Small Craft Advisory 680, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 PM AKDT Wed Sep 18 2019

.ANALYSIS AND UPPER LEVELS...
There is an upper level trough over southern Alaska this
afternoon, with the base of the trough over the southwest. There
is an associated surface low over the lower Kuskokwim basin into
Bristol Bay, and another along the Canadian border just north of
the Alaska Range. This is bringing strong onshore flow and
considerable showers to almost all of southern Alaska. There is a
weak ridge over the central and eastern Bering Sea, with a weak
upper level trough over the western Bering. There is a strong
low south of the western Aleutians moving rapidly eastward.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement through Friday
afternoon. For the low in the Gulf on Friday the models were
previously handling it differently (especially the Nam), but now
they are in good agreement on a solution. Therefore, the forecast
confidence is higher than normal.

&&

.AVIATION...
PANC...VFR conditions are likely to persist. Some brief MVFR
conditions in showers are possible this evening. Gusty southerly
winds this afternoon will ease off a bit this evening.

&&

.FIRE WEATHER...
Strong onshore flow and considerable showers will continue through
tonight. Another system will push into the Gulf Thursday night and
Friday. With this system the low will be in the Gulf, so the low
level flow will be offshore, but it still will bring
precipitation to most of Southcentral.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Friday)...

The wet weather pattern continues across Southcentral through
Friday. Once the current disturbance wanes, there will be a break
in the pattern. Usually after a widespread rain, fog is the first
concern. That should not be the case this time, as breezy
conditions are likely to persist overnight and through the day
Thursday, especially through aligned mountain gaps and along the
Copper River.

Scattered shower activity will redevelop Thursday afternoon along
the mountains and at the coast. This is in advance of the next low
that will bring another round of widespread wetting rain to much
of Southcentral. The low will quickly move northeastward out of
the Pacific and into the southwestern Gulf off Kodiak Island
Thursday evening. Rain should develop ahead of the low in many
areas of Southcentral late Thursday afternoon and evening,
including the Gulf Coast, Kenai Peninsula, and to a lesser extent,
Anchorage and the Mat-Su Valleys. Gusty downsloping winds through
the gaps should cut down on rainfall totals in Anchorage and the
other normal leeside areas. By Thursday night and into the day
Friday, rain overspreads nearly all of Southcentral, as the
low strengthens moving into Prince William Sound. With that said,
the moisture feed with this low is not as robust as with the Wed
rainfall event, so QPF totals area-wide should not be as much.
The notable exception to this will be the Copper River Basin,
which will get quite a bit more rain with this incoming low.

Cooler air, likely the coolest of the season so far, will wrap
around the west side of the low by Friday afternoon and evening.
This will bring snow levels down to the 4,000 to 5,000 foot range,
primarily through the Talkeetnas and Hatcher Pass. While amounts
are still uncertain, several inches are likely above this level
with this storm. Only cold rain is expected in the populated
lower elevation areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 3; Tonight
through Saturday)...

Scattered showers will decrease overnight as a surface trough
associated with a low now inland over the state slides east across
the region. Behind this trough, drier air is sinking south across
the Y-K Delta where satellite imagery is already showing a rapid
decrease in cloud cover for locations north and west of Bethel.
Across the Kuskokwim Valley, the decrease in cloud cover will lead
to radiational cooling, with overnight lows approaching the
freezing mark. Patchy fog will be possible from Bethel to
Sleetmute through the early morning hours of Thursday, a result of
the cooler temperatures and lingering low-level moisture from
recent precipitation. A second shortwave trough and associated
surface low will bring areas of showers to Bristol Bay and the
upper Kuskokwim Valley for Thursday. As this feature lifts
northeast, yet another shortwave trough gaining strength over the
northern Bering will dig southeast across the Y-K Delta Thursday
night and across the rest of the Southwest into Friday. This
system will usher in colder air with widespread low temperatures
near to slightly below freezing across interior locations
overnight by Saturday morning. Showers will also persist through
late Friday with this system, with enough colder air at higher
elevations creating the potential for mixed precipitation. A
ridge then builds across the region from west to east early
Saturday as the next frontal system approaches the coast.

&&

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

Any lingering areas of gale-force winds and showers along the
coastal areas of the eastern Aleutians and AKPEN will continue to
diminish overnight as low pressure tracks farther inland over
Southwest Alaska. Upstream, a second area of low pressure will
quickly slide east through Thursday, with the center of
circulation staying south of the chain. Although conditions will
remain unsettled over the eastern half of the chain, this track
will keep the strongest winds and heaviest precipitation away
from the region. High pressure builds over the western Aleutians
on Thursday, moving east on Friday. Areas of low stratus and fog
will accompany the ridge on its eastward trek. An area of low
pressure will develop over the northwestern Bering on Friday and
track east over top the ridge. This system will race east,
reaching Saint Matthew Island and the Pribilofs overnight Friday
and the west coast of Alaska Saturday morning.

&&

.MARINE (Days 3 through 5)...

On Saturday, a gale force low will be stationary along the North
Gulf Coast. Cold air advection will be pushing high end gale
force winds through the Barren Islands and Shelikof Strait,
although the highest seas will be through the Barrens where the
longer fetch winds will produce greater buildup. The low will move
east toward the Panhandle on Saturday. Attention turns to a
powerful developing low over the eastern Aleutians and eastern
Bering Sea. Uncertainty is higher with this low, but there is
increasing likelihood that gales to near storm force winds will
impact these areas, focused from Dutch Harbor eastwards, with the
peak strength around Saturday night through Sunday. The low will
shift into the Gulf on Sunday evening/overnight and last through
Monday. By this juncture, uncertainty with a developing triple
point low in the Gulf is high, but mariners should be expecting a
potential high end gale force to low end storm force low sometime
on Monday across the entire gulf of Alaska. The next strong low
moves into the western Bering Sea Tuesday and Wednesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Beginning the extended period forecast on Friday evening, there
will be a low bringing widespread rain to Southcentral with a pair
of lows over the Bering Sea. The low over Southcentral will be
driven in part by an Arctic trough to the northwest, so colder
air will be filtering in behind the rain through the remainder of
the weekend. Attention quickly turns to a rapidly developing low
along the Aleutian Chain Saturday night. Currently, this looks to
rapidly deepen into a high end gale force to low end storm low as
it moves from eastern Bering Sea to the Gulf of Alaska Sunday and
Monday. Given the wrapped up nature of the low, the most rain is
likely to develop across parts of Southwest Alaska, Kodiak, and
the North Gulf Coast with much drier conditions inland given the
downslope pattern. Unsettled weather will switch back to the
Bering Sea midweek (23rd-27th) as the storm track moves west
again, giving a break from the rain across most of Southern
Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 150.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...JA