Small Craft Advisory 405, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
406 AM AKDT Thu May 14 2020

.ANALYSIS AND UPPER LEVELS...

A low over central Nunavut Province is currently interacting with
a second low moving westward over the central Bering and causing
an area of stretching deformation across the Alaska Range. This
stretching deformation will result in increase in cloud cover and
likely rain to the northern Copper Basin and Alaska Range
today. Warm temperature anomalies in the stratosphere (10 mb
temperature anomalies) over the Arctic Ocean is helping reinforce
strong upper level high pressure over the North Slope. This upper
level high pressure is enhancing this stretching deformation by
enhancing the northeasterly flow across the northeastern portion
of the state. A third low near 45N 165W is currently moving
eastward through the North Pacific. A thermal trough currently
extends northwestward through British Columbia into Southcentral
Alaska. This thermal trough is currently bringing up-inlet flow as
observations show southwesterly winds inside and around Cook
Inlet.

&&

.MODEL DISCUSSION...

Models are in good agreement with the upper level pattern becoming
more amplified over the next few days. Guidance also shows a high
degree of consistency in the placement and intensity of synoptic
scale features at the surface, thus forecast confidence remains
above average.

&&

.AVIATION...
PANC...VFR conditions will continue to persist throughout the TAF
period. There is a chance for southeasterly Turnagain winds this
afternoon as daytime heating will strenthen the thermal trough
over the Mat-Su Valley and allow Turnagain winds to develop
combined with up-inlet flow. There is still uncertainty on the
exact timing of these winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The basic pattern for the next couple of days will consist of and
upper level ridge over the northern Gulf nudging into
Southcentral, with an upper low over central Alaska. Increasing
moisture and dynamics between these two features is expected over
the northern Copper River basin and northern Susitna Valley,
bringing some precipitation to those areas. Otherwise the southern
mainland will remain mostly dry, though clouds are expected to
increase. At the surface, high pressure will build along the
north Gulf coast today. This will increase the pressure gradient
and bring gusty winds to Turnagain arm and the Knik valley. The
Turnagain winds are also expected to "bend" into Anchorage this
afternoon. The surface high will weaken on Friday and Friday night
and winds will diminish.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Quiet weather conditions will continue across Southwest Alaska
for the remainder of the week and into the first half of the
weekend. However, there will be some ripples in the overall flow
with another stronger upper level wave than the one yesterday that
is expected to pass through Southwest Thursday afternoon into
early Friday. The core of the energy with the wave is expected to
be along the southern Kuskokwim Mountains which is where the best
chance for seeing activity will be. Therefore, likely PoPs remain
in place along the Kuskokwim Mountains. Due to the stronger
nature of this wave and increasing likelihood for activity have
adjusted pops up to chance for much of the Bethel area to Hooper
Bay as well as for Aniak and Sleetmute area with slight chance
pops in place for Dillingham. The activity is expected to begin to
the south of Aniak and east of the Kuskokwim Mountains early
Thursday afternoon before moving westward towards the mountains
and then over into the Bethel area later Thursday afternoon as the
main energy moves overhead. The threat for showers will continue
into the early morning hours Friday before dissipating as the
upper level energy continues moving northwestward. Due to the
daytime heating, longer daylight hours, decent instability, and a
potent upper level wave, this forecast package has the threat of
isolated wet thunderstorms for Thursday afternoon and evening
across the Aniak to Sleetmute area over to Bethel and Hooper Bay.

Behind this upper level wave, an upper level ridge will build into
the region with continued warming temperatures and lower RH
values for Friday and Saturday.

&&

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

Mainly quiet weather will prevail across the Bering and the
Aleutian Chain over the next few days with the stratus cloud deck
remaining mostly in place as the synoptic scale flow remains weak.
This keep the threat for fog and stratus in the forecast for the
Pribilofs although a weak upper level wave moving towards the
Pribilofs currently may allow for the sky to clear for a time
Thursday afternoon and evening.

The next weather system is not expected to impact the western
Aleutians until Friday afternoon and evening with a possible low
end gale force front which the NAM and GFS are both in agreement
with. The ECMWF and Canadian seemed too weak, therefore, a blend
of the GFS and NAM was favored.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

Gulf of Alaska: Weather should be quiet in the Gulf of Alaska to
start the weekend, with no significant weather systems near by.
Late Monday the next round of weather entering the gulf is
expected to bring some weak small craft advisory winds to around
the Barren Islands. We are confident in this forecast.

Bering Sea: A weak frontal system will slowly progress northeast
through the Bering Sea. The low center with this system is
expected to stay in the western Bering. Perhaps some weak small
craft advisory winds are possible with this system, but right now
conditions should largely stay sub advisory level. This is
consistent with yesterday's forecast, so we have increasing
confidence this will remain a weak low and nothing more.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

The long term pattern places two prominent weather features in our
forecast area on Sunday. The first is a ridge of high pressure
pushing north from the Gulf of Alaska into the southern mainland.
The second is a low pressure system located just south of the
western Aleutians. Models are consistent for the second day in a
row depicting this pattern. They are also in fairly good agreement
with each other in the placement of these systems.

One feature they are struggling to resolve is a trough pushing
south through eastern Alaska Sunday into Monday. We will need to
see some better model depiction of how this front moves and
interacts with the ridge over Southcentral in the coming days.
This is one area of the forecast that will likely need refinement.

As models progress the previously mentioned high and low through
next week, high pressure pushes into Canada and southcentral AK
moves to being under low pressure. The Bering remains generally
under low pressure the entire time, save for a quick moving
shortwave ridge pushing east along the Aleutian Chain early next
week. Models also generally agree on the progression of these
systems. With both placement and progression consistent between
models and maintaining continuity since yesterday, we have
increased confidence this will be the weather pattern going into
next week. This would put confidence moderate to high at this point.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...BB