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671
FXAK69 PAFG 182316
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
316 PM AKDT Wed Oct 18 2017

.DISCUSSION...
An active weather pattern continues along the Arctic Coast today.
The main forecast challenge for this shift is movement of two
lows west along the Arctic Coast and eventually southward along
the west coast of Alaska. There is some disagreement between the
models on exactly how this will transpire. Previous shifts have
favored the GFS, a trend that we will continue with our forecast
package this afternoon, particularly after the first 12 hours.

Aloft, at 500 mb, a 493 dam low 350 NM NE of Deadhorse will move
to 100 NM N of Wainwright by 4 AM Thursday and to near Point Lay
by 4 PM on Thursday. By 4 AM Friday the low will be situated over
Kotzebue Sound and by Friday evening it will sit over Nome. By
early Saturday the low will become an elongated double barrel low
with one center over Nome and the other near Scammon Bay. This
double barrel low will rotate around over the Yukon Delta into
early next week. As the low moves southward the flow over the
eastern Interior will become more southerly allowing warmer air to
move in aloft.

At the surface, a 991 mb low near Deadhorse will move to 100 NM
NE of Nuiqsut by 4 am Thursday and to 50 NM NE of Barrow by 4 PM
Thursday. By 4 AM on Friday this low will be 75 NM NW of Point
Lay and By Friday afternoon it will be located over Wales. An 998
mb low 140 NM N of Wainwright will move to 100 NM N of Wainwright
by 4 AM Thursday at 992 MB and to 40 NM E of Point Lay by 4PM
Thursday. By 4 AM on Friday this low will be located near
Shishmaref at 989 mb and by PM on Friday it will be located near
Emmonak at 987 mb.

Arctic Coast and Brooks Range: The low that is currently near
Deadhorse will bring snow and blowing snow to zones 203 and 206
tonight before it moves off to the west. I opted to issue a winter
weather advisory for both zones 203 and 206 for tonight. Winds
will die down by morning as the low moves north and eventually
west. I opted to cancel the ongoing winter weather advisories in
zoned 201, 202 and 205. Winds in these areas have died down and
the reduction in visibility needed for a winter weather advisory
is not expected to continue tonight.

Central and Eastern Interior: An occluded front associated with a
low in the Gulf of Alaska will ride up over the cold air of the
Southeast Interior tonight and Thursday. This will cause 1-2
inches of snow over zones 223, 224 and 226 through Thursday with
NW winds gusting to 20 mph. As we go out into Friday and Saturday
the flow aloft becomes more southerly, this will help to keep
precipitation across the Interior to a minimum.

West Coast and Western Interior: With the low diving
southward Thursday expect increasing showers, particularly along
the coast. The upper low will move to the Yukon Delta by Friday
night where it will remain in place through the weekend which will
help to keep the showery conditions in place. As the low moves
south, the winds will increase through the Bering Strait Thursday
night and and along the coast south of the Bering Strait on
Friday. The exact details of this will depend on the track of the
low, which the models have not been handling that well.

Coastal Hazard Potential Days 3 and 4...Winds could turn NW
tonight,Thu and again Fri over Kotzebue Sound and the Chukchi Sea
Coast, but at this time it looks like strong winds and elevated
surf will remain west of Shishmaref.
&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory for AKZ203-AKZ206.

Heavy Freezing Spray Warning for PKZ240-PKZ245.

Small Craft Advisory for PKZ210-PKZ220-PKZ225-PKZ230-PKZ240-
PKZ245.
&&

$$

OCT 17



000
FXAK68 PAFC 190020
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKDT Wed Oct 18 2017

.ANALYSIS AND UPPER LEVELS...
The main feature on water vapor satellite is a complex upper low
spinning in the eastern Gulf of Alaska. A very zonal jet stream
lies south of the Aleutian Chain, driving the predominant storm
track from the northwest Pacific straight eastward into the Gulf.
While most of the active weather associated with the Gulf system
is out of our area, the main impact on the mainland has been to
draw a colder air mass in from the interior of the state, while
increasing winds along the Gulf coast through favored gaps in the
mountains. Much of Southcentral Alaska is clear as cold advection
dries the air mass out, but the upper trough is currently keeping
a mid-level deck on the west side of the Alaska/Aleutian Range.
Much of the Bering Sea is engulfed in instability showers as
arctic air streams over the warmer water. The next system to
progress across the Chain is beginning to affect the Western
Aleutians.

