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FXAK68 PAFC 191303

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 AM AKST Fri Jan 19 2018

There are 2 main features across Alaska today. The first is a
decaying low over the Gulf of Alaska which continues to produce
showers that are detectable on the Middleton Island radar. A
another low south of the aforementioned low continues to push
eastward along 50N which will not directly impact South Central
but it will interact with the remnants of the low just outside
Prince William Sound. The other primary feature is a
strengthening surface high near the Gulf of Anadyr. This surface
high has been over Siberia will continue to migrate towards the
Central Bering. Anomalously cold air is trapped in this feature
and this will have a major impact on the region as the forecast
period unfolds.


There is good model consensus with the synoptic features and
good run to run continuity. That said, there are a few areas of
uncertainty regarding timing of fog dissipation and pinpointing
the temperatures as we head into a much cooler pattern.


PANC...Fog has moved down Knik Arm and has reduced visibility and
cigs. The latest VIIRS image has a large area of stratus spanning
from Knik Arm and down Cook Inlet which is camouflaging the fog.
Looking at the models, the MOS guidance is anticipating overcast
conditions at PANC for the next 24 hrs with the fog ebbing and
flowing around the airport.



Though there is an upper low currently parked over Southcentral
Alaska, shallow inversions and low level moisture have given rise
to areas of fog from Cook Inlet/Western Kenai, Anchorage Bowl, to
over the Matanuska Valley (especially Knik Arm). Stratus and fog
are more widespread across the Copper River Basin. Except for the
Copper River Basin, where the low conditions are expected to
linger into Saturday, the other aforementioned areas should see a
lessening of these conditions tonight with a little more mixing.
That said, there may be some patchy fog which redevelops/holds
overnight through the Knik Valley.

Pressure and temperature gradients will increase across
Southcentral tonight as cold air butts up against the interior
side of Alaska Range and Copper River Basin and flows through
gaps. Other than along the range, a noticeable increase in
outflow winds will be evident across the northern and Western
Gulf, including the eastern Kenai peninsula overnight which should
continue through Saturday night.


With high pressure settling in over the Bering, dry, northerly
winds will dominate southwest Alaska through the weekend. This
setup will pull down Arctic air over the area dropping
temperatures to around -30 F for interior locations Saturday and
Sunday night. Winds look to be relatively light over the weekend
for low lying areas, but if they do pickup wind chills could
become a concern. However, there remains uncertainty with how
strong the winds will get.


High pressure will dominate the Bering through the weekend with
the eastern side pulling down cold air over the eastern Bering and
Aleutians. These winds look to be around small craft strength
which will be enough combined with the cold air to bring freezing
spray to much of the eastern Bering. Out west, a North Pacific
system will approach the western Aleutians tonight but will be
stalled by the Bering high. This will keep its front and any
precip associated with it over the central and western Aleutians.


.LONG TERM FORECAST (Days 3 through 7)...
The long term begins Sunday night and Monday with a high
amplitude upper high over the Bering Sea and a trough to the east,
which includes a closed low in the Gulf of Alaska and another over
northern Alaska. There is weak ridging in between the two lows
over Southcentral. Strong northerly flow will be over the Alaska
west coast and Alaska Peninsula. During the first half of next
week the Gulf low will slide east, and the Bering high will
weaken. This will allow the north Alaska low to dive southward,
reaching southern Alaska around the middle of next week. The exact
path and timing of this southward traversing low is not agreed
upon exactly by the models, but agreement is good enough for a
fairly high confidence forecast for this time range. The end
result of all of this is a fairly cold regime with temperatures
below normal over mainland southern Alaska. Some precipitation is
possible through the period for the Gulf coastal areas. There is a
chance some precipitation could get further inland, depending on
how the Gulf low behaves, but confidence in this is low.


MARINE...Gale 411 150 177 178 180 130.
Heavy Freezing Spray 160 180 181 185.




FXAK69 PAFG 191354

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
454 AM AKST Fri Jan 19 2018

Snow, blowing snow, low wind chill values and deep cold on tap as
winter returns to northern Alaska.

The 00z model package initialized well against the 00z surface
analysis and verified well against the 06z surface analysis. Model
spread remains minimal in the the short range and is reasonable
in the mid range. Model spread remains large in the extended range
with over 400 mile differences on the position of large and cold
upper level low and attending trough axis by wednesday of next
week. The large spread in the position of these features will have
a significant impact on sensible weather especially over the
western and central interior where cloud cover or lack there of
will moderate temperatures keeping minimum temperatures in the 20s
and 30s below or the lack of clouds will allow temperatures to
fall into the 30s and 40s below. Opted to take middle of the road
solution at this time with cloud cover and temperatures in the
long range.

