Small Craft Advisory 586, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
528 PM AKDT Mon Aug 3 2020

.ANALYSIS AND UPPER LEVELS...The pattern is starting to slow down
after a very wet weekend for some parts of the state. In general,
a broad vertically stacked over the Eastern Aleutians continues to
make for cyclonic flow across the southern mainland. Some parts
along the AK Range received over 5 inches of rain through the
weekend. This has resulted in high water levels across the Copper
River Basin and the Susitna Valley. A Flood Watch has been issued
for the Yentna River. Otherwise, there is widespread moisture
across the area leading to fog and low stratus continuing to hang
on, especially near coastal sites.

The stacked low is in the center of a broad longwave trough. The
trough is supported by a mostly zonal jet that runs from the
Kamchatka Peninsula all the way into SE AK. This jet peaks at
about 110 kts at 30,000' (300mb). There remains enough instability
that hefty showers continue to pop up on radar over SW and
Southcentral AK.

&&

.MODEL DISCUSSION...The models remain in good agreement with the
synoptic pattern. Thus, forecast confidence is moderate to high as
it pertains to the longwave evolution. The biggest difference is
regarding the surface low that develops east of Kodiak late
tonight. The American models (NAM/GFS) continue to hold this
feature further west and keep it stronger even on their 12 and
18z runs this morning. Meanwhile, the EC and Canadian are weaker
and further east over the open Gulf. This will make a significant
difference in how strong the winds get in the marine zones around
the island. For this forecast package, we leaned more towards the
inherited forecast, which is closer to the American models. There
continues to be very good agreement among all the models on the
surface low tracking along the Aleutian Chain through Wed. If this
feature trends any stronger, we may need to increase winds to
gale-force over some of the western marine zones.

&&

.AVIATION...
PANC...VFR conditions should persist. A heavy passing shower could
knock conditions into the MVFR category briefly, but this is not
very likely. The Turnagain Arm winds will persist through the
evening before they diminish. The exact timing of when they die
off could vary by an hour or 2.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday)...

A low moving into Western Gulf is currently bringing moderate
rain across Kodiak Island. Its associated front will track northward,
bringing chances of rain along coastal areas overnight into early
Tuesday morning. Gusty winds along Turnagain Arm and in the Knik
River Valley will persist through the evening before diminishing
overnight. Skies will clear Tuesday and Wednesday across much of
Southcentral with warmer temperatures expected. Brief afternoon
convection is possible near higher terrain. Another front will
move into the Gulf by Thursday, bringing unsettled weather over
much of the region through the end of the week.

&&

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

An upper level ridge over Southwest will slide eastward and
across Southwest Alaska followed by a frontal boundary, which
will provide wetting rains over much of the coastal area starting
Wednesday morning. In response to the upper level ridge, a thermal
trough expanding south of the Kuskokwim Mountains and the Alaska
Range will trigger thunderstorms and scattered to numerous showers
between Tuesday and Wednesday mainly during the afternoon and
evening periods. Overall, anticipate a wet forecast over Southwest
and much of AK Peninsula in the coming days.

&&

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

Satellite imagery and computer models display a broad area of
patchy fog and low level clouds from earlier this morning into
this afternoon, and this situation will continue for the next
couple days as an upper level ridge extends from the Aleutians
northeastward through Kusko Bay. A frontal boundary over the Western
Aleutians will bring a lot of steady rain and small craft
conditions over the region. Although AK-NAM12 and HiRes models
seem aggressive and favor gale force conditions in response to
the frontal system, decided to stick with small craft conditions
for now and monitor the development of the frontal zone. As a
result, expect low level clouds and fog to linger over the region
through the next few days.

&&
.MARINE (Days 3 through 5:Thursday through Saturday)...

Bering Sea/Aleutians

A low over the eastern Bering on Thu will move onshore over the
AK mainland Fri. Forecast confidence has increased that there
should be Gale force gusts from the Pribilofs south towards the
eastern Aleutians and AKPEN through Fri morning. These winds will
be amplified in the terrain gaps. By Fri afternoon forecast
confidence is moderate that a low will enter the western Bering,
with the primary warm front reaching the central Aleutians on Sat.
Winds should be lighter with some small craft advisory winds
possible over the central Aleutians.

Gulf of Alaska

A front will move east across the Gulf of Alaska through Fri night
with a triple point low forming to the east of Kodiak Island Fri
morning. Confidence is moderate on Fri in regards to a barrier jet
setting up along the Northern Gulf Coast north of the low. This
would bring small craft easterly winds to the coastline.
Otherwise, forecast confidence is good that small craft westerly
winds should persist to the south of Kodiak through Fri night. By
Sat the low should move onshore, which will help keep winds light
and westerly across the Gulf.

&&

.LONG TERM FORECAST (Days 4 through 7:Friday through Monday)...

A broad longwave trough extending from Far Eastern Siberia to
Southeast Alaska should continue the cool and wet pattern. The
main, or strongest, upper level low should be over Southwest
Alaska on Friday with a front extending into the Gulf of Alaska.
This system should push east through Sunday, bringing rain and
cooler temperatures to the Alaska mainland. Model uncertainty
increases on after Sunday, with the GFS solution indicating strong
ridging in the wake of the aforementioned system while the
ECMWF/Canadian models have a more zonal jetstream. The GFS
solution would bring a return to summer-like weather of warm
temperatures and sunny skies from the AKPEN to Southcentral,
while the other solutions would likely keep things cooler and
continue the fall-like pattern. Regardless, guidance is in good
agreement on Mon as the next system arrives in the Western Bering.
The exact weather impacts of this system are still uncertain but
long term guidance does indicate a long, deep moisture fetch
coming into the region from the Central Pacific early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FLOOD WATCH: ZONE 145.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...CJ