Flooding in Alaska, September 8 to 9, 2022

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

.ANALYSIS AND UPPER LEVELS...

A complex, deep and now closed off upper level low continues to
drift mainly northeast over the eastern Bering Sea and into
Southwest with time this afternoon. Two embedded upper level waves
visible on GOES west satellite products are rotating around the
main upper center now moving into Southwest, one moving southeast
and crossing the southern Alaska Peninsula (AKPen) and the other
drifting southwest between the Y-K Delta and St Matthew Island.
Farther to the west, a weak front continues to push a band of rain
east across the western and central Bering.

Upstream and to the east, a powerful "atmospheric river" of
moisture extends from the subtropics of the Central Pacific all
the way up into much of Southcentral and out to the AlCan border
in association with a strong 140 kt southwest to northeast
oriented upper jet streak. This concentrated axis of moisture is
also roughly following the contour of a front draped across the
Gulf, with the northern portion of the front spiraling out across
the southern mainland from the parent surface low now moving just
east of Bethel. This surface cyclone is actually deepening as it
heads inland, supporting a strengthening wind field to the south
soon expected to reach gale force range just offshore from the
coast between Bristol Bay and the Kuskokwim Bay area. Gusty winds
are shifting from an easterly to more southerly direction across
Southcentral behind the frontal passage, while weakening winds to
low end gale force persist ahead of the front out over the
eastern Gulf. Areas of steady rain are continuing along the
shearing out front over Southcentral, in particular where aided by
upslope flow into the Chugach and Talkeetna Mountains. Some of
the most intense rainfall is occurring near and north of the
Prince William Sound out to Cordova, where the highest moisture is
streaming northeast into steep terrain.

&&

.MODEL DISCUSSION...

Models are in generally good agreement for the most important
larger features through the upcoming weekend. There are some
differences appearing for the progression of a low and front
expected to cross the Bering between Saturday and Sunday, with the
most recent NAM appearing to be a fast outlier for the front and
an upper shortwave rotating northeast into Southwest by Sunday.
However, this should have only minor impacts to the forecast,
primarily limited to slight timing differences for the onset of
southerly winds and increased rainfall shifting from the AKPen
into Southwest and Southcentral on Sunday.

&&

.AVIATION...
PANC...Gusty southeast winds will persist though the night.
Generally breezy southerly winds will remain over the airfield
through Friday afternoon. Rain will bring in MVFR ceilings at
times through Friday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 2/Tonight
through Saturday night)...

Ongoing showers over much of Southcentral will change little
through the next 24 to 36 hours. A plume of moisture continues to
stream into Prince William Sound, guided by a slow, eastward
moving cold front which extends southward from the Kenai Peninsula
into the North Pacific. This atmospheric river is forecast to
gradually shift eastward through the latter half of Friday.
Correspondingly, the heaviest rain activity is also forecast to
shift eastward and into Yukon. However, Flood Watches remain in
effect across the region and a threat for excess runoff will
persist from the Mat-Su, across the Copper River Basin, into
Northeast and Southeast Prince William Sound through Saturday.

Southeasterly Turnagain Arm winds briefly developed this
afternoon and have since died down. Southerly winds through the
Shelikof Strait will steadily expand northward into Cook Inlet
through tonight. Areas of gales will be possible through the
Shelikof Strait as well as further norther north along the Kenai
Peninsula coast from Ninilchik to Kenai tonight through Friday.
Southeast Turnagain Arms, redeveloping midday Friday, will
gradually bend into Anchorage during the afternoon hours, becoming
southerly into Friday night. By Saturday, low pressure weakens
and more zonal flow is forecast to overspread Southcentral. This
should allow for winds overall to diminish as well as a
significant downward trend in precipitation amounts. By Saturday
afternoon, most locations should dry out with only coastal regions
still seeing occasional light showers with continued southerly
flow into the coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Sunday)...

A longwave trough has shifted across the western Alaska mainland,
with a surface low currently positioned north of the Kuskokwim
Delta. A well-defined shortwave pinwheeling around the low will
bring the next round of moderate to heavy rain showers tonight
through Friday. Gusty, west-northwest winds are also ramping up
for the offshore marine areas and filtering across the the Alaska
Peninsula (AKPEN). Because of the northwesterly component, the
heaviest precipitation will be in the upslope areas along the
Kilbuck Mountains and the Western Alaska/Aleutian Ranges.

