Small Craft Advisory 403, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 PM AKDT Wed May 13 2020

.ANALYSIS AND UPPER LEVELS...

High pressure has moved into the southcentral Alaska. This is well
depicted on visible satellite, which shows clearing through the
area. Clockwise rotation of clouds can also be observed on the
periphery of the high, giving a good indication of the high center
and ridge axis extending from it. At 500MB, this ridge is
extending into Alaska from the Western continental US.

Out west, a weak low continues to slowly move west into the Bering
Sea. The actual low is just north of our forecast area. However,
cloud cover wrapping around the low is keeping the Bering under
those clouds. Another low just south of the Aleutians is also helping
to keep the Bering and Aleutians chain a little unsettled and
under cloud cover. Both of these lows are inducing easterly flow
across the entire Bering, which is producing offshore flow across
southwest Alaska. This is turn is bringing in dry air from the
interior to the region, clearing skies and calming weather there.

&&

.MODEL DISCUSSION...

Models continue to perform well. They are maintaining excellent
agreement both in time from run to run and between different
models. All weather features are handled well with nominal
discrepancies. Forecast confidence remains high.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through early
Thursday. As a low along the Aleutians tracks eastward and passes
south of the Gulf on Thursday, the surface high will shift to the
northwest Gulf. This will initially set up southerly flow up Cook
Inlet, which will affect the airport sometime after 18Z. However,
the ridge will then shift to the north Gulf coast, educing
development of a Turnagain Arm wind. Thus, expect a wind shift
from southwest to southeast sometime around 00Z (Thursday
afternoon). Winds will also strengthen and become gusty.

&&

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

An area of high pressure will strengthen across western Canada,
with an inverted ridge axis from this high extending across
Southcentral over the next few days. As it does so, it'll continue
to strengthen with another mid-level high developing over the Alaska
Peninsula on Friday. This will keep most locations dry with the
exception being the western and northern Susitna Valley, and the
northeastern Copper Basin. Here, a combination of a few
disturbances aloft crossing the area combined with daytime heating
should allow a few showers to develop. The best chance for this
will actually be across the northeastern basin, as soundings show
convective temperatures being breached, allowing MUCAPEs (one of
many parameters that we look at to determine how buoyant the air
is) to climb into the 250 to 500 J/kg range. However, with lifted
indices staying above zero and a skinny CAPE profile seen, no
thunderstorms are anticipated.

The other items of interest will be clouds and winds. As we head
through tomorrow, the strengthening ridge combined with a low
south of the Gulf will help tighten the pressure gradient. As
such, a Turnagain wind is anticipated to make its way into the
western side of Anchorage, with sustained winds of 15 to 20 mph
possible for a few hours. Wind gusts could also approach 30 mph as
well. Otherwise, the building ridge and a low south of the Gulf
allows a temporary Rex Block to develop. With this pattern in
place, a few vort maxes will cross the region, bringing periodic
bursts of ascent and cooling aloft. This will likely lead to
periods of increased cloud cover.

&&

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

Daytime heating has generated enough instability to produce some
showers over the Kuskokwim Mountains that will continue through
the evening hours tonight. Several upper level waves will pass
over the Kuskokwim Delta area Thursday morning moving from
southeast to northwest. This may bring a slight chance for showers
early in the day, but the best chances for precipitation will
continue to be during the afternoon and evening hours when daytime
heating generates instability. By Friday morning, easterly winds
will increase along the Alaska Peninsula as a front associated
with a low in the Gulf approaches. Showers will also develop along
the AKPEN throughout the day on Friday. Conditions will remain
relatively quiet across Southwest for the start of the weekend.

&&

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

The area of low pressure that has impacted the western Bering
over the last few days has moved south of the Aleutians. However,
a frontal band remains in place across the Chain bringing gusty
easterly winds to the central Aleutians. As the low continues to
drift southward, these winds will diminish through the overnight
hours. Conditions will remain generally quiet across the Bering
for Thursday. An upper level wave over the northern Bering will
move from east to west, generating a few showers before dissipating
Thursday evening. By Friday, another front arrives over the
western Aleutians bringing rain and gusty southerly flow. The
parent low will slowly lift toward the northeast on Saturday and
the front will reach the central Aleutians by Saturday evening.

&&

.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 nearby.
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...BB
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...BB