&&

.MODEL DISCUSSION...
Guidance in the short term is in great agreement with the
evolution of the synoptic flow. There are differences with low
positions within the complex Gulf circulation but effects on our
forecast are the same in any solution. The biggest differences in
the models come late Thursday into Friday as the upper level
trough captures some energy from the Gulf over the eastern portion
of the Copper River Basin. Some guidance shows a prolonged light
snow for the McCarthy area while other are much drier. High
resolution guidance will be utilized for this forecast package to
capture outflow winds along the north Gulf coast a little better.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Much of Southcentral AK will settle into a dry northwest flow
pattern through Thursday evening as a large low over the
southeastern Gulf departs towards the Panhandle and a weak upper
trough pushes across the region from the west. This will leave
gusty outflow winds as the primary short term forecast challenge
as most of the area remains precipitation free. Upper level
support for these winds looks to be somewhat lacking, leaving the
surface pressure gradient and cold air advection from the
interior as the primary drivers of this event. This should keep
outflow winds more on the modest side, although strong gusts can
still be expected through the typical coastal gap flow areas
including Valdez, Whittier, and Seward. Winds should begin to
diminish overnight Thursday as an upper low over the Beaufort Sea
pivots to the western mainland and upper level flow shifts around
to the southwest. Meanwhile, the exception to the dry conditions
expected across most of the area will be the eastern portion of
the Copper River Basin, where the combination of the upper trough
approaching from the west and a weak surface low over the
northeastern Gulf should be sufficient to produce a period of
light snow Thursday into Thursday night. Minor accumulations will
be possible within Wrangell-St. Elias National Park including the
McCarthy area through Friday morning.

&&

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

Northwesterly flow will continue to filter cold air into the
southwest mainland through the next couple of days. Gusty
northwesterly winds over the Alaska Peninsula will taper off
through tomorrow. The southwest mainland area will remain fairly
dry through Friday, with only a couple of exceptions. Unstable
northwesterly flow banking up against the Kuskokwim Mountains and
Alaska Range will squeeze out a few showers in the vicinity of the
mountains as the air is orographically forced upward. In
addition, cold air flowing over the relatively warm ocean waters
will continue to steepen lapse rates and support some isolated
showers along the Kuskokwim Coast and Nunivak Island. Otherwise,
expect mainly dry weather with a continual cooling trend to
slightly below normal temperatures by Friday. Slightly more active
weather will return Friday evening as an arctic low will push
southward into the Kuskokwim Delta, bringing a better focus for
some more organized precipitation.

&&

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

The main feature driving the weather over the Bering Sea and
Aleutians will be a northwest Pacific low tracking east towards
the western Aleutians tonight. As the low moves near Shemya
tomorrow morning, it will be caught up in the base of the
downstream longwave trough over mainland Alaska, preventing any
northward movement into the Bering Sea. As has been the pattern
for the last week or so with a zonal storm track over/south of the
Aleutians, the majority of the impacts of this low and it`s
associated front will remain confined to the Aleutians and
southern Bering Sea. Expect a swath of small craft to gale force
winds and rain over the western/central Aleutians and southern
Bering Sea tonight through Friday. As the low departs to the
Northeast Pacific Friday, widespread northerly flow will resume
over the Bering Sea, with increased chances of showers as
instability increases.

Meanwhile further east, small craft force cold northwesterly
winds over the eastern Bering Sea will weaken overnight as
pressure gradients slacken. The area will remain relatively quiet
under a col Thursday before broad northerly flow quickly resumes
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday night through
Wednesday)...

An active pattern is expected through the long range forecast
through the middle of next week. The main challenge surrounds a
storm system which slides east into the Gulf this weekend
bringing the potential for snow across Southcentral into early
next week.

For the start of the weekend, cold air advection keeps
temperatures below normal across the southern mainland into
Saturday morning under dry conditions as low pressure tracks south
of the Alaska Peninsula to the Gulf. Weak disturbances across the
Gulf will initiate rain showers along a surface low near the
northeast coast heading into Saturday morning, with a few snow
showers moving inland over the eastern Copper River. For locations
along the Bering, including the southwest coast, precipitation
chances increase through the upcoming weekend as an upper low
pressure system in the eastern Bering slowly propagates south
toward Bristol Bay. A colder air mass advecting from the northern
Bering changes precip type mainly to snow except along the
Aleutians where a mix of rain and snow prevail under moderate
temperatures.

