Small Craft Advisory 628, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKDT Fri Aug 30 2019

.ANALYSIS AND UPPER LEVELS...
The transition to more active weather is underway. A short-wave
is lifting northward across Bristol Bay and the western Gulf
(including Kodiak Island) with rain spreading northward to the
Kuskokwim Valley and Cook Inlet/Kenai Peninsula. This trough
picked up a surface low which had been spinning over the North
Pacific. This low is tracking northward in vicinity of Shelikof
Strait, with small craft advisory winds being observed across much
of the Kodiak Island waters.

Next in line is a rapidly deepening low approaching the central
Aleutians. This will be a significant weather maker across the
entire region with widespread strong winds and rain through the
next few days. While this storm is strong, it is quite typical for
this time of year. Because the weather has been unusually quiet,
this does represent the first big storm of the season. Conditions
are deteriorating along the central to western Aleutians, with the
rain and winds spreading north and east toward the eastern
Aleutians and Alaska Peninsula.

&&

.MODEL DISCUSSION...
Models are in good agreement with large scale features through
Sunday and forecast confidence is above average. Beyond that,
solutions diverge with track and strength of short-waves rounding
the weakening low over the northern Bering Sea. This will affect
location and intensity of precipitation across the southern Alaska
mainland.

&&

.AVIATION...
PANC...Low-level smoke should largely clear out as Turnagain
winds bend into the airport early this evening. The terminal will
be right on the northern edge of these winds, so there is some
uncertainty in how strong they will get. In any case, these winds
will be brief because down-Inlet pressure gradients will build
during the evening hours in response to a surface low moving up
Shelikof Strait.

Rain and lower ceilings will develop around 06Z and continue
through about 15Z. There will be some southeast winds across the
Kenai and Chugach Mountains, though they will be on the weak side
and they will become more southerly overnight. Thus, any downslope
flow will be weak. However, this flow should help keep ceilings
predominately in the low end VFR category. There is a chance of
MVFR ceilings and vis as the flow becomes more southerly, perhaps
between 09Z and 15Z, but if these conditions do develop they
should be brief in nature. Ceilings will then lift during the day
tomorrow as more unstable air moves in behind this system.

&&

.FIRE WEATHER...
The current system lifting northward across the southern mainland
will likely bring wetting rains to many of the active fires today
through Saturday, from those in Bristol Bay to the Kenai Peninsula
to the Susitna Valley.

The next storm will be much larger and stronger. Gusty east to
southeast winds will be the big story across the Bristol Bay area
Saturday night and Sunday and across the Kenai Peninsula Sunday
through Sunday night. Winds will remain light across the Susitna
Valley corridor. Rainfall will depend heavily on proximity to
mountains as downslope flow plays a key role with this system.
The Swan Lake, McKinley, and Deshka fires will all see fairly
strong downslope flow and will likely remain mostly dry with
frontal passage. Bristol Bay has a better chance of seeing some
rain.

Behind the front, flow will shift to south to southwest on
Monday. The combination of this shift in flow and upper-level
short-waves aloft will bring a good chance of wetting rains to all
of the aforementioned areas.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Sunday)...

Two rounds of rain are on their way to Southcentral this evening.
The first is already moving into the southern Kenai Peninsula,
where Seldovia has already picked up more rain in the last few
hours than the entire rest of the month combined. Homer is rapidly
approaching that threshold. The rain will continue to move north,
reaching Kenai/Soldotna in the next few hours and Anchorage later
on this evening. A concern the past couple days with the more
westward track of this rain was that there would be some
downsloping component to the rain. The latest guidance and current
trends in the radar suggest the rain will be more widespread
area-wide, with at least a tenth of an inch of rain expected most
everywhere, including in Anchorage and on the Swan Lake fire.
There may be some increased breezes as the rain moves through
tonight at higher elevations, but no significant winds...yet. More
good news is that there should be enough rain, with light winds,
to effectively scour out much of the smoke that has been around
Southcentral for quite a while now. Regardless, down-Inlet
northerly flow at the low levels should help to keep any remnant
smoke from the Swan Lake fire out of Anchorage and points north.

