Small Craft Advisory 729, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 PM AKDT Thu Sep 15 2022

.ANALYSIS AND UPPER LEVELS...

All attention is on a powerful storm system (formerly Typhoon Merbok)
which is rapidly deepening as it crosses the western Aleutian Islands
into the Bering Sea. Everything about this system looks highly
impressive. The central pressure is estimated at around 950 mb
and falling. The low will pass near the vicinity of Shemya around
6pm AKDT, which will provide an instrument-based reading to
estimate the central pressure. Based on a combination of real-time
surface and satellite-based observations supplemented by model
data, gale force winds extend roughly 500 miles in all directions
from the low center; storm force or higher winds extend 500 to 800
miles; and a smaller swath of hurricane force winds can be found
on the south to east side of the low center coincident with colder
air wrapping into the system. This core of strongest winds is
tracking northward between Adak and Shemya.

Meanwhile, the overall moisture content of this system is quite
extreme, a combination of advection of warm moist air from the
tropics and dynamical forcing leading to strong upward vertical
motion and heavy precipitation. CIMMS MIMIC satellite-based
precipitable water products show a large area of 1"+ TPW extending
along the Alaska Peninsula, Aleutian chain, and southern Bering
Sea - with a small area of 2"+. To put this into perspective,
this equates to 200 to 300% of normal. The heaviest rain with
this system is spreading across the western Aleutians and into
the Bering Sea, though the greatest impact is from rapidly
increasing winds and seas.

Out ahead of this storm, a vertically stacked low is tracking
onshore of western Alaska in the vicinity of the Y-K Delta.
Rain is spreading from Southwest Alaska to Kodiak and Southcentral
Alaska, with the heaviest rain focused along the frontal system
and ahead of a sharp short-wave on the south side of the upper
low.

&&

.MODEL DISCUSSION...

Models are in excellent agreement with handling of this major
fall storm, with good run-to-run continuity. As a result, overall
forecast confidence in the forecast and associated impacts is
high. The biggest uncertainty is with the exact timing and degree
of storm surge for the Kuskokwim Delta coast. This phenomena is
traditionally difficult to measure and model. However, based on
confidence in the overall track and strength of the storm, it does
look like the most impressive storm surge event in recent memory.

&&

.AVIATION...

PANC...Tight pressure gradients ahead of the low over Southwest
Alaska and associated frontal system will lead to persistent gusty
southeasterly winds this afternoon and evening. This should
largely maintain VFR ceilings despite steady rain, perhaps briefly
lowering close to MVFR at times. Winds will then rapidly cut off
around 09Z as a surface low forms in the northern Gulf, realigning
the pressure gradients. With rain continuing ahead of an upper
level short-wave, the weakening of southeast flow will likely
lead to a further drop in ceilings down to MVFR. Conditions will
then improve as the short-wave exits and rain ends early Friday
morning. There is a risk of localized fog/low clouds in the IFR
category Friday morning, which always poses a difficult forecast
challenge. For now, have opted to keep it out of the terminal.
Lastly, southeasterly winds will push back in Friday afternoon in
response to the major storm system in the Bering Sea. If any low
clouds do linger in the area, this will help to clear them out.

&&

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

The first of two frontal systems expected to impact portions of
the outlook area for the next day or two is already making steady
progress across Southcentral this evening. A band of steady
rainfall currently stretching from the western Kenai Peninsula to
the Mat-Su Valleys will begin to shift east towards the Prince
William Sound (PWS) as an upper level trough slides southeast from
the Lower Kuskokwim Valley rapidly into the northern Gulf. The
influx of upper forcing should foster development of a weak
surface low near Middleton Island along the dying frontal axis.
This will essentially neutralize the southeast to northwest
pressure gradient across the Kenai Peninsula and temporarily shut
off the related southeast gap winds Friday morning. Rain near the
PWS area and winds wrapping the low up, to small craft levels,
will diminish as the weak center drifts into the southeast Gulf by
the afternoon, and as a strong upper ridge axis pushes in from
the west. This will provide a good chance for most of the rest of
Southcentral to dry out for several hours through at least early
Friday afternoon. The main exception will be for Kodiak Island and
the far southern Kenai Peninsula, where more rains associated
with a warm and occluding front will push in from the southwest
late Friday morning, then quickly letting up as the front shears
apart over the Alaska Peninsula later in the day.

Looking into Friday night and Saturday, strong southwest flow is
poised to set up across Southcentral as the large, extratropical
remnant of Merbok lumbers towards the Seward Peninsula and the
strong upper ridge shifts east towards the AlCan border. Models
are beginning to better hone in on a shortwave trough that should
be whisked rapidly northeast across Southcentral late Friday
night. This should push a very quick-hitting round of rain across
the Kenai Peninsula, Mat-Su Valleys and Anchorage area overnight
Friday, shifting rapidly into the Copper River Basin and eastern
PWS as the wave speeds northeast. Despite the wet start, it's
shaping up to become a pleasant afternoon for at least the eastern
half of the outlook area as a slot of cool, dry air noses in
behind the exiting shortwave disturbance, perhaps allowing for a
mostly clear end to the day across the western Kenai Peninsula and
Anchorage vicinity.

