Winter Weather Advisory 18, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 AM AKST Sat Feb 27 2021

.ANALYSIS AND UPPER LEVELS...

A broad jetstream extends from Southwest to Southcentral through
the Gulf of Alaska, followed by another strong jetstream west of
central Aleutians through the western Bering Sea. An upper level
low combined with a steepening upper level trough riding on the
rear edge of the first jetstream are pointing directly to the
western Gulf. Warm temperatures around Bristol Bay through the
Gulf will contribute along with the upper-level support of the low
and trough to bring more rain/snow mix across Bristol Bay through
western Kenai Peninsula today. Stratus cloud deck and fog from
last night continues to expand inland and across Southwest and
Southcentral Alaska early this morning.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement through Monday night. The
general easterly trend of the Gulf low and its associated front
will maintain strong westerlies over the Gulf and intensified
pressure gradients through the weekend. The main impact from this
pattern is winds elevating to gales and storm force over Prince
William Sound, Shelikof Strait and Cook Inlet, while winds west of
Barren Islands including Kamishak Bay is expected to reach
hurricane force by tonight and Sunday.

Models begin to differ starting Tuesday morning when GFS diverges
its surface low way south of Bristol Bay before trekking farther
south of the Gulf through Wednesday morning. Meanwhile, NAM, ECM,
and SREF maintained their low at a close proximity between Monday
night and Tuesday afternoon leading into the western Gulf. Should
GFS changes course in future model runs, then the next Gulf low
could stay either close along the northern Gulf or dives
southeastward by mid-week.

&&

.AVIATION...

PANC...IFR conditions will persist through at least 28/1000Z.
There is uncertainty with the snow accumulation on the backside of
the low over the Gulf, which suggest drying over Anchorage Bowl.
Essentially, flight conditions will improve to MVFR later tonight
through the rest of the forecast period.

&&

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

There has been a change in the evolution of the current storm
system, so quite a few changes have been made to the forecasts for
Southcentral. The key feature causing the change is a short-wave
trough tracking eastward along the southern Alaska Peninsula this
morning. This short-wave is quite a bit stronger than previous
model forecasts indicated it would be. This will lead to
development of a surface low over the western Gulf/Kodiak Island
area by midday. This low will then intensify as it tracks across
the northern Gulf tonight. Prior forecasts were based on a weaker
short-wave tracking farther north, with a weaker surface low
hugging the north Gulf coast and ultimately tracking into Prince
William Sound. Here are the ramifications of this change in key
features:

1. The new system developing over the western Gulf will rob
Southcentral of forcing from the trough (with low level upslope
flow also weakening). As a result, snowfall will be quite light
today, especially over interior Southcentral. In fact, snow could
taper off completely this afternoon. Have canceled the Winter
Weather Advisory for along the Alaska Range in the Copper River
Basin and ended the one for the Susitna Valley later this morning.

2. With the farther south low track, the bulk of accumulating
snow will be confined to portions of the Kenai Peninsula and
Prince William Sound this afternoon before spreading inland from
Cordova and Valdez to the southern Copper River Basin this
evening. With winds developing out ahead of the deepening low,
have issued a Winter Weather Advisory for Thompson Pass for
blowing snow tonight. May also need to issue an advisory for snow
for the southern Copper River Basin.

3. There is quite a bit of uncertainty in how far north the snow
will spread with the new Gulf low. The more the low deepens,
which is the general trend, the smaller the area of snow will be.
For now, bring snow northward across the Kenai Peninsula to
Anchorage and the Matanuska Valley late this afternoon into
evening. However, it is conceivable that snow could remain south
of Anchorage. As cold air advection kicks in behind the low,
overnight tonight still think the western slopes of the mountains
will see a little bit of light snow because we'll still have low
level moisture in place.

4. With a deeper Gulf low, pressure gradients look quite a bit
tighter across the entire region. Cold air advection still looks
quite impressive behind this system tonight through Sunday.
Therefore, have upped winds most places. Of particular note,
issued a High Wind Warning for Kodiak Island overnight tonight
through Sunday morning. Also, upped winds across the Kodiak Island
marine areas, with widespread storm to hurricane force winds gusts
expected.

The low will exit across the eastern Gulf Sunday, bringing an end
to snow across Southcentral. The strong winds will be fairly
short-lived as a short-wave ridge rapidly builds in aloft Sunday
night thanks to continued progressive flow. A frontal system
extending from a hurricane force low along the Aleutians will
enter the Gulf early Monday and quickly spread across Southcentral
on Monday. It once again looks like a a surface low will form
over the western Gulf and keep most of the impacts along the Gulf
coast. However, it does look favorable for all of Southcentral to
see some light snow out of this system.

