Small Craft Advisory 14, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
525 PM AKST Fri Jan 4 2019

.ANALYSIS AND UPPER LEVELS...
A look at the upper level pattern today shows a building ridge
over the eastern Bering. This ridge is flanked by two troughs, one
over the western part of the state and a second, deepening trough
over the western Aleutians. To the south, a strong jet over the
North Pacific boasts 190 knot winds. While this jet has previously
maintained a generally zonal flow over the last few days, it has
since developed a weak oscillation that has allowed tongues of
energy to wrap into the developing surface low over the western
Aleutians.

At the surface, conditions have been fairly quiet across the
Mainland. Radar indicates a few residual showers falling over
Southwest Alaska and the northern AKPEN, a result of the weak
shortwave that moved through overnight. Folks in areas across
Southcentral (especially in the Copper River Basin) likely
noticed the much colder temperatures and fog development this
morning; a result of the continued cold air advection and a
building ridge moving eastward. Areas in Southwest Alaska also
had fog development, particularly along the coast, as the ridge of
high pressure arrives.

While generally quiet weather persists over the Mainland, a 970
mb low continues to deepen over the western Aleutians as it moves
into the southern Bering. The associated front has advanced over
Adak, bringing southerly flow and rain and snow to the western
and central Aleutians, along with some embedded gale-force winds.

&&

.MODEL DISCUSSION...
Model solutions generally agree through the short-term synoptic
pattern. Divergence occurs by Sunday afternoon into Monday as the
second western Bering upper level low phases with the first.
Overall, forecast confidence remains high, and the GFS solution
was preferred.

&&

.AVIATION...
PANC...More fog and LIFR conditions are expected through Saturday
morning. The fog forecast is a little uncertain, and there may be
occasional breaks of VFR. By Saturday afternoon, the fog is
expected to dissipate as some mid level clouds move overhead,
but once again confidence in this is not very high and the
forecast remains rather uncertain.

&&

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

Areas of fog will persist along northern Cook Inlet, the
Anchorage Bowl, and across portions of the Copper River Basin
tonight as an upper-level ridge crests over the southeast
mainland. Upstream, a weakening shortwave will spread scattered
mid and upper level clouds from west to east across Southcentral
overnight and into tomorrow. Although this weakening system is
benign in terms of sensible weather, it is still providing a
couple of forecasting challenges. The first is whether the
increase in cloud cover will help reduce the fog potential through
tomorrow morning. The second concerns the thickness and areal
coverage of clouds and the limiting effect of radiational cooling
and, hence, the low temperatures overnight across the region. IR
satellite shows warming cloud tops, indicative of the weakening
shortwave. Therefore, current line of thinking is that cloud cover
with this system will generally be on the decrease as it
approaches, which will allow for additional radiational cooling
overnight. Another interesting note is the seeder-feeder mechanism
occurring as the higher clouds move over the fog and deposit ice
crystals into the stable fog layer below. This has helped generate
brief bursts of light snow for locations experiencing fog and has
helped to temporarily improve surface conditions while it is
precipitating. Given both of these, the expectation is for
overnight lows to be colder tonight than last night, even for
locations where patchy fog persists.

The ridge builds back in behind this shortwave while a second
shortwave then moves across the region by Monday. Overall, expect
the cold and calm pattern to continue through the weekend with
bands of higher clouds passing through in association with the
shortwaves. Patchy fog will also return late Sunday into early
Monday as the next ridge crests overhead and weak low level flow
remains. Gusty outflow winds will also continue along the coast
through the weekend with the overall pattern remaining the same.

&&

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

Mid and upper level cloud cover will continue to diminish
overnight as a weak shortwave pushes east, though light snow will
still be seen over the Kuskokwim River Valley through the early
morning hours. High pressure builds in over the SW mainland behind
the departing trough with the ridge axis moving over the west
coast by tomorrow morning. Strong subsidence and surface cooling
will lead to increasing fog coverage over the Kuskokwim Delta and
along the Bristol Bay coast, where visibility could drop to a
quarter mile or less at times tonight and tomorrow morning.

