Small Craft Advisory 912, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
719 AM AKST Sat Nov 30 2019

.ANALYSIS AND UPPER LEVELS...

A strong ~170 kt upper level jet stretches from the North Pacific
eastward through Bristol Bay. This in conjunction with a broad
upper level trough over the Bering is leading to the development of
a surface low southwest of Adak. This will bring rain to the western
Aleutians on its north side and is expected to bring significant
impacts to Southwest Alaska. Farther downstream, the negatively
tilted trough axis is centered over the Kuskokwim Delta and Bristol
Bay areas. Combined with a weak surface ridge, much of the snow seen
along the stalled frontal boundary over the past couple of days has
shifted off to the east over the Middle Kuskokwim Valley, though
mostly cloudy skies are persisting in its wake.

A shortwave trough has lead to the formation of a weak surface low
centered over the western Kenai Peninsula. As such, rain is
occurring over Prince William Sound and portions of the Susitna
Valley. Southerly winds with associated warmer air will continue
through this evening.

&&

.MODEL DISCUSSION...

Slight disagreement still exists between the models regarding
precipitation type and timing today in Anchorage. However, the
thinking remains the same in that as long as southerly flow
continues, the rain to snow transition will not occur until this
evening. In any case, precipitation is expected to be light.

With regards to Sunday's system, despite the NAM initially being
an outlier to the south, latest model runs show the GFS, ECMWF and
other foreign models shifting towards this solution. As winds
switch to northerly tonight, cold air will slowly make its way
into the region. Even with surface temperatures a degree or two
above freezing, temperatures aloft should sufficiently cool enough
for snow to develop Sunday morning. While rain may mix in by
afternoon, all models are showing accumulating snow in Anchorage
and the Mat-Su through Monday morning. If the current track
verifies, this accumulation would potentially be significant,
however any subtle shift in the track could greatly change the
snowfall forecast.

&&

.AVIATION...

PANC...While some uncertainty still exists, VFR conditions will
continue through sunrise. MVFR ceilings and lower visibilities are
likely late-morning through this evening as light precipitation
develops over the terminal. Visibilities could further drop to IFR
Sunday morning if snow rates increase. Light winds will persist.

&&

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

An upper trough and associated frontal boundary will move slowly
across Southcentral today and tonight. Cold advection will spread
slowly east across the southern Susitna Valley to Cook Inlet
today. Locations over the southern Susitna Valley which are
currently rain should shift to snow this morning. With southerly
winds aloft, locations mainly north of Talkeetna will see
advisory level snow through tonight. The cold air will lag some
across the Matanuska Valley, Anchorage Bowl and Cook Inlet. Sea
level areas will generally see rain or a mix through this afternoon
and should shift to snow early this evening as the cold air finally
arrives. No significant snow accumulations are expected in these
areas at this time. Northern Gulf locations will see mostly rain or
showers through tonight.

A strong triple point low forms near Kodiak Island Sunday morning
and moves into western Prince William Sound early Sunday evening.
Easterly gales and steady rain will accompany the front as it
heads north into the Gulf tonight and Sunday. Inland areas will
see a shot of overrunning light snow initially with the southerly
flow aloft early Sunday. The picture becomes a little cloudier for
precipitation types and snow intensity during the day Sunday for
the Anchorage Bowl/Matanuska Valley as models differ significantly
on the depth and aerial extent of cold air in these areas.
Current thought is that the cold air will erode some over the
Matanuska Valley/Anchorage Bowl with the onset of strong cross-
barrier flow Sunday. As the low center gets into Prince William Sound
Sunday evening, the strong cross-barrier flow should diminish and
the low level cold air spread east and allow for moisture
advected from the Sound to potentially bring significant
overrunning snowfall Sunday night into Monday. Stay tuned for
updates.

