Small Craft Advisory 233, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKDT Tue Mar 10 2020

.ANALYSIS AND UPPER LEVELS...

A weak upper level ridge is encompassing much of the Bering
leading to relatively quiet conditions. Snow showers are
occurring across the Pribilofs and eastern Aleutians in a cold,
northwesterly flow. Clear skies are seen over Southwest Alaska
under the aforementioned ridge, bringing temperatures well below
zero in many locations. Farther east, an upper level trough
currently extends from the northeastern interior through
Southcentral and the Kenai Peninsula. Scattered snow showers are
continuing ahead of the trough over eastern Prince William Sound.

&&

.MODEL DISCUSSION...

Models remain in good agreement regarding the dry, offshore flow
pattern and strong winds developing through terrain gaps near
Valdez in the near-term. Confidence is above average.

&&

.AVIATION...

PANC...VFR conditions are generally expected, however brief MVFR
conditions are possible this morning with low status currently
indicated on satellite in the vicinity of the terminal. Northerly
winds will continue through the morning, diminishing by
afternoon.

&&

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

The upper-level trough axis over Southwest Alaska yesterday
continues to slowly track east. Currently, this feature is draped
from the Susitna Valley south across the Cook Inlet. Low stratus
is hanging tough this morning underneath the trough, but a
developing drier, northerly flow is beginning to scour away some
of this cloud cover and reduce the areal coverage, as seen via the
nighttime satellite imagery. This clearing trend will continue
from west to east today as the trough moves over the Copper River
Basin later this morning and east of the AL-Can border by early
Wednesday morning. Snow continues to fall from Prince William
Sound to the Copper River Basin east of the trough axis this
morning. The light snow is expected to continue through the
afternoon and evening hours before tapering off. Additional
accumulations of up to 2 inches are expected.

The bigger weather story, however, will be the continued increase
in outflow gap winds through the next 36 hours or so as the
north/south pressure gradient tightens between a surface ridge
over the interior of western Alaska and a trough situated along
the Southcentral coast. Gale-force winds will continue today
across the Barren Islands, Shelikof Strait, Resurrection Bay, and
Passage Canal. This includes locations like Seward and Whittier
where wind gusts exceeding 45 mph are possible through tonight.
Later this morning, winds will quickly ramp up for the city of
Valdez, the Valdez Waterways, Thompson Pass, and the Copper River
Delta. Gales are expected for the waterways through tomorrow,
with gusts approaching 50 mph for Valdez and 70 mph for Thompson
Pass through early Wednesday, with the strongest gusts from mid-
afternoon today through late evening as the 500 mb trough passes
overhead. For these locations, the strong winds will produce
areas of blowing snow. Expect severely restricted visibilities at
times with blowing and drifting snow for Thompson Pass. Gusty
winds are also likely along Broad Pass and portions of the
Matanuska Valley today as well.

Winds will slowly diminish late Wednesday through early Thursday from
west to east as the Arctic high settles along the Al-Can border
and the surface trough weakens and shifts southeast toward the
Alaska Peninsula. An upper-level ridge will approach from the
west, maintaining the clear and dry conditions across Southcentral
through the rest of the week.

&&

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

Cold and clear conditions will persist today and Wednesday as the
quiet, dry pattern continues across Southwest Alaska through the
end of the week. However, there is a chance for snow showers
overnight Wednesday into Thursday morning near Hooper Bay with an
upper level wave moving across the region. By Thursday, an upper
level ridge of high pressure will begin to move over the region,
bringing a warm up in temperatures for the end of the week.

&&

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

Northerly gap flow will persist over the eastern Aleutians and
Alaska Peninsula this morning before beginning to weaken from
west to east later today. The exception being strong winds, which
will continue for gaps south of the Alaska Peninsula through
tomorrow, keeping heavy freezing spray in the forecast. Over the
western Aleutians today, a quick moving system will bring an area
of gale force winds to the Chain before exiting southward
overnight tonight.

Attention quickly focuses on the next storm force system that
enters the western Aleutians and Bering Wednesday morning. This
system will progress eastward over the central Bering by Thursday
afternoon, bringing widespread precipitation to the region.
Meanwhile, models show good agreement that a North Pacific low
will lift farther northward, bringing small craft advisory winds
along the eastern Aleutians and Bering beginning Wednesday
afternoon through Thursday. This system will also help keep the
storm force low from entering the eastern Bering or the Southwest
mainland.

&&

.MARINE (Days 3 through 5)...

By late Wednesday into Thursday, the north/south pressure
gradient will continue to produce winds up to 30 kts with
occasional higher gusts across the Barren Islands, Resurrection
Bay, and the Copper River Delta. These winds will diminish by late
Thursday into early Friday as a trough near the Alaska Peninsula
weakens and moves southeast and a ridge builds over the northern
Gulf. This ridge will remain in place through Saturday with
westerly winds less than 20 kts and seas 7 ft or less expected
across the northern half of the Gulf.

A storm force over the western Aleutians on Wednesday will begin
to weaken and lift northward on Thursday afternoon, keeping an
area of storm force winds west of St Matthew Island and north of
56N is expected through that time. By Friday, this system will
diminish, keeping relatively quiet weather over the Bering. A
North Pacific low will approach the eastern Aleutians Thursday
night, bringing the next round of precipitation to the eastern
Aleutians, including Dutch Harbor, through Friday night.

&&

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

Southcentral Alaska will continue to see clear and dry conditions
with some lingering northerly gap winds for Thursday, albeit much
weaker than earlier in the week. Colder temperatures will
persist, especially across interior locations. A ridge of high
pressure will replace the seemingly persistent trough over the
Gulf by the end of the week. This will shut off the northerly flow
and allow for a warming trend with temperatures rebounding closer
to average through the weekend. Weak shortwaves progressing west
to east along the northern periphery of the ridge may produce
periods of high and mid-level clouds across interior Southcentral
Friday through Sunday.

Relatively quiet conditions will persists across Southwest
mainland with an upper level ridge holding in place through
Friday. A chance for snow showers will return to the Kuskokwim
Delta coast by Friday afternoon as a front in the Bering moves
eastward. The remainder of Southwest Alaska will likely remain dry
through the start of the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE
MARINE...Gale Warning: 121 127-132 138 150 178.
         Heavy Freezing Spray Warning: 119-121 127 129 130 138
                                       139 141 150 155 165
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...LB