Small Craft Advisory 79, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
334 PM AKST Thu Jan 24 2019

.ANALYSIS AND UPPER LEVELS..
A strong weather front is moving rapidly north across the
mainland this afternoon. Rain and snow were reported across
southwestern Alaska and Southcentral. There is a thin dry slot
with showers on the lee side of the front(south) and it is rapidly
filling in as the next front races north. Gusty winds were also
evident across many areas of western Alaska and portions of
Southcentral and the Gulf Coast. A developing low center south of
the western AK PEN was moving north and generating storm force
southerly winds across the Central AK PEN. A rapidly developing
surface low near 50n 151w is positioned under favorable upper jet
support and heading north.

&&

.MODEL DISCUSSION...
Still looks like a NAM/GFS solutions are the best choice for the
surface low developing over the north Pacific and heading into
the northern Gulf tonight. Models seem to be all painting a
similar picture with the low complex heading into the Bering
Friday night.

&&

.AVIATION...
PANC...LLWS conditions will redevelop this evening and persist
into the early morning hours before diminishing. Generally will be
looking at VFR conditions through tonight though ceiling could
fall below 5000 ft during the morning hours with rain. Rain will
likely mix or change to snow in the afternoon hours Friday and
bring a period of MVFR/IFR conditions.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An active pattern ensues through Friday...the strongest system
for the second half of this week races up to the southern Gulf
this evening. This storm system spreads rain and storm force winds
across the Gulf waters and for the north coast tonight; then
continues a northward track pushing inland by Friday afternoon.

There are still a few challenges with this system as models
continue to differ on the track and timing of surface low. The
earlier trends which showed this system re-enforcing warmer air
across the Anchorage Bowl and Matanuska Valley through late Friday
afternoon, now shifts to a cooler pattern advecting across the
area. The support for this solution is fairly solid as the upper
wave lifting the surface front tonight pushes the boundary inland
slightly faster than previously expected. What follows this front
is a colder core low pushing over the Southwest mainland by late
Friday morning. This low continues north through the afternoon
with a significant wave expected to stretch across the southern
mainland during the afternoon and evening. The combination of a
weak remnant surface trough over Southcentral and a short wave
progged to move across is a good set up to see at least one inch
of wet snow on the the exiting side of the system for a few
locations.

There are lingering chances for scattered rain and snow showers
across the Gulf and surrounding coasts on Saturday as one more
upper wave propagates across the gulf with snow chances inland.
Otherwise, Saturday bring a short pause in the active weather
pattern as the next front nudges toward the west Gulf late
Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The longwave pattern features a broad trough across the Bering
and Aleutians with a ridge axis over southcentral Alaska and the
Gulf of Alaska. This pattern will be progressive and unsettled.
For southwest Alaska, expect periods of widespread divergent and
southerly flow aloft. There are numerous shortwaves embedded in
the upper level trough. With this type of pattern in place the
active weather will continue although there will be some breaks
between the precipitation bands/systems. The biggest challenge to
this forecast package is determining the precipitation type. The
thermal profiles from model soundings clearly implicate that there
is some amount of warm air in place on Friday and Saturday that
could result in a complex wintry mix or maybe just rain and snow.
By Saturday, there is additional warming in the boundary layer as
the next front pushes across the Eastern Bering and Bristol Bay.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The longwave pattern features a broad trough from NE Russia that
spans across the Bering and south of the Aleutians with a ridge
axis bisecting mainland Alaska and the Gulf. This pattern does
not break down this forecast period so expect another round of
unsettled weather for the region. At the onset of the forecast
period at the surface, there is a low near Pilot Point, a low
south of Unalaska, a low north of Nikolski and a low near Attu
Island. As time elapses, these systems will push eastward. By late
Fri, a new mature low with an occlusion will move towards the
Western Aleutians with the surface low near Adak by 00z Saturday.
This system will jump the chain into the Bering early Saturday
morning bringing another round of gusty winds, rough seas and
precipitation into the region. The silver lining in this pattern
is that the Aleutians will be relatively warm with temperatures
hovering near or above 40F this forecast period.

&&

.MARINE (Days 3 through 5)...
There is high confidence Saturday night of a gale force low in
the northeastern Bering that will weaken through Sunday as it
pushes north. Sunday night there is also high confidence of a
strong low approaching the western Aleutians that looks to bring
storm force winds to the islands. On Monday those winds will push
north into the western Bering as they weaken.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning Sunday night has a dominant ridge
set up over the eastern part of the state while a low in the
northern Bering exits off to the north. Further west, a weaker
ridge is over the central Bering and a new system will be
approaching the western Aleutians from the North Pacific. This
system will be the main feature Monday as there is high confidence
in it being a deep system that will likely produce storm force
winds. As this system weakens on Tuesday the models struggle with
what happens to its remnants and their handling of the rest of the
domain also suffers. Because of this, ensembles were preferred
for Tuesday through Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119 120 125 155 172.
 Gale Warning 129>132 139 150 160 165 170 171 173 174 176 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...DK