Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 221453
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
553 AM AKST Thu Feb 22 2018

.ANALYSIS AND UPPER LEVELS...
The active weather pattern continues. The ridge that brought the
southern half of AK plenty of stratus and fog for the past day or
so has slid off towards the ALCAN border. The next transient
shortwave is pushing into the Cook Inlet region from SW AK this
morning. Snow and rain overnight for SW AK is starting to wind
down as the back edge of the front is evident on satellite. Fog
and low stratus are once again filling in behind it. But quite a
different story for Southcentral AK as the radar activity has
increased steadily overnight and it is now snowing steadily across
much of the Anchorage Bowl/Kenai Peninsula. The only places
experiencing temperatures warm enough to support rain are over the
souther tip of the Kenai (Homer, Seldovia). The Turnagain Arm wind
has yet to make an appearance. At this point, it looks like it
should be very shallow and short-lived, so precipitation should
remain mostly snow across the Anchorage Bowl.

Meanwhile over the Bering, the next gale-force front is pushing
through the Western Aleutians. Eventually, this will track towards
the West Coast of AK on Fri bringing another round of
precipitation with it.

&&

.MODEL DISCUSSION...
Models are in much better agreement with today`s snow for
Southcentral. There is still a little bit of uncertainty as to
exact amounts due to the orientation of the trough axis and the
timing of the surface low development in Prince Williams Sound.
However, all places will see at least several inches of snow.
Models have then trended much further north with precipitation
with the system across southern AK for Sat. This system looks to
bring more snow to the area. Unlike the current system, this one
should be safely all snow even through the Gulf Coast. There is
some uncertainty in how strong the associated triple- point low
will be over the North Gulf, so we will be watching that for winds
and snow amounts.

&&

.AVIATION...
PANC...Fog and low stratus has blended directly into snow this
morning. LIFR conditions will persist through much of the
morning. The only chance for some improvement would be some
warmer Turnagain Arm wind moving in. But for now, this looks like
an outside chance. So expect minimal improvement until the snow
changes to showers with northwest flow later this evening.&&

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level trough moving over Southcentral this morning will
be the focus for widespread snowfall throughout the area today.
The trough axis is moving slightly slower than initially expected,
so the winter weather advisories for the MatSu Valleys were
extended into the afternoon when snowfall should begin to diminish
from west-east as the trough axis passes overhead. We also added
an winter weather advisory for the Anchorage Bowl, both due to
near advisory level accumulations expected and to highlight
periods of heavy snow during the morning rush hour today. The
lowest confidence in the expected snowfall from this event is
around West Anchorage which be under the influence of a brief
round of Turnagain Arm winds this morning. This is expected to be
a very shallow layer of SE winds that should quickly become SW
within a few thousand feet of the surface. This will prevent any
significant downslope effects from developing, but as temperatures
rise to near-above freezing it is expected to bring decreased
snow ratios (to around 10:1), likely leaving West Anchorage on the
lower end of the expected accumulations today. This trough will
then shift widespread accumulating snowfall into the Copper River
Basin and Gulf coast for this afternoon-evening before all areas
begin to improve around midnight behind the trough axis.

As the upper level trough passes over the North Gulf Coast this
afternoon, it will quickly produce a surface low near Prince
William Sound, bringing a round of strong winds to much of the
Gulf Coast. The strongest of these winds are expected from Eastern
Turnagain Arm into Whittier as a strong westerly jet and shortwave
pass over the intensifying pressure gradient. Some of the guidance
is indicating that this could bring a brief round of high wind
gusts to Whittier, but at this point that gusts should peak in the
60 mph range. These winds will then begin to diminish on Friday
morning as the very progressive pattern continues to bring rapidly
moving systems through the area. Weak high pressure will quickly
move through the Gulf on Friday, ahead of yet another system that
is expected to impact the entire region with continued snow late
Friday into this weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
A front is currently exiting SW Alaska to the east bringing an end
to rain and snow to most of the region this morning as colder and
drier air filters in behind the departing system. Some snow
showers will persist over the Kuskokwim River Delta region this
morning aided by instability from cold air advection aloft. Skies
should clear today as a ridge of high pressure builds in over the
region. As the ridge axis moves over the western coast tonight,
clearing skies, strong subsidence aloft, weak flow, and plenty of
low level moisture will lead to the development of widespread fog
over the coastal and lowland regions tonight and Friday morning.

Another system will rapidly approach the coast by Friday afternoon
with a front sweeping across the region bringing accumulating
snowfall to most locations through Saturday morning. Right now,
due to the speed of the system, about 3-5" of snow is expected,
though higher amounts are likely for communities closer to the
Alaska Range, including Iliamna and Port Alsworth.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
A progressive pattern will continue over the Bering as a series of
low pressure systems punctuated with short periods of ridging in
between will move across the area for at least the next week. The
first system is currently approaching the Western Aleutians from
the SW with snow and gale force winds approaching Attu. This front
will sweep across the Aleutians and Bering Sea before elongating
out as the low complex runs into an Arctic high to the north. The
resultant tightening pressure gradient across the Bering and
strong cold air advection behind the front will result in a wide
spread area of westerly gale force winds spreading across the
entire Bering Sea through the day on Saturday. The long fetch
winds will lead to building seas with waves building up to 25 ft
by Saturday.

&&

.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, which is very good for cold and snow lovers.
Rejoice!

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 111 145.
MARINE...Gale Warning 119 120 121 125 127 130 131 170 173-179 181
         185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...MTL



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