Gale Warning 510, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 AM AKDT Fri Sep 9 2022

.ANALYSIS AND UPPER LEVELS...

The closed upper level low is now situated just east of the Y-K
Delta with a strong, elongated shortwave above the surface cold
front extending from the Susitna Valley, over the Kenai Peninsula
and into the North Pacific. The through is beginning to take on a
negative tilt, which is helping to shift the core of the
atmospheric river (AR) east this morning. Within the AR
precipitable water ranging from 1 to 1.5 inches (150%-250% of
normal) is now pushing east of Cordova via the 100 kt to 120 kt
jet stream. Over the population centers, a band of moderate to
heavy rain showers continued for much of the overnight hours in a
band extending from Fire Island north through Talkeetna. Cooler
air streaming east through Kamishak Gap as well as cold air
advection aloft has increased instability for the Cook Inlet
Region north into the Mat- Su Valleys, which is helping to
reinvigorate shower activity this morning along the western faces
of the Chugach and Kenai Mountains.

Over Southwest Alaska, shower activity and strong southerly to
westerly winds exist throughout the area in the cold sector of the
storm. A shortwave rounding the western flank of the upper low is
helping to enhance rainfall rates over Bristol Bay. Over the
Bering Sea, a shortwave ridge has lead to widespread stratus in
addition to some light showers and moderate westerly winds.

Rainfall totals have been impressive over the Eastern Prince
William Sound over the past 24 to 30 hours. Cordova Airport has
reported 6.5 inches as of 5am with Mt Eyak SNOTEL (1400 ft)
measuring 8.2 inches. The Power Creek station measured a whopping
10.3 inches over the last 30 hours. In general, rainfall rates of
0.3 to 0.6 inches an hour have been observed in Cordova, but are
lightening up now that the core of the AR has shifted east. In
the Valdez area, Sugarloaf Mountain has recorded 3.0 inches and
Thompson Pass has recorded 4.0 inches. Rapid rises in small
rivers and streams have been recorded in Valdez and Cordova, with
a flood advisory and flood warning in effect for each location
respectively.

&&

.MODEL DISCUSSION...

Models are in good agreement overall in regards to the movement of
the atmospheric river east and out of the AOR between noon and 3pm
today. The primary forecast challenge will be timing of shortwaves
as they round the southern edge of the trough, resulting in more
shower activity. Otherwise, models remain in good agreement in the
short term.

&&

.AVIATION...
PANC...Showers will bring periods of MVFR ceilings to the airport
this morning but should return to VFR in the afternoon. Winds will
remain out of the south with occasional gusts to 20 kts in the
morning hours before the Turnagain Arm wind moves back into the
terminal mid-morning. Flow will then transition back to up-inlet
later in the afternoon. Overall, winds will remain elevated (20
kts sustained) and out of the south through the forecast period.

&&

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

A short-wave will lift northeastward across Southcentral this
morning, bringing an end to the atmospheric river and heavy rain
which has been trained on eastern Prince William Sound and the
Copper River Basin since yesterday afternoon. This will bring
another half inch to one inch to these areas before the rain
diminishes. A flood advisory is in effect for the Valdez area,
with localized areas of minor flooding expected. For the Cordova
area, a flood warning is in effect for widespread flooding of low
lying areas along Eyak Lake and nearby rivers and streams. Water
levels in eastern Prince William Sound are generally expected to
crest sometime late this morning or afternoon, then slowly begin
to fall. Flood watches remain in effect for the Copper River Basin
and Mat-Su areas, though rainfall rates in these areas have not
been nearly as significant. Still, area rivers and streams are
rising in response to the rainfall.

Additional weaker short-waves will follow closely behind the main
upper trough and surface cold front, leading to showers across
Southcentral. Deep and strong southwest flow will enhance showers
along the south to west sides of mountain ranges. This includes
the Valdez-Cordova area. While rainfall rates will be much lighter
than what was observed last evening and overnight, there will
still be more localized areas of heavy rain as batches of showers
move through, which will maintain elevated water levels along
area rivers and streams.

There will be a brief break in rain for much of Southcentral on
Saturday as the upper flow briefly becomes zonal. Lighter rain
showers will linger in Prince William Sound and along area
mountains. This is sounding like a broken record, but yet another
upper low and trough will drop out of Russia and across the
Bering Sea on Saturday. The leading edge of this trough will
reach Kodiak Island and Southcentral Saturday night through
Sunday, bringing another round of rain.

Gusty southerly winds will develop across much of the region
today as cooler more unstable air moves in behind the cold front.
Winds will be strongest across the western Gulf and Cook Inlet.
Winds will linger through tonight and then die down on Saturday.
More localized southeasterly gap winds will persist through the
weekend as pressure gradients tighten in response to the storm
system in the Bering Sea.

&&

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

A long-wave trough positioned over the Mainland continues to shift
eastward. An associated surface low located near the Kuskokwim
Delta is influencing a shortwave to track into Southwest Alaska
to bring in another shot of moderate to heavy rain showers
throughout the day today. Gusty west-northwest winds have also
continued to increase for the offshore marine areas and has begun
to impact the the Alaska Peninsula early this morning and is
expected to continue through the day before slowly decreasing in
the evening. As a result, the heaviest precipitation is expected
in upslope areas along the Kilbuck and Kuskokwim Mountains and the
western Alaska/Aleutian Ranges. Saturday is expected to be a
fairly quiet day with rain rates slowing and chances of
precipitation decreasing. However, a few lingering showers will
continue through the night as abundant moisture stays in place
over the region. By Saturday night, a fast moving upper-level
wave will track across the Bering and bring the next round of rain
and winds for the Alaska Peninsula through Saturday. Another
longwave trough will set up over the Bering throughout the rest of
the weekend. This will keep much of the same unsettled weather
occurring across the domain for the next several days.

&&

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

A low will continue to track southeastward in the Bering
throughout the day today. Accompanying the aforementioned low
will be rain and small craft winds along the western and central
Aleutians. As this system exits the area on tonight, a front
extending ahead of a new low located in the Gulf of Anadyr,
brings the next round of rain and small craft winds to the region.
Tonight through Saturday morning, areas of low end gales, with
higher gusts, are possible along the leading edge of the
aforementioned front as it tracks across 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 111 141 145
 		 Flood Advisory 131
		 Flood Warning 135
MARINE...Gale Warning 137 139 140 155 160 175 177 178 180.
FIRE WEATHER...NONE.

&&

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