The quiet, cool break in the weather is likely to extend into
Sunday (barring any pesky low stratus and fog), but there will
certainly be the chill of fall in the air. Assuming it stays calm
and clear enough, much of the Copper River Basin could see the
first frost and/or freeze of the season all the way down to the
lowest elevations, with early Sunday lows dipping into the upper
20s to mid 30s for places including Gulkana and Tazlina.

&&

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

Overall, the forecast remains on track. Showers will taper off
across Southwest Alaska this evening as a low over the Y-K Delta
weakens. Light rain returns early tomorrow morning as a potent
low in the Western Bering pushes its front across Southwest
Alaska. The bigger story with this low, however, are the strong
winds that will wrap around it and push into the Kuskokwim Delta.
A High Wind Warning is in effect for Kuskokwim Delta from Friday
afternoon through Saturday morning. The strongest winds (with
gusts up to 80 mph) are expected nearer the coast, though winds
could still gust up to 60 mph further inland. Given these strong
winds and the long fetch associated with the track of this low, an
extratropical storm surge is expected to affect the Kuskokwim
Delta coast. As such, a Coastal Flood Warning is in effect as
water levels could rise about 5 to 8 feet above the normal highest
tide line. No warnings are in effect for the Lower Kuskokwim
Valley or Bristol Bay; these locations are far enough from the
track of the low that they will be spared from strong winds. Winds
will still be elevated, however, with gusts approaching about 40
mph.

Winds will diminish and conditions will improve from Saturday
afternoon through Sunday as the surface low moves north into the
Bering Strait. Rain showers will linger across the region as a
longwave trough swings over the Bering Sea and towards Southwest
Alaska on Sunday.

&&

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

All eyes are on the extratropical remnants of Typhoon Merbok as it
moves from the Western Aleutians tonight to the Bering Strait on
Saturday. A High Wind Warning is currently in effect for the
Western and Central Aleutians through tomorrow morning, with gusts
up to 90 mph expected as cold air wraps around this low. As the
low continues moving north across the Western and Central Bering
Sea tomorrow morning, hurricane force winds and seas up to 50 ft
will follow it. The next land area to be impacted will be the
Pribilof Islands, for which High Wind Warnings are in effect from
Friday morning through early Saturday morning. With the low center
entering the Bering Strait by Saturday morning, winds should
slowly subside across the Bering Sea through early Sunday morning.
Aside from a few showers, calmer conditions should prevail on
Sunday.

&&

.MARINE...

Merbok remains the main concern through the end of this week. As
this system enters the Bering Sea it is expected to re-intensify
and go through the extra-tropical transition as is ingesting cold
air from eastern Russia. There is very minor uncertainty regarding
the exact track and strength of this storm as the center of the
low expected to pass over Shemya this afternoon. It will then
track from the Western Bering Sea to the vicinity of Norton Sound
on Saturday. Expect widespread storm force winds across the
Bering with areas of hurricane force winds. The hurricane force
winds are most likely from the Western Aleutians to the Central
Bering Sea between this afternoon and Friday evening. Seas are
also expected to build rapidly with widespread 30 to 40 ft seas
across much of the Bering with 40 to 50 ft seas associated with
the strongest winds. This is a major Bering Sea Storm and mariners
are encouraged to enact necessary precautions.

&&

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

Unsettled conditions are expected to continue for Southern Alaska
in the extended period, with multiple waves approaching from the
north Pacific and western Bering Sea. Several bouts of intense
winds and rainfall are expected for both the Aleutian chain and
Southcentral.

By Sunday evening, Typhoon Merbok remnants in the Eastern Bering
will have moved north out of the area, however will still be
inducing onshore flow into the southwest Alaska coastline. The
concern of strong winds and rainfall associated with this low
will have diminished significantly by Monday, however showery
activity across Southwest Alaska is still possible. Following
Merbok in Southwest Alaska, a new low originating from the north
Pacific looks to enter the Eastern Bering Monday. Timing and
positioning of this system has yet to reach a concensus in the
guidance. Thursday's 12Z Canadian and Euro models advance this
north Pacific low across the Eastern Aleutians into the Bristol
Bay area Monday morning. Additionally, a low forming off of the
Kamchatka Peninsula and, with moderate model uncertainty, will
deepen and approach the western Aleutians by Tuesday morning. The
Kamchatka low will move across the Aleutian Chain providing
lingering precipitiation in the Central Bering as well as strong
winds throughout the region through the middle of next week.

From Sunday afternoon, a respite in precipitation for
Southcentral is expected to persist through Monday afternoon. With
some discrepancies, guidance then suggests the introduction and
intensification of a North Pacific low south of the Alaska
Peninsula. This system will provide heavy precipitation across the
Southcentral coastline and eventually move inland. Precipitation
associated with this system is expected to persist into Wednesday.
Southeasterly flow will develop a Barrier jet wind along the
northern Gulf coast, including Prince William Sound Tuesday
morning. Winds up the Kamishak Gap into the Greater Bristol Bay
area will also be a concern Thursday afternoon.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning: 155 187, 191, 195.
         Coastal Flood Warning: 155.

MARINE...Gale Warning: 155, 165, 170, 172-174.
         Storm Warning: 175, 176, 179, 180, 181, 414.
         Hurricane Force Wind Warning: 411-413, 177, 178, 185.

FIRE WEATHER...NONE.

&&

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