&&

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

Showers will begin to taper off from west to east through the
afternoon hours as an upper level wave exits the region. Light
snow accumulations are expected as this system weakens early
today. A surface low lifting over southern Bristol Bay this
afternoon will bring another quick shot a snowfall to the southern
Mainland and Alaska Peninsula through early Sunday morning.
Northerly flow on Sunday will allow for cold, arctic air to filter
in over the Southwest, with temperatures dropping into the single
digits for most locations.

The cold and dry conditions come to an end early Monday morning as
a strong front approaches the Southwest coast. This system will
bring widespread precipitation to the region, with temperatures
quickly warming into the high 20s to mid 30s for interior Bristol
Bay and the Alaska Peninsula. A mix of rain/snow is possible for
locations just above freezing, while areas farther north will stay
all snow. Areas along the coast could see blowing snow conditions
with stronger winds as the front initially moves over the land.
Several inches of snow is expected from Sunday night into Monday
afternoon as the front moves farther inland. The pattern becomes
much more showery late Monday, which will persists through
Tuesday.

&&

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

Widespread precipitation and gale force winds persists across the
western to central Bering through late morning as a low pressure
system tracks back into the North Pacific. Farther east, a low
develops and moves over the southern mainland, which will bring an
area of storm force winds to marine areas south of the Alaska
Peninsula through Sunday afternoon.

The main weather focus continues to be the storm force low
entering the western Bering and Aleutians by this afternoon.
This system could see hurricane force winds gusts as the initial
front tracks eastward throughout the day on Sunday. Sustained
hurricane force winds are expected on the back side of the low
with colder temperatures wrapping into the low center. Thus, a
High Wind Warning has been issued for the western Aleutians,
including Shemya on Sunday. A High Wind Watch has been issued for
Adak as this system progresses through Monday morning. Strong
winds will persists across the Bering through Tuesday when the
associated low begins to track into the North Pacific.

This strong front on Sunday and Monday will create hazardous
heavy freezing spray for a good portion of northern and eastern
Bering Sea extending as far south as the Pribilofs Island and
southeast towards Bristol Bay and the AKPEN.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Bering: A well developed low and front tracks across the Bering
into Southwest Alaska for Tue. Model confidence is good, but
decreases near the end of the period. Widespread storm force winds
and waves with areas of hurricane force gusts with the wind
wrapped low and front as it moves east, diminishing from the west
Tue. Widespread gale force winds take over through Wed. Waves to
47 feet over the Aleutians and Bering, subsiding in the Eastern
Bering by Wed.

Gulf of Alaska: A low and front move across the Gulf for Tue. A
second front is hot on the heels through Wed. Confidence is good
through the forecast. Widespread high end gale force winds and
waves move across the Gulf for Mon and Tue. Gusty outflow winds
spread into the Gulf from Kachemak Bay and the Barren Islands,
down the lower Shelikof Strait and along coastal Alaska Peninsula
for Wed. Waves build to 22 feet move across the Gulf through Wed.

&&

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

The long term forecast period overall looks quieter as an Arctic
trough moves through much of the Mainland for next week. Overall,
a cold pattern is expected for this time frame as rounds of
Siberian/Arctic air move across the northern Bering and Bering
Strait across the Alaska Mainland. The pattern is expected to be
more progressive as well meaning that this cold air will be pushed
through the gaps and passes of the Alaska/Aleutian Range and down
the Alaska Peninsula to the eastern Aleutians. This doesn't look
to be the typical setup for the Southcentral gaps and passes to
really blow strongly but winds will still be on the uptick for
next week. The coldest days as of now appear to be Tuesday and
Wednesday when temperatures will be well below normal across
Southwest and Southcentral Alaska. A weak upper level ridge will
nudge its way into Southwest Alaska Thursday which may bring snow
to the Southwest areas of Alaska. However, another round of cold,
Arctic/Siberian air is lurking behind this ridge that will push
this ridge off to the south and not make much progress into
Southcentral Thursday and Friday. At most, some light snow may
occur for Kodiak and the eastern Kenai Peninsula late week though
temperatures will still remain below normal.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...

PUBLIC...Winter Weather Advisory 131 145.
		 High Wind Warning 171 191.
		 High Wind Watch 187.
MARINE...Hurricane Warning 130 178.
		 Storm Warning 131-132 138 150 175-177 185.
		 Gale Warning 120-121 125 127 129 136-137 139 155 160
					  165 170 172-174 178.
Heavy Freezing Spray Warning 130 138-139 150 160 165 179 180 185.

&&

SYNOPSIS AND MODEL DISCUSSION...CMB
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...MV