Small Craft Advisory 484, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 PM AKDT Sat Jun 29 2019

.ANALYSIS AND UPPER LEVELS...
High pressure continues to dominate the southern mainland but is
now starting to slowly move to the east. The inversion from this
high pressure has trapped smoke from the Swan Lake fire and that
smoke has drifted to areas all around Southcentral Alaska with the
most dense areas on the Kenai Peninsula. Out west, a front is
pushing through the central Bering as it tracks to the east.

&&

.MODEL DISCUSSION...
The main meteorological models are in decent agreement through
the mid term with the major synoptic features. However, the smoke
dispersion models are struggling to handle the smoke from the Swan
Lake fire. How far north to bring the more dense smoke on Sunday
as the flow switches to the south has been a particular area of
struggle. However, most of the models are now bringing in the more
dense areas of smoke over the Anchorage area Sunday.

&&

.AVIATION...
PANC...Smoke from the Swan Lake fire continues to be the main
impact at the airport. As the inversion enhances overnight tonight
smoke will spread from the fire and move over the Anchorage area.
It looks to be dense enough to drop visibilities to MVFR. Early
in the morning winds will shift to the south bringing up more
smoke from the fire and further reducing visibilities at the
airport. The southerly flow and reduced visibilities will last
through much of Sunday.

&&

.FIRE WEATHER...
High pressure over Southcentral Alaska will last through tonight
keeping conditions clear and dry for the Swan Lake fire. The high
pressure has also created an inversion that is trapping smoke near
the surface that will strengthen overnight tonight. This will
result in smoke spreading out for areas around the Kenai
Peninsula and areas farther north including the Susitna Valley.
The most dense areas of smoke will be in the interior Kenai
Peninsula and the Seward area. The Dense Smoke Advisories for
these locations will last through much of the night.

Early on Sunday the front approaching from the west will change
the flow over the area to a southerly direction which will help to
clear out some of the smoke over Seward and interior Kenai
Peninsula, but will drive that smoke to the north to locations
including Anchorage. There is some uncertainty with the smoke
dispersion models on the farthest reaches of the smoke and the
duration that it will linger over some of the more northerly
locations such as the Susitna Valley. However, there is fairly
high confidence in dense smoke moving in over Anchorage on Sunday
resulting in a Dense Smoke Advisory for the area.

Looking out into next week there is good agreement from the models
of high pressure building back in over Southcentral Alaska by mid
week. This high pressure looks to last through the end of the week
bringing the potential for more smoke concerns to interior Kenai
Peninsula for the July 3-5 timeframe. This setup looks somewhat
similar to the current setup which would also mean areas in and
around Seward could see more smoke. There is low confidence in
determining how dense any smoke around the Kenai Peninsula would
be looking that far out into the week.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Monday evening)

Persistence is a virtue they say, and mother nature will be
following that trend as the latest numerical guidance has trended
warmer with afternoon high temperatures for Sunday and Monday.
This looks reasonable given a slower breakdown/less displacement
of the ridge of high pressure dominating our weather.

That said, southwesterly flow ahead of an approaching weak
disturbance trying to round the top of the ridge will be
sufficient to cool 850 mb about 2 air 3 degrees Celsius for all
of southcentral, allowing surface temps to decrease somewhat.
However, this looks to be short-lived though, with more of this in
the long term portion of the discussion.

The other net affect of this incoming disturbance is a switch to
southwesterly flow tonight. Much of the Anchorage Bowl, Matanuska
Valley, and Copper Basin has seen smoke for days now in the lower
valley locations, aided by nocturnal inversions helping to trap
that smoke due. This will allow the smoke density to increase for
the Anchorage Bowl, with our first ever Dense Smoke Advisory
having already been issued for the Anchorage area.

&&

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

Clear and dry conditions will persist across interior Southwest
Alaska through tomorrow as a ridge of high pressure remains
anchored over the southern mainland. The stratus and fog that
developed early this morning across the Bristol Bay region will
likely redevelop during the early morning hours Sunday, aided in
part by the ridge aloft and an increasing moist, onshore flow
ahead of a trough currently over the central Bering. As the
trough moves toward the coast tomorrow, it will suppress the
ridge, allowing clouds and scattered showers to develop along the
coast and over the Y-K Delta. A westerly, onshore flow will also
help lower temperatures a bit as well. This system will quickly
fall apart as it moves into the interior, limiting the development
of clouds and showers south and east of the Kilbuck Mountains to
mainly the higher elevations. Drier conditions return by Monday
for much of the southwest as another surface ridge moves east over
the region. The exception to this will be the Y-K Delta where
some cloud cover and showers will skirt the region late Monday
into Tuesday as a front lifts north across the Bering.

&&

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

A frontal boundary currently west of the Pribilofs will continue
to push east tonight, edging closer to the Southwest Alaska coast
for tomorrow. Behind it, a ridge of high pressure will develop
across the Bering with areas of low stratus and fog through Sunday.
Farther west, a warm front associated with a north Pacific low
will lift north across the western half of the Aleutian Chain
overnight. This system will continue to advance north tomorrow,
with rain overspreading much of the western and central Bering as
the center of the low tracks toward Shemya. This low then slowly
lifts over the western Bering on Monday and Tuesday with
widespread rain and gales persisting from the central Aleutians
north across the Bering, including the Pribilofs.

&&

.MARINE (Days 3 through 5; Tuesday through Friday)...

A front will track across the central Bering Tuesday. As it
interacts with strong high pressure to the east and its parent
low to the west slightly strengthens, localized gales and
moderate showers across will be likely across the region. Winds
will gradually diminish trough the end of the week, however
showers will continue. Further towards the AKPEN, showers are less
likely and the usually marine stratus is more likely northwest of
high pressure.

&&

.LONG TERM FORECAST (Days 4 through 7; Wednesday through Sunday)...

Models are in good agreement that a strong ridge will rebuild over
Alaska through next week. Hot and dry conditions will persist
across Southcentral with temperatures again approaching 80 in the
Anchorage area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood warning 145. Dense smoke advisory 101 and 121.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...AP