Area Forecast Discussion
Issued by NWS Fairbanks, AK

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FXAK69 PAFG 251441 AAA
AFDAFG

Northern Alaska Forecast Discussion...UPDATED
National Weather Service Fairbanks AK
641 AM AKDT SUN SEP 25 2016

Update adds a paragraph on potential for high surf along
the coast from the barrow area southwestward.

.DISCUSSION...

Upper air...
At 500 mb...a 513 decameter low 200 nm northeast of Wrangel
Island will weaken to a 517 decameter low 325 nm north of the
Mackenzie River delta by Tuesday afternoon. A trough extending
from the low southward across the western interior will weaken and
lift out to the northeast by early Monday morning. Another short
wave trough rotating around the low will arrive on the northwest
coast by midnight Sunday night, then move eastward to the Alcan
border by noon Tuesday. A 533 decameter low about 100 nm
southwest of Cape Navarin will weaken to a 545 decameter low over
the outer Alaska Peninsula by noon Monday. A ridge will begin to
build east of the Kamchatka Peninsula late this afternoon,
becoming a high amplitude ridge extending from the Chukchi Sea to
Dall Point and southward by Tuesday morning. By Wednesday morning
the ridge axis will be farther east, extending from the eastern
Beaufort Sea southward to the Big Delta area then southwestward to
Kodiak Island and beyond. This will be followed by a strong
impulse moving eastward along the southern Siberia coast and
arriving over the northern Bering Sea some time between late
Tuesday night and Wednesday morning. The GFS model is a bit slower
than the ECMWF or Canadian model on the arrival time over the
northern Bering Sea, and the Canadian model takes stronger energy
a little farther north than the GFS or the ECMWF.

Surface...
A 998 mb low 200 nm northeast of Wrangel Island will weaken
to a 1005 mb low 300 nm north of Prudhoe Bay by late Monday
night. A 998 mb low 50 nm southwest of Cape Navarin by
early Monday morning as a 1003 mb low. Southwest winds will
begin to increase across the northern Bering Sea Tuesday
as several low pressure troughs approach from the west and
tighten the mean sea level pressure gradient. A strong trough
will arrive over the northern Bering Sea or the Chukotsk
Peninsula sometime between late Tuesday night and Wednesday
morning. Winds over the northern Bering Sea, Chukchi Sea, and
Kotzebue Sound will become westerly after the trough passage.

Rain over St Lawrence Island and the Bering strait coast
will spread to the west coast late this afternoon and this
evening, spreading to the upper Kobuk and Noatak valleys
and nearby areas late tonight and early Monday morning.
Snow amounts of 2 to 6 inches today through tonight are
expected at higher elevations in the Alaska Range. No more
than 1 or 2 inches is expected along parts of the Denali
Park road as far west as Teklanika. Elsewhere over the eastern
interior, mainly isolated to scattered showers are expected today
through Monday morning.

Extended outlook...
GFS and ECMWF models continue the trend which started on
yesterdays runs, building a strong high amplitude ridge
across the Alaska mainland by Friday. GFS and ECMWF models
differ on how long the ridge persists, but much of
the interior may experience pleasant conditions with
above normal temperatures from Thursday through next weekend.
A moderately strong surface low is progged to track east
northeastward across the Arctic Ocean Wednesday through Thursday,
producing moderately strong west to southwest winds along the
Arctic coast.

Coastal Hazard Potential...
There continues to be some differences between the models on the
timing of the strong surface trough incoming from the west along
with the location of the strongest mean`sea level pressure
gradient. The European and Canadian models are moving the trough
to the northern Bering Sea and the Chukotsk Peninsula by late
Tuesday night, while the GFS and NAM are about 6 hours slower. The
GFS and NAM keep the strongest pressure gradient south of the
Bering Strait while the Canadian model and to a lesser extent the
ECMWF model have the strongest gradient north of the Seward
Peninsula. These subtle differences can have significant effects
on the potential for elevated surf and sea levels and coastal
flooding, and the severity of any such conditions that might
occur. The current model solutions suggest that many areas along
the coast from north of Dall Point to the Chukchi Sea coast
northwest of Cape Krusenstern could be at risk.

