Small Craft Advisory 135, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKST Sat Feb 10 2018

.ANALYSIS AND UPPER LEVELS...

The strong front that impacted Southwest Alaska yesterday has
weakened considerably as it continued north and east into the
Gulf and Interior AK. Radar imagery this morning shows a fairly
unimpressive precip shield across the northern Gulf, with the vast
majority of precip confined to the Gulf side of the coastal
mountains. Nevertheless, clouds have overtaken the previously
sunny skies across most of southcentral, and temperatures across
the region have begun to moderate/warm as the arctic airmass is
being pushed out of the area.

The next feature that will bring a renewed push of warmer
southerly air to our part of the world is a low center that has
just crossed over Dutch Harbor and pushed into the Bering Sea,
with a supporting upper level shortwave trough pushing toward
Kodiak Island. This trough will push northward through Cook Inlet
and Southwest Alaska tonight and will provide further moderation
of the arctic airmass, and bring a quick dusting of snow to many
areas from Anchorage northward.

Meanwhile, the western 1/2 of the Bering Sea and Aleutians are
deep within the longwave trough, and are experiencing somewhat
gusty northwesterly winds with snow shower activity across much of
the area.

&&

.MODEL DISCUSSION...

The models over the next few days have come into very good
fundamental agreement, with no models totally thrown out from
consideration during forecast preparation today. That's only half
the battle, though, as now we can begin to factor in the local-
scale influences of the complex terrain at individual locations.
As usual, one of the most challenging locations will be Anchorage,
where the influence of wind coming over the Chugach mountains will
interact with winds moving through Turnagain Arm as well as coming
up Cook Inlet from the southwest. The rain/snow line for this
storm is the primary challenge, and should set up somewhere
between the northern Kenai Peninsula and southern Susitna Valley.
See the detailed discussion below for more information.

&&

.AVIATION...
PANC...VFR conditions and light north winds will persist through
the evening. Ceilings will gradually lower throughout the evening
hours before MVFR conditions and light snow become likely sometime
after midnight. These conditions should persist until sometime
around 6am, at which point precip should diminish and winds will
likely switch to a southerly direction (but stay somewhat light) and
persist throughout much of the day Sunday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Monday evening)

The tale of two storms will be the main theme in the short term.
The first of which will feature a weakening front that'll cross
southcentral overnight. This front will be in a weakening state.
However, enough convergence remains to modestly focus
precipitation development, aided by a pair of disturbance aloft
(one south of the region and the other crossing southern Alaska).
These disturbances will provide a decent amount of ascent aloft
for widespread light snow to cross the region.

After a brief lull in the action Sunday afternoon, the main
weather maker looks to begin impinging on the region Sunday night
into Monday. This system will feature a strong mid and upper
level jet that'll cross the region, providing very strong
isentropic ascent. A veering wind profile will also provide good
low to mid level convergence through the column, with soundings
showing and southeasterly component remaining fairly shallow (~on
the order of 2500 feet or less). Thus, while some rain shadow
effects (an area of less precipitation that occurs on the leeward
side of a mountain due to compressional warming/evaporative
effects) will likely occur, the net result will be a much smaller
terrain- confined footprint with just some lighter amounts in
this region.

The bigger question for coastal locations will be when does enough
warm air push inland to switch any snow over to either a mix of
rain and snow or just plain rain. This system will have mainly
southwest winds from 850 mb and above, with a plentiful moisture
supply, again limiting any down-slope warming effects from the
mountains for the western portions of the Kenai Peninsula,
through the Anchorage Bowl, to the Susitna Valley. Additionally,
Model thicknesses and point sounding show the above freezing air
confined to 900 mb (3000 feet). This would mean locations even in
the mid 30s would likely have snow mix in at times when heavier
precip rates due to less residence time for the moisture in an
above freezing layer (less time to melt since its falling to
fast). Thus, mixed precipitation wording was expanded in the
grids. The highest snow amounts look to occur in the 3 PM Sunday
to 9 PM Monday time frame, mainly for Hatcher Pass and in Susitna
Valley.

&&

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

A series of waves of moisture are moving into Southwest Alaska.
The first moving into the area now is overspreading the area north
of Dillingham and west of Koliganek with a mix of rain and snow,
with the snow occurring where the precipitation is heaviest. Also
a brief rain or snow shower is moving across the Bethel area. As
the wave lifts northward tonight, the warm moist air will
encounter the remains of any near-surface cold air, leading to
overrunning and a quick 1-3 inches of new snow north of Dillingham
into the Lower Kuskokwim Valley. The precipitation tapers to
scattered rain and snow showers for much of the day Sunday.

On Sunday afternoon, the next stronger low approaches the area,
tracking up Kuskokwim Bay and the west side of the Kuskokwim
Mountains late Sunday night into Monday morning. There will be
significantly more moisture with this system, however, it will
also bring a new round of warmer air. Thus, most of the area, even
up into the Lower Kuskokwim Valley is expected to eventually
change over to rain. Before the warmest air gets there however,
another 1-3 inches of heavy, wet snow is possible Sunday night
over the Lower Kuskokwim Valley and Delta with the changeover on
Monday. Further south over Bristol Bay, the changeover to rain
will happen sooner, resulting in less snow before the transition.
The precipitation tapers off Monday afternoon from west to east
with mountain snow showers continuing into Monday night. Yet
another round of precipitation focuses on the Kuskokwim Delta
Tuesday with an approaching front.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Sunday
and Monday)...

A broad, weak low over the central Bering will lift north and
weaken during the day Sunday. The next low begins to impact the
eastern Aleutians Sunday morning before lifting north into
Southwest Alaska Sunday night. Most areas stay warm enough for all
rain. Brief surface ridging moves through the western and central
Bering Sunday. A strong front moves into the western Bering and
Aleutians late Sunday night, traversing much of the Bering by
Tuesday morning. Most areas will be all rain as another punch of
warmer air moves north along with the associated precipitation.
The front will weaken with time, but while it is very strong, high
gale force winds with storm force gusts are expected over the
western and central Aleutians late Sunday night into Monday.

&&

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

The long term forecast is cloudy...both figuratively and also
probably literally. The models have significant differences with
some major details that will affect the Alaskan domain over the
coming week. What can be said with confidence is that a longwave
trough will continue over the northwest Pacific and western Bering
during this period, with a strong East Asian jet stream keep the
storm track south of the western Aleutians. Similarly, a
diminished strength western North American ridge looks to hold on
over the lower 48 and western Canada. This creates the scenario
where the jet stream buckles repeatedly over the north Pacific and
"slings" storms from southwest to northeast across mainland
Alaska. In practical terms, this means that the very
active/progressive pattern we are heading into this weekend will
likely continue through at least next weekend with periodic
rain/snow chances and relatively milder-than-normal temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145.
MARINE...Gale 131 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...AMD
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...AMD