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FXAK69 PAFG 172300

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
300 PM AKDT Thu Aug 17 2017

.DISCUSSION...The 17/12Z 500 mb pattern showed long wave troughing
centered over mainland Alaska, consisting mainly of a 531 dm low
located around 150 miles north of Point Hope. Several short waves
are embedded within this trough, the most prominent rounding the
south side of the trough running through the Gulf of Alaska. A
second low/short wave with a 541 dm low center was located over
the far northeast corner of the NW Territories near the Arctic
Coast. A third, weaker one was observed crossing the Alaska Range
into the southeast Interior. At the surface, the main weather
pattern was dominated by broad low pressure centered over mainland
Alaska, with several embedded weaker lows across the state.

The 12Z deterministic models are in good agreement on the overall
pattern through this weekend, but rapidly begin to diverge by next
Monday on the position timing of the upper air features. However,
the guidance does continue to suggest long wave troughing to
persist across the state into next week. The short wave over the
Gulf of Alaska will swing northeastward today, to move into
British Colombia by late tonight. The upper low north of Point
Hope will swing southward through this weekend, to be located over
the AK Peninsula by Sunday evening. The upper low over the
Northwest Territories will move northwest through the Beaufort
Sea, to merge with another shortwave moving east through the
Siberian Arctic by Saturday.

Periods of rain are expected to affect the west Coast Friday into
this weekend as upper low moves south over the coast toward the AK
Peninsula. Snow levels are expected to fall to around 1500 feet
over northwest Alaska and the Seward Peninsula tonight, to persist
through this weekend. Scattered to numerous showers are expected
for Interior Alaska for the next several days, which will be
diurnally enhanced. Isolated thunderstorms will be possible for
the east Interior through Saturday. Weak short wave over the
Interior, which is bringing a round of rainfall to the White
Mountains early this afternoon will lift north to spread rainfall
over the Brooks Range tonight. For the North Slope, periods of
fog, drizzle and stratus will persist there through Saturday
night due to weak onshore flow, before winds turn offshore from
west to east across the Arctic Coast on Sunday in response a
frontal system crossing the Chukchi Sea, which will bring the next
organized chance of rain Sunday night into Monday.

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


.FIRE WEATHER...No issues under cool, wet weather regime.


.HYDROLOGY...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 and Tetlin
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.


The Flood Advisory for zone 224 in the Tetlin Village Road areas
through Friday.




FXAK68 PAFC 180054

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKDT Thu Aug 17 2017


A broad upper level trough associated with relatively cold air
aloft is supporting widespread showers across much of the
Southcentral region. The sounding this morning indicated steep
lapse rates throughout the depth of much of the atmosphere and
near saturated conditions. This instability was confirmed by a
few lightning strikes near the Kenai Peninsula this morning.
Meanwhile high pressure maintains hold across the central and
western Bering.



Models remain in good agreement in the short term with the strong
upper level forcing from the PV anomaly that now stretches across
much of the state. However, once more there is uncertainty in
where rainfall will develop this afternoon and evening across
Southcentral. While most models keep the majority of showers along
the mountains. However, high resolution models hint at the
possibility of a strengthening Turnagain Arm jet again this
evening, which would produce a convergence boundary with the
southwest low level jet over Anchorage.



Cold air aloft underneath an upper level low will continue to move
in over southcentral leading to a relatively unstable airmass. A
shortwave trough will push through the area this evening and will
act as a forcing mechanism to help initiate widespread rain
shower activity with periods of locally heavy rain across the
region. Rain showers will die down overnight into tomorrow morning
before another more vigorous shortwave trough digs down and
across the Southern Kenai Peninsula on Friday. Rain will spread
north and east up the Kenai and into the Prince William Sound as
well as into the Anchorage and Mat-Su areas by the late
afternoon/early evening hours.

Zonal flow sets-up behind the departing shortwave leading to
drier conditions for Saturday across most of the region, though
showers will develop again over the mountains in the afternoon.


A wet and cloudy pattern persists through the upcoming weekend as
an upper level trough remains over the state. Periods of rain and
patchy fog are the biggest challenges for the upcoming weekend.
Showers are expected to be the prevailing precipitation pattern
as broad surface troughing lacks any notable focus. The airmass
across the southwest mainland is conditionally stable and will
likely have weak surface inversions set up during the late night
and early morning hours allowing patchy fog to develop.

The next potent shortwave develops across the Bering Strait on
Saturday and dives across the Kuskokwim Delta Saturday night with
rain developing along this dynamic set up. This wave moves
southeast through Sunday spreading rain across the valleys to the
western Alaska and Aleutian Ranges by Sunday afternoon.


Low to mid level clouds continue to stream toward the Bering from
low pressure over the Arctic and from another low just south of
the central Aleutians. The ridge over the central Bering blocks
either low from tracking across and holds its position through
this weekend. Areas of fog develops across the stable air mass
associated with the ridge. The lowest visibilities are expected
during the late night and early morning hours, improving slightly
during the afternoon and evenings. Several upper level
disturbances dive south from the Arctic low along the eastern half
of the Bering which will bring light rain to the Pribilof Islands
and the Alaska Peninsula Sunday.

