Small Craft Advisory 60, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 PM AKST Fri Jan 18 2019

.ANALYSIS AND UPPER LEVELS...
There is an upper level ridge over the central and western Bering
Sea. This ridge nudges into the southern Alaska mainland, with
dry conditions and generally clear skies. The exception is in many
of the valleys of Southcentral, where considerable fog and low
clouds are evident. There is an upper level low centered over the
northern Yukon Territory that is slowly moving westward. An rather
weak upper level trough is over southwest Alaska and moving
eastward through the ridge, but not producing any precipitation
over the mainland. There is a rather strong surface low south of
the western Aleutians (central pressure near 965 mb) that is
moving northward. There is a lingering front over the western
Aleutians associated with a dissipating low over the far western
Bering.

&&

.MODEL DISCUSSION...
The numerical models are handling the overall synoptic features
well with good agreement through the short term portion of the
forecast (Sunday afternoon). However, they are struggling with
some of the mesoscale features like low clouds and fog over
Southcentral. Forecast confidence is near normal.

&&

.AVIATION...
PANC...Dense fog will persist due to a strong inversion and light
winds over the Anchorage area. This stagnant pattern will cause
LIFR conditions to remain through Saturday morning. Conditions
will likely improve Saturday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Sunday night)...
A relatively stagnant pattern ensues through the weekend with fog
presenting the biggest challenge as ridging dominates the
southern mainland. The other focus this weekend will be gusty
winds across gap locations along the coast. Overall, drier conditions
continue with cold air advecting toward the coast late Saturday
into Sunday. Scattered showers over the gulf will develop through
Sunday as low pressure is re-enforced with the next diving wave
moving across from the west.

Fog across the Cook inlet is expected to develop further south
overnight edging into Kenai and Homer. Dense fog along the
Anchorage Bowl and locations along Turnagain Arm should improve
by mid day Saturday. This is due to an upper level trough which is
progged to swing across the southern mainland...aiding in mixing
the atmosphere; however, kept a mention of patchy fog through
Sunday afternoon as models have struggled with the timing of the
upcoming synoptic change.

Winds will generally be light across Southcentral except for gusty
gap flow along the coasts. A low over the western gulf this
afternoon drifts south, but a new surface center re-develops
across the northern waters on Saturday. The upper wave on Saturday
will tighten the pressure gradient along the coast with winds
increasing in response. Kept edits conservative for winds as the
surface pattern appears to be relatively weak at this time.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Tonight through Sunday evening)

The main forecast challenge through the period will be patchy fog
development for some interior valleys. This looks to occur as
winds weaken to less than 10 mph for most valley locations, with
increasing heights as a ridge of high pressure strengthens some.
Otherwise, a cold dry pattern will remain in place.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Tonight through Sunday evening)

Another low will pass west of Shemya tonight, with a front
trailing southward. This will bring increasing rainfall with
sustained storm force winds developing west for locations near
Shemya and to the southwest through northwest, with sustained
gales for locations north and east through the central Bering
south and west of the Pribilof Islands, and the central Aleutians.

Winds will then decrease on Saturday to small craft criteria,
before ramping back up on Sunday as a strong low moves northward
through the western Pacific towards the central Aleutians. This
feature looks to bring widespread rainfall with sustained gales
and storm force gusts developing.

&&

.MARINE (Days 3 through 5...Mon through Wed)...

The period will start with a wide swath of gale-force easterly
winds stretching from the Central Bering towards the Western
Aleutians. It is still possible there would be a small swath of
storm force winds through the Central Aleutians, but again, this
would be a small area. Model uncertainty increases rapidly on
Tue-Wed, but it does look like there will be cyclonic flow over
the Bering with a closed high over Siberia. This will likely keep
gale force northeast winds over the Western Bering.

For the Gulf of AK, models continue to grasp at straws. It does
looks like a front will slide into the Western Gulf early Mon
bringing winds up to at least small craft. Another front will lift
through Tue pushing winds to small craft again, but with an
outside shot of some gales. The pattern will remain active through
the middle of the week, though we will not be able to nail down
any specific surface low's location or strength until later.

&&

.LONG TERM FORECAST (Days 3 through 7...Mon through Fri)...

The long term forecast starts out with a surface low situated near
the Central Aleutians, bringing gale force winds across the
Bering on Monday. Models have a similar position, with the GFS a
bit further east. The GFS has been the most consistent model in
the past 24 hours with this system, increasing confidence for
early Monday morning. By Monday afternoon into Tuesday, a weak
low in the eastern Gulf looks to bring stronger winds to the
eastern Gulf and coastal areas, but models have yet to lock on to
an exact position.

Tuesday is when the models start to diverge a bit with the low
over the Aleutians as some models try to split the low.  Things
become messier in terms of low position and winds in the Gulf due
to the model disagreement. For this reason, forecast confidence
will remain low for this system for now. It seems it is about 3
days out when models come into better consensus...so stay tuned.
Regardless of how the low system moves, models all seem to be in
better agreement with cyclonic flow over the eastern
Bering/southwest AK and stronger winds entering the Gulf for
Wednesday.

By Thursday, a highly amplified ridge sits over the ALCAN border,
with a deep trough over the Bering and Aleutians. Models are in
agreement with the big picture, however, they vary vastly on the
development of any system in the base of the longwave trough. In
any case, this pattern is likely to bring warm, southerly flow,
with increased precipitation along coastal areas of Southern AK.
This is a good setup to potentially bring atmospheric rivers to
the area. The middle of next week (21-25 Jan) will see a much more
active pattern.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Fog Advisory 101 125 141
MARINE...Storm 178. Gales 170 173-177 411 413.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...MO/AH