Small Craft Advisory 319, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKDT Thu Apr 19 2018

.ANALYSIS AND UPPER LEVELS...
The anchor of the long-wave pattern is still the decaying closed-
low over the Bering Sea. The upper-level jet wraps around the
system to the south, pointing into the Gulf of Alaska. Within the
jet, a short-wave trough is beginning to amplify as it interacts
with the stalled frontal boundary that has been stretched across
the area for the past couple days. Northerly flow aloft over the
Al-Can border is currently hampering northward progression of this
system, the two opposing flows should battle it out the next two
days.

The stretched out front is rapidly losing strength as the system
begins to deepen to the east. Most of the shower activity
associated with the front is rapidly falling apart across the
southwest mainland, leaving mostly middle and high-level clouds in
its wake. The development of the compact low in the eastern Gulf
is shifting the pressure gradient around, acting to really
diminish the gap winds in most areas.

&&

.MODEL DISCUSSION...
Now that we're in 24 hours of the event and the low has started to
develop, guidance has really converged on a solution for the
system. The low will dissipate rapidly once it reaches Prince
William Sound late tonight. The only discrepancy among the models
at this point is the amount of showers that come over the
mountains once the system moves inland.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

East to southeast flow aloft over southcentral Alaska will keep
widespread rain showers along the coast with few making it inland
to the lee side of the mountains. Higher elevations will see snow
showers, but most of the precipitation is expected to be rain. As
flow aloft becomes weak tonight and a trough digs from the eastern
interior into the Kuskokwim valley tonight, it will bring in a
better chance of precipitation to the inland areas from the
western Kenai Peninsula to the Susitna Valley, including the
Anchorage and Matanuska Valley. Most of the precipitation that
does fall even tonight should be in the form of rain but there is
a chance that a little snow will mix in with the rain in some
locations. Friday and Friday night will also see some chance of
precipitation inland as flow aloft will turn south to southwest.
This will especially enhance the precipitation over the Susitna
Valley. With highs in the 40s, the precipitation should be rain
for most areas, but as one gains elevation north of Talkeetna,
there may be some accumulating snow. Look for gusty Turnagain Arm
winds and Knik River winds to develop Friday and persist well into
the night Friday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Thu and Fri)...
Weather will remain fairly benign for the next couple of days. A
slowly decaying upper level low will be the primary weather maker
as it pulls away into the Gulf of AK. This system continues to
pump enough moisture and instability into Bristol Bay from the
southeast that showers have persisted through the night. Showers
will linger today stretching from Iliamna back through Pilot
Point. Some afternoon instability could enhance these showers a
bit by this evening.

Friday will see a shift in the weather patter. While most of the
systems have been coming from the west or south over the past week
or so, a cold trough is preparing to plunge over the area from the
northeast. This is courtesy of a closed Arctic High that continues
to retrograde south and west in the vicinity of the Chukchi Sea.
So Friday will start out fairly mild and dry for most locations.
But as the cold air and upper level energy start to make their way
over the AK Range by late Friday afternoon, expect showers to form
on the westward slopes of the Alaskan and Aleutian Ranges. Showers
will slowly expand out across all of SW AK as the trough continues
to dig and forms a closed low near the coast. While the airmass
will certainly be cooler and will likely provide some snow to
higher elevations, most of the precipitation will still be driven
by diurnal temperatures (rain during the day, mixing with or
turning to snow at night).

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Thu and
Fri)...
One low over the Eastern Aleutians/AK Peninsula will slowly pull
away as another hugs the south side of the chain over the next
couple of days. Some cold air advection on the backside of the
first low is helping to mix down some stronger winds this morning.
While some gale-force gusts are expected through the very early
morning for Atka and over towards Dutch Harbor, this should be
short-lived. Showers will continue for the Eastern Aleutians and
the AK Peninsula today, especially on the Bering side where
upslope conditions will persist. The remnants of a decaying front
over the Western Aleutians could lead to some patchy fog.

The next low pressure center approaches the Western and Central
Aleutians from the North Pacific tonight. Previously, this system
was progged to head a bit further north and bring slightly
stronger winds and heavier precipitation to the islands. The track
now holds it a bit further south. Thus, only expect a chance for
rain with maximum small-craft winds across the region.

&&

.LONG TERM FORECAST (Saturday through Wednesday)...
On Saturday the remnants of the upper low dropping down through
the interior of the state will produce widespread precip across
the upper Kuskokwim Valley and interior Bristol Bay zones before
the low heads out into the Eastern Bering on Sunday. Bristol Bay
locations should be warm enough for rain, but the lower Kuskokwim
Valley from Aniak to Stony may stay cold enough for a few inches
of snow accumulation. Meanwhile gusty, cloudy, and cool
conditions across southcentral will slowly dissipate by the end of
the weekend.

The first half of next week will feature a continuation of the
pattern we are in right now, which means southeast flow across
most of southern Alaska with rounds of steady rain/fronts along
the coastal areas and cloudy cool but mostly dry conditions inland
and in the favored downslope areas.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 174.

&&

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