Small Craft Advisory 406, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...
A rather benign synoptic pattern continues to unfold today. Much
of the southern mainland of AK is caught between systems and
remains mostly under the influence of weak ridging. The primary
weather makers are an elongating deformation axis stretching from
west to east along the Alaska Range, a broad area of low pressure
in the North Pacific, and a weakening low over the Central Bering.
For the majority of the area, this made for a very quiet morning
with just passing mid-high clouds in much of Southcentral. The
deformation axis along the AK Range is producing widespread cloud
cover and some light rain. There was some stratus/fog near the
Gulf Coast and over portions of interior SW AK. The low over the
Bering is making for some light rain west of the Pribilofs, but
winds remain very light.

There is no jet activity in the upper levels (300 mb-30,000') over
the southern mainland. The best activity is associated with an
extraordinarily strong high pressure center over the Arctic and
some deep cold air over Northern Canada. The high over the Arctic
has an unusual nearly concentric jet around it at about 80 kts. A
few weak fingers at about 60 kt will try to usher some of the
energy down from Canada into Eastern AK/Copper River Basin over
the next couple of days.

&&

.MODEL DISCUSSION...

Model agreement remains high this afternoon with no significant
weather in place over Southern Alaska. In the short term, cloud
cover and its associated impacts such as the land-sea breeze will
remain a forecast challenge. There is also a chance for isolated
showers over the high terrain, particularly over the eastern
Alaska Range. However, models are in agreement that showers should
remain over the high terrain underneath weak upper level steering
flow. Overall forecast confidence remains high.

&&

.AVIATION...
PANC...VFR conditions will persist through the TAF period. The
pressure gradient is setting up to bring gusty southeast winds
off the Turnagain Arm this afternoon through the evening. The
exact timing and strength are still somewhat uncertain, with up
Inlet south-southwest winds dominating prior to the onset of
the southeasterly Turnagain winds.

&&

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

Thus far, the Turnagain winds haven't made it into Anchorage.
Model guidance continues to show both the Turnagain and Knik River
Valley winds developing though, so we'll leave them in the forecast
while tempering them down some in strength. A few showers may also
develop along the Alaska Range in the northern Susitna Valley.
Slightly better coverage is also possible across the northern
Copper River Basin, as this area will be sandwiched between an
area of high pressure that briefly develops over the Kenai
Peninsula for Friday evening into Saturday, and a low retrograding
from Fairbanks towards Siberia, and another low moving from
northern Canada to the Yukon. This allows a shear axis to develop
across the northern Susitna Valley and Copper Basin. Lift from
these features and increasing moisture in an upslope flow regime
may allow a few showers to develop. Thunderstorms aren't
anticipated though as instability is meager.

For Kodiak Island, a disturbance seen in both model and satellite
data is approaching from the south. This may allow a few showers
to develop through Friday morning. Then, as high pressure builds
across the Kenai, glancing effects from this high should decrease
any onshore effects, with mainly dry conditions developing by
Saturday.

Saturday night into Sunday is where things get slightly more
interesting. The GFS model continues to be the most aggressive
with dropping energy south towards the northern Copper Basin,
while other numerical guidance shunts this off further to the
east. At the same time, a weakening shortwave trough approaches
Southcentral from the west. This induces a southwesterly flow
aloft from The Mat-Su valleys and Kenai Peninsula westward,
allowing for increased cloud cover with the precipitation
potential increasing for these areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Quiet conditions once again dominate Southwest Alaska this
afternoon. A series of weak upper level waves are tracking
overhead, which will lead to extensive cloud cover. Precipitation
on the other hand, will be minimal, with just a few showers and
possibly a thunderstorm this evening from the Kuskokwim Valley to
the Kuskokwim Delta.

Chances for precipitation taper off overnight tonight into Friday
as an upper level ridge begins to move from Southcentral into
Southwest Alaska. This will bring more stable conditions under
the high pressure system, along with dry weather through the
beginning of the weekend. Offshore flow and warm temperatures
will continue over the next few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Weather remains calm and quiet across the Bering today as a North
Pacific low keeps any significant weather south of the forecast
area. The stratus cloud deck will remain in place, with the
potential for fog development for the Pribilofs again tonight.
This quiet pattern will persist through Friday afternoon when the
next strong system approaches the western Bering and Aleutians.
A small swath of low end gale force winds and widespread rain
will accompany this system as it slide over the central Bering
and Aleutians by Saturday afternoon as it begins to weaken.

&&

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

The mid-range forecast for the marine zones looks fairly quiet.

Gulf of AK: A weak front will linger over the extreme southern
Gulf of AK on Sat. This will support small-craft easterly winds
early, but these will quickly diminish through the day. The next
feature will be a front approaching the area from the Alaska
Peninsula on Mon. At this time, this front looks to support at
least small-craft winds, but could trend stronger as it gets
closer in time.

Bering Sea: Initially, the strongest system will be located over
the Western Aleutians. It will contain a gale-force (easterlies)
front that will slowly drift north on Sat. A much stronger, bigger
system will develop late Sun-Mon along the Central Aleutian chain.
It will bring at least small-craft level winds. However, there is
some indication that it could support gales. This system will
traverse along the Aleutian chain going into early next week
before it moves into the Gulf of AK.

&&

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

To start the long term on Sunday, the primary feature is a broad
low pressure system over the central Aleutian Islands. Other
notable features include a weak ridge over the Gulf, a deformation
zone over the Alaska Interior, and a cold air trough over the
Canadian Northwest Territories. The main weather impacts from the
Aleutian low will be widespread precipitation over the Aleutian
chain Sunday, extending up into the AKPEN and southwest mainland
Monday morning as the ridge over the Gulf breaks down. Winds are
expected to remain on the lower end due to the broad nature of
this storm. As the low slowly occludes and pushes inland,
widespread precipitation due to southerly moisture advection is
expected over Southcentral Monday night into Wednesday evening.
Models remain consistent with the evolution of this feature with
the heavier precipitation favoring coastal areas and the high
terrain.

Models continue to struggle with pulses of upper level energy off
of the backside of the trough over the Northwest Territories.
Both the intensity and propagation of these features should be
monitored as it may aid convection near the Alcan border and along
the eastern Alaska Range. The GFS and EC remain consistent within
their own model solutions, but differ greatly between the two
models. The GFS has failed to adequately resolve these features
over the past few weeks so guidance will likely tend towards the
EC, but the pattern should be monitored closely.

Moving into midweek, models indicate the next system arriving in
the western Aleutians Tuesday morning. Currently there is a high
degree of uncertainty in both the strength and location of the
surface low. However, despite their discrepancies the impacts of
the system will be gusty winds and widespread precipitation over
the Aleutian chain through Thursday night. Southerly moisture
advection over the Akpen and southwest mainland is expected to
begin late Wednesday night and into Thursday morning. Forecast
confidence remains high at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 177.
FIRE WEATHER...None.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ/MSO
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...CJ/MSO