Gale Warning 379, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
522 AM AKDT Thu Aug 22 2019

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low, recently translated eastward from near
the western Aleutians, is currently located south of Akhiok. This
is really the only large-scale feature of significance affecting
anywhere in Southern Alaska. This low is moving into the Gulf.
Associated rainfall is moving into Kodiak Island, with some
lingering rain showers continuing along the Alaska Peninsula
through the Shumagin Islands.

Ridging ahead of the low is building across the northern Gulf
over to Prince William Sound, both in the upper and lower levels.
This ridging has helped cause easterly winds to develop through
Turnagain Arm and other mountain gaps through the Kenai Peninsula.
This has lead to the Swan Lake fire smoke beginning to be fanned
towards the west and northwest, including moving over Anchorage.
So far, however, the denser smoke remains in the mid-levels with
relatively little of it at the surface. The easterly winds are
moving the smoke out of the valleys along the Seward Highway, so
the Dense Smoke Advisory for the eastern half of the Kenai
Peninsula has been allowed to expire.

&&

.MODEL DISCUSSION...

The models are in decent agreement through Friday. By Saturday,
however, the GFS diverges from the other models by bringing an
upper level low eastward much more quickly through Bristol Bay and
the Alaska Peninsula. The other models cut off the low much more,
allowing it to remain over the eastern Aleutians and Alaska
Peninsula into Sunday.

Forecast challenges remain with small-scale features. Smoke from
the several wildfires in Southern Alaska is the primary challenge.
Timing out where the smoke will go and how dense it will be
remains a point of very low certainty. With the surface low moving
into the Gulf now, how far north the associated precipitation gets
will also be a challenge. It's highly unlikely to impact much
further inland than the immediate Gulf coast, but rain anywhere
would be welcome. Forecast confidence is above average.

&&

.AVIATION...
PANC...Swan Lake fire smoke remains the primary hazard impacting
the airport today. The Turnagain Arm wind will also impact the
Cook Inlet waters off the coast of the airport. There is a chance
that wind gets into the airport this afternoon and evening, but
exactly when that would happen would be a low-confidence forecast.
The Turnagain Arm wind should help keep visibilities near VFR by
mixing and diluting the smoke. Once that wind weakens tonight,
thicker smoke may impact the airport.

&&

.FIRE WEATHER (Southcentral)...

With nearly steady or slowly rising relative humidity values
across Southcentral and temperatures that will remain in the lower
60s to around 70 range area-wide, the main weather factor
impacting the fires around the area will be the winds. The
prevailing winds will be southerly to easterly through the day
today, which will direct wildfire smoke generally northward
towards the Susitna Valley. There remains significant uncertainty
for Anchorage as to whether or not the Turnagain Arm wind will
keep the Swan Lake fire smoke from moving into the city.

Tonight, the winds will flip around to the north to northwest
through the low to mid-levels. The winds are not expected to be
anywhere near as strong as the latest wind storm, but nevertheless
will keep things dry over Southcentral. This should result in the
smoke returning to the eastern Kenai Peninsula/Western Prince
William Sound/northern Gulf.

Isolated to scattered showers will be possible across much of
Southcentral on Friday afternoon and evening with the passage of a
trough. It should be emphasized that many of us will not see any
rain at all, and those that do will only get a brief period of
rain. Any showers have the potential to locally impact the winds
if they move near or over any of the fires.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Saturday night)...
Areas of dense smoke will persist across the interior and western
Kenai Peninsula today. Increasing low level easterly winds across
the Kenai Peninsula will transport smoke from the Swan Lake fire
westward toward Cook Inlet. Thus, Kenai and Soldotna can expect
reduced visibilities as well. Expect some improvement in
visibilities this afternoon. However, smoke could remain dense
right near the fire, depending on fire activity. Otherwise, smoke
from the many active wildfires in Southcentral has now blanketed
the entire region and will continue to move around with the winds.
Thus, areas of smoke and haze will persist.

The low south of Kodiak will head into the Gulf today, with a new
low forming along the triple point and deepening over the central
Gulf this afternoon. This low will be stronger than previous
forecasts indicated, so have upped winds across the Gulf offshore
and coastal waters. The bulk of rain with this system will remain
over the Gulf and Kodiak Island, though expect a few showers to
reach western Prince William Sound/eastern Kenai Peninsula. A
stray shower may make it over the Kenai Mountains to the western
Kenai, but most areas will remain dry. Weak ridging over the
remainder of Southcentral will lead to another dry day with hazy
sunshine.

The Gulf storm will quickly exit to Southeast Alaska on Friday,
while a broad and weak upper level trough drops down from the
Arctic. There is some decent instability in the low levels (thanks
to continued above normal surface temperatures). This should be
enough to trigger isolated to scattered showers Friday afternoon
and evening, especially along the mountains. Very weak cyclonic
flow will persist on Saturday, though upper forcing looks weaker
than Friday. Therefore, expect mainly some isolated showers along
the mountains with dry conditions elsewhere.

As for the wind forecast, there will be some localized gap winds
today, primarily Turnagain Arm and the Kenai Peninsula. As the
Gulf low tracks to the eastern Gulf tonight and Friday, winds
will shift back around to a north to west direction across
Southcentral. Gusty offshore flow will develop in Seward/Resurrection
Bay and Whittier/Passage Canal. This overall flow will persist
through Saturday then become light and variable. This wind shift
will once again transport smoke from the Swan Lake fire south and
eastward across the interior and eastern Kenai Peninsula.

&&

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

Quiet weather will continue today, upstream of an approaching
upper trough near the Bering Strait. Temperatures will trend a bit
cooler than yesterday, as afternoon cloud cover increases and
north flow strengthens. A weak wave ahead of the trough could
trigger a few showers over the mountains later today, then expect
enhanced shower development Friday as the trough axis pushes
through Southwest. There is a slight chance of thunderstorms
Friday primarily in the mountains, though depending on midlevel
winds some cells could drift away from the ridges. Uncertainty is
considerable with this feature. Models have waffled between
bringing the trough as an open wave, and a low closing off at the
base of the trough which wouldn't be as good of a trigger for
convection. This system will continue to be monitored/refined over
the next few forecast cycles. In any case, the cooling trend will
continue through Saturday under persistent north flow.

&&

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

Small craft advisory level winds and/or seas will continue
through tomorrow in portions of the Aleutians. In particular, a
vertically stacked low exiting towards the Gulf will drive
elevated winds on the south side of the AKPEN today, while fresh
swell is evident in the western Aleutians. Otherwise, expect
generally benign conditions under broad north flow in the Bering
over the next several days, with widespread fog likely by Friday
as cold air advects over warmer waters. South of the chain,
stronger gap winds are possible Friday into Saturday.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...
Westerly flow is expected across the Gulf through the long term
period. There is moderate confidence that a low will develop over
or just offshore of the Alaska Peninsula Sunday. However there is
low confidence regarding the exact track and strength of this low
as it moves eastward into the Gulf for Monday. Meanwhile over the
Bering, there has been consistent indication that a ridge of high
pressure will persist through early next week.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...
Unfortunately there continues to be only muddled signals in the
long-term weather pattern for next week. We have high confidence
that a longwave trough will be in place over the state with a
ridge of high pressure over the western Bering. Model solutions
continue to vastly disagree in the timing and strength of several
shortwaves that move across Southcentral early next week. This
translates into an unsettled, cloudy, and showery weather pattern
that will likely persist through the middle of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Smoke Advisory: 121.
		 Flood Advisory: 121 125.
MARINE...Gale Warning: 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...KO