Small Craft Advisory 330, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKDT Sun Apr 12 2020

.ANALYSIS AND UPPER LEVELS...
The longwave pattern features a ridge axis over Southeast Alaska
and extending into the Yukon Territory. There is a negative-tilted
shortwave near Kodiak. There is a closed low north of the Pribilof
Islands with the trough axis over the Central Aleutians and there
is a more robust closed low south of Attu Island. The radars at
Kenai and Middleton Island have been active overnight with the
system that is approaching the Anchorage Bowl. Gale force winds
were detected in Shelikof Strait and near the Barren Islands with
ASCAT (Advanced Scatterometer) data early this morning. A second
area of gale force winds were detected south of the western
Aleutians which are being generated from a 970 mb low that is
south of Shemya. There is a 90 knot jet core near Kodiak and a
160+ knot jet core south of Attu Island. The current snow depth at
the the NWS Office in Anchorage is 15 inches while Valdez City
has 75 inches of snow on the ground.

&&

.MODEL DISCUSSION...
Models are in decent agreement through Monday. The timing of the
precipitation shield up Cook Inlet and across the Kenai peninsula looks
reasonable with the GFS, NAM and Canadian. However, there are
some minor differences in the exact timing of the precipitation
starting across the Kenai Peninsula and Anchorage Bowl. There are
2 additional systems upstream and the precipitation shields look
good with those systems as well.

&&

.AVIATION...
PANC...As of 10z the ceilings at PANC and PAMR were VFR. Homer's
ceiling was at 7,000 ft. The radars at Middleton Island and Kenai
clearly show a band of moisture with the much advertised feature
in the Gulf. The expectation is that the ceilings will continue to
decrease this morning and there is the potential for a rain/snow
mix at PANC. Its possible that PANC will go briefly into MVFR
territory, but its a tough call.

&&

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

The warm front is upon Southcentral Alaska early this morning
bringing rain to Kodiak and a mix of precipitation types
elsewhere. A dry lower level of the atmosphere, combined with some
downsloping is keeping the precipitation at bay for the majority
of the Cook Inlet region, but that is expected to change as the
day wears on. While a rain/snow mix is expected, snow amounts are
expected to be very low as anything that falls should melt
quickly.

The Susitna Valley is the are with the best chance for
accumulating snowfall. Temperatures aloft will be colder there
then the rest of Southcentral and flow aloft will be southwest,
which is a favorable direction for there. Temperatures will be
warm enough to put a damper on accumulations for the daytime
hours, but overnight there should be enough cooling for the snow
to accumulate. After weighing all these factors, ended up going
with total snow amounts around 3 to 7 inches through Monday
morning in the Susitna Valley with this system.

For the Prince William Sound area, the precipitation should start
as snow this morning and then switch to rain by afternoon. The
snow may mix back in overnight as temperatures cool diurnally, but
temperatures aloft are still warm enough to give rain the upper
hand for areas near sea level through Monday.

&&

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

Snow showers can be spotted around inland locations of Southwest
Alaska this morning, with a few rain showers along coastal areas.
By this afternoon, a surface low develops off the Kuskokwim Delta
Coast, which will bring a much more organized round of
precipitation to the region. As warmer air begins to move in with
this system, many locations that initially started off as snow
will transition over to rain or a mix of rain or snow as the band
of precipitation continues to move eastward through Monday
morning. By Monday, the bulk of the showers will have tapered off
across the region as a shortwave ridge moves in across Southwest
Alaska.

&&

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

Over the western Bering and Aleutians, a front continues to
dissipate, resulting in a showery pattern. Meanwhile, a surface
low in the eastern Bering this morning will bring a period of rain
and snow, as well as stronger winds, to the eastern Aleutians and
Alaska Peninsula before the front moves inland by the afternoon
hours. Shortwave ridging moving in behind this feature will give a
brief period of quiet weather before the next North Pacific
surface low moves across the eastern Aleutians, allowing strong
winds and precipitation to return.

&&

.MARINE (Days 3 through 5)...
(Tuesday through Thursday)

Monday

Bering Sea/Aleutians…

The well established area of high pressure residing over the eastern
Pacific will continue to influence the Bering with consistent
southerly winds and a tap of warmer, moist air. Tuesday morning a
1000 mb low over the central Bering looks to be on its way to
degrading into an open wave under an upper level trough. Wednesday
looks to be more of the same with the ridge holding its ground
and a series of minor disturbances moving across the eastern
Aleutians and eastern Bering. However, a bigger pattern shift
looks to be advertised for Thursday as a stronger low pressure
system materializes in the northwest Pacific and pushes through
the Bering. The first impacts look to be a strong push of
southerly moisture into the Aleutians Thursday morning and
spreading across the Pribilofs by Thursday afternoon before the
parent low pushes deeper into the Bering Thursday night.

Gulf of Alaska…

The resident high pressure in the eastern Pacific will continue to
push strong southerly flow into the Gulf of Alaska. The frontal
boundary stretched across the north Gulf coast Tuesday morning
will slowly erode as the low pressure in the Bering moves beyond
60N. This will usher in a relatively quiet Tuesday afternoon for
the Gulf of Alaska under the upper ridge that remains in place.
Late Tuesday night another upper level disturbance pushes into the
southwest Gulf and will extend its impact to the north Gulf coast
and the remainder of the Gulf Wednesday morning into mid-morning
Thursday. The remainder of Thursday looks to be quiet again as
high pressure regains a foothold in the wake of this disturbance.

&&

.LONG TERM FORECAST (Days 4 through 7)...
(Wednesday through next Saturday)

Beginning Monday night, the entire forecast area will come under
the influence of the trough currently established over the Bering
Sea. While the main low center will remain to the south of the
Aleutian Islands, several waves will develop on the eastern side,
moving northward over Kodiak and into Southcentral. This will
bring periods of precipitation for both Southwest Alaska and
Southcentral as these waves move northward. With a southerly fetch
of warm air, for the most part this looks like precipitation will
mix across the mainland. However, with winter still hanging on by
its teeth in some of the more northern regions, this could mean
another round of snowfall for the Lower Kuskokwim Valley and the
northern Susitna Valley.

Beginning Thursday, the operational model runs are in fairly good
agreement on an upper level ridge building in over eastern AK,
with a broad longwave trough settling in over the Bering Sea.
However, as the models progress, there is some difference in the
orientation of the ridge axis over eastern AK. GFS indicates a
broader, flatter ridge, which EC shows a much sharper ridge with
the axis slightly further west. This will make a difference in the
sensible weather at the surface, in particular for Southcentral
and areas eastward. Should the GFS solution set up, the broader
ridge will allow waves to move across the forecast area, thus
bringing the possibility of rain to the eastern half of the state.
However, should the EC solution eventuate, the sharper ridge will
keep weather systems further west and the eastern half of the
state will remain dry.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 131.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ALH
MARINE/LONG TERM...LF