Small Craft Advisory 848, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 PM AKDT Tue Oct 30 2018

.ANALYSIS AND UPPER LEVELS...
An upper level shortwave trough positioned over the Copper River
Valley continues to drift eastward today over the Yukon Territory.
Filling in behind this trough is an upper level ridge of high
pressure with a negatively tilted axis extending from Saint Paul
Island down into the southwestern Gulf of Alaska.

Satellite imagery indicates clearing skies over Southcentral
today as the incoming ridge begins to influence the southern
mainland. Vertical temperature profiles retrieved from atmospheric
soundings overnight showed very dry low-level conditions across
Southcentral, however a moist low-level airmass remains in place over
Southwest Alaska. This has led to the development of fog across
parts of Southwest today, which will likely become more widespread
overnight as the building ridge brings subsidence (sinking air)
across the area.

&&

.MODEL DISCUSSION...
Overall models remain in fairly good agreement with the exiting
trough and the building high. By Thu afternoon, models indicate a
North Pacific low developing just south of the central Aleutians
with a leading front entering the western Gulf by Thu evening.
Model solutions disagree slightly with the position of this low by
the early weekend, as the NAM/EC brings the low into the eastern
Bering while the GFS keeps it on the south side of the eastern
Aleutians. At this time, the GFS solution was preferred.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
For the short term, overall conditions will remain cool and dry.
There will be large areas of clear skies around southcentral with
some clouds persisting this evening in the eastern Prince William
Sound region due to a weak low south of Icy Bay which is heading
eastward. The clouds will clear as this low moves away from the
area. The Copper River Basin is the trickiest area tonight as
there will be some areas of stratus and patchy fog around. The
Basin could become completely cloudy in the lower elevations due
to the stratus. The upper level ridge will build into the region
Wednesday and help clear out the area of remaining clouds. The
Copper River Basin will remain the big question mark for cloud
cover. While the overall synoptic pattern is good for clearing
out the area, there is a possibility the stratus hangs in there
longer than expected. This will also impact temperatures as they
will be significantly warmer should the stratus persist. Leaned
toward the clearing solution with the forecast and therefore the
lows near Glennallen are forecast to drop below zero by Wednesday
night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The longwave pattern features a ridge over the Central Aleutians
that is bisecting the Bering and there is a major shortwave trough
over central Alaska. There are a few lingering showers on the
radars at Bethel and King Salmon as of 3:58 PM local time. With
the ridge building into the southwest expect a drying trend into
Friday for the region. With the ridge moving in and the recent
moisture, areas of fog are expected tonight. There is another low
that is currently south of the chain and that system will track
toward the region however the leading edge of the precipitation
shield is not expected until Friday morning.

Veniaminof Volcano, which is on the AKPEN and located near 56.2N
159.4W is currently active. Emissions were observed earlier today
in satellite imagery and in the Perryville FAA webcam. The hotspot
from the volcano is detectable in infrared imagery.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The longwave pattern features a ridge over the Central Aleutians/Bering
and there is a major shortwave trough over Kamchatka, with a
second area of low pressure south of the Aleutian chain. This low
will inch its way closer and kick out a negatively tilted shortwave.
Gale force winds are expected with this system. The first rains
are expected at Nikolski and Unalaska. Then the precipitation
shield slowly moves north and east. The aforementioned ridge is
blocking its forward progression but the precipitation shield
doesn't completely stall out, it just moves really slowly.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The extended period begins Friday evening with an upper-level
ridge of high pressure over the mainland pinching off from the
main flow as it retreats to the north. This movement is in
response to the eastward progression of a mature low situated near
the AKPEN. An occluded front attached to the low lifts into the
northern Gulf from the southwest, with areas of easterly gales
moving toward the southcentral coast. The front is then expected
to weaken as it stalls along the coast for Saturday. Guidance
remains in decent agreement through Saturday, with the NAM, GFS,
and EC all suggesting this outcome. The outlier is with the
Canadian which tries to bring a weaker low along with the
occlusion through the Gulf to the southeast coast by Saturday.
Based on the overall consensus of the other models (deterministic
and ensemble), this is has been discounted in favor of the others.

Uncertainty ratchets up quite a bit after Saturday. The 06z and
12z GFS runs were on par with the EC runs and its GEFS
counterpart bringing the parent low into the Gulf on Sunday. The
EC was faster by about 12 hours with the track of the low toward
the southeast coast while the GFS weakened the low into more of
an open wave. The 18z GFS, however, has the more EC-like closed
low, but with the track hugging the southcentral coastline. So
while exact track and timing is still uncertain, there is high
confidence of inclement weather along the southern coast and
northern Gulf through Sunday.

Any remaining confidence in the models going into early next week
is then completely eroded by how the GFS and EC handle the upper-
level dynamics upstream over the Bering. The EC wants to build a
ridge between the aforementioned low and the long-wave trough over
the Bering. The ridge is then shifted east over the northern Gulf
and southern mainland for Monday and Tuesday as the Bering low
deepens and the ridge amplifies. The GFS, on the other hand, has
kept consistent with a zonal pattern, allowing the upper-level
shortwaves to progress east across the forecast area. The first
comes across the Aleutians beginning Saturday, reaching the
western Gulf on Monday and continuing to impact the southern
mainland through mid-week. The second impacts Southwest Alaska
late Monday with the third moving into the eastern Bering on
Tuesday. Overall, the long-term forecast beyond the weekend is
weighted more toward the GFS solution given that it is more in
agreement with ensemble guidance and consistent with previous
runs.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 155 172.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
LONG TERM...TM