Small Craft Advisory 667, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKDT Tue Aug 28 2018

.ANALYSIS AND UPPER LEVELS...

As the trough that has been dominating the Gulf of Alaska shifts
inland over Southeast Alaska, a second upper level trough advances
toward the Aleutian Chain. An upper level low near the base of
the long-wave trough is moving over the western Aleutians. The low
is sitting in the left exit region of the jet stream with winds
approaching 105 kts at 300mb. This favorable position is
encouraging the low to slightly intensify. The associated trough
is producing an unorganized front on the leading edge which is
spreading over the central Aleutians. This front is bringing
widespread cloud cover and ample moisture over the Bering Sea and
YK-Delta. Ahead of the front, a ridge is moving over Southcentral,
the AKPen, and southwestern Mainland Alaska. As the ridge builds,
skies are clearing throughout Southcentral and the AKPen which
can be seen in visible satellite imagery. However, clouds are
moving in slowly ahead of the front with the promise of increasing
cloud cover over the AKPen and southwestern Mainland Alaska.

&&

.MODEL DISCUSSION...
Models continue to be in good agreement regarding synoptic
features through the first 36 hours. As the low moves into the
northwestern Bering Sea, the EC is faster in moving the low
eastward, while the NAM and GFS lag behind. Models eventually come
into generally good agreement with the placement of the low in
the Bering, however there is still some localized discrepancies.
In the east, models are not handling a short-wave trough that is
moving into the western Gulf. The NAM is forming a closed low
while the EC and GFS are a bit more broad with low pressure as the
system moves eastward.

&&

.AVIATION...
VFR conditions and light winds will continue. There is a chance of
some fog and/or low stratus late tonight and early Wednesday
morning, but the chances are low enough that only VCFG was
indicated.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
(Tonight through Friday)

After a partly to mostly sunny day across much of Southcentral,
look for some patchy fog to develop tonight along the Cook Inlet
northward through portions of the Anchorage Bowl and Mat-Su
Valleys tonight, as light winds and good radiational cooling
conditions set up under partly cloudy skies. Another relatively
nice day is in store for the region tomorrow, with seasonably warm
temperatures expected. Some mid to high-level cloud cover will
develop as moisture advects into the area from the west on
Wednesday, as moisture aloft begins to increase ahead of our next
storm system, with that cloud deck lowering heading into the
evening. This is in part due to a front entering the area, which
will also cause the Turnagain Arm to increase which will also
affect the upper hillside.

With this lowering cloud shield rainfall will also accompany the
front, spreading from southwest to northeast as the evening/night
progressing and having reached the Copper River Basin by
Thursday. A weak lee side low will develop just east of Kenai
during the day, with this low trekking east near the coast on
Friday, with wrap-around moisture keeping showers in the picture
for a good portion of the region on Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Low level flow across Southwest Alaska will become light and variable
this evening as the ridge of high pressure passes overhead. With
a stable air mass in place, this means that clouds will hold in
across most of the Kuskokwim Delta and Kuskokwim Valley. This will
limit radiational cooling, so have raised minimum temperatures for
tonight. The Bristol Bay region will start out mostly clear, but
expect increasing clouds as a warm/occluded frontal system approaches
from the Bering Sea.

The storm center will be far to the west over the west-central
Bering, so the frontal system will be elongating and weakening as
it reaches the Southwest coast midday Wednesday. Thus, expect a
quick shot of light rain as the frontal system moves through
Wednesday afternoon through Wednesday evening. A cold front will
then usher in a cooler showery regime with gusty westerly winds
overnight Wednesday night through Thursday night. There will be a
long fetch of southwest winds across the Bering Sea which sometimes
leads to coastal storm surge. However, in this case, the strongest
winds and highest waves will remain well out in the Bering, so
water levels may be a bit higher than normal high tide, but
flooding is not expected.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A vertically stacked low will lift northward across the west-
central Bering Sea tonight with a warm and occluded front
spreading north and east across the bering and a cold front
sweeping eastward underneath the low center. Expect widespread
small craft winds with isolated areas of Gales while the bulk of
rain will found along and ahead of the low center and warm and
occluded fronts. Winds will strengthen behind the cold front on
Wednesday leading to a larger area of Gales across the southern
Bering Sea. The unstable air will lead to isolated to scattered
showers and gusty winds along the Aleutian chain over to the
southern Alaska Peninsula through Thursday night. Meanwhile, the
low will weaken and track eastward across the Bering.

&&

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

An area of low pressure located between the Bering coast and the
Pribilofs Friday morning will continue to keep things somewhat
unsettled across Southwest Alaska to start the day Friday.
Meanwhile, cooler air over northern Alaska will try to skirt
Alaska Range communities, which will set off some rainfall
activity primarily in the mountains as well on Friday. Overall,
any rain activity will be light and scattered in nature with most
of Southcentral remaining cloudy, but dry on Friday. The weakening
low over the eastern Bering moves southeastward into the Alaska
Peninsula and opens up into a broad surface trough by Friday
evening. This should keep any precipitation for the Alaska
Peninsula and Kodiak to drizzle or very light rain as it passes
by. The remnants of the low move into the Gulf by Saturday and
weaken.

Meanwhile, offshore light northerly winds will develop across much
of Southern Alaska on Saturday, which will allow for a nice, dry
weekend for all of mainland Southern Alaska with very seasonable
temperatures expected. A fairly weak front associated with another
attempted influx of cooler, fall like air will approach from the
north on Sunday afternoon and evening. As is typical, this feature
will be largely moisture starved, and upon reaching the Alaska
Range, should largely rain itself out, leaving relatively little
precipitation for Southcentral. However, there is some
disagreement in the models as to how much moisture makes it into
communities further south, so uncertainty remains high.
Nevertheless it appears likely that Sunday will be the cloudier of
the two weekend days.

Labor Day on Monday still features considerable uncertainty with
the forecast, with the GFS keeping at least some shower activity
across Southcentral, while the EC keeps the area nice and dry once
again. This is largely due to differences between how the two
models are handling the development of an initially weak upper
level wave. As the GFS appears to slow the energy down and
redevelop it for no apparent reason, the EC appears to be the more
plausible solution at this point. Regardless, both of the two
models dry out Southcentral once again by Tuesday. For Southwest
Alaska, ridging building in from a large area of high pressure
over the Bering means Saturday straight through Tuesday looks
nearly entirely dry, with the exception of the possibility of some
isolated shower activity from the dying front that moves in from
the northwest Sunday afternoon and evening.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 179 185 412-414.

&&

SYNOPSIS AND MODEL DISCUSSION...RJA/BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
LONG TERM...JW