Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 141441
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
541 AM AKST Thu Feb 14 2019

.ANALYSIS AND UPPER LEVELS...
The large scale upper level flow is rather "uncomplicated" this
morning, at least in terms of how complex Alaska synoptic patterns
can be. There is a very large storm force low over the far western
Bering Sea with the center in between Attu Island and the
Kamchatka Peninsula. This massive and sprawling low, from west to
east, is approximately 1800 miles across...essentially covering
the entire Bering Sea. Compared to the lower 48, this low would
cover the expanse from Washington state to Lake Michigan. The
associated warm occlusion is now tracking through the Pribilof
Islands, and will reach the west coast by late this morning. Ahead
of this large low is a shortwave ridge axis which is moving over
Southwest Alaska. Over Southcentral, there is a strong upper level
trough extending into the Gulf of Alaska. This is promoting a
moderate strength offshore flow wind event along the coast. Some
highlights with this offshore wind event have been hurricane force
gusts at Kodiak and Thompson Pass, with gusts into the 50 MPH
range around Seward and Valdez. North winds are also blowing
through parts of Anchorage and the Mat Valley, and overall, this
pattern is finally clearing all remnants of fog and low stratus
out of the region.

Finally, rounding out the main weather features, is a developing
basal shortwave trough rounding the base of the larger low. This
feature, on satellite, is developing a well defined baroclinic
leaf, and is currently in the process of merging with a secondary
subtropical jet streak. This is expected to develop into another
secondary storm force low over the Bering Thu night and Friday.

&&

.MODEL DISCUSSION...
The numerical weather models are locked into the general pattern
through about Saturday morning before divergence in the track of a
low moving toward the Alaska Peninsula and into the mainland
diminishes confidence greatly. However, before that point, there
remains some uncertainty with a now powerful basal shortwave
trough which is expected to round the base of the larger trough
over the western Bering Sea and move north toward the Bering
Strait Thu night and Friday. This feature has been shown by the
numerical models for a few days, but there has been quite a bit of
variance over the last two days. With that said, satellite imagery
is now picking this feature up around 42N 169E, which is
currently a split northern stream shortwave merging with a
southern stream subtropical jet. There are effectively three
"moving parts" phasing together into one, with the southern stream
subtropical jet being the main instigator for potential rapid
development over the Bering Sea. The overall trend, doing a
dprog/dt in every model, has been for a much stronger and wrapped
up low as it tracks north through the Bering, instead of a frontal
zone, which the models had been simulating for multiple days. For
this forecast package, the first strong low will be captured by a
blend to the GFS (the strongest solution with a 954 hpa storm
force low) which will introduce stronger storm force winds along
the backside of the low over the western and central Bering Sea
Fri.

Thereafter, another NPAC low moving north toward the mainland Sat
into Sun will be the next significant weather feature. This will
bring snow threats to large parts of the southern Mainland
including Southcentral for the weekend, but high uncertainty
exists after Saturday evening as all the models depict a different
track with varying impacts. Consistency wise, all the model
solutions are changing with each run, so there is currently no
"favored" solution. The signal in the noise? Increasing snow
threats for the weekend.

&&

.AVIATION...
PANC...VFR conditions prevail through Friday afternoon. Gusty
northerly winds this morning will diminish this afternoon.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Saturday night)...
Strong gap winds will continue along the north Gulf coast and
Prince William Sound today. These winds are supported by an upper
level jet streak on the front side of a high amplitude ridge,
tight surface pressure gradients, and colder air sitting inland.
As we head through the day all of these forcing mechanisms will
either exit the region or weaken. Thus, expect a downtrend in
winds region-wide. With such a cold air mass in place, temperatures
will generally hold steady or slowly fall.

A strong trough and jet streak associated with the deep Aleutian
low will temporarily deform the upper ridge, but pass over
Southcentral tonight producing no more than a few mid to high
level clouds. Increased northerly flow and subsidence as the
short-wave drops down the east side of a re-amplifying ridge will
lead to re-strengthening of coastal gap winds, though they won`t
be quite as strong as they are this morning. Winds will then die
down for good on Friday as the upper ridge builds in from the
west.