The GFS-based storm surge model indicates that the maximum
storm surge in Norton Sound would occur Wednesday afternoon,
with surge values of 4 feet or greater along the coast from east
of Port Safety to Kotlik along with west winds to 40 kt across
Norton Sound producing waves to as high as 8-10 feet offshore.
Surge values of 6 feet or greater would occur along the coast
from Cape Darby and Stuart Island eastward. The highest surge
values, from around 7 to almost 7.5 feet, would occur from the
Unalakleet northward to Norton Bay. Along the Chukchi Sea Coast
northwest of Cape Krusenstern, gale force south to southeast winds
would begin Tuesday evening, producing high surf conditions along
that section of the coast through Wednesday morning when the winds
switch to southwest and start to decrease. The GFS-based storm
surge indicates storm surge values rising to 2 to 2.6 feet by
Wednesday afternoon over the entire Chukchi Sea and in Kotzebue
Sound. A sea level rise of 2 feet would make flooding possible in
the Kotzebue area.

The ECMWF and Canadian solutions suggest somewhat stronger west
winds across the Chukchi Sea and Kotzebue Sound after the
trough passage as compared to the GFS model, but lighter
winds and probably less storm surge across Norton Sound than
indicated by the GFS model.

Quick look at the just arrived 06Z runs of the GFS/NAM runs
is they do not look much different than the 00Z runs, and they
look quite similar at Sep 28/18Z. The 06Z Canadian run also looks
similar to its predecessor, keeping the strongest pressure gradient
and winds north of Norton Sound ad the same time. Maximum storm
surge from the 06Z run of the storm surge model is about 6.7 to
7.0 feet along the eastern shore of Norton Sound and in Norton Bay
Wednesday afternoon...about a half foot lower than the 00Z run.
Maximum storm surge at Nome is expected to be about 3.7 feet at
19Z Wednesday. At Kotzebue, the 06Z run has upped the maximum
storm surge to 3.0 feet at 03Z-05Z Thursday, 0.3 foot higher than
the previous run. Given that the Canadian and European models are
suggesting stronger west winds in the Chukchi Sea and Kotzebue
Sound after the trough passage than the GFS, there should be a
good likelihood that the maximum storm surge at Kotzebue will be 3
feet or a little higher.

It is still to early to issue any coastal flood watches or high
surf advisories at this time, but it should be emphasized that
there is potential for coastal flooding and/or high surf
conditions along many parts of the coast from north of Dall Point
to the Chukchi Sea coast northwest of Cape Krusenstern. Subsequent
model runs will need to be evaluated closely to assess the
potential for high surf and coastal flooding. Based on currently
available model information, the greatest risk of flooding
would be in areas along the southern and eastern shore of Norton
Sound, and in the Kotzebue area after the winds shift to westerly
behind the trough passage, pushing more water into Kotzebue Sound.
High surf conditions appear likely on the Chukchi Sea coast northwest
of Cape Krusenstern. Not to be overlooked is the possibility of
flooding or beach erosion at Shishmaref when the winds shift to
westerly after the trough passage. Maximum storm surge there
is progged at around 2.5 feet Wednesday evening as winds become
west around 25 kt.

There is potential for High surf conditions to develop along the
arctic coast from the Barrow area westward Wednesday afternoon
as moderately strong south to southwest winds shift to a
more westerly direction, creating a storm surge of about 1.5
feet in the Barrow area and as high as 2.5 to 3 feet in
areas farther southwestward along the coast.

&&

.HYDROLOGY...
None.


&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory for PKZ210.

&&

$$

RF SEP 16














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

&&

.HYDROLOGY...


&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory FOR PKZ210.
&&

$$

Sep 16


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