Small Craft Advisory 488, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 AM AKDT Mon Jul 1 2019

.ANALYSIS AND UPPER LEVELS...

A significant difference occurred in the weather yesterday across
southern mainland Alaska. As a cold front approached the area,
southwesterly winds took over. The southwesterly winds transported
most of the Swan Lake fire smoke northeastward into Anchorage and
the Mat-Su Valleys, along with portions of the Copper River Basin.
In addition to lowering visibilities and worsening air quality,
the smoke also acted as a shade-maker across Anchorage, which kept
high temperatures from getting nearly as warm as it got to on
previous afternoons. The mid-levels of the atmosphere also cooled
as the upper level high responsible for the very warm temperatures
was shunted south over the North Pacific. Thus, despite another
day with a lot of filtered sun, many areas were a good 5 to 10
degrees cooler on Sunday than on Saturday.

Currently the upper level high has shifted south and west and is
now centered southeast of Sand Point over the North Pacific. A
shortwave trough is crossing the northern portion of the area,
which caused some thunderstorm activity across the Susitna Valley
and Copper River Basin. This trough and associated surface cold
front have mostly completed doing all the cooling they're going to
do. Out west across the Bering, a new North Pacific low centered
just east of Shemya currently has good upper level support in the
form of a shortwave trough. The low is producing gale force winds
west of Adak, along with moderate to at times heavy rain.

&&

.MODEL DISCUSSION...

The models remain in excellent agreement, so high-resolution
models were used for most of the forecast, which generally has
very few changed. Forecast confidence is excellent. Forecast
challenges remain similar as the past several days. With zero rain
expected for most of Southcentral, determining Swan Lake fire
smoke distribution, density, and when remains a significant
challenge. This is largely due to small changes in wind direction
that can be difficult to predict, with other local effects such as
sea breezes. Across Southwest Alaska and the Gulf, marine stratus
and fog remain the primary challenge.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. General
wind flow today will be out of the northwest, which will help to
keep any dense smoke away from the airport today. However, haze
and smoke that does not significantly reduce visibilities will
still be possible at times, especially in the morning.

&&

.FIRE WEATHER...Swan Lake Fire...Western Kenai Peninsula:

Slightly cooler conditions are expected in the area of the Swan
Lake fire through Tuesday. With no precipitation in the forecast,
higher relative humidity values in the area will be the result of
the cooler temperatures. General wind flow will be out of the
northwest, which will take most of the smoke into the Kenai
Mountains and towards Seward. More on that below. Winds may
approach 10 mph at times around the fire, but with marginal
temperatures and relative humidity values well above criteria, red
flag conditions are not expected around the fire for at least the
next two days.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wed night)...
Weak upper waves moving across a now flattened upper level ridge
will produce isolated to scattered convection today over interior
Southcentral, primarily from the northern Susitna Valley
southeastward across the Copper River Basin. Otherwise, clouds
will transit the region this morning, with increasing sunshine
during the afternoon hours. With a surface high building upstream
over Southwest Alaska, low level offshore flow (northerly) will
strengthen today and persist for the next few days. This will
lead to locally gusty winds along the coast, namely in
Seward/Resurrection Bay and Whittier/Passage Canal. The offshore
winds look strong enough to prevent development of sea breezes,
so these coastal locations will see warmer temperatures. Away
from the coast, temperatures will be similar to Sunday.

The upper level ridge will gradually strengthen and build
northward over Southcentral on Tuesday and Wednesday. With the
ridge center well to the south, mid to high clouds moving over
top of the ridge will continue to cross Southcentral, especially
inland areas of the Susitna Valley and Copper River Basin. This
creates a bit of a tricky temperature forecast as thicker cirrus
clouds during the day will keep temps a bit cooler. Nonetheless,
expect warmer temperatures each day, with temperatures climbing
back into the 80s on Wednesday for much of the region. In fact,
with persistent offshore flow temperatures in Seward, Whittier,
and Valdez could reach 80 degrees as well.

As far as the smoke forecast for the Swan Lake fire, persistent
low level northwesterly flow will carry the bulk of the smoke
southeastward across the interior Kenai Peninsula down to Seward
and out into Prince William Sound and the northern Gulf. Based on
this, have issued Dense Smoke Advisories for the Kenai Peninsula
zones through 1pm Wednesday. The worst conditions/lowest
visibilities will generally be late at night through the morning
hours as smoke gets trapped near the surface with night-time
inversions. The dense smoke could also become more widespread as
subsidence inversions strengthen beneath the upper level ridge.
The advisories may need to be extended further out in time, as the
pattern and general flow look like it will hold through the
remainder of the week.

As was the case last weak, there will be a lee trough in Cook
Inlet thanks to broad westerly flow across the Aleutian and Alaska
Ranges. This will induce up-Inlet southwesterly flow each
afternoon and evening. A shallow southerly flow in the vicinity of
the fire will likely carry some smoke northward to Anchorage each
night. Therefore, have kept a mention of smoke in the Anchorage
and Matanuska Valley forecasts.

&&

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

Mostly clear skies will persist this morning, with potential
patchy fog developing along the AKPEN/Bristol Bay coasts in weak
onshore flow under a ridge. While much of Southwest will remain
clear this afternoon, clouds will increase in the Kuskokwim Delta
with showers developing ahead of a front. This pattern will
continue Wednesday as an upper level trough moves over the
region. An upper ridge will begin to build in from the south
thereafter, bringing mostly sunny skies and warmer temperatures
through the end of the week.

&&

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

Moderate showers and gusty southerly winds will slowly track
across the western and central Aleutians today as it stalls
behind strong high pressure. Winds will gradually diminish through
Wednesday as the front weakens, although showers will continue.
The front will retrograde Wednesday evening into Thursday as a
strong upper ridge builds in from the southwest. More benign
conditions are expected across the eastern Aleutians through the
forecast period, remaining under the influence of the
aforementioned ridge.

&&

.MARINE (Days 3 through 5)...
Wednesday through Friday will be characterized by high pressure
rebuilding over southern Alaska. There is high confidence in this
setup that will result in winds below gale force throughout the
marine areas.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Models are in good agreement that a strong ridge will rebuild over
southern Alaska the middle of this week and remain in place into
the weekend. Hot and dry conditions will persist across the
southern mainland with above average temperatures and generally
light winds expected.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Smoke Advisory 121 125.
MARINE...Gale Warning 170 173 174 175 176 177 179 181 185.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1158 AM AKDT Mon Jul 1 2019

AKZ101-111-121-125-141-145-032000-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Copper River Basin-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass,
Glennallen, Eureka, McCarthy, Paxson, Slana, Talkeetna, Willow,
and Cantwell
1158 AM AKDT Mon Jul 1 2019

Elevated water conditions will persist for Eagle River. For those
near or attempting to cross Eagle River, be prepared for very
high and swift water. Please see weather.gov/aprfc for the latest
conditions and forecasts.

Generally across Southcentral, high river conditions are expected
along the Matanuska, Skwentna, Klutina, Sixmile, Talkeetna, and Kenai
rivers. Water levels are expected to continue and/or rise into
Action Stage and Bankfull conditions through the weekend. No
significant impacts are expected along these rivers.

A Flood Warning has been issued for the Yentna River at Fish
Creek. Please see the Flood Warning for that area for further
details.

CVB/MSO