Small Craft Advisory 11, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
554 PM AKST Thu Jan 3 2019

.ANALYSIS AND UPPER LEVELS...
There is an upper level trough along the Canadian border that is
exiting the area. Showers associated with this trough are tapering
off over the eastern Copper River basin. There is an upper level
ridge building over Southcentral. A weak upper level trough and
associated surface low are along the southwest coast stretching
across the eastern Alaska Peninsula. This feature is weakening as
it moves northeast into the state. Generally cyclonic flow is
over much of the Bering Sea and Aleutians, producing some showers.
There is a rapidly developing cyclone south of the western
Aleutians, just beginning to move northward.

&&

.MODEL DISCUSSION...
The numerical models are in decent agreement through Saturday
afternoon. There are some detail differences in how the front
along the southwest coast splits and falls apart tonight and
Friday, but this has minimal impact on the forecast. There is
still some spread in the models with the low that moves across the
Aleutians into the southern Bering Friday afternoon. However, the
timing is quite similar and the models are getting closer with
each run, so confidence in the forecast is increasing.

&&

.AVIATION...
PANC...LIFR conditions are possible in fog into early Friday
afternoon. There is fairly high confidence in fog development
around the area but somewhat lower confidence in timing and how
far conditions will drop.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through )...

Clear and dry conditions will prevail through the short-term as
an upper-level shortwave over the Copper River Basin and a surface
low spinning east of Middleton Island both continue to slide east
away from the forecast area tonight. A building ridge upstream
will settle in tonight, allowing much colder air from the west to
advect across Southcentral. Weak flow and low-level moisture may
help in the formation of fog as temperatures cool from northern
Cook Inlet into the Mat-Su Valleys and along the Copper River
Basin tonight through tomorrow morning. Visible satellite already
shows a band of fog and low stratus along Cook Inlet stretching
from Knik Arm south to Nikiski.

A second shortwave moving across Southcentral tomorrow night will
spread bands of mid-and high-level clouds across the region. The
increase in clouds may help bump up the overnight low
temperatures, or at least keep them steady, for a time as they
move overhead. Even with these clouds, however, temperatures
across the region will trend much colder than normal and result in
the coldest temperatures of the season thus far.

The other story is the outflow winds along the coast due to the
increased north-south pressure gradient. The gradient, and hence
the outflow winds, will remain through the forecast period, but
will slowly subside through the day tomorrow as a ridge axis
crests overhead. These winds will then ramp up again over the
weekend as the interior surface high strengthens while a trough
deepens in the northern Gulf. In general, though, these winds will
not be extremely strong due to a lack of any well-organized
surface low in the Gulf.

&&

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

An upper level shortwave trough over Bristol Bay continues to
lift residual moisture over the Southwest coast, producing light
and showery precipitation over Southwest. This trend will continue
through the evening hours as the trough progresses eastward
bringing a few inches of snow to the Y-K Delta. Behind this
system, a ridge of high pressure will move in over Southwest
through Saturday. As this ridge builds, fog is likely to develop
over the Y-K Delta and coastal areas along the northern AKPEN as a
result of subsidence, clear skies, and light onshore flow
advecting moisture into the area. Fog could possibly linger
through the late morning hours of Saturday as well. By Saturday
afternoon, a front associated with a strong Bering low moves
eastward, bringing relatively warmer air from the south. By the
time the front reaches the Southwest coast Saturday evening, the
front will be occluded, and enough cold air will already be in
place that despite relatively warmer southerly winds at the
surface, all precipitation will likely fall as snow. As the front
moves inland, precipitation will taper off through the day on
Sunday.

&&

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

An active pattern has returned to the Bering as a strong Kamchatka
low moves in from the west, pushing a large swath of gale-force
winds over the western Aleutians Fri morning. The low center will
continue to deepen some as it moves north across the Aleutians,
leading to an area of easterly storm-force winds sweeping north
across the NW Bering. At the same time, winds will ramp up on the
backside of the low as cold air gets drawn down from the north and
wraps around the low center, leading to a large swath of
northwesterly storm force winds over the western Aleutians Friday
night through Saturday morning.  As the front continues eastward
and reaches and Pribilofs by Friday night, snowfall combined with
strong winds will create blowing snow and the potential for low
visibility conditions for the Pribilof Islands Friday evening into
Saturday morning. The low will lift to the north Saturday night,
causing winds to shift to the south as it departs. Just as this
low departs, another strong low follows closely behind, bringing
amplified southerly flow to the western Aleutians Sunday morning.
The associated low center lands just north of Shemya by Sunday
afternoon, draping a pocket of storm-force winds over the western
and central Aleutians for Sunday. Meanwhile, nearly the entire
western Bering will see southerly gale-force winds beginning
Sunday morning and lasting through the evening hours.

&&

.LONG TERM FORECAST (Days 4 through 7)...
An Arctic high over northern AK and Canada will continue to bring
cold dry conditions and offshore flow to Southcentral and
Southwest AK through early next week. Expect temperatures to be
below climo normal for the first time this winter for most of
Southcentral. The last few runs of the GFS and EC have positioned
the axis of the high over the Yukon, indicating very cold
conditions in the Copper River Basin. Meanwhile, models continue
to indicate a storm force, or high end gale force low impacting
the Aleutians and western Bering Sea Sunday, then winds should
diminish and precipitation will transition to a showery pattern
Monday as the low shifts northward into the central Bering Sea.
Expect a series of weak fronts associated with the low to bring
onshore flow and potential for precipitation to the AKPEN, Bristol
Bay, and Y-K Delta areas early next week. There's uncertainty
with the timing of these waves given the dependence on the exact
progression of that main low, which has differed between various
models and runs. High pressure over Southcentral will erode by
mid next week, bringing increasing chances for precipitation by
the second half of the week.

&&

.MARINE (Days 3 through 5)...

A storm force low will move into the western Bering Sea Sunday
then track north toward Eastern Siberia by Monday. The front will
bring a swath of southerly gale/storm force winds to the Central
Bering Sea before the front exits the north Bering Sea Monday
night. Much quieter weather moves into the Bering Tuesday as the
storm track shifts south.

Over the Gulf waters, the weather regime will remain quiet as
north/northwest offshore winds remain below gale force. A weak
front moves into the Gulf waters on Tuesday, but the front will
be weakening and also currently looks to remain below gale force.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storms...177 178.
		 Gales...170 171 172 173 174 175 176 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/KVP
MARINE/LONG TERM...MM/JA