&&

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

The remnants of a fast moving occluded front are already tracking
through Southwest this morning and will clear the Alaska Range and
exit by this evening. High pressure briefly sets in before a fast
moving and compact low along an arctic front quickly moves into
Southern Bristol Bay tonight then spreads inland Sunday morning.
This will bring accumulating snow from the Alaska Peninsula north
to the Lower Kuskokwim Valley, although the fast moving nature of
the low will likely limit accumulations to around 2-4" for most
locations. This snow will be relegated to areas east of the
Kuskokwim Mountains. In conjunction with this will be increasing
cold air advection and a strengthening pressure gradient which will
allow northerly winds to increase Sunday into Monday. In conjunction
with the new snow, 6-15" of snow from the previous storm will allow
for plenty of blowing and drifting and reduced visibility,
especially north and east of King Salmon toward the Alaska Range.
Winds will likely remain below 35 MPH, so a blizzard warning was not
issued, but the impacts will remain the same with hazardous travel
and potential brief periods of whiteout conditions. The low will
exit Monday morning and conditions rapidly improve through the day.

&&

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

A moderate strength gale force low along the North Pacific jet
stream is already moving into the Central and Eastern Aleutians
this morning, with a mix of snow across the Central Aleutians.
Over the warm sector, expect mostly rain along the Eastern
Aleutians with some light snow accumulations across the Pribs as
the low tracks into Bristol Bay. The low moves inland Sunday
evening, leaving in its wake broad westerly cold air advection and
widespread snow showers over the Bering and most of the Aleutian
Chain which will last through Monday.

&&

.MARINE (Days 3 through 5)...

On Monday, low pressure will move into the central Aleutians,
although there is some uncertainty if this low will track north or
south of the Aleutians as it continues eastward. If this low
deepens enough, there is a chance that storm force winds may
develop around the core. For now, it looks to produce westerly
small craft winds with embedded gales from the western Aleutians
Monday towards the AKPEN Tuesday. A small craft to possibly a gale
force low in the northern Bering looks to drop towards the southeast
Monday then gradually weaken Tuesday with uncertainty as to when it
will dissipate or where exactly its remnants will end up.

A front associated with a low over Prince William Sound will
produce small crafts and gales Monday into Tuesday. While the
front then weakens and moves out of the area, offshore flow will
continue to produce locally strong winds near terrain gaps along
the Gulf coast.

&&

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

By Monday, a cold upper level trough over Southwest Alaska will be
shifting east into Southcentral with the accompanying strong cold
air advection supporting snow showers and strengthening offshore
north to northwesterly winds. Once the trough reaches the northern
Gulf coast, a cutoff upper low will develop vertically stacked
over the surface low near Prince William Sound. Both features
will then track slowly to the east southeast Monday night and
Tuesday with snow showers gradually tapering off and skies
clearing. The resulting cold, dry pattern will persist through
midweek with another upper level trough swinging in from the
northwest around Wednesday and prolonging the offshore flow.

To the west, a more active storm track will persist near the
Aleutians supported by cold air over the Bering and a strong
westerly jet. A developing surface low near the central Aleutians
on Monday will continue east through the southern Bering Monday
night with upper level ridging out ahead of it building in over
the Southwest Alaska coast. The surface low will then turn
southeastward, crossing the western Alaska Peninsula Tuesday
morning and digging south of the central Gulf Wednesday. As this
low phases with the cold upper trough over southern Alaska, an
upstream high amplitude upper level ridge will build in over the
western Bering Monday night and Tuesday, moving east to be
centered over the central and eastern Bering Tuesday night and
Wednesday.

A warm front pushing up the west side of the ridge will reach the
eastern Bering Wednesday night and southwest Alaska coast Thursday
morning. The associated cold front will follow more slowly due to
several waves developing along it and lifting north through the
Aleutians Wednesday and Thursday. A cold upper level trough
moving off of the Kamchatka Peninsula Wednesday night and over
the western Aleutians Thursday will spin up a deep vertically
stacked low over the Bering Thursday night and Friday. As this
larger and more consolidated low develops it will strengthen the
frontal system along the Southwest Alaska coast and push it inland
Thursday night and Friday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 145 161.
MARINE...Gale Warning: 120 125 131 132 138 150 155 172 174 176.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
MARINE/LONG TERM...CK/JER