Small Craft Advisory 429, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Mon May 28 2018

.ANALYSIS AND UPPER LEVELS...
A trough sits over Southcentral, with a upper level low tracking
eastward through the eastern Gulf. Radar imagery from early this
morning shows the remaining showers tapering off over the Susitna
Valley and Kenai Peninsula. Meanwhile out west, an upper level
ridge over the Bering is starting to shift eastward into southwest
Alaska, with a vertically stacked low entering the western Bering
and Aleutians from the west. The associated front currently over
the central Aleutians and Bering continues to bring wet and windy
conditions to the area as it progresses eastward.

&&

.MODEL DISCUSSION...
Models are in good agreement with the brief shortwave ridge
tracking over the southern Mainland today, and the low over the
eastern Gulf moving inland over southeastern Alaska by Tuesday
morning. Models are also in good agreement with the timing and
strength of the front tracking through the eastern Aleutians and
Bering this morning, and into the southwest coast by tonight.
There are some model differences as to how far north this system
track into the northern Gulf coast region, and how long showery
conditions will linger in it's wake on Wednesday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist though this
morning. Gusty southeast winds will pick up over the Anchorage
bowl early this evening through Tuesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Weak high pressure will build over the area today. Some lingering
shower activity is expected, and will be diurnally enhanced.
Isolated thunderstorms are possible in the central Copper River
basin, the Susitna valley and the Talkeetnas this afternoon and
evening. The remainder of the area will see more stable conditions
compared to yesterday as weak ridging moves overhead and the upper
level flow shifts to the southwest. A weak front will move into
Kodiak Island from the southwest tonight. This front will weaken
and slow down as it approaches the north Gulf coast on Tuesday,
with just remnants remain as it moves into the mainland Tuesday
night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Isolated showers continue to drift slowly south southwest across
portions of the Kuskokwim Delta region, with a skinny line of
showers also making a run at King Salmon. Otherwise varying
degrees of cloud cover blanket a good chunk of the region. Some of
this, especially in areas of channeled terrain, are likely a
result of evaporative transpiration. Patchy fog has not been
observed thus far and given the trends in model guidance and
observations, this was removed from the forecast this morning.

Otherwise, look for a weakening area of high pressure to cross the
region. Seeing that the ridge is degrading as it gets the squeeze
play between a deepening low entering the western Bering and
another area of low pressure in the northeastern Gulf of Alaska,
subsidence will not have as much of an effect in clearing out the
skies as normal. Still, a fairly nice day with dry conditions are
anticipated for much of southwest Alaska, with perhaps just a few
random showers developing with daytime heating and
orographically-induced effects near the mountains. Our next
weather maker will be a front in the occlusion process as it
approaches the coast the coast tonight, reaching the Alaska Range
by Wednesday. Fairly widespread precipitation will develop along
this front, with winds remaining in check, except at higher
elevations where it may be a little bit breezy.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

An area of low pressure moving into the western Bering will
continue to slowly deepen through the next 24 hours, while it's
surface front, currently bisecting the central Bering and
Aleutians, continues towards the coast of Southwest Alaska.
Widespread precipitation has and will continue to accompany this
feature, with sustained gale force winds and gusts a little short
of storm force continuing. Precipitation will become more
scattered in nature following frontal passage, and with the front
beginning to occlude, cold advection behind it isn't that
impressive either. Thus, look for minimal changes in temperatures
compared to what we'd typically see for this time of year. Another
short-wave ridge will build across the western Bering/chain with
decreasing cloud cover and drying conditions by Wednesday, with
winds remaining relatively uneventful through the period behind
the departing system.

&&

.LONG TERM FORECAST (Days 3 through 7...Wed through Sun)...
This portion of the forecast period is showing signs of
uncertainty. We will walk through it chronologically. The front
associated with the broad trough over the Bering will be inland
over SW Alaska. This will make for a cool, wet, and breezy start
to Wed. While some of the moisture will be scrubbed from the
system by the Alaska and Aleutian Ranges, it will still trigger
widespread showers over most of Southcentral come Wed afternoon.
Then the upper level trough itself will advect over the entire
southern half of the mainland by Thu morning. This system has
trended slightly further north over the past 24 hours, so
precipitation might not be quite as heavy as originally thought.
However, it will bring reinforcing shot of cooler air, so snow
levels could drop as low as 2000' by Thu morning over the West
Coast.

The ridge from the North Pacific still tries to push up into the
mainland late Thu into Fri. However, models have shifted in a
significant way with this feature. Yesterday, the EC stood alone
in developing a closed high over the southern mainland for several
days. Now, it really only pushes it through as a transient, flat
ridge. Meanwhile, the GFS has trended towards a much stronger
ridge that would bring Summer-like weather to the state for the
first weekend in June. Ultimately, the difference is likely due to
how they handle the upper-level (30K') jet. The GFS carries a weak
finger up and over the ridge while it pushes the majority of the
energy well south into the North Pacific. The EC meanwhile keeps
it just south of the Aleutians. This would open the door for a
much more active pattern across the entire area.

All models advertise a fairly strong low over the Central
Aleutians next weekend. But because of the difference in jet
placement, the GFS has it much weaker in the mid-980 mb range while
the EC pushes it into the lower 970 mb range. The EC keeps
progressing this system into the mainland, bringing another round
of cloudy and wet weather. Due to the lack of clarity in the
models, little change was made to this portion of the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...411-413 170 172-174 178 185.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/TP
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...MO