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FXAK69 PAFG 162118

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
118 PM AKDT Wed Aug 16 2017

The longwave trough situated along the west coast of Alaska will
shift east by Thursday and remain over mainland Alaska through
early next week. This will result in periods of rain or showers
and temperatures near or slightly cooler than normal persisting
across northern Alaska. No significant weather impacts are
anticpated at this time through the weekend.

Models intialized well and are in good agreement through Friday
with position and strength of features. Differences in model
solutions begin to develop by the weekend but the general
consensus is for the longwave trough to persist over Alaska into
next week.

A short wave trough over the Interior will move north to the
central Brooks Range tonight and to the northwest Arctic coast
Thursday night. Upslope flow will enhance the preciptation as it
crosses the Brooks Range with 1 to 1.5 inches of rain possible
between the Dalton Highway and Anaktuvuk Pass and up to 1 inch
possible on North Slope. The snow level will remain above 4500
feet so the bulk of the precipitation will fall as rain and runoff
into the rivers. Elsewhere across the forecast area, several weak
features will allow shower activity to continue over much of the
area. A thermal trough stretching Arctic Village to Chicken may
kick off some isolated thunderstorms in the eastern Interior
through Friday.

The pressure gradient will remain generally weak over most of the
forecast area. Marine winds will stay below small craft the next
few days, although northwest winds may increase to 25 kt in the
Bering Strait on Friday. Persistent northwest winds 15-20 kt
across the Chukchi Sea will bring incresing waves to the Seward
Peninsula coast at Shishmaref Thurday night and into the weekend,
but high surf or significant coastal erosion is not expected.

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


.FIRE WEATHER...No issues with the cool and wet pattern.


.HYDROLOGY...Rivers and streams in the central Brooks Range, such
as in the Koyukuk river basin, are rising from recent rains. The
additional rainfall tonight and into Thurday will bring water
levels to near bankfull on some smaller streams but flooding is
not anticipated at this time. A similar scenerio is expected on
some of the north draining rivers as well. Recent cooler
temperatures aloft in the eastern Alaska Range and Wrangels has
significantly slowed down glacial runoff into the Tanana River.
The Tanana River near Tok has dropped considerably during the past
24 hours and overland flow causing flooding near the Tetlin
Village Road and high water along the Alaska Hwy should begin to
recede some during the next few days.




AUG 17

FXAK68 PAFC 170037

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKDT Wed Aug 16 2017


Water vapor imagery indicates an upper level PV anomaly digging
southward from the northern Bering Sea to the Alaska Peninsula.
This upper level forcing caused widespread rainfall across much
of the forecast area today from the Alaska Peninsula to the north
Gulf of Alaska coast. To the west of this, high pressure is
dominating the western and central Bering and supporting a broad
stratus deck with occasional glimpses of blue skies. In between
these two features, a low level jet is bringing gusty gale force
winds across much of the Central Bering.



In the short term, models are in fairly good agreement with the
GFS favored for the majority of the forecast. There are some
minor mesoscale differences between the models on the intensity of
the low level jet along Cook Inlet and through Turnagain Arm this
evening, but generally these differences are relatively small.
There are also the usual minor differences among the models in the
intensity and location of precipitation, but with fairly strong
forcing and consistency in models, these differences are fairly


PANC...CLoudy and rainy conditions across the Anchorage airport
will persist for much of the afternoon. There is a chance that a
weak Turnagain Arm wind will develop this evening, which should
help to lift ceilings and switch flow to a more southerly or
southeasterly direction. However, the timing of this switch is
still uncertain.


A typical cool and rainy August pattern will continue over
much of Southcentral through the weekend with a few breaks in
between. A cold upper level low dropping south out of the Bering
Sea is leading to broad moist southwesterly flow over the region
and widespread precipitation. Rain will continue overnight as the
upper low continues to dig south maintaining the overall
southwesterly flow into southcentral. Precipitation looks to taper
off some by tomorrow morning as the first shortwave moves off to
the northeast. Cold air aloft underneath the low will promote
instability leading to showery conditions, especially over the
mountains and the Prince William Sound. With enough daytime
heating, isolated thunderstorms may form over the Talkeetna
Mountains and the Copper River Basin tomorrow afternoon. The
pattern turns messy by Thursday night as several shortwaves
spinning around the base of the upper trough pass through the area
leading to continued chances of rain and showers over most of the
region throughout the day on Friday.


An upper level trough slowly passing over the region will bring
continued cool, cloudy, and rainy conditions to the area east of
Bethel through Thursday. There is an area of clearing that is
developing along the southwest coast which, along with onshore
flow, will aid in the development of patchy fog/low stratus
overnight tonight. One limiting factor in this development is the
lack of extensive low clouds over the Eastern Bering which will
prevent any fog from becoming too dense or widespread. On Friday
a weak shortwave moving over the area will bring continued
cloudcover, however any rainfall from this should be light and
spotty in coverage.


Rain and gale force northwesterly winds will continue streaming
from the Central Bering into the Eastern Aleutians/AK Pen as a
deformation band passes through. Most of the rainfall from this
system will be focused along a narrow area with the deformation
band, which the models are all in good agreement will setup from
just east of the Pribilof Islands southward into the AK Pen. As a
result small changes in the location of this band will have big
implications in how much rain will fall, but at this point there
is reasonable confidence that the heaviest rainfall will occur
along the Bering coast of the AK Pen. Starting on Friday, the
extra tropical remnants of Tropical Storm Banyan will have
weakened substantially as it moves into the Central/Eastern
Aleutians. There is very good confidence that this system will
have weakened to small craft or less by the time it approaches the
chain, with no significant impacts expected to the area.


