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000
FXAK69 PAFG 162252
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
152 PM AKST Sat Dec 16 2017

.DISCUSSION...

Aloft...At 500 hpa...Ridge over the Eastern Arctic Coast will move
back into Canada by this evening. 502 dam low over Nunivak Island
this afternoon will weaken to 512 dam as it moves across the
Interior on Sunday. The trough will continue to slowly move east
to be over the Alcan border by Monday morning. Ridging will build
over the west coast ahead of a strong front which will develop
over the Bering sea by Monday morning. A 485 dam low will move
into the Western Bering Sea by Sunday morning and to the western
Gulf of Anadyr by Tuesday morning at 493 dam. A strong shortwave
associated with the low will move to the Eastern Bering Sea by
Monday afternoon, and over the West Coast by Tuesday morning.
Moist southwest flow aloft will remain over much of the state
through Wednesday.

Surface...A 973 mb low over Nunivak Island will move to
Kodiak by Sun morning, while a 997 mb low develops over Kotzebue
Sound. A 945 mb low will move to the Western Bering Sea Sun
morning with a weather front moving to the Central Bering Sea and
Central Aleutians by Sun night. The low will move E in the
Northern Bering Sea and the front will move to the West Coast by
Mon afternoon.

Arctic Coast and Brooks Range...Cloudy, light snow or flurries,
fog, Blowing snow, and persistent winds of 20 to 30 mph will
continue through Sunday. Winds will diminish a bit Sunday to 10
to 20 mph, and the blowing snow will diminish as the winds come
down and the falling snow tapers off. Temperatures will continue
to cool.

West Coast and Western Interior...Snow showers tapering off around
the Yukon Delta as the 973 mb low near Nunivak Island moves to
the south tonight. Temperatures cooling through Monday morning.
The next weather system will be moving to the West Coast and St
Lawrence Island Monday and it will be quite messy with models
currently indicating snowfall of 6 to 12 inches, freezing rain in
the lower Yukon Delta and on St Lawrence Island, and winds gusting
to 60 mph on St Lawrence Island and the Bering Strait Coast. Local
gusts to 70 mph along the Bering strait on Monday. Winter storm
watches have been issued for zones 211 213 and for the Yukon
Delta. Watches or warnings will likely be issued for northern
and inland zones tomorrow. Strong southerly winds and precip will
ramp up Monday morning and continue into Tuesday morning. Snow
will then spread inland on Tuesday. Southerly winds will continue
for the west coast through the week.

Central and Eastern Interior...Light snow around the interior
will taper off as a decaying front moves to the south slopes of
the Brooks Range by this evening. Another chance of snow Sunday
for the Middle Tanana Valley as a weak low moves east across the
area. Expecting light amounts of snow. Temperatures will continue
to cool. A strong weather front will move into the interior
Tuesday and Wednesday that could produce 3 to 6 inches of snow.

Coastal Hazard Potential Days 3 and 4...Strong south winds moving
to the West Coast Monday may impact the area. There is some
potential for a minor ice push on the south facing coastal areas.


&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch for AKZ210-AKZ211-AKZ212-AKZ213-AKZ214.

Heavy Freezing Spray Warning for PKZ210-PKZ220.

Small Craft Advisory for PKZ210-PKZ220.

Brisk Wind Advisory for PKZ225-PKZ230-PKZ235-PKZ240-PKZ245.
&&

$$

JK DEC 17


000
FXAK68 PAFC 170110
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
410 PM AKST Sat Dec 16 2017

.ANALYSIS AND UPPER LEVELS...

A broad upper low stretching from the Bering Sea to the eastern
border of Alaska continues to dominate the regional upper air
pattern. The main upper low center remains over the eastern Bering
Sea with a series of weak embedded shortwave troughs in southwest
flow tracking toward the North Gulf Coast. One of these is clearly
visible on satellite imagery with an associated deformation band
moving into Prince William Sound. Behind this shortwave, there is
an extensive region of post-frontal unstable air, clearly evident
in satellite imagery with a large region of convective cumulus.
Some lightning has been observed on the GOES lightning detector in
this unstable air mass, including over Kodiak Island. Finally, a
powerful low is beginning to develop just south of the Kamchatka
Peninsula. This low will become a major weather "player" over the
next few days as the storm force front moves across the Bering
Sea.

