Small Craft Advisory 705, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 PM AKDT Wed Sep 25 2019

.ANALYSIS AND UPPER LEVELS...

A rapidly moving low is currently pushing through southcentral AK.
It is an interesting feature, as it has a well defined cold front
associated with it. A sharp clearing line can be seen on satellite
and radar behind it. This is something that is not common for Alaska,
so is interesting to watch from a meteorological perspective. Behind
the low, high pressure is beginning to build into southcentral AK.
Out in the Bering Sea, a broad area of low pressure exists, sending
a frontal system east across the Bering Sea and Aleutian Islands and
is approaching southwest AK. Overall, flow is beginning to transition
from a faster moving zonal flow to a slower, more amplified pattern.

&&

.MODEL DISCUSSION...

Models are in good agreement through the short term. There are
minor differences between them, but no striking differences that
would cause concern for the forecast. Therefore, we have high
confidence in both the models and the forecast through the end of
the week.

&&

.AVIATION...

PANC...Showers moving through the area are clearing quickly.
Behind them weather will be VFR. Gusty winds should be calming
this evening. Early tomorrow morning winds will shift to a light
northerly direction.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A strong shortwave trough is tracking east across Southcentral
this afternoon and evening. This system brought graupel and hail
to parts of Anchorage, as well as mixed snow down to the surface
in Talkeetna and along the Parks Highway corridor. As the cold front
associated with this trough moves into the Copper Basin, expect a
gradual turnover from rain to snow, with some possible light
accumulations. With cold air advection and a strong pressure
gradient, offshore winds will increase quickly this evening along
the North Gulf Coast as gap winds develop in favored areas. These
winds peak overnight and will gradually weaken during the day
Thursday.

Offshore flow along with ridging building in Thu night will allow
for developing inversions with likely the coldest temps of this
fall season, especially in normally cold locations. High pressure
dominates into the weekend, keeping high and dry conditions across
Southcentral.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Scattered rain showers over the coastal areas of Southwest will
continue through the early evening tonight as the low pressure
that brought gusty conditions yesterday continues to move
eastward. Behind the low, a ridge of high pressure will build in
for Thursday allowing for a general clearing trend. Coastal areas
around Bristol Bay may briefly see foggy conditions early tomorrow
morning as weak onshore flow is expected. By Thursday afternoon,
the next front associated with a weakening Bering low approaches
Southwest, bringing gusty southerly flow and rain for the end of
the workweek.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

High pressure will bring fair skies and light winds to the eastern
Aleutians through Thursday. Farther west, a large, weakening low
is approaching the northwestern Bering. Its associated front will
bring southerly gale force winds and moderate rain to the western
and central Aleutians this evening. The front is expected to move
eastward overnight, and will reach the eastern Bering Thursday. An
additional area of small craft winds will push across the Bering
Friday as the low center lifts northward.

&&.MARINE (Days 3 through 5: Saturday through Monday)...

Near-gale force southerly winds associated with a strengthening
low may develop Saturday afternoon south of the Alaska Peninsula,
through Bristol Bay, and on southeast winds through the Barren
Islands. High seas associated with those southerly winds are
possible south of the Alaska Peninsula. Those winds will weaken
Saturday night. No gales are expected on Sunday. A front moving
over the western Bering Monday may produce widespread small-craft
winds, but gales look to be isolated right now through the Western
Aleutians. Another strengthening low in the Gulf may lead to gale
force winds developing late Monday afternoon and especially into
Monday night.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through
Wednesday)...

The models are in good agreement that a low will develop along a
front south of Dutch Harbor Saturday morning. The low will
continue strengthening as it moves northward into Bristol Bay and
up the west coast of the state through Saturday night. The low
will continue strengthening until it reaches the Seward Peninsula
Sunday morning. The eventual strength of the low as it tracks
across Southwest Alaska will be a very important factor further
east across Southcentral as to the strength of the winds,
particularly through the gaps as the low moves by. The stronger
the low becomes, the stronger the winds will be Saturday night
into Sunday morning. Pattern-wise, the low will be following a
nearly ideal track for bringing strong Turnagain Arm winds into
Anchorage. Thus, this will need to be a point of stronger focus
for the forecast in the upcoming days, as there is potential for
strong winds, especially if the low ends up being stronger than
currently forecast.

Regardless of the winds, the low will be a big rainmaker for
nearly all of Southcentral as a slow moving cold front moves
eastward this weekend. In advance of the front, the low will tap
some unseasonably warm air, which will ensure that dropping
freezing levels will not be a concern with this round of
precipitation, even at mountaintop. Rain will begin in Southwest
Alaska during the day Saturday, remaining steady into Sunday
morning. Many areas of Southwest Alaska may exceed 2 inches of
rain through the entire storm. While most areas of Southcentral
will get less rain, this should be a soaking rain eventually for
all of Southern Alaska. Even in the typical leeside areas, the
winds should diminish by Sunday evening to allow for rainfall in
those areas.

Lingering rainfall activity will continue into Monday
across the Copper River Basin and Prince William Sound, as a new
low develops in the Gulf Monday afternoon and evening. Things will
quiet down by Monday over Southwest Alaska and the eastern Bering
as a new front moves into the western Bering. While uncertainty
increases significantly by Tuesday regarding the track and timing
of winds and rains with the Gulf low, all of the longer range
models have a strong low somewhere in the central Gulf Monday
night and Tuesday, so there is some degree of confidence of high
winds and seas through the gaps and the coastal waters. How the
front in the Bering behaves Tuesday and overall weather on
Wednesday for Southern Alaska remains highly uncertain.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...BJB
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...JPW