Flood in Alaska, October 12 to 16, 2018

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 PM AKDT Thu Oct 11 2018

.ANALYSIS AND UPPER LEVELS...
A short-wave trough is digging southeastward across the north-
central Pacific causing re-amplification of the upper level flow
in the wake of the strong storm system which barreled into mainland
Alaska yesterday. Thus, we are back to a high amplitude trough
over the Bering Sea/North Pacific and a building ridge from the
East Pacific up to Southcentral Alaska. A separate short-wave
trough and weak surface low are tracking northward up the east
side of the trough into the eastern Bering Sea, with a cold front
trailing southward into the Pacific. Rain and southeast small
craft to gale force winds are spreading north and east across
the southern Alaska Peninsula/eastern Bering Sea and into
Southwest Alaska.

Across the remainder of the Bering Sea and Aleutians uniform
instability and weak short-waves or lows embedded in the flow
are leading to areas of showers and gusty winds. Out ahead of
the building ridge instability combined with weak short-wave
aloft are also producing some showers in Southcentral Alaska,
mainly in upslope areas of eastern Prince William Sound and the
Alaska Range. Otherwise, it's a quiet day with partly cloudy
skies across the remainder of Southcentral.

&&

.MODEL DISCUSSION...
Model agreement is not as good as it has been in recent days,
but all solutions do show the same general idea. The first notable
difference is with eastward progression of the cold front into
Southcentral on Friday. The GFS is most aggressive bringing rain
into Anchorage and the Matanuska Valley. Based on amplitude of the
ridge, with due south flow aloft and low level southeast (gradient
flow) across the Kenai and Chugach Mountains, seems more likely
the front will remain solidly west of Anchorage as most of the
other solutions indicate.

There is much better agreement with track of the low across the
Gulf of Alaska on Friday, but minor differences in exactly where
it moves onshore of Prince William Sound. This will affect the
details of winds in Prince William Sound and Turnagain Arm. Haver
generally trended toward the higher resolution guidance for now
which usually has a better handle on low track once it begins to
interact with the coastal mountains.

Lastly, there are track differences with the deep low tracking
along the Aleutian chain Friday night and Saturday. There has been
a general trend toward a further south track, but not quite ready
to buy the NAM/GFS which are the farthest south. Will stick closer
to the ECMWF/Canadian idea of keeping the low right near the
Aleutian Islands.

&&

.AVIATION...
PANC...VFR conditions will persist. The primary forecast challenge
is winds. The Turnagain jet will strengthen tonight through Friday
as a front approaches from the Bering Sea. It looks like the jet
will initially come straight out of Turnagain Arm, so the terminal
will be right on the northern edge. There may be some fluctuations
of periods with gusts greater than 10 kts and periods with no
gusts at all. As surface temps warm tomorrow think the unstable
low level lapse rates will support more wind bending into the
terminal, but still expect the strongest winds to remain south of
the terminal as models all forecast Talkeetna surface pressure to
remain higher than Anchorage.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (through Sunday afternoon)...
A break between weather systems will give most places a
seasonably warm afternoon and evening. However, the next front
will quickly move into the region from the west late tonight and
briefly park itself over the western Kenai Peninsula. The pressure
gradient will tighten in response and kick up gap winds around
mainly Turnagain Arm. Tomorrow afternoon, a quickly developing
disturbance will ride the jet stream from the south, bringing a
compact surface low pressure and front into the northern Gulf of
Alaska. As that system slams into the coast, a decent gale-force
barrier jet will form. Once that system exits on late Saturday, a
short period of benign conditions will fall in on Sunday.

So what does this mean for sensible weather? Gap winds will begin
to ramp up overnight as the front approaches. Rain will spread
west to east over Cook Inlet, coming to a stop near the western
Kenai Peninsula. The north gulf coast will also see steadier rain
with onshore flow. Winds will this system will mainly be through
Turnagain Arm, and to a lesser extent the upper Hillside. South
and west Anchorage may see some increased flow as well. Overnight
Saturday, the barrier jet will really ramp up, nearing storm-
force as more intense rain affects the coast. The jet will also
affect Prince William Sound. As this system moves inland on
Saturday, nearly all locations across the area will see a period
of light to moderate rain. Above average temperatures will
continue.

&&

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

A low situated in the eastern Bering will lift north tonight.
An associated warm front draped along the southwest coast will
continue to push rain into the southwest interior this evening as
it weakens and occludes. Behind the front the rain will taper to
scattered showers. Scattered showers will continue into Friday,
primarily over coastal locations, as a weak shortwave rounding
the base of the longwave trough moves over Southwest Alaska.
Farther east, over the southwest interior, a few breaks in the
clouds are possible Friday evening and night as the region sits
between the shortwave to the west and a second surface low moving
northeast across the Aleutian and Alaska Ranges. By late Friday,
another round of more widespread rain moves toward the AKPEN and
southwest coast as yet another shortwave ejects from the main
circulation over the Bering and moves east. This system lifts
north through the day Saturday as an occluded front associated
with a surface low south of Dutch Harbor brings rain and strong
southerly winds to the AKPEN for the weekend.

&&

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

A low in the eastern Bering will lift north tonight as a cold
front slides across the Bristol Bay and AKPEN. Broad, cyclonic
flow dominating the Bering will keep showers in the picture
through the forecast period. A shortwave crossing the Bering
Friday will increase winds to small-crafts from west of the
Pribilofs east to the mainland coast through the first half of
Saturday.

Farther west, an occluded front will slide across the western and
central Aleutians beginning Friday afternoon with rain and weak
gales. The front will continue its eastward track through the
weekend, with weak gales developing around the AKPEN by Sunday,
as the associated surface low moves to a position south of Dutch
Harbor.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday Night through
Thursday)...

A sprawling low centered south of Dutch Harbor Sunday evening will
be the driver for the weather all across Southern Alaska through
the long-range forecast period. The center of the low will track
northward and move to about halfway in between the Pribilofs and
Cape Newenham by Tuesday morning. Beyond that the model
differences increase markedly. The GFS and Canadian turn the low
center back to the southeast and move it into the southwestern
Gulf by Thursday afternoon. Meanwhile the EC parks the low in the
same place for about 60 consecutive hours through Thursday
evening, gradually weakening as it sits and spins. Along this
track, most of Southern mainland Alaska will be east of the low
center, which means it will be subject to nearly continuous flow
off the Gulf and the Bering. This will support a cloudy and wet
weather pattern with scattered showers likely each day. An
associated front located east of the low is expected to continue
to progress into Southern mainland Alaska on Monday. That will
make it the wettest day of the week and also the windiest day
through the gaps.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 130-137 150 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...RA/JW