&&

.MODEL DISCUSSION...

The numerical weather models are in excellent agreement with the
general synoptic pattern for the next 4 days with little to no
discrepancy. All the models are progging a powerful storm force
front expected to cross the Bering Sea then move inland across
Southwest Alaska Monday and into Southcentral Tuesday. Agreement
is very good and all the models are depicting a similar scenario
with the exception of the ECMWF which shows more overrunning snow
across parts of Southcentral Tuesday. For now, a more "moderate"
approach will be favored since the GFS/NAM are depicting a
stronger cross-barrier gradient which would act to limit some
inland snow amounts for select.

&&

.AVIATION...
PANC...VFR conditions are expected to prevail through most of the
TAF period in a showery southwest flow regime, however occasional
MVFR conditions will remain possible, although unlikely, through
early Sunday morning before shower threats end.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Snow and rain showers will persist over the coastal areas tonight
as a shortwave embedded within the base of an upper level low to
the west sweeps through the Northern Gulf/Prince William Sound
area. Precipitation should stay mostly to the east of the Chugach
Front this evening, though a few stray snow showers may make it
into Anchorage and the Mat-Valley. Showers will taper off some
overnight before a surface low moves into the gulf tomorrow
bringing another round of precipitation to the coastal areas
during the day. Cold air will wrap around the backside of the low
which should help keep temperatures cool enough for the
precipitation to stay as mostly snow. The surface low will weaken
and move off to the east allowing a ridge of high pressure to
build in over the area by Monday. Fog could develop over the Cook
Inlet area, including Anchorage and the Mat Valley, on Sunday
night/Monday morning as the ridge axis moves overhead, allowing
strong subsidence to combine with clearing skies and cold surface
temperatures. Areas without fog on Monday will see mostly clear
skies though temperatures will be noticeably cooler than the
previous weeks, though around average for this time of year.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The eastern Bering low will track south along the southwest Alaska
coastline tonight before being absorbed into the low forming in
the western Gulf Sunday morning. As it passes, it will bring
light snow showers to southwest Alaska. Ridging will then build in
over the area on Sunday clearing out the showers and also helping
with fog formation over the area Sunday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Ridging is building in over the western Bering and tracking east
which will help to diminish the remaining showers over the
eastern Aleutians and Alaska Peninsula. Behind the ridging a new
system will deepen in the far western Bering to sub 960 mb.

The front with this system will move over the central Aleutians
Sunday afternoon bringing storm force winds as it passes. It will
then make it to the eastern Bering by Monday morning as the winds
begin to slowly diminish. This system will pull up enough warm air
from the south for the majority of the precip with the front to
fall as rain. Behind the front, cold air will move back in
bringing snow showers in its wake.

&&

.LONG TERM FORECAST (Days Tuesday through Saturday)...

On Tuesday, all of southern Alaska will be under the influence of a
weakening weather front moving from west to east. In general, a
front oriented southwest to northeast should not cause too much
downsloping, so the usually drier areas have a pretty good chance
of seeing some precipitation from this storm. Most coastal areas
will probably warm up enough to see rainfall, but from Anchorage
northward the airmass might just be cold enough for a few inches
of snow at lower elevations. Details are uncertain, so stay tuned.
We are more confident that the northern Susitna Valley and
Hatcher Pass areas should see some significant accumulations of
snow from this storm.

