Flood Watch 3, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
548 AM AKDT Sun Aug 7 2022

.ANALYSIS AND UPPER LEVELS...

The upper-level longwave trough axis remains in place over the
state, keeping active and dynamic weather across the southern
mainland. As one shortwave trough exits Southcentral to the
Yukon, the next one moves in over the northern Bering and
Southwest Alaska. At the surface, a weak low in the northern Gulf
is tracking out of Prince William Sound, allowing scattered
showers to clear out for Cordova and Valdez as onshore flow
ceases. With the shortwave exiting east into the Yukon Territory,
showers across Southcentral Alaska as a whole are decreasing in
coverage and intensity, with lingering activity primarily over the
Copper River Basin. However, very isolated showers continue over
the Susitna Valley and Cook Inlet. Despite precipitation clearing,
broken mid-level cloud cover continues to blanket much of
Southcentral as a moist environment remains in place.

The second shortwave beginning to move through the area is
associated with a developing surface low centered just south of
St Lawrence Island pushing a series of fronts into Western Alaska.
A warm front out ahead of the low is producing widespread
stratiform rain as it moves inland over the Kuskokwim Delta, where
one to two tenths of an inch of accumulation have been reported
over the past six hours. A cold front trails close behind,
stretching out of Far Eastern Russia and sweeping south across the
northern Bering with a surge of sub-zero (Celsius) 850 mb
temperatures on course for the southern Mainland early in the
week.

Out west, a North Pacific low is approaching the western
Aleutians, with its rain-producing front remaining just southwest
of the islands, over the marine area. Otherwise, conditions over
the rest of the Bering Sea/Aleutians are calm, with widespread
stratus, under the influence of high pressure. Some areas of fog
are present over the eastern Bering, including the island of Saint
Paul, where visibilities have frequently been as low as one
quarter mile overnight.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement through the short term
with several shortwaves and fronts moving through the long wave
trough. Model differences with the timing and placement of
shortwaves tracking across Southwest and Southcentral Alaska will
introduce uncertainty in the timing of precipitation and
enhancement of winds.

&&

.AVIATION...
PANC...VFR conditions are expected through this afternoon before
southerly winds develop over the terminal, with gusts as high as
25 knots possible. Rain showers also develop this
afternoon/evening, which are expected to bring ceilings down to
MVFR. MVFR ceilings and gusty southerly winds will persist into
Monday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Tuesday night)...

A deep upper-level low over the Chukchi Sea is sending waves of
vorticity across southern Alaska, and will persist into the week
as the low drops south into the Bering Sea. Current dry conditions
will diminish as widespread rain enters Southcentral this
afternoon. Rain will persist in Southcentral until Tuesday as
precipitation moves out of the area to the east. Precipitation to
approach Kodiak Island from the south by Tuesday morning and will
persist through Wednesday afternoon.

Near-freezing temperatures associated with the dropping low will
push through the Alaska Range around Paxson by Monday bringing
cooler temperatures to Southcentral. No snow is expected with this
wave of colder air as sources of precipitation have already moved
out of the area by this time. Gusty up-inlet flow expected along
the western Kenai Peninsula into the Anchorage Bowl expected to
kick-off this afternoon followed by a Turnagain Arm wind shortly
after. Both are expected to diminish by late Monday night.

&&

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

Parts of Southwest Alaska will see heavy rain and strong winds
today as a shortwave and its associated surface low move into the
Yukon Delta. As the front moves across Southwest Alaska through
Monday, many locations will see storm total rain amounts around
one-half to one inch, with lower amounts across interior Bristol
Bay and the Middle Kuskokwim Valley. Gusty westerly to
southwesterly winds will also accompany this front, with sustained
winds up to 35 mph and gusts up to 45 mph. There remains a
potential for coastal surge and beach erosion as waves impact the
shoreline, but the forecast surge height has continued to trend
lower as the system slows.

