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fxak68 pafc 220100 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
500 PM akdt Wed Mar 21 2018

Analysis and upper levels...
there is a full latitude upper level ridge along about 165 degrees
west longitude. This ridge is becoming "sharper" and nudging
eastward. There is an upper level trough over the eastern Gulf of
Alaska. Strong northerly flow through the atmospheric column is
situated in between these two features over much of southern
Alaska. Conditions over southern Alaska are rather dry, especially
over southcentral. There is a large and deep upper low centered
near the Kamchatka peninsula. An associated north to south
oriented front is over the central to western Bering and central
to western Aleutians. A surface low is rapidly forming along this
front over the western Bering.


Model discussion...
the numerical models are in good agreement, and show good
continuity, through the short term portion of the forecast (friday
afternoon). Therefore little change has been made to the forecast
and confidence is higher than normal.


panc...VFR conditions will persist. Breezy north winds will
diminish around midnight tonight as the surface gradient relaxes
and the atmosphere decouples.


Short term forecast southcentral Alaska (days 1 and 2...Thu and
the pattern overall will be relatively slow to change. However,
small differences in how the upper-levels reorient this evening
into Thu will result in noticeable changes at the surface. Gusty
winds will continue to be the main issue. Winds have already
started to blow through many of the usual spots (thompson pass,
Seward, the mat valley, Valdez, etc.). in fact, Thompson Pass
already hit a gust to 90 mph. The ridge and upper-level energy
that is helping to support these winds will shift ever so slightly
tonight. This will produce a bit of a more northeast (as opposed
to north) offshore gradient. This should allow the winds to
increase through the favored terrain of the mat valley and the
town of Valdez. With this in mind, we have extended the High Wind
Warning until noon Thu. Even by then, it looks like winds will be
slow to subside through the afternoon (instead of swift drop-off)
as the thermal gradient slowly weakens and energy shifts east.

Blowing snow will continue to limit visibilities through Thompson
Pass. Conditions should slowly improve as most of the loose snow
gets scoured away. For the Matanuska Valley, it looks like there
is too much "hardened" snow for much blowing snow. In general,
this snow pack should limit any blowing dust. However, we can't
entirely rule out some localized dust in places where the snow is
now gone.

The ridge and offshore flow should continue to make for clear
skies across just about all of southcentral. With abundant March
sunshine, even with cold overnight temperatures, things should
warm up quite dramatically during the afternoons. The next chance
for precipitation is a decaying front approaching the Alaska Range
from the west Fri. However, this feature effectively falls apart
before it even makes it over the mountains. Thus, we have opted to
back off/slow down the already meager pops in the forecast.


Short term forecast southwest Alaska (days 1 and 2)...

High pressure aloft and at the surface is keeping the pattern dry,
but the flow is weak enough along the Kuskokwim Delta and parts of
the Alaska Peninsula to support low stratus and fog. Offshore flow
ahead of an approaching front will help some of that break up

The next front rolls into southwest Thursday evening then moves
east toward the Alaska Range before stalling and falling apart on
Friday. This front will bring light snow (perhaps 1-2" along the
coast with amounts dwindling as it moves inland) but the fast
movement will limit accumulations.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...

The main story for the Bering remains to be a powerful developing
bent-back occlusion which will rapidly deepen over the western
Bering Sea tonight and Thursday. Storm force winds around the
backside of the low will approach 60 knots with gusts as high as
75 miles per hour over the northern Bering Sea offshore marine zones with the
associated gale force front moving into the eastern Bering by
Thursday afternoon.

Strong southwesterly cold air advection will wrap around the
Bering Sea behind the low.


Long term forecast (days 3 through 7)...
the extended portion of the forecast beginning Friday night will
see outflow winds diminishing across southcentral as the Bering
ridge shifts eastward to the Mainland and breaks down by Saturday.
This will bring about a return to a more progressive pattern
going into next week as several systems track across the region
from west to east. The next good chance for precipitation across
southcentral looks to occur on Sunday as a low moves into the
eastern Bering and a front extends across the Alaska Peninsula
into to the Gulf. Forecast confidence decreases after Monday as
guidance begins to diverge more substantially. There is a general
consensus on a more active and somewhat wetter pattern for the
southern Mainland going through next week, however the individual
details remain unresolved at this time as models continue to
struggle after day 5.


Afc watches/warnings/advisories...
public...High Wind Warning 131.
Marine...storm 185 412. Gales 126-128 170 172-181 411 412 414.



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