Gale Warning 729, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
532 AM AKST Sat Nov 25 2023

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Monday night)...

A large vertically stacked low centered near the southern Alaska
Peninsula this morning is weakening as it drifts eastward. The low
anchors a high amplitude trough which extends well down into the
Pacific. A corresponding high amplitude ridge is positioned
downstream over the far eastern Pacific up across Southeast Alaska
and into NW Canada. In the deep southerly flow in between, a
plethora of short-waves are tracking northward across the Gulf of
Alaska and into Southcentral, bringing fairly widespread showers
to the region. The upper levels have cooled quite a bit in the
last 24 hours, leading to a more unstable airmass and the showery
character of the precipitation. Low levels remain quite warm and
above freezing, leading to rain as the predominant precipitation-
type. Temperatures in a few locales were able to drop back to
around freezing, namely the Willow area in the southern Susitna
Valley. Thus, the threat for localized areas of freezing rain
continues.

Meanwhile, an atmospheric river is streaming northward toward
Southcentral, with a surface stationary front now moving northward
as a warm front. Steady stratiform precipitation is now visible
over the Gulf south of Middleton Island. This precipitation will
overspread eastern Prince William Sound this morning and head into
the Copper River Basin this afternoon.

At the same time, the upper low will open into a trough and the
long-wave trough will begin to lift north and east. This will lead
to development of a surface low which will track across the Gulf
tonight and into western Prince William Sound early Sunday. This
is a big departure from the forecast 24 hours ago, which indicated
a weaker, further east low. The ramifications of this include a
stronger barrier jet along the north Gulf coast and Prince William
Sound and precipitation backing westward out ahead of the low
center to the Kenai Peninsula, Anchorage, and the Mat-Su tonight
into Sunday.

Precipitation-type is once again one of the big forecast challenges
with this system. Weak warm air advection (and perhaps some
localized downslope warming) will affect the southern Copper
River Basin tonight. The southeastern basin, roughly from Chitina
to McCarthy looks warm enough to see mostly rain. As you head west
to the Richardson/Edgerton interchange and south along the
Richardson Highway toward Thompson Pass, the airmass looks just
cold enough to support snow initially, but there could be a
transition to rain (and perhaps even freezing rain) late tonight
through Sunday morning as low level southeasterly flow increases.
As you head further north and west across the basin, expect
precipitation to fall as snow.

The remainder of Southcentral will undergo little change in the
low level thermal profile before steady precipitation arrives.
However, marginally cooler air will accompany the increasingly
consolidated negatively tilted upper level trough as it approaches
Southcentral Sunday morning. This, combined with cooling of the
column due to melting should be just enough to lead to a
transition from rain to snow tomorrow from Anchorage north to the
Mat-Su. The speed of the trough will accelerate, so expect light
accumulation. Meanwhile, with the strongest forcing focused on
Prince William Sound, expect roughly 1 to 3 inches of rain for
coastal communities.

As the upper trough exits Sunday night, a fast moving long-wave
trough and leading occluded front will move in. This will track
rapidly across the region Sunday night through Monday, bringing
another round of precipitation. Colder air will accompany this
trough and front, leading to more snow - especially for inland
areas of Southcentral. Strong southeast flow ahead of the front
will produce some localized warming and lead to a rain to snow
transition for coastal locations. This includes Anchorage, where
Turnagain Arm winds will likely bend into south to west Anchorage
Sunday night keeping surface temperatures above freezing. There
looks to be a fairly strong short-wave aloft and cold air
advection up Cook Inlet looks quite robust, so there is still
good opportunity for snow accumulation Monday morning. In fact,
this could be one of those instances where gusty southerly winds
and snow occur at the same time, leading to low visibility. As it
stands now, the worst of the snow and wind looks to arrive just
in time for the morning commute. Stay tuned as we continue to
work out the details.

