Small Craft Advisory 819, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKDT Tue Oct 27 2020

.ANALYSIS AND UPPER LEVELS...

A positively tilted upper-level trough stretching from the Yukon
to the AKPEN continues to slide southeast this afternoon. A drier
airmass is working in behind it, as indicated by the diminished
cloud cover on satellite over the Susitna Valley and northwest
Copper River Basin. Low stratus and fog lingers from north of Knik
Arm south to Anchorage and Turnagain Arm this afternoon, but is
not as dense or widespread as this morning. A surface low remains
southeast of Cordova this afternoon with scattered showers from
Prince William Sound to the Copper River Delta.

Farther to the west, a ridge extends from the northern Pacific
north to the eastern Bering. Northwesterly flow around the ridge,
along with a trough sliding south, is keeping low clouds over much
of Southwest Alaska this afternoon, especially over the Kuskokwim
Delta and upslope sides of the mountain ranges. For the Aleutians
and Bering, a 1002 mb low northwest of Attu is extending a front
over the central Bering/Aleutians with widespread low clouds and
fog.

&&

.MODEL DISCUSSION...

Model agreement remains good through Friday with only outflow
winds the predominant weather story for Southcentral. By Friday,
models begin to diverge with regard to a developing triple-point
low east of Kodiak Island. This low will spin up along an occluded
front extending northeast from another low south of the eastern
Aleutians. Most guidance (GFS/EC/Canadian) has this feature
moving northeast, to a position near Cordova by late Friday into
early Saturday. The 06z and the 12z NAM runs showed a more
westward solution, potentially bringing precipitation to mush of
the Kenai Peninsula and toward the Anchorage Bowl. However, the
18z run has drifted back to the east and is more in line with
other models. Nevertheless, there is still plenty uncertainty with
the evolution of an arctic low dropping south from Russia
Wednesday into Thursday, and it is this feature and its
interaction with the aforementioned eastern Aleutian low that will
drive the development and placement of the Gulf low. Given this
uncertainty, forecast confidence for the next system to impact
Southcentral is below average.

&&

.AVIATION...

PANC...Low stratus over the terminal will allow MVFR to IFR
ceilings to persist through the evening and overnight hours along
with light winds. Fog may redevelop over the airport complex by
early Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tue
afternoon through Fri afternoon)...

An upper level shortwave currently positioned over Kodiak Island
is pushing eastward across the northern Gulf. At the surface,
weak low pressure continues to spin in place over the Prince
William Sound area. This low will continue to weaken and remain
nearly stationary through the evening, allowing for residual rain
showers to continue overnight for communities along the northern
Gulf coast. Meanwhile, low level stratus and fog that developed
over the Anchorage Bowl and southern Susitna Valley this morning
still remains in the area. Given that an upper level ridge is
moving in overhead and "trapping" this moisture, current thinking
is that the stratus clouds will lower again overnight creating
foggy conditions again for Wednesday morning. This fog could also
become more widespread over the Cook Inlet area, and may
potentially spread southward over the Kenai/Ninilchik area by Wednesday
morning.

Overall, conditions should remain clear and cold across much of
Southcentral for Wednesday. The weak surface low currently in the
vicinity of Prince William Sound may bring cloud cover and
scattered snow showers to the southern Copper River Basin
Wednesday into Thursday, but little to no accumulation is
expected. By late Thursday, the focus shifts to an inbound upper
level trough with an embedded shortwave digging southward over
the state while a Bering low moves into the northern Gulf from
the west. Given that ample cold air will already be in place
across much of Southcentral, we'll certainly be keeping an eye on
the track and strength of these two features. If all of the
ingredients and timing come together just right, it could be the
next possible chance for snowfall in Southcentral by the end of
the week.