Progressive upper level flow will then drive a weakening front
to Southcentral Friday night through Saturday. While most of the
upper level forcing will exit to northern Alaska, isentropic lift
will likely produce widespread light snow from the Kenai Peninsula
to Anchorage and the Mat-Su. As the upper ridge then exits
eastward Saturday through Saturday night a storm will move up
from the Pacific, tracking somewhere into the eastern Bering/SW
Alaska. While there is uncertainty in the track of the low, the
amplified pattern and deep southerly flow are favorable for
widespread precipitation, with snow transitioning to rain along
and near the coast. We`ll have to wait and see how much of a role
downsloping will play in the precipitation distribution and
amounts.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Today
through Saturday night)...
The very active weather pattern continues with the next frontal
system approaching the Kuskokwim Delta this morning and expected
to bring areas of snow and blowing snow to coastal locations. The
front will swing inland across the Kuskokwim Delta this afternoon
but then weaken rapidly this evening as it is sheared away from
its parent low far to the west. The next front will be stronger,
reaching the Southwest Alaska coast Friday morning and then
holding together better as it swings inland Friday afternoon and
evening. Along the coast strong winds and blowing snow will be a
concern with several inches of snow possible further inland as
well. The next low will cross the Alaska Peninsula Saturday and
then track inland across Southwest Alaska Saturday night through
Sunday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Today
through Saturday night)...
A strong low in the far western Bering will track north today
while its associated frontal system continues to push north and
east across the northern and eastern Bering. The next rapidly
deepening North Pacific low will track north across the west
central Aleutians this evening and then continue north across the
Bering along the 180 degree longitude line tonight through Friday.
This very strong low looks like it will bottom out in the mid
950s with an area of storm force winds accompanying it. Its
associated frontal system will swing through the central Bering
tonight and into the eastern Bering Friday morning. A couple of
troughs to the south of the low will swing through the western and
central Bering Friday night through Saturday night. A surface low
will track north to the Alaska Peninsula Saturday and into
Bristol Bay Saturday evening. Another rather strong low and
associated frontal system will cross the western Aleutian and push
into the Bering Saturday night through Sunday.

&&

.LONG TERM FORECAST (Days 4 through 7)...
Beginning the extended period forecast on Sunday morning, there
will be a complex upper trough centered over Bristol Bay with a
Pacific ridge to the east. In between will be a warm front and a
surface low which will be tracking into Southern Alaska. The low
will likely take a track up Bristol Bay with a secondary low
eventually forming along the N Gulf Coast Sunday afternoon and
evening as the trough shifts east. Threats for snow are high with
this system, but the exact track of the low will strongly
modulate the degree of inland downslope flow as well as potential cold
air advection enhanced snow on the back-side of the trough.

Attention quickly turns to the Bering Sea as a strong low tracks
from the Western Bering Sea Sunday morning and moves eastward to
the Southwest coast by Sunday night/Monday morning. This storm
will likely be at least storm force across the Bering Sea (winds
at least 50 knots) and a potential surge of winds/high surf to the
coast. Stay tuned for this storm...

The strong Bering Sea storm moves inland through Tuesday, bringing
the threat for inland snow as the upper level flow flattens and
becomes more westerly. It does seem that the upper level flow will
become more progressive with a series of fast moving waves riding
along the baroclinic zone, but each of these successive waves by
midweek are not much more than noise in this pattern, that far
out. The takeaway? Expect threats for inland snow, near climo
temperatures, and general unsettled weather to last through late
week.


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Gale Warning 121 127 131 132 155 165 170 172 173 174 175
                      176 177 179 352 414.
         Storm Warning 178 180 181 185 411 412 413.
         Heavy Freezing Spray 121 127 180 181 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
MARINE/LONG TERM...JA


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