.LONG TERM FORECAST (Days 3 through 7)...

An active wet pattern continues in the long range forecast
Saturday through late next Tuesday. A merger of two low pressure
systems is anticipated for this upcoming weekend. A broad trough
deepens over the western coast and shifts east through Saturday.
At the same time a fast paced North Pacific low races along the
Aleutian Chain, taking a northward turn Saturday night. The merger
of these two systems will likely bring moderate to heavy rainfall
along the Gulf coastal areas and Southcentral. Models differ in
timing and track of this merger, therefore a broad brushed
solution of the ensembles of the ECMWF and GFS were used to update
the inherited forecast. For the Bering and Southwest mainland, a
short break with drier conditions settles in Sunday as a ridge
builds behind the eastward moving trough over the mainland. On
Monday rain moves back into the southwest coast from a fast diving
low off of Kamchatka, and another North Pacific front moves into
the western Bering/Aleutians along an eastward progressive
subtropical jet streak.


MARINE...Gale 155 170 171 179.




FXAK67 PAJK 162256

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
256 PM AKDT Wed Aug 16 2017

.SHORT TERM...A weakening short-wave with an associated low has
moved into the northern Panhandle and we should begin to see
pressure rises shortly this evening as ridging develops behind.
This will translate into small craft winds that due to begin
shortly in northern Lynn Canal, peaking at 30 kt southerly later
tonight. Rain will all but transition to showers as cooler air
migrates aloft behind the front.

A longwave trough deepening across western Alaska will spark a
slight chance of thunderstorms spreading east from the zone 52
later tonight into Thursday. Yakutat and Sitka have been included
in the threat area.

Examining satellite imagery stretching from the gulf westward
across the North Pacific, one can detect a virtually uninterrupted
stream of deep moisture. From afar, it aligns into an even arc of
some sort. But upon closer inspection, one can pick out kinks
within the arc. These are what the models are desperately trying
to detect. We look to have another low weaker low embedded in the
southwest flow on Thursday, again tracking very similarly to the
low today.

Friday, more showers follow within broad onshore flow. While
models indicate more concentration east of Yakutat, a low due to
form across the northern gulf will likely keep showers numerous
over the northeast gulf as well.

High and low temperatures will be less interesting than the tiny
diurnal changes with this pattern. Dress no differently daytime
than nighttime, just take the rain jacket.

NAM/GFS were used for through Friday with a nudge toward GFS
Friday night. Forecast confidence is good for showers and

.LONG TERM...Beyond Thursday`s storm, a weaker shortwave
approaches the southeast gulf on Friday. With solutions
indicating varying degrees of intensity, we drew a moderately
weak system from which we can build if we need to strengthen as
it moves into the Panhandle. If anything, the system will help
reinforce some cooler air aloft and keep showers entrenched
across Southeast in the zonal flow that sets up through Saturday
night. The short-wave`s passage across the southern Panhandle
leaves weak ridging over the Inner Channels and weak troughing
over the gulf. This pattern begins to amplify later in the weekend
as a western gulf system arrives, possessing the deeper tropical
moisture from former Tropical Storm Banyan. Therefore we have
increased southerlies and southeasterlies over the gulf at this
time, but both ECMWF and GFS raises winds even further to 25 kt.

The system pressing into the western gulf will have originated
within a very moist region of the western Pacific. Satellite
imagery already indicates Banyan showing a slight eastward
component within its current trajectory. This will increase as it
approaches the main jet region. GFS ensembles are showing a high
probability of Integrated Vapor Transport in excess of 250 kg/m/s
across the gulf and southern Panhandle late Sunday into Monday.
However, just as quickly this feature ejects to the east. Thus,
this setup is showing signs of atmospheric river development and
will indeed cause rivers to rise. However, at this stage, even if
it does develop, any flooding impacts are still to be determined.
In any event, what we can say with certainty is that these
successive storms are priming soils in the Panhandle for potential
impacts later in the season. In line with its tropical origins,
we raised snow levels up to around 10K feet by late Sunday as

Beyond this, we lowered temperatures over the waters in line with
the mainly mid to upper 50s we have across the land areas. We used
a blend of NAM, ECMWF, GFS Friday, followed by ECMWF and GFS for
the weekend. Confidence was too low to make changes beyond Sunday.
Forecast confidence high for wet weather and high that Monday`s
eclipse will not be viewable from Southeast AK.


.AVIATION...Very moist lower level conditions for all of SEAK
except Yakutat area. South of the Yakutat area, widespread MVFR to
IFR conditions through the day with the lower clouds and rain and
areas of mist/fog. Think the ceiling which have lifted slightly
during the day will lower again as the next weather front moves
inland late Wednesday night. Expect the lower cloud heights up to
about 2000 feet, and visibilities lower to the 2 to 5 mile range
as well.


.HYDROLOGY...Rivers slowly continuing to rise with rains
continuing for the remainder of the week. No flooding hazards at
the present time, however the development of an atmospheric river
aimed mainly to southern half of the panhandle late Sunday then
being drawn northward Monday to Tuesday by a potential secondary
wave in the flow will bear watching. Rain totals over 2 to 4
inches possible across the panhandle through Sunday night and that
is missing the AR`s return northward to the southern panhandle.
Will be watching the river levels and rain rates for the rest of
the week.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ012-013-042.
Small Craft Advisory due to Seas for PKZ022-041-043-051-052.




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