&&

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

A very quiet period of weather is expected across Southwest Alaska
the next several days. A mix of sun and clouds are expected
through Thursday, with clouds gradually clearing with time. A bit
of fog is also possible the next two nights across portions of the
Kuskokwim Delta. The Kuskokwim Mountains should work to block the
Bering moisture from encroaching further east, so the Lower
Kuskokwim Valley and Bristol Bay will be faster to clear than the
Kuskokwim Delta the next couple days.

A polar air mass will slowly move southward out of the interior
and into Southwest Alaska the next couple days. This will
initially result in slowly dropping temperatures area-wide through
Thursday. On Thursday night into Friday, a strong low is expected
to track well south of the area. There is quite a bit of
uncertainty as to where the northern extent of the associated
precipitation will be. The forecast trend has been north with
time, and thus the chances of snow are not zero for primarily the
Bristol Bay area, with rapidly increasing chances of snow the
further south you go. What is highly certain, however, is that the
low will effectively pull much colder air southward across the
entire area, so by far the coldest temperatures of the season will
be seen over the entire area from Friday night into the weekend.
In the meantime, another highly certain portion of the forecast
is that the winds will kick up, especially along the coast. This
means mariners can expect light freezing spray along coastal
waters Thursday night into Friday.

&&

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

A weak front over the central Bering this evening will gradually
weaken as it moves across the Pribilofs on Wednesday. Behind it, a
much stronger North Pacific low is expected to approach the
central Aleutians Wednesday night, track to south of Nikolski
Friday morning, then turn south away from the Aleutians back into
the North Pacific. The leading front associated with the low will
move across the Eastern Aleutians and Alaska Peninsula Thursday
afternoon and Thursday night, persisting along the AKPEN through
Friday.

The low will produce storm force winds along the Central
Aleutians, mainly west of Adak Wednesday night, weaken a bit
through Thursday, then reintensify to storms north and west of
Nikolski Thursday night. Gale force winds will overspread much of
the Southern Bering by Friday. The low will bring much colder air
southward across the Eastern Bering, but the modifying influence
of the waters should limit snow totals, or keep the precipitation
mostly as a cold rain for most Aleutian communities.

&&

.MARINE (Days 3 through 5)...

Northwesterly storm-force winds are likely across the southern
Bering as well as the central and eastern Aleutians Friday as
storm tracks south of the Chain. Storm-force gusts remain likely
across the bays and passes of the eastern Aleutians and southern
coastal AKPEN into Saturday. Winds and waves over this area will
diminish by Sunday as a gale-force front looks to move over the
western Aleutians.

For the Gulf, a low will spin up east of Kodiak Island with the
potential for southerly gales over the northeasterly Gulf by late
Friday into Saturday. Northwesterly gales are likely late
Saturday into Sunday across the northern Gulf, especially through
bays and passes, in the wake of the low.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Models are coming into better agreement with the development of a
~990 mb low in the western Gulf on Friday, moving into the
northern Gulf to the Southcentral Coast by Saturday morning. With
colder air in place, this system is likely to produce snow from
parts of Prince William Sound north to the Copper River Basin by
Saturday. There is still uncertainty in the track of this system,
however, and any westward wobble would mean the potential for snow
for a greater portion of the Kenai Peninsula as well as the
Anchorage Bowl and Mat Valley.

This system looks to then looks to move east as a ridge settles in
over the interior. The result would be a return to a cold,
northerly flow with gusty winds along the gaps of coastal
Southcentral through early next week. There is plenty of
uncertainty as to the strength of this ridge and whether a couple
upper-level disturbances may produce some scattered snow showers
across parts of Southcentral, especially over the mountains, next
week.

Out west, scattered snow showers are expected across Southwest
Alaska Saturday as a weak disturbance drops south. Lingering wind
gusts across the eastern Bering/Aleutians and AKPEN will also be
trending downward as a low south of the region shifts east. A
front looks to clip the western Aleutians by Sunday as high
pressure sits over the rest of the Chain. Another round of snow
showers is possible by midweek across Southwest as another system
drops southeast from Russia.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 175 176 178
		 STORM 177
FIRE WEATHER...NONE.

&&

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