By Saturday, rain rates are expected to slow down along with
subsiding winds as the low weakens and the main trough lifts
northward into the Brooks Range. We will see a short-lived break
on Saturday, but a few linger showers will continue through the
night hours as abundant moisture stays in place. Meanwhile, a fast
moving upper level wave crosses the central Bering Sea, bringing
the next round of rain and winds for the AKPEN between Friday
afternoon and Saturday.

For the rest of the weekend, another longwave trough sets up over
the Bering Sea. The trough pattern will likely keep unsettled
conditions going as multiple pieces of energy ahead of the
southwesterly flow ejects across the forecast area. A new low
associated with this trough is expected to move southeastward
from eastern Russia. A front extending ahead of this low brings a
rinse and repeat of widespread rain and gusty winds.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Sunday)...

A low will move southeastward tonight, bringing rain and small
craft winds along the western and central Aleutians. As the system
exits the area on Friday night, a front extending ahead of a new
low from eastern Russia brings the next round of rain and small
craft winds. From Fri night to Saturday morning, areas of low end
gales with higher gusts may occur along the leading edge of the
front as it passes the western Aleutians.

&&
.MARINE (Days 3 through 5: Sunday through Tuesday)...

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 is located over the Northern Bering from
Sunday through Tuesday. Forecast confidence is average with there
being a small corridor of low end gale force winds along the
southwestern periphery of the low on Sunday, and then gales would
likely diminish to small craft by late Sunday night. Expect small
crafts across the rest of the Bering Sea and Aleutian waters,
with winds diminishing through Tuesday as the low tracks
northeast. Seas 10 to 13 feet are possible closer to the low and
subside to 6 to 8 feet by Tuesday morning.

&&

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

Model agreement in the extended period is poor beyond Monday,
having worsened since yesterday. For Monday, model agreement looks
decent with the occluded front moving across Southern Alaska. The
orientation of this front, and the broad southwesterly flow
associated with it, continues to favor widespread rain over most
of Southern Alaska, including inland areas that typically
experience downsloping. Prince William Sound will likely see the
most rain with this event.

Confidence declines into Tuesday. Models differ on the timing of a
shortwave that will help develop a surface low in the Gulf of
Alaska. The Canadian model is quickest to develop this low,
followed by the GFS and ECMWF. The GFS continues to be the most
aggressive regarding the strength of this low. Whereas the other
models are hinting at localized areas of small craft winds, the
GFS is suggesting that winds could reach gale force. Though the
GFS is an outlier, it has also shown consistency from yesterday's
run to today's in spinning up a strong, compact low.

The next feature of note is a broad and deep surface low that
pushes into the Western Bering Sea and Aleutians early Wednesday
morning. Model agreement is notably poor through Thursday, with
major differences in the track and the potential for multiple low
centers embedded within this broad feature. Still, this continues
to look like a system that will pack a punch. Deterministic models
continue to show minimum pressure at around 970 mb and broad
swaths of gales. The Canadian model, in particular, is hinting at
storm force sustained winds around the backside of the low. Though
the Canadian model is noticeably stronger today, it seems to be
most consistent with the track of this low as compared to
other models. The GFS, in contrast, is keeping the low further
west in the Bering as it occludes and multiple low centers
develop. Meanwhile, the ECMWF has the low entering the Bering Sea
from the south and curving northwards as it runs up against the
ridge in the North Pacific Ocean.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch 111 131 134 141 145.
MARINE...Gale Warning 130 138 139 140 160 177 178 180 411 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS/EZ
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MF
MARINE/LONG TERM...AF/KC

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
934 PM AKDT Thu Sep 8 2022

AKZ101-091400-
Anchorage-
Including the cities of Anchorage, Eagle River, Indian,
and Eklutna
934 PM AKDT Thu Sep 8 2022

...PERIODS OF MODERATE RAINFALL WILL CONTINUE ACROSS ANCHORAGE
THROUGH FRIDAY MORNING...

Persistent southerly flow will produce rain across the Anchorage
with another 0.25 to 0.75 of rainfall possible.

The additional rain tonight could push small streams and creeks
to, or slightly above, bankfull. Smaller rivers could see water
levels rise to bankfull; however, water levels along larger rivers
are expected to remain below bankfull.

In urban areas, ponding of water and localized flooding on poorly
drained roadways is also possible. Please use caution when
driving or when recreating along or on area waterways.