Small Craft Advisory 228, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
411 PM AKDT Sun Mar 8 2020

.ANALYSIS AND UPPER LEVELS...
In the upper levels, a high amplitude blocking ridge is sitting
to the south over the north central Pacific Ocean. A series of
low amplitude (weak) short-wave troughs are tracking from west
to east across the Alaska Region in fairly zonal flow. The
strongest of these waves is tracking into Southcentral today,
with widespread snow for all areas except the Copper River Basin.
A weak triple point low at the surface is also transiting the
northern Gulf. This morning, as it passes south of Anchorage, it
did produce some downslope flow in Anchorage and the Matanuska
Valley. However, as the low continued eastward the downslope flow
ended and shifted to broad south to southwest flow, which is
enhancing snow rates in upslope areas of the Chugach and Talkeetna
Mountains. There is also a low level deformation band (of heavier
snow) on the back side of the low, which has been tracking slowly
eastward from Cook Inlet to the mountain through the day. The
heaviest snow totals have been in the Susitna Valley, where a foot
more of snow has been reported over the past couple days.

Snow is tapering off across Southwest Alaska as the short-wave
exits eastward, with snow lingering along the west side of the
Alaska and Aleutian Ranges in westerly upslope flow. As weak
short-waves drop down from the Arctic, cold air advection has
ensued in the Kuskokwim Valley and Kuskokwim Delta.

Out over the Bering Sea, a broad trough is tracking eastward
across the heart of the southern Bering Sea, with a weak surface
low near the Pribilof Islands. Precipitation with the trough is
mostly showery, while with the surface low there is an area of
steady light rain and snow across the eastern Bering Sea and
Pribilof Islands. Winds are generally small craft advisory or
lower across the region, though Arctic air filtering southward
behind the trough, leading to strengthening winds from the
central Bering to central Aleutians.

&&

.MODEL DISCUSSION...
Model guidance is in excellent agreement with large scale features
over the next few days and forecast confidence is above average.

&&

.AVIATION...
PANC...The heaviest of the snow will shift east of the terminal
by evening, so generally expect improving ceilings and
visibility. However, broad cyclonic flow will persist in the upper
levels, with weak waves dropping down from the northwest. Thus,
periods of light snow will likely persist through at least Monday
morning (possibly beyond). It will be challenging to identify the
best time periods of snow due to the weak forcing mechanisms.
Generally expect MVFR conditions to prevail, with low end VFR
possible when it is not snowing. VFR conditions should then settle
in during the day Monday as low level northwesterly flow
increases and drier air begins to advect into the area.

&&

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

An upper trough continues to shift eastward from northwest AK, and a
leading shortwave is tracking through Southcentral. A surface low in
the Gulf associated with the shortwave continues to wrap moist
southeast flow into Southcentral, overrunning cold air at the
surface farther inland. This will maintain light stratiform snow
over the Anchorage Bowl/Mat-Su Valleys/Kenai Peninsula tonight.
Snow will slowly taper off to flurries Monday, with a few
exceptions. Weak cold air advection/upslope flow over the
Anchorage Hillside and Palmer area could maintain light snow
through Monday night. In the Susitna Valley, especially from
Willow north, steady south flow aloft and a surface trough will
maintain snow a bit longer than elsewhere, possibly into Monday
night as well.

These features will exit to the east Monday, and the parent upper
trough will dive southeastward towards Southcentral AK Tuesday.
Light snow will spread into the Copper River Basin, as south flow
aloft ahead of the trough increases moisture advection. Expect a
few inches of snow along the Richardson Hwy Monday night.
Otherwise, weather Tuesday will transition to cold, dry conditions
with strong offshore winds out of the usual gaps and passes as
dense air spills southward under the trough. In particular, expect
gusty winds in Thompson Pass Tuesday, likely blowing surface snow
around and causing low visibility. Winds will also ramp up in
Broad Pass, the Valdez waters, over the Barren Islands,
Resurrection Bay, Passage Canal and the Copper River Delta.

&&

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

Skies will gradually clear, particularly over the Kuskokwim Delta,
through this evening under a weak ridge. A cold front will track
across Southwest Alaska Monday. Highs in the single digits and
lows into the minus teens are likely through Tuesday with lower
wind chill values. No precipitation is expected.

&&

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

A positively tilted upper level trough extends from the Seward
Peninsula southwestward through the western Aleutians. This is
currently bringing rain and/or snow showers across the central and
eastern Bering and will track to the southeast overnight. It is
expected to phase with another low in the Gulf as it approaches
the eastern Aleutians. This in conjunction with high pressure
building over the North Pacific will strengthen the pressure
gradient west of its center. As such, strong northwesterly wind
gusts potentially up to 50 mph will develop out of bays and passes
on the Pacific side of the eastern Aleutians and Alaska Peninsula
on Monday. Winds will diminish from west to east Tuesday as the
low departs, however gusty winds will continue west of Kodiak
Island through Tuesday.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...
Forecast confidence in the advertised marine forecast is high.
The only question is with strength of winds as a front tracks from
east to west across the Bering Sea Wednesday through Thursday. For
now, forecast advertises high end gale force southerlies, though
there is potential for storm force southerlies, particularly for
the western Bering and western Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7/Tuesday through Saturday)...
Beginning Tuesday, an Arctic trough will cross Southcentral and
the Gulf of Alaska, with cold and dry air settling in across the
entire southern mainland. Offshore gap winds will be in full
force along the Gulf coast and Prince William Sound, including the
communities of Seward, Whittier, and Valdez. While low level cold
air advection and pressure gradients look fairly impressive, the
upper level jet is much weaker than in the wind event this past
week. Thus, expect overall winds to be weaker than what was
observed last week, particularly in the Valdez to Thompson Pass
corridor. Blowing and drifting snow and low visibilities will
likely plague Thompson Pass again as winds kick up Monday night
and Tuesday. This may necessitate issuing a blowing snow advisory
or blizzard warning, depending on how much new transportable snow
is in place. There will also be some localized gusty winds across
the rest of Southcentral and Kodiak Island, as winds accelerate
through mountain gaps and bays. Winds will gradually diminish
across the region Wednesday through Thursday, while temperatures
continue to fall.

The Bering Sea and Aleutians will start out quiet on Tuesday as a
high amplitude upper level ridge builds in from the west. The
ridge will then shift eastward through Friday, with a storm system
tracking from near the Kamchatka Peninsula on Thursday way up into
the high Arctic on Friday. A trailing front will bring gale force
southerly winds to much of the Bering Sea, with potential for
storm force winds over the western Bering and Aleutians. With the
high amplitude ridge holding over the Gulf and Southcentral and
another ridge likely building into the Bering Sea next weekend,
the forecast looks dry (and quiet) for much of the forecast area.
In particular, after the current snow event ends over the southern
mainland, it could be dry for a week or more. After temperatures
bottom out Thursday and Friday of this week, expect steadily
increasing temperatures after that.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145.
MARINE...Gale 130 131 138 150 170 172-174 178.
		 Heavy Freezing Spray 130 139 150 165 179 185.
FIRE WEATHER...NONE.

&&

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