Flooding in Alaska, September 3 to 7, 2022

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
529 AM AKDT Sat Sep 3 2022

.ANALYSIS AND UPPER LEVELS...

A broad longwave trough continues to track over the Alaska
Peninsula and into the Gulf this morning. A shortwave rotating
around the aforementioned trough and into Southcentral is causing
widespread rain showers over Price William Sound and portions of
the Kenai Peninsula. Its associated occluded front is extending
across the northern Gulf and along the coastline. Further west,
another upper-level low is tracking towards the western Aleutians
and is helping to build a ridge of high pressure over the central
Aleutians. Satellite Imagery is showing a blanket of marine
stratus over the central Bering as a result.

&&

.MODEL DISCUSSION...

A trough rotating over the Alaska Peninsula and into the Gulf is
continuing to elude the models on its smaller scale features. Due
to inconsistent run-to-run consistency forecast confidence
continues to be low. Models were starting to trend precipitation
further east over the past couple days, but runs are now pulling
precipitation chances further west with a greater threat for
convection over the northern Gulf and over the mountains inland of
the coast. However, the timing of all of this is still in
question. Overall, forecast confidence continues to be low.

&&

.AVIATION...

PANC...VFR conditions are expected to continue. An easterly wave
of low pressure will impact the region early this morning with
increasing rain chances and lowered ceilings. There is quite a bit
of uncertainty in the track of the low currently over the
northern Gulf and as it heads inland and weakens. This will impact
the extent of cross-barrier flow and distribution of rainfall in
the Anchorage area. As a result, forecast confidence is low with
regard to how far the ceiling will drop. At this point, have
maintained low end VFR ceilings, but there is potential for MVFR
conditions to settle in this morning. A more unstable airmass will
move in during the day today and there should be at least a weak
southeast flow in the low levels. As a result, expect increasing
ceilings after about 18Z along with a chance of some gusty
southeasterly Turnagain Arm winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3...Sat
through Mon)...

The broad upper level trough will slowly drift east over the Gulf
through Sunday morning. This should allow some drier air to slowly
work its way in from the north. However, there is a lot of
moisture in the atmosphere that will need to be overcome. The
Anchorage sounding this morning indicates 0.83 inches of
precipitable water, just confirming how juicy it is. The surface
low that tracked towards Cordova on Fri will fall apart near
Prince William Sound today. The winds in the Sound will be tricky
to nail down as it does so, but speeds should all remain below
small craft levels (23 kts). The remnant energy associated with
the low will then track inland. This energy combined with some
daytime heating/instability will allow for some breezy southeast
winds to mix down to the surface. Besides the typical gaps, the
most noticeable spots for these winds will be W Anchorage, the
Knik Arm, and the Copper River Valley. We also can't rule out a
slight chance of some thunderstorms along the mountains today,
especially over the Copper River Basin and Kenai Peninsula.

Another weak surface low will take nearly an identical track to
Friday's low and move right into the Cordova area on Sun. This
will once again produce a weak barrier jet and bring steady
precipitation back into Prince William Sound late this evening
into early Sun. As the upper level trough exits the Gulf to
the east, showers will slowly dissipate across the entire area by
Sun afternoon. While a shortwave ridge attempts to nudge in from
SW AK and bring some clearing skies, yet another shortwave slides
quietly into the area from the north. So while there may be a few
breaks in the clouds, they are looking less likely due in part to
this feature. Either way, besides some stray showers, there looks
to be a mostly dry window across much of the area from mid-day Sun
to mid-day Mon. Just in time for the Holiday Weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Tuesday morning)...

Gusty northwesterly winds and rain chances across the Alaska
Peninsula will continue to decrease during the day today as deep
northwesterly flow weakens. Chances for rain will increase again
this afternoon for the inland Bristol Bay area and Lower Kuskokwim
Valley in relation to a low stationed over the Gulf of Alaska.
The entire Southwest region will see a shift towards calmer
weather by Sunday as a high-pressure ridge enters from the west.
Expect drier conditions, light winds, and clearing skies to unfold
for the remainder of the weekend. By Monday afternoon, rain and
clouds will return to the region as a result of a low extending
south off of Northern Bering low.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Tuesday morning)...

Gusty winds across the Eastern Bering and Aleutians will continue
to diminish during the day today as deep northwesterly flow
weakens. A ridge building over the Bering will promote clearing
skies (where their isn't stubborn low stratus) and drier
conditions today for the central and eastern portions of the
domain. On the other hand, the Western Aleutians are seeing more
active weather as a low and its front track along the Aleutian
Chain. Rain and gusty winds will continue spreading east to the
Central Aleutians today before tapering off Sunday morning as the
low dives south into the North Pacific. In general, expect most of
the rain and stronger winds with this system to stay confined to
the Western and Central Aleutians and their surrounding waters. On
Sunday morning, high pressure is advertised to blanket the entire
domain before the pattern quickly becomes more active. The
pattern change will be a result of multiple upper troughs tracking
from eastern Russia to and across the Bering Sea. The first
trough and embedded low are expected to enter the Western Bering
later on Sunday morning and continue east. The second trough will
enter from the west on Monday and track east through at least
Tuesday morning. Nonetheless, expect unsettled weather to unfold
across the domain starting on Sunday.

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

Confidence is fairly high that winds will be less than 35 kt and
seas less than 12 ft from Monday through Wednesday. However, there
is a slight chance that northwesterly gap winds could approach
35 kt south of the Alaska Peninsula on Wednesday.

&&

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

Model agreement is fairly good on the synoptic scale through Friday.
Overall, expect mostly unsettled conditions as a longwave trough
redevelops over much of Alaska and the Bering Sea from Tuesday
through Friday. The exception will be the Aleutian Chain, where
much quieter weather is expected until early Thursday due to
higher pressure.

For much of the forecast area, active weather begins on Tuesday as
a vertically stacked low and its front push into Southwest Alaska.
This looks to bring steady rain across the region through Tuesday,
with rain gradually tapering off on Wednesday. Temperatures will
likely dip in Southwest Alaska as northerly flow helps bring in
cooler air. Southcentral could see some rain on Tuesday as this
front marches eastward, but the bulk of rainfall will likely come
on Wednesday as a low develops at the triple point near Kodiak
Island. Then, southerly to southeasterly flow looks to bring
sustained rainfall to the North Gulf Coast from Wednesday through
Thursday. Model confidence is lower from Wednesday onwards, though
the general consensus suggests the triple point low will move
towards the Northern Gulf. The GFS and Canadian models seem to
favor a solution that will likely keep the bulk of rain confined
to the coast, with the potential for gusty gap winds along the
Alaska Peninsula behind the low.

For Thursday, attention shifts towards a low and its front moving
into the Bering Sea from Russia. This looks to be a quick-moving
feature, with winds and rain intensity diminishing from Thursday
through Friday as the low weakens. The front will reach Southwest
Alaska by Friday, bringing the possibility of coastal flooding
due to sustained southwesterly flow. Details will become clearer
as we get closer to this event; while the pattern is favorable for
coastal flooding, winds do look a bit low.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...Sprinkles
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...Fenrich
MARINE/LONG TERM...KC