Small Craft Advisory 826, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKDT Wed Oct 24 2018

.ANALYSIS AND UPPER LEVELS...

Low pressure in the central Bering Sea, and an impending front in
the Gulf of Alaska are the weather makers today. A weakly
vertically stacked, upper-level closed low spins in the western
Bering Sea just north of Adak. This low is advecting cool air from
the Arctic over the western Bering and over the Aleutians. The
low sits at the base of a positively tilted trough, oriented from
the northeast to the southwest over the Bering Sea. An 85 knot
jet max is coming down from the north, passing over the western
Aleutians, and then dipping well south of the AKPEN. As this jet
max moves southward, it is phasing with cold air advection
entering the western Bering Sea from the Kamchatka Peninsula, thus
aiding frontal development along the western and central
Aleutians. The developing front is bringing rainy and gusty
conditions to the Bering Sea and Aleutian Islands. Western
mainland Alaska remains fairly calm with some patchy fog and areas
of light scattered showers.

Meanwhile, the eastern side of the state is in the lull
between storms. A weak shortwave persists over the northern Gulf
of Alaska in the flow on the leading edge of the storm which is
strengthening in the North Pacific. A front is pushing ahead of
the storm into the northern Gulf maintaining the southeasterly
flow into Southcentral. Showers continue to fall throughout
Southcentral as the front moves northward.

&&

.MODEL DISCUSSION...

Models are showing a general pattern shift over the state through
the weekend as the upper level trough moves eastward and northerly
flow prevails through the Bering. Aloft, models are mainly in sync
with the timing and placement as the upper level low over the
western Bering drops into the base of the trough. On the east side
of the trough over the Gulf of Alaska, the timing is mostly the
same but there is some slight difference in the sharpness of the
shortwave moving northward into the mainland.

Surface features are in less agreement between models, although
they are coming together with the 18Z run. This mostly concerns
the low moving northward into the Gulf of Alaska. GFS has been
most consistent with the track and strength of this low, with the
other model runs varying on location in particular. At the time of
writing this discussing, the 18Z NAM had started trending toward
the GFS solution, keeping the low center further from the coast.
This will make a difference with winds along the coast as the
further south solution has the front weakening and moving onshore
more rapidly.

&&

.AVIATION...
PANC...VFR conditions will persist at the airport, with the
potential of an isolated shower as the air remains unstable behind
yesterday's front. Southeasterly winds have ceased, but northerly
winds are expected to increase overnight as the next low
approaches. These winds should remain below 25 kts.

&&

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

Both the Kenai and the Middleton Island radars continue to ping
into showers this afternoon. The latest AMSU (Advanced Microwave
Sounding Unit) data from satellite image has 150-200 percent of
normal TPW (Total Precipitable Water) from Prince William Sound
to Talkeetna. Suffice to say, the moisture has been slow to clear.
Expect rain showers will continue to linger into the evening in
the wake of the recent storm passage. All the while a new low with
a central pressure 969 mb will move north across the Gulf of
Alaska and toward the Kenai Peninsula and Kodiak Island. The
models are not in sync with this feature, although there is
general consensus that the track will be more southerly than the
previous storm and that the pressure gradient will bring another
round of gusty winds and rain into Southcentral. Gale force winds
will move into the Prince William Sound area tomorrow afternoon
and evening. Gusty winds anticipated again Thursday night for
Turnagain Arm and the Hillside area of Anchorage.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Wednesday night through Sunday)

Quiet weather through Friday in southwest Alaska. Some showers
may be present as some weak disturbances move through the area,
but not much precipitation expected out of them. The one item of
note is some patchy fog that has been persisting through the day.
With fog already in place, expecting it to persist in low lying
areas through the night. Northerly low level winds will increase
overnight, inhibiting its development in all but the most
sheltered areas. Going in the weekend, a trough will move south
into the area, bringing cold air with it. This means any
precipitation that could fall this weekend will fall as snow.
Little accumulation is expected. One exception is in the lower
Kuskokwim Valley where there is the possibility of some
precipitation moving in over the Alaska/Aleutians ranges, so some
light snow accumulation may result in that area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Wednesday night through Sunday)

The polar jet continues to charge south through the Bering Sea.
Weak surface lows embedded in the flow will enhance winds as well.
Therefore, keeping winds strong, including maintaining the storm
force winds tonight over the western Aleutians. The push of cold
air is evident with dewpoints falling along stations in the Aleutians
and ice pellets being reported at Dutch Harbor. This push of cold
air is a bit stronger than initially expected, so we are now
forecasting some spotty mixed precipitation over the western and
central Aleutians the next couple of days. This northerly push
will slide east and weaken slowly as it goes, with it reaching the
coast and diminishing Friday into Saturday. The next system to
watch is a low moving through the north Pacific Saturday, though
right now it is forecasted to stay south of the Aleutians.

&&

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

The synoptic pattern will change little over the next few days,
before a pattern shift commences as we head into next week.
A series of low pressure systems will lift north into the Gulf by
Saturday, and then absorb the remnants of Thursday's low, as they
rotate around each other in a Fujiwhara-like fashion. This will
keep a periodically wet theme continuing through the weekend for
southwest and southcentral Alaska, before lifting northeast into
Canada by Monday. This will also allow for our warm afternoon
temperatures to continue.

However, the models have come into better agreement regarding a
pattern change for the start of next week, although there's still
room for improvement in the model continuity. Additionally, this
is the first day out of the last few days that this has occurred,
thus it should be taken with a grain of salt. None-the-less, the
current numerical forecasts modestly agree with a Siberian low
shifting into the northern Bering while deepening on Monday. This
storm, should it develop as forecast, would act as a "kicker" to
the long-wave trough that has dominated the Bering for what seems
like weeks now, allowing it to deamplify and move eastward.
Another storm will also lift into the western Gulf south of the
Aleutian chain, and there are questions as to how much will these
storms interact or phase. So, while it doesn't look like any
major pushes of Arctic air will develop, these changes will play a
role in the possibility of this down the road.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119 175 176 351 352.
         Gale Warning 120 125 131 132 139 165 170 171 172 173 174
                      177 178 179 180 181 185 411 412 413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...LF/RA
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...PD