Small Craft Advisory 800, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
505 PM AKDT Tue Oct 16 2018

.ANALYSIS AND UPPER LEVELS...
The large anchor-low over the eastern Bering Sea remains
vertically stacked today with the 250 mb jet stream running south
of it and then bending northward into the eastern Gulf of Alaska.
This jet stream has moved eastward over the past day and was the
main source of the strong winds and heavy rainfall over areas of
Southcentral Alaska since then. Areas around the north Gulf coast
and Prince William Sound region saw 3 to 5 inches of rain in a 24
hour period. Today there are just some showers that now persist
in the southerly flow aloft now that the jet stream has moved off
to the east.

&&

.MODEL DISCUSSION...
Models are in good agreement with the low placement and intensity
situated in the eastern Bering and seem do be doing well through
tomorrow night.

Differences arise with the North Pacific low that moves northward
into the Gulf Thursday afternoon. The GFS and NAM are not only in
good agreement with each other, but have been more consistent on
a run-to-run basis as well. The GFS model, in particular, has
been consistent in showing the low with a stronger and more
northwesterly track which fits into the overall pattern the best.
The 12Z EC is starting to catch up with the other models compared
to older model runs, but has been the most inconsistent.

&&

.AVIATION...
PANC...Gusty southeast winds will persist through the evening and
then diminish toward morning. Showers will move through the
region through tomorrow, but should become less numerous
throughout the period. Ceilings and visibilities should remain VFR
even during these showers, though the possibility exists for a
brief drop to MVFR conditions in a shower.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tonight through
Friday)...

The main weather concern through the end of the work week surrounds
the rapidly deepening low that lifts north over the Gulf Thursday
night into Friday. This system is expected to bring storm force
winds around the core of the low along with moderate to heavy rain
along the coastal locations.

Showers will continue to develop across Southcentral and the Gulf
as a weak trough remains over the region through Wednesday. Gusty
southerly winds persist through late tonight, then taper down as
the pressure gradient relaxes along the diminishing front across
the southern mainland. Isolated thunderstorms are possible across
much of the gulf and near the coasts with greater instability over
the area waters through Wednesday night.

The next front moves up along a strong short wave entering the
southern Gulf Thursday morning. The surface low deepens to 985 mb
as it approaches Kodiak Island Thursday afternoon. Gale force
easterly flow across much of the gulf will increase to storm
force as the another low quickly follows Thursday night. The
initial front brings steady rain back to Kodiak Island the eastern
Kenai with rain spreading to the north coast by Friday morning.
Showers will move inland ahead of the front, however strong north
to easterly flow limits rainfall amounts. The heaviest rain is
expected across the eastern Kenai and along Prince William Sound.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Tonight
through Friday)...
A low south of Nunivak Island will weaken as it drifts to the
vicinity of Cape Newenham Wednesday afternoon, then remain nearly
stationary as it continues to dissipate through Thu night. Numerous
weak waves embedded within the flow around the upper low will
combine with broad-scale atmospheric instability to produce
showers. The lack of a strong feature makes it difficult to
pinpoint timing and location of showers. Thus, have largely used
a broad-brush approach to the precip forecast with scattered
showers across most of Southwest Alaska the next few days. Showers
will begin to diminish Friday as the low dissipates and weak
ridging builds in ahead of a deep low crossing the Gulf of
Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Tonight through Friday)...
A weakening low will remain parked near the Southwest Alaska coast
the next few days. Weak cold advection on the back side of this
low will produce a band of gusty gale force northerly winds
across the central Bering Sea on down to the central and eastern
Aleutian Islands. The cold advection will help to focus showers
through this same corridor. Temperatures may be just cold enough
to support snow or a rain snow mix over the northern Bering, but
the remainder of the area will remain safely all rain.

The low and upper level wave pattern will slowly shift eastward,
with a trough moving overhead of the western Aleutians Thursday
night through Friday. With ridging out ahead of this trough the
surface low will remain west of the Aleutian chain, though an
occluded front will track eastward across the western Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7)...

At the onset of the forecast period, the synoptic pattern
features a low near Kenai, a second low south of the Aleutian
chain and a low near Kamchatka. Anticipate a very wet and progressive
pattern. There is good general consensus between the
GFS/EC/Canadian but the best performing model is thought to be the
GFS for this forecast period. Expect back to back lows to bring a
hefty amount of precipitation to Kodiak, Prince William Sound and
the Anchorage Bowl. Even though the third week of October is
considered to be part of the cold season for Alaska, the
temperatures with both storms will remain well above freezing in
the low elevations. Higher elevations on the Kenai and other
portions of South Central can expect a fresh blanket of snow this
forecast period but significant accumulations not expected. The
entire forecast period, the GFS has above normal temperatures at
500 mb from Kenai to the Yukon Territory to Juneau.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood advisory 125.
MARINE...Gales 173-176 412-413.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
LONG TERM...PS