Small Craft Advisory 942, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
637 PM AKST Wed Nov 28 2018

.ANALYSIS AND UPPER LEVELS...

A long-wave trough extends over the eastern Bering into the
southwestern Gulf of Alaska. Cold air advection is occurring on
the backside of this trough. This cold air is wrapping around an
occluded low over the eastern Aleutians, that sits at the base of
a shortwave that is embedded in the main long wave trough. A
surface front sits on the leading edge of this shortwave and is
pushing further inland over Southwest Alaska. This front has
brought significant snowfall through the Lower Kuskokwim Valley in
areas around Iliamna.

A second shortwave, over the Barren Islands, is embedded in the
main parent trough. This shortwave has strong vorticity
associated with it and is creating showers through much of the
northern Gulf and Southcentral. While the precipitation started
as rain this morning, it quickly shifted to snow as a cold air
burst moved over the western Gulf and up Cook Inlet. Radar from
this morning through early afternoon showed a single, persistent
band of snow over Anchorage which happens to be the area with the
most consistent snowfall.

&&

.MODEL DISCUSSION...
While models continue to agree on the overall synoptic pattern,
they are different enough with some significant details to keep
us from digging into the details to deeply on any of the models.
Enough has been changing from run-to-run that one has to look at
the overall picture the models are painting and adjust the
forecast from there. This situation with the models continues into
early next week where there is some information that can be
gleaned from them, but it is more general patterns and not any
specifics.

&&

.AVIATION...
Snow showers will end by this evening and then the question
becomes whether there will be any post-precipitation fog. While
guidance is mostly saying 'no,' it has taken very little to
produce fog and stratus around the airport in recent weeks
(especially after precipitation) that this is included in the TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
(Tonight through Saturday)

A challenging forecast has be the theme all day for the afternoon
through late evening hours, as a very nice mesoscale convergence
band set up along Turnagain Arm, and extended into the lower Mat-
Su Valleys and parts of the Anchorage Bowl. This was aided by a
mesoscale low that developed along the Kenai Peninsula/Prince
William Sound region, and an approaching upper trough. Snowfall
amounts have been quite variable with conditions significantly
changing over just a few miles. Case and point, by 3 PM the
downtown area had seen around one quarter of an inch, to near 6
inches on the south side of town near the Seward Hwy/Huffman Rd.
intersection Here at the forecast office we measured 3.2 inches,
while just 0.5 miles north of Lake Hood that amount dropped to 1.5
inches, and this is over about a 2.5 mile distance. Needless to
say, those are two good examples of the weather variability.

The short term challenge is when does this band shut off tonight
as high pressure begins to build across the region, moving
directly overhead tomorrow. Right now it looks like the latest
hi-resolution models show things calming down around 10 PM, if not
an hour or two earlier. The next impact we look to develop is some
patchy fog for Thursday night into Friday morning for all area
valleys and along the Cook Inlet. None of the models currently
suggest this occurring, but from a pattern recognition
perspective, with a strong ridge and subsequent steep low-level
inversion, some fog should develop for this period, with a
potential for reduced visibilities where the snowfall accumulated
today.

Also during this time period, a fairly strong storm system will be
pushing into the Kodiak Island region, with winds increasing and
widespread precipitation developing. This will all move northeast
for your day Friday, with the initial precipitation falling as
snow during the afternoon and evening, changing to a mix of snow
and freezing rain Friday night, as warm air a few thousand feet
above the ground moves over sub-freezing surface temperatures.
Then on Saturday, enough top-down warming, aided by a weak warm
front, will allow precipitation to change over to all rain. The
exception to this may be the Copper basin, where typically it
takes longer to scour out he cold valley air.

&&

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

Colder air continues to advect across interior portions of
Southwest Alaska through the evening and overnight hours as a
trough lifts out of the area and a mid-level ridge settles in.
With winds weakening under the ridge, areas of fog are also
possible through the evening and overnight hours, especially over
the Kuskokwim Delta.

The active weather pattern returns tomorrow as an occluding front
stretching over the central Bering races toward the Kuskokwim
Delta coast Thursday afternoon and moves inland Thursday night.
With this front weakening and not advecting much warm air aloft,
precipitation is expected to fall primarily as snow with light
amounts along the coast and higher amounts over the hills inland.
Areas of blowing snow are also likely in association with strong
frontal winds. At this time, given the fast-moving nature of the
front and a slight warming in the vertical, an advisory has not
been issued; however, this will be revisited by the night shift to
determine if an advisory is warranted. A second front then lifts
north towards the area from the southwest on Friday and Friday
night. This system have tap into sub-tropical moisture from the
North Pacific, bringing quite a bit more precipitation. This
system, however, will also pump much warmer air into the region
allowing any snow during the onset of precipitation to quickly
change to rain as the warm front slides across the southwest.
Colder air may linger a bit longer over the Kuskokwim Delta which
may yield a brief period of freezing rain for places like Bethel
late Friday before the changeover the rain. This system lifts
north for Saturday as a ridge builds behind it.

&&

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

A surface low near Sand Point will continue to track east toward
the western Gulf overnight. Upstream, a ridge is building over
the central Aleutians while a warm front slides across the
western Aleutians and Bering. This frontal system will produce
areas of widespread gales as it slides east and occludes
overnight then moves over the eastern Bering Thursday.

As this front moves into Southwest Alaska late Thursday, a second
system will rapidly develop over the North Pacific south of the
central Aleutians. Again widespread gales and pockets of storm-
force winds will develop as this system gains strength and lifts
across the Aleutians and into the eastern Bering on Friday. A
ridge briefly builds over the eastern Bering by early Saturday as
the low continues to quickly lift north. The next North Pacific
low then lifts over the central and eastern Aleutians and enters
the southern Bering for the weekend.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday Nov 30th through
Tuesday Dec 4rd)...

The upper level pattern for the extended forecast will features a
longwave trough centered over the northwest Bering Sea, and a
ridge over the central/eastern Gulf of Alaska. Between the two
features, an active jet stream with several embedded disturbances.
Guidance are all keying in on a potent disturbance coming off east
Asia, which will interact with baroclinic zone draped over the
northwest Pacific. The result will be a reshaping of the long-wave
as a stronger surface system develops south of the Western
Aleutians over the weekend. That system will becoming the new
center of the long-wave and shape the pattern into the middle of
next week.

Saturday begins under the southwesterly jet aloft, an embedded
trough, and plume of moisture from the Pacific. Unfortunately for
snow lovers, a bit warmer air mass will also be accompanying. As
that system exits a frontal boundary will stall from the western
Gulf through the southwest mainland. The pressure gradient around
this time will favor southeasterly winds through Turnagain Arm and
possible into Anchorage itself. As the previously mentioned
stronger system spreads into the Bering Sea we will return into a
deeper layer southeasterly flow pattern, meaning drier and warmer
with above average temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 155 165 170 172-181 185 411-414.

&&

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