The low pressure currently south of the central Aleutians moves on
a slow easterly track through Saturday bringing rain to the Chain.
This system shoves southeast Saturday night with a building ridge
from the North Pacific amplifying across the western Bering
through Sunday.


.LONG TERM FORECAST (Days 3 through 7)...
The active wet pattern remains intact through the long term
forecast period as we head through the middle of next week. There
in a North Pacific low tracking eastward parallel along the
Eastern Aleutians before it moves into the Gulf by Sunday
afternoon. This system remains almost stationary in the northern
Gulf before exiting into the lower eastern Gulf by Tuesday, and
then tracks into the North Pacific Ocean by Wednesday. The models
struggle with the timing as this low tracks along the Aleutian Chain
before entering the Gulf. Look for the bulk of the moisture to
remain along the northern Gulf coastal communities. Meanwhile, the
Bering Sea has a ridge of high pressure building into the region
before a North Pacific 997 mb low tracks near 45N 167E with an
associated front entering the Western Aleutians by Monday morning.
By Tuesday evening the high pressure moves southward, and the
aforementioned low moves into the western Bering. The models have
difficulty as this system tracks northward. The WPC keeps this
system as a broad area of low pressure, while the ECMWF has a more
consolidated system near Saint Matthew Island by Wednesday
evening. As we go into Thursday the WPC remains broad brushing
this system, but the ECMWF keeps it consolidated as it tracks into
the eastern Bering. Therefore, look for marine layer stratus/fog
with the high pressure ridge before transitioning to widespread
showery regime as the low pressure system enters the Bering. The
Southwest Alaska area will transition to an offshore drier flow
briefly before transitioning back to onshore flow by Tuesday. This
will advect marine layer stratus into the region with showery
regime setting up by Wednesday. Since there continues to be model
discrepancy with the timing/placement of the different synoptic
features, the forecast confidence will be barely average through
the long term forecast.






FXAK67 PAJK 172307

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
307 PM AKDT Thu Aug 17 2017

.SHORT TERM.../Tonight through Saturday/As of 1430 this afternoon,
current water vapor satellite imagery shows a shortwave trough in
the northeastern gulf, which appears to be lifting out of the
area. A front associated with the shortwave is bringing areas of
heavy precipitation across many parts of the panhandle today.
Downstream in WV imagery, we can see an even more moist pocket
that will impact the panhandle beginning this weekend and
continuing into the early part of next week. Additional heavy
precipitation can be expected with a possible atmospheric river
setup later this weekend.

After the shortwave exits tonight, models then indicate that a
weak ridge of high pressure will build into the southern portion
of the panhandle tomorrow before another shortwave quickly steals
the thunder and impacts the area this weekend.

Winds have been strong in a few locations, namely Clarence Strait
and Cross Sound. We also have small craft advisories in a few marine
zones along the coast that are in effect through tonight, mainly
from the Dixon Entrance to Cape Decision and Cape Decision to Cape

There weren`t many drastic changes to the forecast. Many elements
were tweaked slightly using the GFS. PoP was slightly increased
into the weekend using the SREF.

.LONG TERM.../Saturday through Thursday/ Extended forecast
continues the trend of wet and cool through next week. A broad
upper level trough remains planted over Alaska through much of the
period supporting SW flow at most levels of the atmosphere. This
is briefly broken up mid week as an upper low breaks off from the
main trough and dives SE.

Main concern is for rainfall Sun into Mon as deep tropical
moisture from former tropical storm Banyan gets advected into the
area. Current model performance on this system is not that great
right now. First off, the low that is drawing this moisture
northeastward is not being handled very well by the various
forecast models. ECMWF and Canadian are trended slower then what
the GFS and some of the ensemble means are showing for low track.
Further more the track of the moisture plume itself is still up in
the air. Currently it appears that the moisture will first arrive
with a front on Sun followed by a second surge of moisture on Mon
morning associated with a following shortwave. Recent model runs
are starting to trend that second surge farther south with most of
the moisture impacting British Columbia rather then SE Alaska.
Elected to not do much with the current forecast to await to see
if the southward trend holds out.

Into mid next week, the upper low dropping southeastward toward
Haida Gwaii will cut off the SW flow in favor of a more
southeasterly flow over the area. Showers expected to remain in
the area but will not be as heavy as recently. Also the farther
north you get the drier it will be. Mainly used WPC guidance for
updates where needed.


.AVIATION...Overall flying weather better over the northern
panhandle on Thursday, but still a lot of rain and local fog
associated with precipitation. Heavier rains have moved south and
east during the day. Low level jet moving into the southern
panhandle late this afternoon causing some low level wind shears.
Wind shear should taper off in the evening. VFR to MVFR for the
day on Friday.


.AJK Watches/Warnings/Advisories...
MARINE...Small Craft Advisory for PKZ041-042.
Small Craft Advisory due to Seas for PKZ043.




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