Small Craft Advisory 168, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
435 PM AKST Wed Feb 21 2018

.ANALYSIS AND UPPER LEVELS...
A low complex over the eastern Bering Sea is progressing northward.
The associated frontal boundary extends from near Saint Lawrence
Island south to near the Y-K Delta Coast, then southwest across
the eastern Aleutians. Widespread cloud cover under broad
Southwesterly flow aloft extends across much of Southern Mainland.
Generally lower clouds with areas of snow and rain were evident
west of the Alaska Range/Aleutian range. With the exception of
lower stratus over the Copper River Basin and near Knik Arm,
higher clouds were the norm across Southcentral. Brisk to strong
southerly winds were observed in advance of the front over the
eastern Bering Sea and along the Alaska Peninsula. Showers and
brisk/strong westerly winds were evident on the backside of the
front across the Bering Sea and remainder of the Aleutians.

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.MODEL DISCUSSION...
Available models (ECMWF not available) overall are relatively
good agreement. The main challenge will be in the mesoscale
details across Southcentral which will favor the use of higher
resolution models and forecaster experience.

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.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. Expect IFR to LIFR conditions in snow developing
after midnight tonight.

&&

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

The frontal system and southwesterly flow aloft currently over
Southwest Alaska will push east tonight, spreading precipitation
into southcentral Alaska during the early morning hours. While
there is some uncertainty regarding the strength and extent of
Turnagain Arm wind developing overnight (which would warming
temperatures somewhat), with the dry air mass in place, even if
precipitation in the Anchorage bowl does start out as a patchy mix
of light snow, rain and freezing rain it should quickly
transition over to all snow as precipitation rates increase and
wet bulb evaporation cools the lower atmosphere.

The heavier precipitation will start between 4 and 6 am Thursday
and continue through late morning. With strengthening south to
southwesterly 850 and 700 mb winds and good cold air advection
with the upper level trough following the frontal system, the
eastern Susitna Valley, Talkeetna Mountains and Matanuska Valley
will be favored for higher snow amounts with this event.
Additionally, the western Chugach will also benefit from
orographic enhancement as winds turn more westerly from mid
morning onward.

Behind the upper trough, west to northwesterly flow aloft and
accompanying colder air mass will push across southcentral Alaska
Thursday night and Friday. While up-slope flow will cause snow
showers to persist longer along the Chugach mountains...elsewhere
showers will taper off with clearing skies and increasing west to
northwesterly winds, strongest where channeled through terrain
gaps.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The front that has moved in over the Kuskokwim Delta will continue
its track east tonight spreading precip throughout southwest
Alaska. This system has brought up enough warm air for most places
to see rain initially. However, as more cold air moves in behind
the front, most areas will transition to snow. Thursday will see
ridging move over the area from the west that will clear out the
precip and allow for fog formation Thursday night. Then on Friday
another front will push in from the west bringing more snow to
southwest Alaska. Snow will continue but diminish into Saturday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
There is an active pattern over the Bering with a front moving
through the eastern Bering and ridging over the western Bering
following behind the eastward moving front. The front will move
out tonight with the ridging building in over the central and
eventually eastern Bering. Behind the ridge a new system will move
in over the western Aleutians on Thursday. This new system will
track into the central Bering and eastern Aleutians Thursday
night. Friday afternoon it will exit the Bering to the east
leaving the Bering and Aleutians in cold westerly flow.

&&

.LONG TERM FORECAST (Saturday afternoon through Wednesday)...
An active pattern continues through the extended time frame. For
Southcentral Alaska, snow-bearing systems with a day of cold and
clear between and near normal temperatures. For Southwest Alaska,
the same frequency of systems exists, but more rain and above
normal temperatures.

A new front and developing triple-point low pressure system enters
the Gulf from the North Pacific Ocean. Guidance has come into much
better agreement with timing and track of the system, taking it
right up to Middleton Island by Saturday morning. The front
associated with the system looks to push all the way up Cook Inlet
and across the north gulf coast, even into the Copper River Basin.
All these areas should see snow spreading from south to north late
Friday night into Saturday morning. Snow should end by the
afternoon hours, returning the area back to a colder outflow/gap
wind pattern heading into next week.

Early next week, the upper pattern remains very progressive from
the southwest. While exact timing of each embedded system is not
feasible at this time, we can expect a new system to traverse the
area about every other day. At this time it looks like Mon/Wed/Fri
for snow chances over the area. The bulk of the warm air stays
south in the Gulf, but guidance is hinting at a more amplified
pattern late next week, which could bring a brief warm-up,
however, the pattern remains progressive w/o any intact warm
frontal passage.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145.
MARINE...Gale 119-125 127 130 131 170 173-179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...BC
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...MTL