Conditions improve as we head through Monday. Winds will continue
to diminish through the day. Rain will linger along the mountains
and in southern Bristol Bay/northern Alaska Peninsula. Other
locations should see rain gradually taper off as cold, dry air
accompanies an upper level low into the region. This will help
drop temperatures into the high 30s on Monday night. Tuesday will
see clearing skies, though cold temperatures overnight will keep
highs below 60. Fairly quiet weather over most of Southwest Alaska
will be juxtaposed with an atmospheric river pushing towards the
Alaska Peninsula late Monday night into Tuesday. There remains
uncertainty with the placement of this feature, so there is lower
confidence in rain amounts at this time.

&&

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

Stratus and areas of fog continue to blanket the Bering Sea. Most
locations can expect the weather to stay quiet as a ridge remains
over the Bering Sea and much of the Aleutian Islands. One
exception to this will be in the Northern and Eastern Bering,
where small craft winds will persist through Monday morning as a
front tracks into Southwest Alaska. While winds will begin to
diminish Monday morning, rain will linger through the day along a
front that extends from Bristol Bay through the Central Bering.
The Western Aleutians will also see winds increase and a chance
for rain as a North Pacific low's front grazes the area today.
More rain is on the way from Monday night through Tuesday as an
atmospheric river extends across the Eastern Aleutians. The
placement of this feature remains uncertain, so there is lower
confidence in rain amounts at this time.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...

Gulf of Alaska: Confidence is high for small craft winds or less
and seas less than 20 ft for Tuesday through early Wednesday.
Between Wednesday and Thursday, a low pressure center is expected
to enter the southwestern Gulf and shift towards the central Gulf
with time. Winds could peak at or near gale force around portions
of this system. However, significant uncertainty remains for the
track and timing of the low pressure, and confidence is low for
timing and location for any winds reaching gale force between
Wednesday and Thursday.

Aleutians/Bering: High pressure is anticipated to have the
dominant influence over most of the Bering throughout the period,
where confidence is moderate to high for small craft winds or less
and seas less than 20 ft to prevail. From Wednesday to Thursday,
there is some possibility for stronger winds to pick up on the
back side of a North Pacific low moving south of the Alaska
Peninsula and into the Gulf. The best chance for any winds to gale
force would also be in the vicinity of the Alaska Peninsula.
However, uncertainty for the track, strength and timing of this
system remains significant, and confidence for any higher winds in
this area between Wednesday and Thursday is low.

&&

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

There remains a decent signal for longwave ridging to become the
dominant player over most of the Bering Sea and Aleutians during
the course of the extended range, where seasonably benign
conditions will generally persist. Models are continuing to
struggle quite a bit with the remainder of the pattern details,
particularly across the Gulf and Southcentral vicinity. A fair
amount of this uncertainty appears to stem from the poor run-to-
run consistency for the handling of two separate upper level
troughs, especially leading up to the start of the extended range
between Tuesday and Wednesday. Previous model cycles projected a
shortwave moving southeast across the eastern Bering and AKPen
later phasing with another upper trough moving over the Interior,
becoming one large upper low over Southcentral on Wednesday.

More recent operational runs have backed off on this scenario
markedly, instead keeping two distinct troughs moving over the
southern mainland and southern AKPen respectively, separated by a
shortwave ridge over the Gulf by Wednesday. This is leading to
wild fluctuations in especially the GFS for the entire pattern
progression during the latter half of the week, yielding overall
poor forecast confidence for just about all of the extended
period. Shifts in the ensemble guidance have been less pronounced,
but still are hinting at the slower progression of the trough
moving across the Bering and resulting delay (or complete
avoidance) of the merger with the other upper trough sweeping
across the southern mainland. Still, there remains a decent
consensus for longwave troughing to eventually become established
over far eastern parts of the domain, with the trough axis setting
up roughly along the AlCan border through the end of the week.
This synoptic setup would favor a steady drying/warming trend with
persistent westerly to northwest flow, especially across
Southwest where upper ridging over the Bering will likely
expand/move east with time. Again though, other details for
individual systems are not at all easy to parse out at this point,
leading to the overall decreased forecast confidence.

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

&&

SYNOPSIS AND MODEL DISCUSSION...CQ
SOUTHCENTRAL ALASKA...CL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
MARINE/LONG TERM...AS