Area Forecast Discussion
Issued by NWS Anchorage, AK

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067
FXAK68 PAFC 070155
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
555 PM AKDT Thu Aug 6 2020

.ANALYSIS AND UPPER LEVELS...

A vertically stacked 985 mb surface low situated over the Eastern
Bering is slowly drifting northeast this morning. An occluded
front attached to the surface low is stretched from Interior
Southwest Alaska south over the Western Gulf. Widespread rain is
falling along the front from the Southern Kenai Peninsula and
Kodiak Island north and west to the Middle Kuskokwim Valley.
Behind the front and closer to the center of the low, the
precipitation has transitioned to scattered showers from Bristol
Bay to the Kuskokwim Delta.

Downstream, the ridge axis in place over the Alaska Range
yesterday evening is now retreating to the north as the upper low
moves east and the frontal system lifts toward the northern Gulf.
Scattered shower activity continues to linger across portions of
the Susitna Valley this morning, enhanced by a weak upper-level
wave lifting northwest and phasing with the larger upper-level low
to the west. To the south, showers are developing from Western
Prince William Sound across the Kenai and Chugach Mountains and
parts of the Anchorage Bowl out ahead of the main frontal system.

Farther west, widespread clouds, showers, and northwesterly gale-
force gusts persist from the Pribilofs to the central and eastern
Aleutians as weak upper-level waves move north to south,
pinwheeling around the main low.

&&

.MODEL DISCUSSION...

Models remain in good agreement through 00z Saturday (late Friday
afternoon AKDT). The NAM and GFS continue to both weaken the
upper-level low over Kodiak Island Friday morning as it
transitions to an open wave over the northern Gulf through Friday
before moving toward the Alaska Peninsula by Saturday morning.
The weakening of the upper-level dynamics translate to a weaker
solution for the evolution of the developing surface low near
Kodiak Island Friday and the transition of this feature to an
open trough by Saturday morning. The GFS and NAM also track both
features slightly farther south, away from the Southcentral
coast. This has continued to ratchet up confidence that winds
will remain below gale force outside of the Barren Islands with
widespread rain confined to the coast.

Guidance then breaks down after 00z Saturday, specifically with
the track and strength of the next low as it moves from the North
Pacific through the Gulf late Saturday into Sunday. The 18z GFS
signaled a shift in both the upper-level and surface lows
slightly west compared to the NAM, EC, and previous runs of the
GFS. The 00z GFS run continued this trend with a track of a
stronger surface low 200 miles west of the other solutions,
tracking the center of the low 100 miles east of Kodiak Island
late Saturday to just south of Cordova by Sunday morning. The 06z
GFS run now has jogged the track of the low back to the east
slightly with the 06z NAM farther west than its 00z run. Both
models are in better agreement as of the 06z runs, but still
nowhere near in lock step. The upper-levels are a mirror of this,
with the 00z GFS now spinning up a vigorous upper-level low by
Saturday afternoon with other 00z solutions keeping this feature
a positively-tilted open wave. However, the 06z NAM is now trying
to transition this wave into a closed low over the Northern Gulf
similar to the 06z GFS.

The differences between the various model runs would yield
significant differences to the sensible weather across
Southcentral, with the GFS solution bringing stronger winds
(widespread gales and maybe even a storm-force barrier jet over
the coastal waters) and heavier rain to the most of coastal
Southcentral, with the NAM now bringing at gales to the Northern
Gulf and other solutions keeping the brunt of the impactful
weather east of Prince William Sound. Given these run to run
differences, forecast confidence is below average and little
change was made this morning to the weekend forecast.

&&

.AVIATION...

PANC...Expect cloudy but VFR conditions with some isolated rain
showers near the airfield through the evening. Southeasterly winds
have diminished and made the turn down inlet, keeping light winds
out of the north through the night.

&&

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

The frontal system continues to push slowly northward into the
Gulf with moderate to heavy rain continuing along the windward
sides of Prince William Sound and the Eastern Kenai. Radar is
showing very light returns on leeside of Chugach with a few
showers across the Anchorage Bowl and the Mat-Su Valleys and only
trace amounts of rain reported thus far. Seward is also seeing
lower amounts of rainfall due to southeasterly low level flow
with mostly light rain reported. Winds are blowing 20 to 30 knots
up the Turnagain Arm, however Anchorage and areas nearby are
seeing light northerly flow due to down-Inlet pressure gradients.
The Knik winds have been around 10 to 20 knots and bending
westward towards the Wasilla area. Finally, winds in the Copper
River Valley are 20 to 30 knots.

