Small Craft Advisory 590, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 PM AKDT Thu Aug 10 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level low centered over Bristol Bay. Plenty of
clouds and showers are associated with this low over most of
southern Alaska and into the eastern Aleutians. There is an
associated weather system developing in the southern Gulf that is
moving towards the north. There are upper level ridges over
northwest Canada and the western Bering Sea, weakly attached
through central Alaska. A large and deep upper low over the Arctic
is spurring a strong Polar jet over the northern tip of Alaska and
into the surrounding Arctic Ocean. An associated cold front is
poised to the northwest of the state and moving towards the
southeast.

&&

.MODEL DISCUSSION...
The numerical models are in reasonable agreement through the short
term portion of the forecast (Sunday afternoon). There are some
important differences in how they handle the timing and associated
moisture with the cold front moving through southern Alaska on the
weekend. This leads to uncertainty in the forecast and confidence
is below normal.

&&

.AVIATION...
PANC...VFR light rain showers with generally light winds will
persist into Friday. Winds are expected to turn out of the S-SE
this evening, however with a weak pressure gradient along
Turnagain Arm these are expected to be much weaker than what
occurred Wednesday evening.

&&

.FIRE WEATHER...
The primary fire weather concern today will be along the Copper
River where gusty southerly winds are creating conditions that are
expected to approach, but not quite reach Red Flag criteria this
evening. There may be some isolated areas along the river that
reach criteria, however with upper level cloudcover streaming in
from the south these are not expected to be long lasting or
widespread enough to warrant a warning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A low pressure system currently centered over the North Pacific
will move north into the Gulf of Alaska tomorrow bringing
widespread rain to most of Southcentral. A deep plume of moisture
being pulled north up for the subtropic with precipitable water
values near 1.5" will collide with the Gulf Coast by Friday
morning, leading to heavy rain for most coastal locations. This
moisture will spread north and west into the the Kenai, Anchorage,
and Mat-Su areas by Friday afternoon with precipitation from this
system lasting through most of Saturday.

The next system will come down from the north on Saturday night
bringing with it much colder air and another round of
precipitation. There are still some model discrepancies regarding
the timing of this system into southcentral, with the EC/Canadian
bringing the low in faster than the GFS/NAM. But all models show
much colder air filtering in, with 850 mb temps dropping to near
the freezing mark by Sunday morning across most of the region.
This means snow levels will drop below the crest of most of the
major mountain ranges in the area, with snow levels ranging
between 3,500 - 5,000 ft in the Kenai, Chugach, and Talkeetna
Mountains by Sunday morning. It does appear that precipitation
from this front will begin tapering off by the time snow levels
are expected to be at their lowest so major snow accumulations are
not expected in the most popular mountain recreation areas,
though a dusting of snow may be possible.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Cloudy and showery conditions will persist over the southwest
Mainland through tonight, as the upper low over the eastern Bering
keeps the area in moist southeasterly flow. There will be just
enough instability during the evening hours that isolated
thunderstorms cannot be completely ruled out this evening, but
the chances remain low. The upper low will weaken and shift south
of the AKPen by Friday morning, bringing a brief reprieve from
the widespread showery conditions.

By Friday evening a cold front will start to push into the
southwest Mainland from the Bering Strait, bringing another round
of rain and gusty winds to the area. This strong upper trough
digging southward will also bring the first cold air punch of the
season to the area starting late Friday night. The front will
spread rain and gusty winds from west to east Friday afternoon
into on Saturday evening, possibly with snow levels dropping to
3500 ft or so. The cooler air mass will sit over the mainland
through the remainder of the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Much quieter conditions persist over the western half of the the
Bering, as surface high pressure sits over the western Bering
through Saturday morning. A weakening low pressure system sits
over the eastern Bering, keeping showery and gusty conditions over
the area through Friday morning when the low dissipates. Light
northerly flow sits between the two systems, with spotty
precipitation and low clouds over the central Bering.

Not much change is expected until a cold front drops in from the
Bering Strait on Friday afternoon. This will bring rain and gusty
northerly winds to the north and northeastern Bering Sea Friday
evening through Saturday night. Gale force winds are expected
mainly along the coastal waters of the Kuskokwim Delta late
Friday night through early Sunday morning.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended forecast period Sunday evening, there will
be an exiting dynamic arctic trough moving east through the Gulf
of Alaska with a series of smaller scale troughs extending from
the North Pacific to the Bering Sea. Showers and rain will be
ending across most of Southern Alaska as offshore flow ramps up as
the upper wind regime becomes more westerly behind the departing
trough. Rather quiescent weather with mostly diurnal showers and
clouds looks to dominate through early Wednesday before the
pattern amplifies once again. It seems rather likely that another
arctic trough will dig into the Eastern Bering Sea and merge with
a southern stream trough over the Western Bering Sea, setting in
motion another period of wet weather as Alaska quickly transitions
into the typical wetter pattern for mid-August.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL/DEK
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JA