Small Craft Advisory 179, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
418 PM AKST Fri Feb 21 2020

.ANALYSIS AND UPPER LEVELS...
A large upper level low is centered on eastern Russia and has a
trough extending through the Bering Sea toward the western
Aleutians. Southwest flow aloft is over mainland Alaska. The jet
stream is running well south of the Aleutians and makes landfall
south of Southeast Alaska which is keeping the whole state in a
colder airmass.

At the surface, the main low is located near Prince William Sound
with colder air moving off the ice edge in the Bering Sea into
Southwest Alaska and eventually filtering up Cook Inlet from the
south. Another low is south of Attu and is in a favorable location
in the trough and along the jet stream to intensify over the next
day.

&&

.MODEL DISCUSSION...
Models remain in good agreement through the weekend. However there
is a good bit of variance in solutions for the low moving into
the Gulf of Alaska region on Monday. This low will likely remain
somewhere over the Gulf so the forecast issues are more with the
marine areas and right along the Gulf coast for Monday and beyond.

&&

.AVIATION...
PANC...Snow will persist through the night and into Saturday
morning. It looks like ceilings and visibility may improve a
little toward morning but VFR conditions look unlikely until at
least noon Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

An upper level trough centered over the Barren Islands will
continue to slowly lift north and east over the Kenai Peninsula.
This feature is beginning to advect much colder air from
Southwest Alaska. This has already started the gusty westerly,
offshore flow through gaps and passes along Shelikof Strait, the
Barren Islands and into Kachemak Bay. With a moist environment
remaining from the last warm front, the cold air advection is
helping to maintain widespread light snow along the Kenai
Peninsula, the Anchorage Bowl, across the Mat-Su Valleys and
into Prince William Sound.

A 986 mb low will close off from the trough and proceed to the
southeast Saturday morning, crossing the Kenai Peninsula and
entering the northern Gulf Saturday night. As this low propagates
to the southeast, offshore winds will reach their peak. Cold air
advection, combined with a strengthening pressure gradient across
the AKPEN waters and into Cook Inlet, look to produce storm force
winds across the Barren Islands late Saturday into early Sunday
morning. Precipitation chances remain likely for portions of the
Kenai Peninsula, Susitna Valley and Prince William Sound through
this time.

The low will be driven towards the Panhandle as the upper level
trough flattens and weakens due to a zonal flow pattern aloft
that will propagate the trough from the west to the east Sunday
afternoon. This will strengthen the pressure gradient over Prince
William Sound and will increase offshore flow through Thompson
Pass, the Copper River Delta and the Valdez waters as cold air
drains from the Copper River Basin. Precipitation chances for
Southcentral begin to wane with likely chances remaining over the
windward side of Kodiak, the Chugach and the Kenai Mountains, the
Upper Hillside and into Sutton and Eureka as upslope flow becomes
favorable for these areas to continue to receive snow. This will
remain until dry air overtakes Southcentral Sunday night, thereby
eliminating most chances for precipitation across the region.

&&

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

The snow shower activity ongoing across much of Southwest Alaska
this afternoon is expected to diminish from west to east overnight
tonight. Thus, areas along the Alaska Range will take the longest
to clear out. Clearing skies are expected from west to east, along
with diminishing winds as high pressure builds across Southwest
Alaska. Much of this weekend will feature bright, sunny, but cold
conditions over much of the area. Periods of cloudier weather are
possible within that time, but most of the area will see plenty of
sunshine. All threats of precipitation through Monday will end
with the conclusion of the current ongoing snow shower activity.

The main forecast concern this weekend therefore will be the cold
temperatures. The air mass building in will allow some areas of
the Kuskokwim Delta and Valley to be unable to reach zero for
highs, while lows at night in those same areas drop into the teens
to 20 below. A strong low and associated front well south of the
Alaska Peninsula and Kodiak Island on Monday will cause increasing
offshore north to northeasterly wind flow during the day, but
associated cloud cover currently looks to remain from King Salmon
south. By Monday, some moderation of the cold air mass, despite
the offshore winds, should bring temperatures above zero for most
of Southwest Alaska, which should preclude any wind chill
concerns.

&&

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

High pressure over the eastern Bering currently will move eastward
into Southwest Alaska this weekend, and strengthen as it does so.
meanwhile, a strong low will approach the central and western
Aleutians on Saturday. A track a bit further south than previous
runs suggest the winds may not be quite as strong across the
islands than previous forecasts. Any gales should remain south of
the chain. Rain and snow showers are possible through the islands
with temperatures in the mid 30s, but any snow accumulations will
be minor. On Saturday night into Sunday, the low quickly drops
south and all effects will diminish. A weak front will approach
the western Aleutians on Monday, but other than gusty
southeasterly winds, and some rain showers, the effects from this
feature will also be minimal. A low over the southwestern Gulf
will cause increasing northeasterly winds around the Alaska
Peninsula on Monday. Differences in track will determine if that
area sees any precipitation.

Elsewhere across the Bering, a quiet period with light winds and
cold temperatures are expected through Monday. A few compact lows
are possible on Saturday, but will have minimal impact. The
strengthening high pressure over interior Alaska will lead to
gradually increasing easterly winds across much of the Bering by
Monday.

&&

.MARINE (Days 3 through 5 Mon through Wed)...
The main low will be in the Gulf of Alaska region or just a little
south of there during this period. Gales are expected with this
low with a decent chance of storm force winds. There is a bit of
uncertainty with this system as its projected track and strength
remains uncertain.

The Bering Sea region is a bit more settled as generally westerly
winds will be over the area with the potential for occasional
gales.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Next week will start off with well below normal temperatures over
most on southern mainland Alaska with high pressure over the state
and a low to the south of the Gulf of Alaska. This low will make
some headway northward through the middle of next week which will
bring most temperatures back to seasonal levels for the middle to
late parts of next week. There may be some periods of snowfall in
Southcentral with this low monday through Wednesday timeframe, but
at this time and snow should not make it much farther west than
just west of the Alaska Range. However, there is a bit of
uncertainty to the track of this low so the areas of snowfall are
subject to change.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 121.
MARINE...Storm 130. Gales 130-132 138 139 176-178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...EZ