&&

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

A weakening upper level low over the eastern Aleutians will
bring in warmer air with gusty southerly winds continuing through
this afternoon. Meanwhile, the front of a Kamchatka low will sweep
across the western Bering Sea this afternoon with its southerly
gale-force winds reaching the western coast of Alaska by Sunday
morning. Confidence is high that this front will bring gale-force
wind, with localized areas of embedded storm force wind. Behind
the aforementioned front, strong cold air advection will bring
gusty westerly winds following the front through this weekend and
into next week. One challenge with this front, and an area where
model variation is greatest, is identifying where a triple point
low will develop along the occluding front. Any developing low(s)
will lead to increased winds in their immediate vicinity,
increasing the complexity of forecasting where stronger winds will
occur.

The front is expected to reach the Pribilof Islands by Saturday
night and then the Southwest coast by Sunday. With this front,
heavy to moderate precipitation will fall across the Chain but
then lose steam as the front weakens bringing light precipitation
to Southwest Alaska, with the exception of the Bristol Bay region
receiving moderate precipitation. Temperatures across the entire
region will trend downward through this weekend and into next week
as colder air builds in behind the front. Rain is more likely to
linger along the coast than toward the interior, but even there
temperatures in most locations will decrease enough behind the
front for rain to transition into snow by Monday.

-ER

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)...

An upper level Western Bering low moves across to the Western
Alaska Coast through the period. The associated long wave trough
sweeps across the Aleutians and Bering before extending over the
Southwestern Mainland into the Gulf of Alaska by Friday. Cooler
temperatures spread over the region with this long wave trough.
Several short waves move through over the Aleutians through the
end of the week, and develop surface lows near the Eastern
Aleutians for both Tuesday and Thursday. Another short wave
supports a surface low over the Western Aleutians Thursday through
Friday. Forecast confidence decreases through Friday with the
transient lows. Periods of locally heavier precipitation and gusty
winds will be the order of the day along the Aleutians and Alaska
Peninsula through the forecast period. Areas of high surf are
possible Tuesday and again Thursday and Friday around Bristol Bay.

In the East, the long wave trough spreads over the Western Gulf
Tuesday night, with a moderately strong surface low sliding past
Kodiak Island Wednesday and Thursday. This brings strong winds
from the Alaska Peninsula to the Southcentral Coast through
Friday. Locally heavy precipitation occurs with the low through
the forecast period, beginning along the coasts and spreading
inland as the low tracks to the Southcentral Coast.

Kutz

&&

.AVIATION...

PANC... Wind direction will be variable most of today but brief
periods of southeast gusts to 15 to 25 kts remain possible. VFR
conditions with intermittent rain showers are also likely into the
late morning hours with temperatures remaining above freezing
today. Isolated showers will persist for the afternoon hours.
Precipitation returns this evening as rain and as temperatures
remain a few degrees above freezing. Mainly rain will fall
overnight with a mix of rain/snow late Sunday morning. There is a
low chance of freezing rain during this time but the majority of
the precipitation through Sunday morning will fall as a rain/snow
mix. Colder air will continue to filter into the area Sunday
leading to high confidence of the main precipitation type turning
to all snow Sunday afternoon. MVFR conditions are also expected
most of Saturday night and Sunday.

DJ/CQ

&&

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
135 PM AKST Sat Nov 25 2023

AKZ132-271200-
Northeast Prince William Sound-
Including the cities of Valdez and Thompson Pass
135 PM AKST Sat Nov 25 2023

...PERIODS OF HEAVY RAIN ON SNOW EXPECTED FOR VALDEZ THROUGH
SUNDAY...

An approaching warm front and associated atmospheric river will
bring heavy rainfall to Valdez. One to two inches of rain is
expected for Valdez from Saturday through Sunday night before a
transition to snow by Monday morning. Heavy rain on snowpack may
result in ponding water on roadways. Additionally, rain falling on
frozen or snowpacked roads may lead to slick driving conditions.