Small Craft Advisory 709, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKDT Thu Sep 8 2022

.ANALYSIS AND UPPER LEVELS...

A quasi-stationary, high amplitude trough exists from the central
Bering Sea to Southcentral. Moisture originating from
approximately 35N latitude can be seen on satellite streaming
directly into Southcentral via a 140 kt to 160 kt jet stream. The
surface low associated with this trough is currently over Bristol
Bay. All three operational radars depict widespread light to
moderate rainfall from Bristol Bay east into Southcentral. With
southerly winds aloft and strong dynamic forcing, there is little
in the way of downsloping along the western faces of the Kenai and
Chugach Mountains. The warm front is currently along the Gulf
Coast, with increasing surface winds being observed through
Prince William Sound and into Turnagain Arm. Over the Bering,
fairly quiet conditions exist with broad northerly winds and light
rain showers. Farther west, a front is just grazing Shemya.

&&

.MODEL DISCUSSION...

Overall models are in good agreement over the next few days. Some
minor differences exist in the evolution of the upper level
features associated with the trough and how progressive the system
is come Friday. Particularly, the NAM is much faster in moving
the current system north and out of the region late Friday, while
the other solutions are slower to do so. However, both solutions
have a minimal impact to the forecast overall.

&&

.AVIATION...
PANC...Light northerly winds early this morning will give way to
Turnagain arm winds around 10am this morning. These will peak
around noon with southeasterly gusts between 25 and 30 kts. A
short period of gusts up to 35 kts is possible. In the afternoon,
up-inlet flow will combine with the Turnagain Arm wind to bring
due southerly winds around 15 kts. Southerly winds are expected to
persist through the night, with the Turnagain arm wind
redeveloping tomorrow morning. Rain is expected to linger through
Friday with MVFR ceilings possibly developing after noon today.
Moderate to heavy rain at times may lower visibility, but it
should be predominately VFR.

&&

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

The forecast for the current fall-like storm impacting Southcentral
with widespread rain and wind is largely on track, with just some
minor changes to the forecast. Rainfall totals overnight were
generally on the light side, but rain rates will increase this
morning as a leading warm front pushes onshore. The heaviest rain
will occur this afternoon through Friday morning across eastern
Prince William Sound (Cordova/Valdez areas) as a strong short-wave
trough lifts toward Southcentral, acting on the atmospheric river
aloft. While the short-wave starts out neutrally tilted, it does
look like it will take on a negative tilt as it nears eastern
Prince William Sound and the Copper River Basin. In addition, once
the warm front pushes onshore later this morning, low level flow
will shift to southerly, leading to strong upslope flow along the
south sides of mountain ranges. Flood watches remain out for much
of Southcentral with the likelihood that at least some of these
will be upgraded later today.

The cold front with this system will march slowly eastward across
the western Gulf and Cook Inlet/Kenai Peninsula this afternoon
through tonight. This will bring a break to the steady rain for
these areas, but this will be short-lived as additional short-
waves move in from the west on Friday, bringing another round of
rain. Weaker short-waves will follow through Saturday night. Fast
zonal flow will settle in behind the current high amplitude,
making it a challenge to lock on to timing of specific short-waves
and associated precipitation. Nonetheless, generally expect the
wet weather to continue with only brief breaks of drier conditions.
Persistent strong south to southwest low level flow through
Saturday will also continue to enhance precipitation along south
to west facing mountain slopes.

Meanwhile, gap winds will peak this morning as the warm front
moves onshore. Gusty winds will then spread inland behind the
front. Passage of the cold front tonight through Friday will
further reinforce gusty winds thanks to increased instability and
vertical mixing. As a result, windy conditions will continue for
much of the region through Friday night before finally diminishing
on Saturday.

&&

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

Persistent wet weather will continue to dominate the forecast
area today and Friday. The longwave pattern features a tough over
the Bering. There is a 150 knot jet moving into Bristol Bay. On
satellite imagery (CIMSS MIMIC PW) there is a plume of moisture
extending from Pedro Bay to Kodiak and extending into the North
Pacific. There is a second plume of moisture over the western
Aleutians. The models depict the 500 mb low over Southwest Alaska
today and most of Friday with multiple shortwaves rotating
through. The heaviest precipitation will be in the upslope areas
along the Kilbuck Mountains and the Western Alaska/Aleutian
Ranges.

&&

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

Back to back lows will impact the chain. This morning there is an
area of low pressure over Bristol Bay that is expected to lift
northward toward the western Alaska. Rain and gusty winds are
expected for the eastern Aleutians between today and Friday.
Meanwhile, on satellite imagery there is a second feature over the
Western Aleutians that the models track into the Central
Aleutians. Expect another round rain and gusty winds.

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

Gulf of Alaska: Expect winds less than 35 kts and seas less than
20 feet for the Day 3 through 5 period.

Aleutians/Bering: A low tracks into the western Bering Sea
overnight Friday into Saturday. Forecast confidence is average
with there being a small corridor of low end gale force winds on
the southern periphery of the low Friday night through Saturday
evening as it tracks eastward. Winds diminish to small craft
Saturday night. Expect small craft winds through the rest of the
Bering, gaps and passes along the Aleutian Chain, and on the North
Pacific side of the Chain throughout the rest of the period. Seas
10 to 15 feet are possible closer to the low and subside to 8 to
11 feet Monday morning.

&&

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

Models remain in decent agreement, with good consistency between
yesterday's and today's runs. The extended period begins on Sunday
with an occluded low pushing its front into Southern Alaska.
Widespread rain continues to look likely along its front. A ridge
in the North Pacific Ocean will likely keep flow oriented
southwesterly along the front, at least initially. As mentioned in
yesterday's long term discussion, this looks to support good
moisture flow into interior Southwest and Southcentral Alaska. By
Monday, rain looks to transition into more of a showery regime for
Southwest as the upper level low moves overhead. For
Southcentral, flow should become more easterly as the ridge is
displaced. Depending on the strength of the flow, this could favor
heavier rain along Prince William Sound and the Eastern Kenai
Peninsula, with precipitation diminishing inland due to
downsloping. With the surface low hovering around the 60N latitude
line, this could also favor enhanced easterly winds through
Turnagain Arm and Knik River Valley. However, details are too
uncertain to say for now.

As we head into Tuesday, models are hinting at the development of
a triple point low in the Gulf of Alaska. This, in conjunction
with a ridge moving east across the Aleutian Chain, could lead to
gusty northwesterly gap winds along the Alaska Peninsula. Still,
much uncertainty remains with regards to this low. While both the
GFS and Euro models suggest it could linger in the Gulf through
late Wednesday, the GFS is suggesting a much more compact low
with stronger winds. The much bigger story will be a strengthening
low pressure system that pushes into the Western Bering Sea and
Aleutians early Wednesday morning. Models have come into better
agreement since the previous runs, but there still remains
considerable uncertainty regarding the evolution of this feature.
What models can agree on, however, is that it will be deep.
Deterministic models all agree that pressure could drop to around
970 mb by Wednesday evening, with broad areas of gales and
plentiful moisture. Overall, this low will be one to watch as we
head into next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch 111 131 135 141 145.
MARINE...Gale Warning 119 120 125 128 130-132 138-140 150 160 351
352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...DAN/KC