Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 191413
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
513 AM AKST Sat Jan 19 2019

.ANALYSIS AND UPPER LEVELS...
A quiet weather pattern has descended across most of the forecast
area this morning as an upper level ridge extends over the
central and eastern Bering. This is bringing dry offshore flow
over most of the mainland leading to mostly clear skies and cold
temperatures to the area. Strong subsidence and a surface
inversion is leading to widespread fog and stratus over the Copper
River Basin with dense fog continuing over the Anchorage area,
the Knik Arm, and filtering down the Cook Inlet. The only action
is seen over the far western Aleutians as a strong North Pacific
low is pushing a front north across the islands with scatterometer
data showing widespread gale force winds with embedded storm
force winds impacting the area.

&&

.MODEL DISCUSSION...
Models are in good overall agreement through the end of the
weekend, now all locking onto a North Pacific low moving north
towards the Aleutians by Monday morning. The EC/GFS are in lock
step with the low after Sunday keeping it just south of Atka while
the 06Z NAM has deviated from the other 2 models, bringing the
low north into Bristol Bay. This solution was thrown out as the
outlier. Forecast confidence is near normal.

&&

.AVIATION...
PANC...Dense fog will persist through the early afternoon as a
strong inversion is still in place over Anchorage. A slight shift
in the pattern will lead to the fog bank ebbing and flowing over
the Anchorage Bowl with conditions expected to switch between
LIFR and MVFR conditions. In the afternoon, visibility will
slightly improve, maintaining the patchy nature of the fog bank
over the terminal. Overnight on Saturday, fog is expected to
return as the surface inversion redevelops.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Sunday night)...
A weak upper level short-wave trough will drop southeastward
across Southcentral this afternoon through tonight, bringing weak
cold air advection. Expect the increase in low level flow will
create just enough vertical mixing in the boundary layer to erode
some of the dense fog currently sitting over Anchorage, Turnagain
Arm and parts of the Matanuska Valley and western Kenai Peninsula.
Thus, expect improving visibilities by this afternoon. However,
do not expect winds to mix down to the surface for most areas, so
a weaker surface-based temperature inversion will persist and
suspect at least patches of fog will remain around into Sunday.

A stronger short-wave will drop down into the northern reaches of
Southcentral tonight (up along the Alaska Range), bringing notably
colder air to inland areas. For locations which are cloud and fog
free, this will lead to quickly dropping temperatures tonight.
Meanwhile, the weak cold advection will combine with tightening
pressure gradients to strengthen gap winds along the Gulf coast
and Prince William Sound.

As the short-waves continue off to the east on Sunday, a reverse
in flow will ensue, as a short-wave ridge builds up from the Gulf
followed by an upper trough and frontal system moving up from the
Pacific. This front will approach Kodiak Island Sunday night,
bringing rain and snow along with strengthening winds. Clouds will
gradually increase across Southcentral Sunday night into Monday.

&&

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

High pressure over the Bering combined with an Arctic high inland
will cause offshore winds and cold conditions to continue into
Monday. During the day Monday, a front associated with a low
shifting towards the central Aleutians from the North Pacific,
will bring snow and increasing temperatures in southerly flow.

&&

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

A storm force low is nearing the western Aleutians, and winds
have already gusted near 50 kt in the coastal waters based on the
10Z ASCAT pass. Besides storm force winds in the western
Aleutians, this low will bring gales to the western Bering, and
precipitation to the western and central Aleutians. It will then
shift northwest out of the area. Another low will push in from the
North Pacific later today into early Sunday. Initially it will
bring gales to the central Aleutians, then expand to a large
swath of gales and precipitation over much of the Aleutians and
the Bering Sea by midday Sunday. With cold air in place, expect
precipitation over the Aleutians to initially be snow or a
rain/snow mix, then transition to mostly rain over the central
and eastern Aleutians Monday.

&&

.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 uncertainty
will remain 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 101 111 125 141.
 MARINE...Gales 170 172 173 174 175 176 177 179
          Storms 178
          Heavy Freezing Spray 127 150 160 165 180.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA/MM
MARINE/LONG TERM...MO/AH


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