The biggest challenge of the upcoming forecast is the storm system
which tracks into the gulf on Saturday. This system is currently
expected to lift a frontal boundary to the northern waters by
Saturday afternoon ahead of a surface low. This low moves across
the central Gulf Saturday night and deepens to 970 mb. This warm
core system will spread rain across the Gulf on Saturday, with
showers along the Gulf coast starting initially as snow mixing
with or changing over to rain through Saturday night. The upper
level low associated with this front begins to lift north towards
Sunday morning, which brings the potential for snow to develop
across inland areas of Southcentral. Models continue to struggle
from run to run on the timing and track of this system, therefore
only small changes were made to increase precipitation chances for
the latter part of the weekend. With high uncertainty during the
mid range forecast, kept changes minimal through the middle of
next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130-132 150 175-178 352 411 413.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RD
LONG TERM...KH


000
FXAK67 PAJK 182304
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
304 PM AKDT Wed Oct 18 2017

.SHORT TERM...Tonight through Friday...As of 1430 this afternoon,
a broad trough with an embedded shortwave has positioned itself
over the gulf, with the shortwave beginning to lift out of the
eastern gulf. This system has produced heavier cells with a decent
amount of lightning over the gulf and near Haida Gwaii throughout
the day, sometimes popping up along the panhandle this morning. A
dry slot associated with a strong jet aloft, which we can see on
WV satellite, has increased instability in the upper levels,
encouraging development with these cells. There is still a
possibility of some thunderstorms over the eastern gulf waters
along the panhandle through this evening, as cold air with the
circulation remains aloft over relatively warm ocean waters. We
can also still see what appears to be a CU field over the water
on visible satellite.

As this shortwave continues to lift northeast, we expect the
winds to increase at the northwestern head of the associated
surface low. Gale force winds will be possible along the western
edge of the offshore waters late tonight before this surface low
weakens tomorrow. Small Craft Advisory level winds will also be a
concern for the eastern gulf marine zones tonight and continue
for Northern Lynn Canal tomorrow. An additional weak surface low
will develop over the northern gulf near the Yakutat Bay area late
Thursday into Friday morning before quickly dissipating and
exiting the area. A weak ridge of high pressure is then expected
to develop over the eastern gulf Saturday before another shortwave
develops over the western gulf.

Models are generally in good agreement up until Friday regarding
the set up of surface level features for our area. On Saturday,
the model spread increases for the shortwave that begins to
develop over the western gulf during the day.

.LONG TERM.../Friday through Tuesday/Minor changes were made to
longer range forecast. Models are generally good agreement on the
large scale features but have important differences on smaller
scale features and tracks of surface low pressure systems. For
Friday showers will remain over the area as the main upper trof
moves east. The showers will begin to diminish some with more
upper and mid level ridging ahead of the large system moving out
of the North Pacific.

The system of concern in long range is a complex low expected to
move into the gulf by the weekend as mentioned above. Models are
getting in better agreement with a deep 960s low moving into the
gulf but there are still differences in the details. This low will
spread strong gales from the Southeast over the gulf along with
large sea heights of 20ft. Over the inner channels the winds over
the northern half may be out of the north from 15 to 20kt with
the southern half seeing southerlies to near 25kt due to the
track of the main surface low. Temps will be an issue with some
cooler air moving in with the north surface winds and a threat for
snow but that threat will remain over the far northern inner
channels and along the Klondike and Haines highways. For the most
part the forecast is a wet one with good onshore flow over the
weekend and into next week.

Later next week, there is a lot of uncertainty with any track and
continued to use WPC from Monday into Tuesday. We will need to
monitor a system that could have the remnants of tropical cyclone
Lan embedded with it on Monday or Tuesday. The GFS is faster and
further to the west with this while the EC and GEM are slower and
further to the southeast.

In the mid term used a mix of the 00zEC and 00zNAM with about
average forecaster confidence.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-022-034-036-041-043-051.
Small Craft Advisory due to Seas for PKZ042-052-053.

&&

$$

Voveris/ABJ

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