Gale Warning 234, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKDT Fri Mar 20 2020

.ANALYSIS AND UPPER LEVELS...

An upper level shortwave trough is exiting the Copper River Basin
to the east this morning. A flat ridge with embedded 115 kt jet
streak extends over the Kuskokwim Valley. A much stronger
shortwave trough is moving over the western Bering. The fast
progressive flow associated with that jet streak is keeping the
weather highly changeable from day to day.

Much of Southcentral and Southwest are under mostly cloudy skies.
Clearing is mostly prevalent over the Kenai Peninsula and western
Susitna Valley. Fog and low stratus are prevalent over much of
Bristol Bay and the Susitna Valley. Remnant low stratus persists
over portions of the Copper River Basin and Prince William Sound
as a result of yesterday's frontal passage. Temperatures are
hovering at to a couple degrees above freezing in many areas with
south to southwesterly winds helping keep temperatures up this
morning.

&&

.MODEL DISCUSSION...

Model agreement remains very good through late Saturday night, as
a large area of high pressure south of the Alaska Peninsula, a
front tracking over the northern Bering and into mainland Alaska,
and a new high building in over the Bering are the primary players
in the forecast. Many areas are starting out the day at their
warmest levels, so temperatures are going to be a challenge today,
especially as they relate to the movement, formation, or
dissipation of the low stratus that has enveloped much of the
area. Nevertheless, overall forecast confidence is high.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist today through
tonight. The next round of snow will begin Saturday morning, and
will persist through much of the afternoon. MVFR conditions are
expected early in the morning, but IFR conditions are likely for
much of the day. Winds will remain light during Saturday's snow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level wave that brought rain and snow to the Cook Inlet
region yesterday is in the eastern Copper River Basin as of early
this morning and will be well into the Yukon Territory by late
morning. This will bring in zonal flow aloft over Southcentral
Alaska today. This translates into fair skies and a break in the
precipitation before the next system arrives tomorrow.

For Saturday, a moist airmass will move into the region from the
southwest as a cold upper level trough digs into the area from
the northwest. Flow aloft will be southwesterly and temperatures
aloft will be cold enough to support snow for most of the region.
However, precipitation-type will be the biggest challenge for
tomorrow. It looks as if Kenai and northward (including the Copper
River Basin) will see snow. Temperatures will be in the mid 30s
across much of the area and that will mitigate the snow
accumulation. Homer should warm enough to make 40 degrees, but
temperatures aloft will be cold enough for snow and dewpoints look
to remain in the upper 20s.

This system will also move through rather quickly leaving Sunday
as a clear and dry day with the potential for gusty northerly
winds to develop through the typical areas of channeled terrain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Today through Saturday evening)

The main feature of interest will be an approaching shortwave
trough from the west. Warmer air and increasing moisture just
above the surface will precede this feature, with perhaps a weak
area of surface low pressure developing near Bristol Bay ahead of
these features. The net effect will be with the increased ascent
(lift) associated with the incoming trough, and the theta
advection (amount of warm air and moisture moving into a region),
widespread precipitation development is anticipated by Friday
night while continuing into your Saturday. Most of this will be in
the form of snow, but a few areas closer to the coast may see a
brief mix or change over to rain as you get an added heat flux of
the ocean. As for snowfall, accumulations will be light, with most
areas likely seeing 2 to 4 inches of accumulation.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Today through Saturday evening)

A shortwave trough crossing the central Bering today will allow
for showery type precipitation to develop here. Further south, an
approaching disturbance in the southwesterly flow ahead of the
trough will cross the eastern Aleutians by this evening.
Increasing amounts of lift (ascent), warmth, and moisture (both
just above the surface) will interact to foster an increased
coverage and intensity of rain and snow, especially for eastern
portions of the Bering, the eastern Aleutians, and the Alaska
Peninsula (AKPEN). Mixed precipitation is possible for all of
these areas, with a higher likelihood that rain will be the more
predominant type as you get closer to the AKPEN and Aleutian
chain.

Further west, high pressure will build across the region behind
the departing trough, with decreasing cloud cover and dry
conditions expected.

&&

.MARINE (Days 3 through 5)...
A ridge of high pressure in place of the eastern Bering will keep
things generally quiet through the early part of next week. A
weakening North Pacific low moves northward over the central
Aleutians Sunday on Monday. This will create some elevated
southeasterly winds through gaps and passes along the central and
eastern Aleutians through Monday. As this system weakens
significantly and the high moves eastward, generally quiet
conditions area expected across the Bering for Tuesday.

Meanwhile for the Gulf, a building high over the eastern Bering
and a departing low centered over the southeastern Gulf will
generate elevated winds through Sunday in usual gaps and passes,
including Kamishak Bay and the Barren Islands. Winds will
diminish throughout the day on Monday as high pressure moves in,
leading to generally quiet conditions over the Gulf for Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There is moderate confidence that a strong upper level ridge will
be in place over the southern half of the state early next week.
A series of progressive upper level shortwaves will eventually
force the ridge to weaken and move southward by midweek, opening
up a zonal flow pattern aloft. By the end of the week, models
suggest a strong low will approach from the Bering from the west,
through there is little confidence in track at this time. Due to
uncertainty with models in the long term, we have low to moderate
confidence in the forecast. With them continuing to depict a ridge
to start next week, we are gaining confidence in that feature.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warning: 185.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...KO