Small Craft Advisory 618, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
402 AM AKDT Tue Aug 27 2019

.ANALYSIS AND UPPER LEVELS...

A ridge of high pressure continues to fill in across the Southern
Mainland with its axis stretching from the Kuskokwim Delta east
along the Alaska Range. Dry and partly cloudy conditions exist
across most of Southcentral with a general northerly flow. The
northerly winds have been enhanced across the Copper River Valley
and Delta as well as through the bays and passes of Prince
William Sound due to the increased pressure gradient between the
building ridge and a vertically stacked low inching closer to the
Alaska Peninsula.

Farther west, areas of low stratus persist across the northern
half of Southwest Alaska due to a moist onshore flow wrapping
around the northern periphery of the ridge. A weak trough over
the AKPEN did provide enough instability to touch off few
scattered showers and thunderstorms along the Aleutian Range
during the evening and overnight hours, but these have since
diminished.

A high amplitude ridge stretching from the north Pacific northward
over the central Aleutians and Pribilofs is keeping widespread low
stratus over the region. A few scattered showers are still
lingering along the Bering side of the southern half of the AKPEN
and eastern Aleutians due to the northerly flow around the front
side of the ridge. A frontal boundary remains draped across the
western Bering in association of a low west of Shemya.

Smoke from multiple wildfires continues to impact much of
Southcentral, reducing air quality and visibility from the Kenai
Peninsula to the Mat-Su Valleys. Smoke from the Swan Lake fire has
reduced visibility along the Sterling Highway to one quarter mile
or less at times. Thus, a Dense Smoke Advisory has been issued for
this area through 1pm Wednesday.

&&

.MODEL DISCUSSION...

Models are in good agreement through the short term. There are
some minor timing differences with the movement of a front and
development of a surface low over the western Gulf on Friday, but
nothing that would affect the overall forecast.

&&

.AVIATION...
PANC...Smoke from the Swan Lake fire will continue to linger over
the airport complex through mid-morning due to a light southerly
wind and developing inversion. This will reduce visibility to
MVFR conditions at times through mid-morning. As winds become
more northerly later this morning, the smoke will retreat to the
west and south of the terminal with VFR conditions persisting
through the remainder of the forecast period.

&&

.FIRE WEATHER...

Southcentral...Warm and dry conditions will persist across much
of the Southern Mainland through the next couple of days as high
pressure builds overhead. The northerly, offshore flow will begin
to diminish later today, allowing for the development of weak
diurnal sea breezes and southerly up-valley winds across the
MatSu. A weak shortwave will traverse Southcentral on Wednesday,
but any shower activity from this system will be isolated in
nature and confined to the mountains. No wetting rains are
expected at fire locations.

Southwest...High pressure will keep conditions seasonably mild
and generally dry across Southwest Alaska. Clouds will persist
across the Kuskokwim Delta with an onshore flow. A light,
diurnally driven sea breeze is possible along the Bristol Bay
coast. The chance for widespread wetting rains will increase by
the end of the week as a front approaches the coast.

&&

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

Weak upper ridging is expected to increase across Southcentral over the
next couple of days. Weak embedded troughs propagating west may be
enough to kick off a few showers along the Talkeetna Mountains
today and Wednesday. There is also an outside chance of a few
showers along the Chugach and Kenai Mountains on Wednesday. Other
then that, there is not much in the way of precipitation through
mid week.  The low level flow will shift to the north this
morning with the strongest outflow winds over the eastern/southern
Kenai Peninsula and Prince William Sound. This will help to delay
the onset of afternoon seabreezes today in these areas allowing
afternoon temps to get fairly warm. The flow should be fairly
slack tonight. Westerly flow aloft will produce lee side
troughing over the Cook Inlet Region on Wednesday. This will bring
increasing southerly flow from Cook Inlet to the Susitna Valley,
strongest over the Inlet. Also, a weak Turnagain Arm wind is
expected Wednesday.

&&

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

A ridge of high pressure over the eastern Bering continues to move
east over Southwest. With weak onshore flow, low level fog and
stratus are expected along the coastal areas of Bristol Bay and
the Kuskokwim Delta this morning. Several embedded upper level
waves will allow cloudy skies and showery conditions to persist
through Wednesday. Winds will increase Wednesday evening along
the coast as a front nears bringing precipitation and gusty
southerly flow to the Southwest coast for the end of the
workweek.

&&

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

High pressure over the eastern Bering begins to shift eastward,
making room for an inbound low. The front associated with the low
reaches the central Aleutians this evening bringing rain and gusty
southerly winds with low end gales possible. As the low lifts
northward, it strengthens and produces gale force winds in the
eastern Bering late Wednesday into Thursday before moving inland
early Friday.

&&

.MARINE (Days 3 through 5)...
Thursday through Saturday
Model solutions are in poor agreement beyond Thursday. There is
the potential for a gale force wind across the central Aleutians
late Friday. The low may move through the eastern Aleutians
Saturday with gales possible from the central to eastern Aleutians
to the Alaska Peninsula into Bristol Bay. Confidence is very low
beyond Thursday.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Overall model agreement is good until Friday. At that point the
EC and GFS part ways with a significantly different solution with
respect to the low approaching the central Aleutians. The GFS
supports a gale force low south of the Aleutians while the EC is
much weaker. The upper level solutions also differ significantly
as the GFS has a closed low while the EC maintains the trough
structure from the main low in the eastern Bering. The EC does
intensify this low Saturday, although the solution is more
progressive than the GFS and would bring the strongest wind and
heaviest rain to the Alaska Peninsula Saturday night. The GFS does
elongate the front to the Alaska Peninsula through Saturday. The
models are all holding the low more west than previous runs, and
are 10 to 20 MB deeper Saturday night into Sunday than the
previous solutions. At this point we will trend to the more
intense and more west solutions. This will continue to keep
southwest Alaska more cool, wet and windy, and the southern
mainland will remain on the drier side, but temperatures should
continue to cool. Continuing through the weekend into Monday,
model agreement only gets worse. The GFS moves a strong low north
of the Seward Peninsula while the EC and WPC solutions hold a
moderately strong low over the Yukon-Kuskokwim Delta where the GFS
has a ridge of high pressure. Confidence in the forecast beyond
Thursday is very low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Smoke Advisory 121.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...SA