Ongoing blizzard on the eastern North Slope expected to continue
into Saturday as cold arctic high pressure builds over the
western North Slope and a low moving north over the McKenzie
Delta increases the gradient to produce 60 mph wind gusts over the
eastern North Slope. Winter Weather Advisories for snow and
blowing snow on the north slope have been updated and extended to
include low wind chill values to 55 below zero as a much colder
airmass from Siberia moves south and east over the western north
slope and western interior today and over the rest of the north
slope tonight and Saturday as well as the central interior
Saturday night and Sunday. Periods of light snow are expected to
continue along a stalled front over the central interior with
generally 1 to 2 inches of snow expected through tonight and again
on Saturday as various weak upper level waves interact with the
stalled frontal system. Additional light snow is expected over the
interior as the cold airmass moves south and east and interacts
with the stalled front and eventually pushes the front into the
eastern interior.

Much colder temperatures are expected to move south and east out
of Siberia and over the western North Slope, the west coast and
western interior today, and over the rest of the North Slope and
central interior Saturday and Saturday night. The 850 mb
temperature was around 8 below on the 12z sounding over Fairbanks
this morning. 850 mb temperatures will continue to fall over the
next several days with 17 below expected Saturday morning and 23
below at 850 mb expected Sunday morning and around 20 below
expected monday morning. Model differences on the position of the
core of the cold airmass results in 850 mb temperatures between
16 below and 26 below by Wednesday of next week over Fairbanks.
850 mb temperatures in this range and combined with 1000 mb to 500
mb thickness values in the 495 dm to 492 dm range would support
30s and 40s below with a lack of clouds or in the 20s to 30s below
with clouds. The models differ on how long the cold airmass
remains over the interior with the Canadian and ECMWF having the
cold air retreat to the west as warmer air moves north into the
central and eastern interior while the GFS keep the cold airmass
entrenched over the interior through next weekend. Continue to

Coastal Hazard Potential Days 3 and 4...None.


Blizzard Warning for AKZ204.

Winter Weather Advisory for AKZ201-AKZ202-AKZ203-AKZ206-AKZ218.

Heavy Freezing Spray Warning for PKZ210.

Small Craft Advisory for PKZ210.

Gale Warning for PKZ245.

Brisk Wind Advisory for PKZ215-PKZ230-PKZ235-PKZ240.



FXAK67 PAJK 191509

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
609 AM AKST Fri Jan 19 2018

.SHORT TERM... A low to the south of Haida Gwaii will move
northwest through late tonight into the Southeast gulf. This
should bring some rain and snow showers further northward into the
panhandle. Another low is tracking to the west along 50 N which
will pull the first low to the southeast Gulf.

New model runs have not changed the trajectory much for the
dissipating low over the southern panhandle, but seem to be more
consistent in defining the low. Some minor changes to pressure
were made using a NAM/EC blend and winds increased slightly using
the NAM with the passage of the low, with small craft advisories
remaining in the outer coast and Northern Lynn increasing to gale
force by tomorrow night.

A NAM/SREF blend was used for POP, backing off some of the
precipitation over the panhandle for tonight and brining it back
in tomorrow. There will be quick changeovers from rain to snow
tonight, as a band of WAA squeezes in ahead the low. Again, there
is the potential for some patchy fog to develop tonight in
Petersburg, Ketchikan, and Metlakatla.

While there is a decreasing trend in temperatures for the short-
term period, both maximum and minimum temps had to be increased
slightly to be within the model spectrum spreads. The GFS wanted
to remain slightly warmer, as this could affect the rain/snow
transitions into tonight. The CPC outlook for the next 10 days
shows precipitation and temperatures to remain slightly below
normal. Forecast confidence remains average to above average with
good continuity in the pattern over the next several days.

.LONG TERM.../Sunday through Friday as of 10 pm Thursday/
Models in a little better agreement on the larger scale features
through early next week. Upper trof will dig S into the W gulf
over the weekend then base of the upper trof will shift E into the
central gulf early next week as another shortwave digs S into the
W side of the upper trof. After Tue however, the models differ on
where the various shortwaves within the trof go. Does appear that
the mean trof axis will reform further W late in the week however.
Blended in latest WPC for most parameters starting 12z Mon. Left
most parameters alone for Sun.

Did warm temps some especially Sun-Tue. Initial colder airmass
coming in from the N will stay over the far N area. Colder air
will slowly wrap around sfc low over the central gulf into the
area early next week, but this cool down will be slower. Later in
the week, as upper trof reforms to the W, the cooling trend will
likely end and some moderation in temps is likely by late week.

Did keep in likely POPs over much of the central and S area early
next week as a couple of systems move in from the SW. Some
decrease in POP is possible for latter part of the week as main
low level flow may become more offshore, but still a lot of model
differences to resolve that far out.

Looking like the most wind will be with any lows over the gulf.
There may be some outflow as well over the N third of the area,
but it does not look particularly strong due to lack of a
strong sfc high over the Yukon. However, any stronger lows that
would move into the E gulf could increase outflow more than
current forecast has it.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012.
Small Craft Advisory due to Seas for PKZ041>043-051-052.




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