Small Craft Advisory 104, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
328 PM AKST Thu Jan 30 2020

.ANALYSIS AND UPPER LEVELS...
A broad upper-level trough over the Alaskan mainland has a closed
center over the northwestern Gulf with a positively-tilted trough
axis extending southwestward across the Gulf and into the north
Pacific. Upper-level ridging is currently prevailing across the
the Bering/Aleutians. A second upper-level trough is currently
over the northern Kamchatka Peninsula with its trough-axis moving
eastward toward the western Bering/Aleutians. The polar jet
currently extends southeastward from the central Bering to near
47N 155W and then northeastward to southeast Alaska. The
subtropical jet remains south of 45N over the north Pacific
through 140W and then extends northeastward to over southern
British Columbia.

At the lower levels, snow showers continue to move northwestward
across the central Gulf to the southern Kenai Peninsula, Prince
William Sound and southern Chugach. King Salmon radar shows
widespread light snow slowly moving southwestward across the
Kvichak Delta.

&&

.MODEL DISCUSSION...
Models are generally in good agreement at both the surface and
upper-levels with respect to the overall synoptic pattern across
the area through the weekend. Some of the high-resolution models
were used to try and refine the forecast around the snow showers
in the eastern part of the area.

&&

.AVIATION...
PANC...VFR conditions with light winds are expected through
tomorrow. The biggest question will be whether a snow shower or
two is able to make it across the Chugach Mountains this afternoon
or early evening. Even if some snow showers do make it to the
terminal, conditions should remain MVFR or better.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A stalled surface low centered in the northern Gulf is currently
bringing widespread snow showers to the eastern Kenai Peninsula,
Portage Valley and Prince William Sound areas. These will continue
into early Friday morning and may be heavy at times, briefly
reducing visibility. It is possible that some may reach Anchorage
this evening, however would likely be brief and not cause any
significant impacts. Snow showers will gradually taper off through
Friday for most areas, although they are expected to continue
across Prince William Sound into Saturday. As high pressure
strengthens over the interior on Saturday, gusty northerly winds
will develop in the usual Chugach terrain gaps, however weak
dynamics aloft will inhibit surface winds from increasing to
anything significant. Skies will clear Saturday afternoon through
early Sunday morning, leading to colder temperatures across
Southcentral. A front will move into the area Sunday afternoon,
bringing the next chance of snow to the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
An Arctic trough over Southwest AK will continue shifting east. A
deformation band associated with a Gulf low continues to bring
light snow in the Alaska Range/foothills, which should diminish
Friday morning. While temperatures remain well below normal over
the Southwest Mainland, winds have diminished. Friday will be
another very cold dry day, with a pattern change on the horizon. A
shortwave trough moving east through the northern Bering will
bring a transition to south winds and warmer conditions Saturday.
Precipitation is possible in coastal areas. Interior areas such as
the middle to lower Kuskokwim Valley and the Nushagak River
Valley will be slower to warm up and will likely remain dry
Saturday. Sunday looks to be a much more significant warm up... a
North Pacific low expected to track south of the western Aleutians
will swing an elongated front northeastward across the Aleutians
toward the west coast. This will bring a swift warm up and a
heftier bout of precipitation which should make it inland. Highs
Sunday will generally be around 20 degrees warmer than Saturday in
most areas. Precipitation will remain snow Sunday, though there's
a chance of changing over to rain along the AKPEN late Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
An Arctic trough axis over Southwest AK continues shifting
eastward. Snow showers over the eastern Aleutians/AKPEN will
continue to dissipate tonight. A trough in the western Bering will
shift eastward, bringing enhanced snow showers to Adak/Atka and
east to Unalaska and the Pribilof Islands Friday. A storm force
low will approach the western Aleutians from the western Pacific
Friday into Saturday. Winds will shift to southerly, then easterly
over the Aleutians. The low will have a moisture fetch reaching
nearly into the Tropics, bringing steady rain to the Aleutians
Saturday. The associated front will lift north into the Bering
Saturday into Sunday.

&&

.MARINE (Days 3 through 5 Sunday through Tuesday)...
Bering Sea and Aleutian Coastal Waters: A ~950 mb low will be
located near the Rat Islands Sunday afternoon, with sustained
high-end gales and storm force gusts expected on the backside of
this system across the western Aleutians and Bering. The low then
moves to just west of the Pribilof Islands by Tuesday, with
sustained winds having decreased below gales by then.

As for seas, given a long dynamic fetch, seas of 25 to 30 feet are
anticipated on Sunday across the western Bering and south of the
Aleutian chain west of Dutch Harbor decreasing into the 5 to 15
feet range north of the chain and east of 175W. These waves then
subside further for Monday and Tuesday as the storm system
weakens.

Gulf of Alaska: A sustained gale force front will enter the Gulf
on Sunday from west to east, before the front stretches out
parallel to the coast on Monday as it weakens. As it does so
winds will decrease towards the small craft variety, with a few
gales still possible near the Copper River Delta. By Tuesday winds
will remain below 20 knots. As the front crosses the region, waves
will build into the 15 to 20 feet range for Sunday into Monday
before decreasing on Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7, Monday through Thursday)...
A pattern change looks to commence over the next several days as
a fairly strong storm system reaches the west central Aleutians
Tuesday morning, moving into the Pribilof Islands by later that
evening. As it does so a warm front will surge north and east
bringing increased warmth and moisture aloft, which translates
to temperatures moderating towards normal for the middle portion
of the week for the central Aleutians eastward through southwest
and southcentral Alaska. Precipitation potential will also
increase ahead of these features.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 130.
 Gale Warning 120 131 132 136 138 150 155 176>178.
 Heavy Freezing Spray 121 129>132 136>139 141 150 155 165 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...PD