Small Craft Advisory 1012, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKST Tue Nov 12 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tuesday through Thursday evening)...

Two weak upper-level troughs are moving across Southcentral this
morning. The first is over the Copper River Basin and is beginning
to lift northeast, exiting the region later this morning. The
second is moving over Shelikof Strait and will continue to track
slowly to the east today, eventually reaching the Copper River
Basin by Wednesday morning. Both features will interact with a
weak surface low near Middleton Island, drawing moisture northward
due to a southerly wind flow out ahead of the surface low and the
incoming trough to the west. The result will be an area of
precipitation, mainly in the form of snow, stretching from the
Copper River Delta north into the Copper River Basin and west
toward Cordova and Valdez through early Wednesday afternoon.

Snow amounts look to be slightly higher than initially forecast,
with 1 to 3 inches around the Glennallen area and 2 to 4 inches
farther south and along the Edgerton Highway. The most challenging
part of the forecast remains how far west the band of steadier
snow pivots for Valdez to see measurable snowfall and how quickly
temperatures fall around Cordova to allow for any accumulation.
Right now, enough dry air may linger around Valdez to limit
accumulation to around an inch or so. For Cordova, 3 to 5 inches
of snow is possible as temperatures fall below freezing this
evening.

Gap winds will also persist through Thursday morning as the
troughs move east of Prince William Sound and a ridge of high
pressure builds to the west in their wake. The result of these
features will be an increase in both a pressure gradient as well
as an increasing thermal gradient due to cold air advection. These
gradients are already driving up the winds for Seward and
Whittier this morning and will also allow for an increase in winds
through the gaps around eastern Prince William Sound later today.
The winds will likely peak Wednesday morning then remain rather
consistent in magnitude through early Thursday morning before
quickly diminishing, especially for Seward and Whittier, as the
flow weakens and shifts to the south in advance of the next front
approaching from the west. Wind gusts will likely approach 40 to
45 mph for locations such as Resurrection Bay, Passage Canal,
Valdez Narrows, and Thompson Pass.

Fog is also lingering across Knik Arm this morning, with Wasilla
and Palmer reporting lower visibilities. The fog should begin to
erode by mid-morning.

At the end of the short term period Thursday, a frontal boundary
from a dissipating low in western Alaska will push into the
region, bringing chances of precipitation to Kodiak and setting
the stage for snowfall for the rest of Southcentral late Thursday
into Friday.

-TM/JR

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...

Another day of relatively benign weather for much of Southwest
Alaska. A front is moving across the western Bering and
Western/Central Aleutians and the Pribilof Islands this morning
and will reach the Eastern Aleutians and Southwest coast this
afternoon. Snow or a rain/snow mix is likely initially for the
Pribilof Island and Eastern Aleutians but is expected to quickly
transition to all rain. Along the Southwest Coast, mostly snow is
expected, except for a chance for a rain/snow mix along the
Kuskokwim Delta coast from Kipnuk to Toksook Bay, including
Nunivak Island.

The next system crosses the Aleutians into the southern Bering
Sea Wednesday afternoon and approaches the Southwest coast by
Thursday. There is still a rather large model spread with this
next system, with nearly 200 miles between the northern most and
southern most solution by 15Z Thursday (6AM Thursday). The 06Z GFS
and 06Z NAM are tracking at similar speeds and are in okay
agreement until about 10Z Thursday (1AM Thursday) when they start
to diverge, with the NAM taking a more northern track. What the
models do agree on is the strength of the system and the
likelihood of precipitation spreading across Southwest Alaska
Thursday/Friday.

&&

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

The previously-mentioned North Pacific low that will enter the
Bering late Wednesday will be impacting Southwest Alaska by the
start of the extended forecast period. Widespread precipitation
and gusty winds are still expected, though the track and intensity
of the low are still uncertain. Most inland locations will
primarily get snowfall, but places along the southwest coastline
could see a rain/snow mix due to warmer air aloft.

As the front of the low moves eastward towards Southcentral, a
shortwave trough will jet across the Bering and begin to interact
with the low late Friday. This interaction will help with the
development of a triple point low forming just south of the Alaska
Peninsula and situating itself in the northern Gulf of Alaska.
Most of the energy from the system will shift down to the triple
point low, resulting in a widespread snowfall event for most of
Southcentral and a heavy precipitation event along the Gulf
coastline.

The system will slowly shift eastward towards Southeast through
early Sunday as an upper-level trough digs in behind it. Strong
offshore directed gap winds are expected as an Arctic air mass
helps drive in what will likely be the coldest temperatures
southern Alaska has experienced so far this season. Lastly,
another shortwave trough will pass over the Aleutians starting
late Saturday and situate itself over the central Gulf by Monday
afternoon, providing another round of precipitation for the chain
and Gulf coast, as well as a continuation of offshore-directed gap
winds through the early portions of next week.

&&

.AVIATION...

PANC...Vicinity fog is possible this morning, though any fog that
does develop is likely to remain north of the terminal.
Otherwise, VFR conditions and light winds will persist.

&&