Small Craft Advisory 87, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 AM AKST Mon Jan 28 2019

.ANALYSIS AND UPPER LEVELS...

A fairly deep upper-level trough extended from eastern Siberia
southward towards the far western Aleutians, with a deep surface
low anchoring this trough southwest of Shemya. An occluded front
stretched across the western portions of the chain, with this
morning's scatterometer pass showing sustain storm force winds
continuing just north of here. Another trough extended south
across the eastern Bering into the north Pacific, with ridging in
between. To the east, an even stronger ridge was poking up from
the Panhandle towards the Yukon border. This has resulted in a
deep southerly flow developing across the Alaska Range into
southcentral, with warm advection continuing. Also of note is this
mornings water vapor imagery, which shows a tropical fetch
surging north into the Gulf, with a surface low located well
offshore to the southeast of Kodiak.

&&

.MODEL DISCUSSION...

The models remain in good agreement in their surface and upper-
level forecasts.

&&

.AVIATION...
PANC...Mainly VFR conditions with light winds are expected to
persist through 04z as some periods of snow and rain develop.
However, ceilings and visibilities will be reduced in the heavier
bursts of precipitation.

By this evening, precipitation coverage is expected to increase in
coverage and intensity, with wet-bulb cooling effects allowing
temperatures to cool to freezing or below. This looks to occur
after 04z. Earlier guidance indicated a freezing rain to snow
potential for the evening into the overnight hours, and this
remains possible. However, the latest guidance is trending
towards a mainly snow event with light accumulations possible. In
either case, MVFR ceilings with IFR visibilities are anticipated
and this was inserted into the amended TAF package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The big forecast challenge remains precipitation type today
through Tuesday evening from the Kenai Peninsula northward
through the Susitna Valley, including the Anchorage area and
Matanuska Valley. The first low that was moving through the
western Gulf of Alaska from south to north has elongated as a
second low, following almost the same track, catches up to it and
absorbs it. With warm air spilling into the area with these lows
there will be a mix of precipitation types. While the overall
pattern over the Anchorage area looks like there may be some rain
or freezing at times, the more recent model runs have been hinting
at more snow as the precipitation type. Part of this is due to
the warmest of the air aloft sliding in just east of Anchorage
allowing the city to remain in slightly colder air while Palmer
and Wasilla may get slightly warmer air (and the terrain in this
pattern actually allows more low level warming in the Matanuska
Valley than Anchorage). That is how close the edge between
precipitation types looks for today through Tuesday. Tuesday will
see slightly warmer temperatures across the region which may
switch most of the precipitation to rain over Anchorage, the
Matanuska Valley, and the rest of the Cook Inlet region. This is
also quite uncertain and will be watched to see how it develops.

Higher elevations of the Kenai and Chugach Mountains will see all
snow and areas of heavy snow at that. Thompson Pass will be an
area that will likely see snowfall overnight in the 12 to 18 inch
range with some 20 to 30 mph wind gusts to help further reduce
visibility in the heavy snow.

Areas of the Richardson Highway in the southern part of the
Copper River Basin will also see significant snowfall overnight
tonight.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Monday
through Wednesday)...
Low pressure to the east is expected to send moisture into
southwest Alaska today into tonight, making for a few inches of
snow in eastern zones 161 and 152. There could be some areas of
mixed precipitation as the surface warm layer holds on in
southwest Alaska. After this system passes, weather is pretty
quiet the next couple of days. Gradual cooling will continue as
cold air filters in from the northeast.

&&

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

A powerful low is still forecast to move through the western
Aleutians, with storm force winds forecasted through tonight for
the western Aleutians and Western Bering Sea. Unfortunately
Shemya, our one observation in the western Aleutians, is not
operating. This means it will be difficult to verify the strength
of the low unless we get a ship observation or a satellite pass.

This low is expected to weaken as it progresses east through the
Aleutians. By Wednesday it becomes pretty stretched out with a
disorganized center. Wind speeds should be mainly small craft
with maybe a few gales by then. Late Wednesday the next low enters
the western Aleutians.

&&

.MARINE (Days 3 through 5)...

With a couple of strong frontal systems lifting up from the south,
gale to possibly storm force winds look likely in the Aleutians
and Bering Wednesday afternoon through Friday. The strongest
winds will initially start in the western Aleutian and then
become more widespread and expand to the north and east with each
subsequent frontal system. Conditions in the Gulf will generally
be quieter, with the exception of along the northern Gulf coast
where strong north to northeasterly outflow gap winds will
develop Thursday and Friday. Stronger frontal systems will begin
impacting the Southwestern Gulf around Saturday as the active
southwesterly Aleutian/Bering storm track shifts east to direct
the stronger frontal systems into the eastern Bering and
eventually western Gulf through the weekend.

&&

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

By Tuesday evening an elongated vertically stacked low will
stretch across the southern half of the Bering, the Aleutians and
the Alaska Peninsula with a flat upper level ridge extending over
the eastern Gulf, southeast Alaska Panhandle and British Columbia.
Further to the north, an upper level ridge will stretch from
eastern Russia across the Bering Strait to the Seward Peninsula
with an arctic trough centered over the Beaufort Sea. The center
of the arctic low will dig south through the Yukon and Northwest
Territories Wednesday and Thursday, while the upper level ridging
shifts over western Alaska and amplifies. This will put
southcentral Alaska under northerly flow aloft with a vorticity
maximum and accompanying strong push of cold air and north winds
surging into the area Thursday and Thursday night. Cold offshore
flow with strong gap winds will persist along the northern Gulf
Coast through Friday and then begin to gradually diminish
Saturday.

A north Pacific frontal system will approach the western Aleutians
on Wednesday. A frontal wave further to the south along the same
system will rapidly deepen Wednesday night as it tracks north to
push a strong front across the western and central Aleutians on
Thursday. The front will continue northeast, though weakening
gradually as it does so, to cross the eastern Aleutians Thursday
night and reach the Alaska Peninsula and eastern Bering Friday.
Another front will follow behind it Friday and Saturday with the
track of its main surface low shifting further east in response
to the eastward progression of the large supporting upper level
trough and strong jet stream to the south.

By Saturday, the axis of the upper level ridge will shift east to
be centered over mainland Alaska with what looks to be a fairly
strong overrunning frontal system pushing into Southwest Alaska
Saturday and continuing into southcentral Alaska late Saturday
night and through Sunday. In response, the upper level ridge will
flatten and be pushed east with a second system following Sunday
and Monday and the active southwesterly storm track likely
continuing through midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 175 176 177 178
         Gale Warning 119 120 125 130 131 139 170 172 173 174

FIRE WEATHER...NONE.

&&

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