As we head towards Friday the upper trough will move inland and
weaken with decreasing gap winds and precipitation. Expect showers
for most of Southcentral tomorrow with best chances from Kodiak
Island to the Prince William Sound area and higher terrain inland.
Although there is decent instability inland mainly along the
Alaskan Range and Talkeetna Mountains, we do not expect much in
the way of thunderstorms as forcing is weak and the shortwave
remains well south near the coast.

Saturday the upper trough dissipates by afternoon but with lingering
moisture and weak forcing, chances of showers remain possible
across most of the Southcentral area. Our focus then turns to a
strong jet moving into the Gulf Saturday afternoon into Sunday.
This will cause a Fall-like low pressure system to form over the
Gulf with winds to near Gale force in the offshore zones late
Saturday night through Sunday morning. By Sunday, there is low
confidence in the forecast with models all over the place with the
progression of the system. However for now we leaned towards the
NAM, EC, and Canadian solutions with the GFS being the (strongest)
outlier.

&&

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

Satellite shows clouds and showers moving across Southwest Alaska,
with isolated areas of convection spreading inland around Bristol
Bay and south-southwest of the Kuskokwim Valley this afternoon.
Meanwhile, wetting rains associated with a weakening front will
gradually taper off from the area starting Friday morning. At 500
mb, a shortwave meandering over the Kusko Delta and south of the
Kuskokwim Mountains between Friday morning and Friday night could
bring increasing cloud coverage and slight chance of showers,
otherwise improving weather conditions will resume as soon wet
weather conditions departs from the area.

&&

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

Strong northwest winds and gusts reaching mid-range gales swept
across the Eastern Bering Sea, portions of the Central Aleutians
and through Central Bristol Bay since this morning. As the
weakening low crosses Alaska Peninsula (AKPEN) tonight, winds and
seas will also gradually subside across the Bering Sea and
Aleutians starting late tonight into Friday afternoon.

A well-defined upper level trough over the Western Bering will
pass through the Aleutians followed by a frontal boundary starting
Friday morning. This next frontal system will boost winds to
small craft conditions and will at least bring steady rain across
the area over the weekend. Otherwise, low clouds and stratus will
emerge again as the new front enters the forecast waters in the
coming days.

&&

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

Gulf of Alaska: Models are having a hard time handling a North
Pacific low moving into the Gulf on Sunday. Most models have the
system tracking into the Western Gulf, while the NAM solution is
keeping a weaker solution farther east. There is also a bit of
spread on the strength of this low pressure system, but most
models are in consensus that winds will be in the gale force
range, with the GFS trending towards a higher solution. By Monday
morning, this system weakens, with quieter conditions expected for
Tuesday. Stay tuned for future forecasts as this system continues
to be evaluated and updated.

Bering Sea: Models are in good consensus with a front tracking
over the Western Bering and Aleutians Sunday into Monday morning,
with winds likely to stay below gale force level. A weak front
associated with the system will continue to track eastward towards
the Eastern Bering and Aleutians through Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Thursday)...

The overall synoptic pattern on Monday keeps a low approaching
the Western Aleutians and Bering, with a low over the Gulf of
Alaska. Models have are in pretty good agreement for the system
out west, keeping showers in the forecast as the front moves
eastward through Tuesday, stretched across the Bering and
Aleutians. Southwest Alaska will remain relatively dry until this
front approaches the coast for Wednesday, but models are still
struggling with timing and position of precipitation moving in
through Thursday.

For Southcentral, a weakening low in the Gulf will keep a chance
of showers in the forecast on Monday, before tapering off on
Tuesday. However, the uncertainty is this low position keeps
precipitation a forecast challenge. A front approaching late
Wednesday associated with a low south of the Aleutians will be
the next chance for widespread rain, but models are having a much
harder time handling this system the farther east it travels.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES: 150, 155
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PS
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...AH



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