The next system approaches the area by tomorrow evening as an
occluded front sweeps across the region, attached to a parent low
out over the western Bering Sea. The front is expected to fall
apart as it comes ashore, though it will still bring accumulating
snowfall and gusty southerly winds, especially near the coast.
The area will see a quick break as a ridge builds in over the area
out ahead of the next system. It will be deja-vu again as another
front sweeps east across the Bering and approaches the west coast
by Monday evening.

&&

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

The Bering Sea and Aleutians will be impacted by a series of
strong North Pacific storm systems through the weekend. The first
system is already moving across the western Aleutians north into
the western Bering with gale and some embedded areas of storm
force winds seen wrapping around the system. The low center will
loop NW across the Western Bering overnight, while an area of
storm force winds to the north of the low sweeps north across the
western Bering. Cold air wrapping into the low will enhance
westerly surface winds south of the low which should bump up into
storm force category over Attu and Shemya in the morning hours
then taper off through the day. An occluding front will sweep east
across the Bering and impact the Pribilofs and Central Aleutians
tomorrow morning bringing a quick burst of gale to storm force
southerly winds and precipitation, which should remain mostly
mixed or all rain with warm air advecting north ahead of the
front.

Riding quickly on its heels is another, even stronger, North
Pacific low, taking a similar track to its predecessor, though
just a bit further west. Models have been pretty consistent with
the track and strength of this system, dropping it below 960 mb as
it crosses just west of Shemya Sunday morning. A large swath of
storm force southerly winds, supported by a strong upper level
jet, will move across the western Aleutians and Bering Sunday
morning. Currently, the strongest core of winds are expected to be
over the Rat Islands, just east of Shemya, and move east over
Adak and Atka by early Sunday afternoon. Cold air wrapping into
the backside of system could help to enhance wind gusts in these
areas, with hurricane force gusts possible for a short period of
time over Adak. The front will again sweep east across the Bering
and Aleutians Monday while snow showers enhanced by cold air
advection behind the low will develop over the western Aleutians.

&&

.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 mid to late next week. Monday into Wednesday, several
shortwaves will rotate around a low situated over the Bering Sea,
bringing periods of precipitation and increased winds to the
Aleutians. These waves will push weakening fronts toward the
mainland resulting in onshore flow and potential precipitation
over the AKPEN, Bristol Bay, and Y-K Delta areas. Southcentral
will remain mostly dry during this time period, as any approaching
boundary will dissipate by the time it reaches the Alaska Range.
Late Wednesday into Thursday, a potent low from the southwest will
approach the western Aleutians. Models have been fairly
consistent with the basic pattern, but there's uncertainty as to
how strong winds will get in the Aleutians...the low is expected
to rapidly deepen with potential storm force winds, however a
slight southerly shift in its track would indicate primarily gales
for the Aleutians. On that note, models continue to point to the
potential for wetter conditions in Southcentral late next week,
but this will be contingent on the progression of the low
upstream.

&&

.MARINE (Days 3 through 5)...

Monday evening, the Bering Sea will be "quieting down" behind two
strong storm force lows. The remnants of the last storm force low
will be moving into the Bering Strait. This period of time from
Monday night through Wednesday will be characterized by very broad
low pressure with a couple of weak fronts moving in from the
North Pacific. At this juncture, these lows look rather weak and
will likely remain below gale force.

The next front of interest comes late Wednesday night as it moves
north into the western Bering Sea and east toward the Central
Aleutians. This front will likely support strong gales to low end
storm force winds.

Over the Gulf waters, a weak front will move onto the North Gulf
Coast Monday night, but winds will continue to remain below gale
force as the storm track remains well south of the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 178 185.
  Gale Warning 165 170 171 172 173 174 175 176 177 179 181.
  Heavy Freezing Spray Warning 185.
FIRE WEATHER...NONE.

&&

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