For the 2nd half of next week, models continue to indicate the
resurgence of a strong ridge over the eastern Gulf, that produced
deep-layer (and warm) southerly flow across most of southern
Alaska. This is the same type of pattern that we`ve been stuck
under for much of December so far. The orientation and strength of
the ridge will dictate what coastal locations are most likely to
get slammed by heavy precipitation (somewhere from Kodiak to
Cordova), as well as how warm and windy interior locations like
Anchorage will get. Stay tuned.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 173-178 411-414 119 120 130-132 138 150 155 165
170-172 179-181 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...AD


148
FXAK67 PAJK 162341
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
241 PM AKST Sat Dec 16 2017

.SHORT TERM.../Saturday through Monday as of 1 PM Saturday/

The weather front affecting the Panhandle has pushed through the
northern Panhandle and has almost made its way through the
southern Panhandle. Behind the front widespread shower activity
is expected through the evening. An area of interest is the far
northern gulf and the eastern gulf over the next 24 hours where
steeper lapse rates (faster cooling with height) will create a
more unstable atmosphere and allow for a chance of thunderstorms
in those areas. Sunday night through Monday a surface low will
track southeast through the gulf acting to keep the Panhandle wet.
Shower activity will diminish Monday from north to south as the
low tracks southeast. Surface high pressure builds over the gulf
and Panhandle behind this system which will act to dry southeast
Alaska out as Monday progresses.

Winds are primarily a concern this evening as mainly over the
northern Panhandle and with tip jets near Cross sound and Cape
Decision. Strongest winds will be near Skagway and gales will be
present in northern Lynn canal and near Cross Sound.

The seasonably warm above freezing temperatures decrease through
the short term. However, with the coolest temperatures being
Monday not much snow is expected across Southeast Alaska since
this will be the drying day for the Panhandle.

Not many significant changes were made in the short term. Primary
guidance was a blend of the GFS and ECMWF with some input from the
NAMNest and ARW to handle the shower activity behind the front.
POP was increased to allow for more widespread shower activity
this evening. Confidence is above average in the short term.

.LONG TERM.../Monday through Friday/ As of 10pm Friday

Yet another anomalously strong upper ridge continues to be
advertised by both operational models and their ensembles to
rapidly build over the Gulf of Alaska next week. Cool upper trough
over the northern Gulf early on Monday will quickly be shunted off
to the southeast ahead of this building ridge. Lingering shower
activity on Monday will likely be in the form of snow even to sea
level for the northern and interior inner channels from Haines
and Juneau as far south as Petersburg. Only light precip amounts
are expected and this will primarily be a case of the cold air
chasing the moisture so any snow accumulations would be minor.

Still looks like a very brief offshore flow even late monday into
Tuesday. Northerly winds will develop over the northern inner
channels on monday as the weak low offshore slides to the south.
Light northerly flow through the low levels will promote at least
some clearing by Monday night and Tuesday although will need to
watch for fog in places that are sheltered from the wind as the
ground will be saturated.

Low level flow becomes more NW`ly on Tuesday night into Wednesday
which is not necessarily a clear sky direction at times. Could see
some low level moisture and stratocumulus try to make a return
along the outer coast, however Wednesday should be another dry
day.

Upper ridge begins to shift east by late week with a series of
storm systems and their associated deep plumes of moisture riding
up the west side. At this time it appears the ridge will hold
strong enough to keep the deep fetch of moisture mainly to our
west, with only weakening fronts approaching from the west by
Thursday/Friday. Unfortunately for snow lovers, we are hard
pressed to find any significant hope for snow in the future as
freezing levels rise to around 5 kft by late week under the strong
ridging aloft.

&&

.AVIATION.../As of 130 pm Saturday/ Most of the Panhandle is
behind the front besides the far southeast Panhandle. Shower
activity will be widespread behind this system through tonight.
This will keep conditions primarily MVFR; however, as stronger
showers pass through expect short periods of IFR conditions. In
addition to the lower ceilings and visibilities associated with
showers, westerly low level windshear will exists over much of the
panhandle through tomorrow morning. Shower activity will begin to
diminish late tomorrow.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Warning until 3 PM AKST this afternoon for AKZ018.
Strong Wind from 6 PM AKST this evening through late tonight for
AKZ018.
MARINE...Gale Warning for PKZ012-022.
Small Craft Advisory for PKZ013-021-031-032-034-036-041>043-051-
052.
Small Craft Advisory due to Seas for PKZ053.

&&

$$

JB/DEL

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