Small Craft Advisory 384, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKDT Wed May 6 2020

.ANALYSIS AND UPPER LEVELS...

A strong ridge is centered over the North Pacific near 30N/170E
(roughly 1500 miles south of the Aleutian Islands). A fairly
strong sub-tropical jet (~100 to 130 kt) extends over top of this
ridge and then dives southward into northeast Pacific. High
amplitude troughs with embedded closed lows are positioned over
the Western Bering and Northeast Pacific. As would be expected,
this is where the most significant weather is focused. The storm
over the western Bering has occluded and is in a weakening state.
It's associated occluded front is also weakening as it tracks
eastward across the Central Bering Sea and Aleutians. The
northeast low is more impressive, with cyclogenesis ongoing at the
triple point north of the original low center. This is in
response to a short-wave and jet streak moving up the east side of
the low/trough. This will ultimately develop into a major storm
system which will affect the Gulf beginning tonight. In the
meantime, there are a couple features rotating around the north
side of the trough, a weak vertically stacked low over the eastern
Gulf and a short-wave over the Southwestern Gulf. Weather impacts
from these are minimal.

Conditions over the southern mainland of Alaska are quiet, with
mostly clear skies underneath a ridge of high pressure. Satellite
imagery does show some cumulus clouds forming along the mountains
as the surface heats up. The short-waves passing south of the
ridge may provide just enough forcing to produce isolated showers
this afternoon, but most of the region will remain dry.

&&

.MODEL DISCUSSION...

Models are in excellent agreement with handling of large scale
features over the next few days and forecast confidence is
generally high.

&&

.AVIATION...

PANC...VFR conditions will persist. A Turnagain Arm wind will
develop this evening into the overnight hours in response to
tightening pressure gradients out ahead of a front cross the Gulf
of Alaska. While the core of Turnagain Arm winds will remain over
northern Cook Inlet, could see some gusty southeast winds affect
the terminal in response to warmer temperatures (and lower
pressure) over the Mat-Su. On Thursday, the Turnagain Arm wind
will bend solidly southward as it exits Turnagain Arm, with much
lower pressure to the south over the western Gulf. Normally, this
would mean the airport would remain out of the southeast winds.
However, this time of year with more surface heating and
instability, suspect the airport could see periods of southeast
winds.

&&

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

Sunny skies will continue today under high pressure. There remains
a slight chance of isolated thunderstorms across the northeastern
Copper River Basin this afternoon and evening as weak instability
is possible with daytime heating. Clouds will increase across
Southcentral Thursday as a strong front moves into the northern
Gulf. Rain will develop over Prince William Sound in the morning,
before tracking westward Thursday afternoon. Gusty easterly winds
will accompany the precipitation along the eastern Kenai
Peninsula and Kodiak Island. These winds will gradually diminish
into the weekend, though rain will continue. Little to no rain is
expected in Anchorage or the Mat-Su given the cross-barrier flow
pattern over the mountains.

&&

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

Isolated convection continues to percolate this evening and will
get cutoff once the sun sets. Thursday morning, an inverted trough
pushes across the Alaska Peninsula from the southeast bringing gusty
easterly to northeasterly winds to the region (AKPEN to Bristol
Bay). The winds will taper off by Friday morning. A warming trend
will continue through the end of the week with most interior
locations reaching highs in the low 60s.

&&

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

A front currently over the Western Bering continues to move
eastward this afternoon bringing rain and gusty southerly winds to
the Aleutian Chain. The ASCAT (Advanced Scatterometer) pass from
this afternoon detected gale force winds west of Attu Island.
However, this front is expected to weaken and the front will stall
near the Pribilof Islands Thursday. In its wake, a new potent low
will approach the Bering. This low will have a slightly different
track and the expectation is that widespread gales will sweep
most of the Bering. Rough seas and a widespread precipitation
shield will accompany this system.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...

Bering Sea/Aleutians...

A strong low pressure system will be moving northeastward across
the Western Bering/Aleutians on Saturday. There is an increasing
confidence that widespread Gale force south-southwesterly winds
associated with this low will develop late Friday night into early
Saturday morning across the Western Bering/Aleutians. Confidence
also remains high with a second area of Gale force southerly winds
developing in the central Bering/Aleutians on Saturday. As the
low gradually weakens while moving northeastward, there is above
average confidence that these Gale force winds will ease Saturday
night into Sunday morning. A second low pressure system is
expected to approach the western Bering/Aleutians Monday
afternoon, though confidence in the wind forecast remains low as
there is a high degree of uncertainty in the intensity and
placement of this low.

Gulf of Alaska...

Forecast confidence generally remains low Saturday through Monday
due to discrepancies on the placement of synoptic scale features.
However, winds are expected to remain below gale force during the
period.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...

The pattern will remain amplified Sunday through Wednesday as
upper level high pressure continues to extend northwestward from
the Canadian Rockies to the Alaskan North Slope. This pattern will
allow for warmer temperatures and overall dry weather. As the snow
continues to melt and sunlight becomes more abundant and longer in
duration, the potential for convective rain showers focused along
area mountains will increase. The upper ridging will help set the
stage for convection by minimizing the potential for widespread
cloud cover and allowing ample diurnal heating at the surface.

Upper troughing will persist across the Bering/Aleutians and
extend southeastward into the North Pacific. Due to this pattern,
the predominant storm track will be through the Bering and into
the North Pacific. A weakening low pressure system is expected to
move northwestward from the central Bering into Siberia Sunday
into Monday. A second low is expected to move eastward along the
Aleutians Monday afternoon through Wednesday. However, forecast
confidence remains low at this time due to large discrepancies in
the placement and intensity of this low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC....Flood Warning 155 (Kuskokwim River at Kwethluk)
Flood Advisory 155 (Kuskokwim River Akiachak to Napakiak)
MARINE...Gale Warning 352, 351, 119, 120, 131, 130, 138, 137, 132, 150, 160, 155.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...ED