air off the water
should also limit much blowing snow potential, however, if some
colder air can work in the heavier bands over the southern half of
the island, visibilities could drop for a while.

Then attention shifts to a shortwave moving quickly from west to
east across the southern mainland late tonight into early Sat.
This trough has trended slightly further south and stronger than
in previous model runs. The atmosphere will be primed this evening
with some moisture riding up potential temperature surfaces
(isentropic lift) helping to saturate the column. This could even
result in some flurries across the Cook Inlet region during the
evening hours. This lift coupled with the trough axis inching
closer will result in light snow starting to fall first in the
Susitna Valley after midnight. As southwest flow gets established
in the column through early Sat morning, snow should continue for
the Susitna Valley and start to spread east. By early-afternoon, a
finger of the 20,000 ft jet will bring further upper-level
support to the trough across much of Southcentral. As this
interaction occurs it will provide the best window for moderate
snow and quick accumulations for Anchorage and Palmer (generally
1-3"). The best lift for this event does appear to be at the
upper-end of the snow growth zone (-12C to -20C), so this could
hamper accumulations to a degree. But unlike Monday's snow, there
should not be any Turnagain Arm wind or precipitation type issues
to contend with.

The trough moves through quickly bringing an abrupt end to the
snow (possibly lingering a bit longer on W facing slopes) Sat eve
and switching us around to an offshore flow pattern once again.
Thus, expect gusty winds through Seward, Whittier, and Valdez to
end the weekend. These winds should not be nearly as strong as
earlier in the week as the cold air advection is quite weak.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
A surface low skirting south of the AKPEN is pumping moisture
north into the Bristol Bay coast and Kuskokwim River Valley which
is combining with an upper level trough to the west and weak
deformation zone over SW mainland to produce an area of snow over
the area. The snow will spread north into the lower Kuskokwim
River Valley tonight before tapering off through the morning
tomorrow. Northeast flow will ensure temperatures across most of
the area will remain below freezing keeping precip all snow,
with total snow accumulations between 2-4 inches. Ridging builds
in behind the departing system tomorrow afternoon with skies
clearing some leading to cooler temperatures Saturday night. The
next system approaches the area Sunday morning with a weak warm
front bringing a moderate increase in temps and another round of
snowfall. An upper level shortwave trough will favor snowfall over
the Kuskokwim Delta this time, with 2-3 inches expected to fall
by Monday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
Beginning Friday night, ridging will build over the Eastern
Bering/Aleutians while a gale force warm front will sweep across
the Western and Central Bering/Aleutians. The front will bring
strong southerly winds and a line of heavy precipitation, mostly
rain before weakening some as the front becomes elongated and
detached from the parent low well to the NW over NE Siberia.
Gale force westerly winds aided by strong cold air advection from
the north will wrap in behind the front over the western Bering
during the day on Sunday. The long fetch of these westerly winds
will lead to rising sea states with waves building to 20-25 ft
during the day on Sunday across the Western Bering Sea.

&&

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

The next upper level trough will swing into Southwest Alaska
Sunday evening then weaken rapidly as it continues east into
Southcentral Alaska on Monday. A transient upper level ridge will
follow, building over Southwest Alaska on Monday and then sliding
east across Southcentral Alaska Tuesday. A very strong westerly
jet will push across the Bering Monday and Monday night to propel
a fast moving open wave system across southwest Alaska Tuesday. A
surface low will spin up in the western Gulf Tuesday evening as
the frontal system crosses the Alaska Peninsula. With the surface
low tracking from the Barren Island north up the outer coast of
the Kenai Peninsula and the support of a strong upper level trough
and good cold air advection, this system look like it has good
snow potential for much of Southcentral Alaska Tuesday night and
Wednesday.

Following a brief break, the next fast moving frontal wave will
spin up to a rapidly strengthening closed low as it crosses the
southeastern Bering and moves into Bristol Bay late Wednesday
night and Thursday morning. The system will track onshore into
Southwest Alaska on Thursday and then swing east over the western
Gulf and Southcentral Alaska Thursday night and Friday with rather
large, cold upper level low digging south behind it.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 155 172 177 178 411-413.

&&

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