Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 191410
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
510 AM AKST Tue Feb 19 2019

.ANALYSIS AND UPPER LEVELS...

A ridge building over the eastern Bering is progressing eastward
with several shortwaves moving within the flow on the leading edge
of the ridge. These short-waves are moving over the Kuskokwim
Delta and along the western Gulf of Alaska. A second feature to
note is a trough that is digging just off of the coast of the
western Aleutians. The jet stream is slightly meridional with one
segment of stronger winds northwesterly oriented over the central
Gulf and AK-Pen. The strongest winds however, are a swath of 160
kt northerly winds flowing south of the western Aleutians at
30,000 ft. The placement of this jet streak creates favorable
upper level support for a strengthening North Pacific surface low
that is advancing toward the western Aleutians. Over southwest
Alaska and the Gulf, a cold front is pushing inland ahead of a
ridge at the surface. This front is advecting cold air over
Southcentral and is bringing precipitation to the area which can
be seen by radar.

&&

.MODEL DISCUSSION(through Thursday Morning)...

Overall model consensus is good through the period as the front
moves through the Gulf and with the initial position of the North
Pacific low. The first sign of model divergence is when the EC
moves the low towards the northern Bering much quicker than the
North American models on Tuesday. Models return to a similar
pattern Wednesday morning as the low enters the northern Bering.
The next system is a Kamchatka low that approaches the western
Aleutians Wednesday night. Here, models again are in decent
agreement both with the intensity and location of this system as
it moves along the western Bering through Thursday.

&&

.AVIATION...

PANC... The snow that fell during the overnight hours last night
has ended except for some scattered flurries. Pockets with lower
ceilings and visibilities will continue lifting through the
morning as a cold front exits the area. Behind the front, the
skies will clear with VFR conditions under mostly sunny skies
expected by late morning. The next round of snow arrives
Wednesday afternoon through Thursday morning, which will have the
potential to produce plowable amounts of snow.

&&

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

The progressive pattern will continue over Southcentral Alaska
the next few days. The surface low which was over the Pribilof
Islands early Monday is now nearly to Yakutat and turning winds
to the north-to-northwest over most of Southcentral. Winds will
increase rapidly through Thompson Pass this morning and the one
to two feet of transportable snow on the ground will likely cause
significant visibility restrictions as soon as the winds increase.
Therefore a Winter Weather Advisory due to the expected blowing
snow has been issued through 6 AM Wednesday even though there will
be overall clearing conditions today.

An upper level short wave stretches from the Susitna Valley to the
Kenai Peninsula early this morning and is heading eastward. There
has been re-development of snow showers under and ahead of the
short wave. While intensities are generally light, it is delaying
the clearing until it passes. Behind the wave, winds are kicking
up and skies are clearing which will be the general story for
Southcentral today through tonight.

The next system will begin to move in from the southwest making it
to Kodiak Island late tonight, and then continue to move rapidly
northeastward into Southcentral Wednesday and Wednesday night.
This will be a snowy system for many areas, and precipitation
type will not be an issue as colder air in the region keep it all
snow. When the system initially moves into Southcentral Wednesday
afternoon and Wednesday night, it will develop snow in a very
similar pattern to the system from over the weekend. Snowfall will
be heaviest on the Western Kenai Peninsula near the Inlet, with
lighter amounts closer to the mountains. Anchorage will see a
similar situation with west Anchorage seeing more snow than the
eastern parts of the city. The Susitna Valley will begin to see
greater amounts of snow Wednesday evening as well.

The real interesting part occurs late Wednesday after midnight
through Thursday morning when a weak low moves into Prince William
Sound, and there is cold advection over Cook Inlet and the
Susitna Valley. This will reverse the snow pattern and produce
heavier snowfall in the more eastern areas with the potential for
periods of heavy snow throughout the Cook Inlet region,
especially in Anchorage through the Matanuska Valley. If the 500
mb low swings through rapidly Thursday morning as models currently
depict, the snow will shut off by noon Thursday, but not until
after what will probably be a significant snowfall.

&&

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

In the wake of the recent frontal passage, a surface ridge has
moved into Southwest Alaska. The latest nighttime microphysics
satellite image shows an area of fog around Bethel and extending
northward. But the fog hasn`t been able to make it south of the
Kuskokwim mountains. With the surface ridge in place, today will
be the respite between storms. Looking down range, there is a
mature low south of Adak. The latest AMSU (Advanced Microwave
Sounding Unit) has the richest portion of the moisture plume south
of Adak, Unalaska and Sand Point. Expect the edge of the
precipitation shield to continue to push north and east today,
but it won`t actually reach SW AK until tomorrow morning. The
thermal profiles of the atmosphere suggest that this will be a
mixed mode event meaning, expect some snow and potentially a
change in precipitation type at least for the Bristol Bay area and
the AKPEN. Portions of SW AK that are further inland look
substantially colder, and those communities (such as Bethel,
Aniak and Sleetmute) can expect a fresh blanket of snow.

&&

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

The latest ASCAT/Advanced Scatterometer pass has detected gale
force winds south of the Aleutians and in the Western Pacific.
There is a mature low south of Adak with a well-defined occlusion
and warm front. The latest GOES Airmass image already has the
associated cirrus shield over the Pribilof Islands and clouds
wrapped behind the low near Shemya with the cold front extending
into the North Pacific. The storm is expected to intensify as it
moves northeastward bring gale and storm force winds to the
Bering, as well as rough seas and another round of precipitation.
All of the major operational models suggest that there is a
thermal ridge that will develop bringing a wedge of warm air into
the Bering. There is no doubt that cold air will continue to
advect on the back side of this frontal boundary. However, this
storm absolutely has a warm sector. Rain and snow will accompany
this system.

&&

.MARINE (Days 3 through 5...Thu through Sat)...
A very active storm track continues across the Western Bering Thursday
through Saturday. Models have come into agreement that a mid-960
mb low will move up the Western Bering on Thursday, bringing
southerly gale force winds through most of the Bering Sea. Another
low pressure system is expected to develop on its tail in the
Northern Pacific, crossing the Aleutians and moving into the
Bering on Friday afternoon. Winds with this next system are
expected to remain near max gale force, but northwesterly winds on
the western side of the system may increase to storms by
Saturday.

Of note in the Gulf of Alaska, high pressure will build in behind
the low expected to move into Prince William Sound on Thursday.
This has the potential to cause strong northwesterly outflow
winds across the Western Gulf and through the Barren Islands
Thursday afternoon. Right now winds are expected to peak below
storm force but this bears watching as the two systems develop.

&&

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

The westerly storm track will continue across the mainland on
Wednesday and Thursday before a ridge builds in over the eastern
mainland on Friday. The ridge will shift the weekend`s storms to
a more south to north track through the Bering Sea. The storm that
moves through the mainland on Weds and into the Prince William
Sound on Thurs will bring snow across the region, with the
potential for 6 inches or more through the Mat-Su Valley. By
Friday, the ridge will be settling into place, which means that
any weekend potential for precipitation will be kept from the
Kuskokwim Mountains westward.

Although there is certainty in the ridge building over the
mainland through the weekend causing a change in the general storm
track, some uncertainty still exists in the strength of the ridge.
This could make the difference in whether some precipitation makes
it further inland Saturday and Sunday, or whether it is mainly
held to the coast. Regardless, it does look like an influx of warm
air is likely, so precip type for the western mainland will
likely be a forecast challenge as we move into early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 131.
MARINE...Gales: 120 130 131 160 165 170 172>176 178 351 413 414.
         Storm: 179 180 181 185 412.
         Heavy Freezing Spray: 181.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RJA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...LF



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