Small Craft Advisory 936, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 AM AKST Sat Dec 5 2020

.ANALYSIS AND UPPER LEVELS...

A long wave trough extends across much of mainland Alaska this
morning with a negatively titled ridge across the Bering and into
Southwest. The Southwest is largely calm and clear this morning.
However, as the ridge is being forced north by a trough over the
North Pacific, high winds, waves and a wintry mix of precipitation
are being seen on observations across the western and central
Aleutians. Low stratus and fog still continue to be a concern from
Palmer to Anchorage throughout the day. Light snow has been
falling in Anchorage for a few hours this morning, however,
northerly winds aloft will keep cold temperatures and dry
conditions across much of Southcentral for most of the day.

&&

.MODEL DISCUSSION...

Models remain in better agreement with a storm force low moving
into the Gulf Monday morning. The 00Z and the 06Z GFS have sped
up towards the faster NAM solution, which is the preferred
solution for this forecast package.

&&

.AVIATION...

PANC...MVFR to LIFR conditions are possible with stratus and fog
during the morning hours. Light winds will prevail.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Areas of low clouds, fog, and flurries have redeveloped overnight
in northern Cook Inlet with the Palmer to Anchorage corridor in
the thick of it. This fog and stratus can be seen best on the
satellite 'Nighttime Microphysics' setting. This also shows the
region of clouds packed against the Chugach and Kenai Mountains as
it pushes southward into the western Kenai Peninsula. There is
also widespread stratus in the lower elevations of the Copper
River Basin. Today's biggest forecast uncertainty for all these
areas is whether there will be enough drier air moving in from the
north to erode the low clouds and fog to bring significant
clearing during the daytime hours.

The next system can be seen on satellite spinning up south of the
Alaska Peninsula this morning. This low was analyzed at 949 mb
last evening and is expected to bottom out around 942 mb this
morning before slowly filling, so it is quite strong. The front
wrapped around this low will reach the southwest Gulf of Alaska
this evening and bring some strong winds and precipitation to
Kodiak late tonight into Sunday. The precipitation will start as
snow, but likely only last a few hours before it turns to rain.

This front will then reach the north Gulf coast later in the
morning Sunday and then inland after that. At this time it looks
like the rain and snow will remain mostly over the coastal areas
with just about all the inland areas on the lee side of the
mountains downsloping out for Sunday. Precipitation type may
become a tricky forecast for Sunday night into Monday for
Anchorage and the Matanuska Valley.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Cool northerly winds will continue to dominate the Southwest
region, with winds becoming gusty across the Kuskokwim Delta and
parts of Bristol Bay by tonight. Overall, conditions will be clear
and dry throughout the weekend. Precipitation will make its way
up the AKPEN to Bristol Bay tonight and into tomorrow and reach
the rest of the Southwest by Monday. Downsloping should warm
temperatures slightly tomorrow and tomorrow night, but are
expected to remain cold enough to keep precipitation as all snow.
Snow accumulations should not exceed 3 inches.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A strong North Pacific low continues to track east, just south of
the Aleutian Chain. This system is expected to reach the southern
Gulf early next week. Gales will dominate the eastern Bering and
Aleutian coasts through the weekend, with storms along the Pacific
side of the Chain from Dutch Harbor and east. High pressure sets
up to the southwest of the Chain, leaving the western portion of
the Bering/Aleutians in southerly flow by Monday, while elsewhere
remains in northerly flow behind the low. Cold arctic air will be
re-advected into the Bering once the low moves out. Heavy
freezing spray will be an issue along the Southwest Coast with
light freezing spray across the eastern Bering.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska: Strong winds will persist until Monday night.
Over Kamishak Bay and southern Cook Inlet (adjacent from Anchor
Point), strong northerly winds and hazardous wave heights peaking
near 11 to 13 ft will diminish by Monday night into Tuesday
morning. As the low races through the northern Gulf, it loses its
energy. Its strong winds begin to weaken, too, by Monday evening.
Winds will gradually drop near 20 kt between Tuesday and Tuesday
night. Solutions become a bit sketchy by Day 5, so will focus
mainly on Days 3 and 4 which will be a bit quieter for the
forecast area.

Bering: Minimum gales possible over the eastern Bering Sea Monday
into Tuesday. The focus shifts to a strong low approaching the
central Aleutians on Wednesday. There will be a wide swath of
gales with the system and front as it sweeps north and east across
the Bering/Aleutians Wednesday through Friday. There is also a
strong possibility of embedded storm force winds will accompany
this system. Stay tuned for updates as confidence increases.

&&

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

The model's performance between Monday and Tuesday (Day 3 and 4)
shows the placement of synoptic features in close proximity,
except GFS is a bit distant from most solutions. After day 4,
there's a lot of uncertainty among the models, yet they do pick up
the next potential storm from the Northwest Pacific around the
middle of the week. Shortwaves from the upper low will make their
way across Southcentral and the AKPEN through at least Tuesday
morning. Southerly winds bringing warm air advection to
Southcentral will yield to a rainy pattern for the first two days.
Over Southwest Alaska, a nearly stationary long wave from Yukon
River runs southwestward to Nunivak Island on Monday morning.
Models pick up on this upper level feature and a nicely uniformed
area of northerly flow, including a broad area of cold air
advection over the Kuskokwim Delta. Meanwhile, warm air advection
continues to extend over the AKPEN and fills part of Bristol Bay
through Tuesday. An upper ridge over the Bering will either shift
eastward or settle over the central Bering until the longwave
over Southwest AK phases out. All in all, the forecast confidence
is moderate to high for Days 3 and 4.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warning: 139 140 160 180 181
		 Gale Warning: 119 120 130 131 132 138 139 160 165 170 171
					   173 174 175 176 179 180 181 185 352 412 413
					   414
	 	 Storm Warning: 150 155 172
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CRK
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...CB