On Saturday, the rain will continue moving north and will slowly
end from south to north, mainly from Anchorage north. There will
be a break in the precipitation Saturday evening.

The second round of rain will move into Southcentral on Sunday.
This second round will be very different than this first round in
a number of important ways. First, it will have more moisture, as
the front is associated with a low that is currently rapidly
strengthening over the Aleutians. This will mean much more rain
along the coast, from the southern end of the Kenai Peninsula
through Seward and up to Whittier, where over 2 additional inches
of rain are possible through Sunday night. Second, the front will
be strong, and as such, there is high confidence that there will
be a significant downsloping component to the precipitation, so
areas in the lee of the Kenai and Chugach mountains could very
well get less rain with this second round of rain than the first.
Third, this strong front will cause the winds to come up much more
than this first round. Winds along Turnagain Arm could gust to 65
mph. At mountain-top level, gusts could approach 55 mph. Along
the Anchorage Hillside and down the Knik River Valley up into
Palmer, gusts could get up to 45 mph. Finally, gusts could get up
to 35 mph at the openings to the valleys around the Swan Lake
fire. Weakened trees from the burning could come down as a result
of these winds. It is important to remember that this kind of
weather is very typical for this time of year, but since this is
our first windstorm in months, it appears to be atypical this
year.

&&

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

A weakening trough over the Southwest mainland will keep rain in
the forecast through this evening for the Bristol Bay region. The
remainder of Southwest will continue to see showers associated
with a weakening frontal boundary. There may be a brief break in
the rain and cloud cover overnight after these systems have moved
out of the area, but before the next big storm approaches.

Tomorrow will see a shift in the weather pattern. A strong Bering
low will bring cloudy conditions, rainfall, and strong
southeasterly winds to the coast by the late afternoon hours.
Though this is not an unseasonable storm pattern for this time of
year, this will be the first big shift seen across Southwest
Alaska in a while.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A gale-force low will track over the western Aleutians this
afternoon and will spread into the Bering through the night. The
strongest winds will remain on the western side of the low center
near Atka, which are likely to reach storm-force through tomorrow
afternoon. Out in front of the low center, gale-force winds will
continue to move eastward into the eastern Bering and Aleutians
before reaching the Southwest coast by tomorrow afternoon. Rough
seas and heavy rainfall are expected as this strong low
progresses through the weekend.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

The most significant weather feature continues to be the broad
gale force low in the Bering Sea on Monday. The track of this low
is the same as it has been, heading northeastward across the
Bering before drifting northward to the Arctic. A wide swath of
gales over the Eastern Bering to Unimak Pass will diminish on
Monday and by Tuesday, winds are expected to be westerly and sub-
gale-force.

Monday, winds will have weakened in the western Gulf, but a
barrier jet will bring minimum gales to the north Gulf Coast and
into Prince William Sound. These should rapidly diminish on
Monday, leaving Tuesday sub-gale force across the region.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Friday)...

By Sunday night and Monday morning, the upper-level flow will
become more directly southerly, which will allow for increasing
chances of much-needed rain across Southcentral. Copper River
Basin will be the driest area in this flow, although they should
expect to see rain to their south and over the Wrangell Mountains.
Although not unseasonably strong, this will be the first such
storm to move our way this fall, so expect a significant change in
the weather.

West of the Alaska Range, the low is forecast to move northward
through the eastern Bering and toward the Arctic. This should
bring gusty southeasterly winds to the coastal areas and the
Aleutian Islands. Again, this isn't unseasonably strong, but a
significant change from what we have been seeing. Of more concern
is the prolonged westerly flow across the Bering beginning Monday
and ending Wednesday. Right now models are not indicating problems
with storm surge for the west coast, but this bears watching as
we head into early next week.

After Tuesday and Wednesday, models are in surprisingly good
agreement for the end of the week. The long wave trough remains in
place, which will keep an active weather pattern with low pressure
systems moving through the forecast area through the end of the
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...
GALES... 150 155 160 165 170-175 178-181 185
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...LF