Small Craft Advisory 538, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
341 PM AKDT SUN AUG 28 2016

.SHORT TERM...A low to the southwest of Haida Gwaii continues to
push extensive cloud cover over the panhandle from the south this
afternoon. Over the southern zones and as far north as the central
outer coast, there are rain showers as well. Cloud cover has been
one of the more difficult aspects of the forecast today, as wind
direction varies dramatically with height and clouds are advecting
in from different directions. Nice breaks have opened up over the
northern panhandle this afternoon and for a while it appeared that
similar such breaks were going to open up over the central inner
channels. Reliably clear skies from Cape Fairweather north this
afternoon with variable cloud cover over the remainder of the
northern half and occasionally large breaks over the southern
half best describes the situation. General trend for tomorrow will
be for increasing overcast from south to north.

The low southwest of Haida Gwaii will continue to rotate showers
over the southern panhandle tonight. Numerous small vorticity
maximums will reinforce the shower activity and the areal coverage
will expand north through the day tomorrow with most frequent
showers along the coast mountains and southern zones. Locations
west of Cape Fairweather should see another dry day tomorrow. A
vort max moving over the southern inner channels late tonight is
expected to produce locally heavy showers, which will weaken as
they spread north tomorrow morning. No hydrology concerns at this
time.

The Haida Gwaii low will intensify somewhat overnight as it
wobbles around the far southeastern gulf, producing small craft
winds along the northern half of the low - over coastal marine
zone 41 as well as Clarence Strait tonight. Farther north, outflow
winds resulting from a strong northerly pressure gradient there have
already been reported vicinity Alsek River. Have northeasterly
small craft winds in the forecast for coastal zone 51 late tonight
through much of tomorrow as a result. On the inside, the same
strong northerly gradient will produce gusty conditions in
Skagway and small craft winds over Lynn Canal tonight through
tomorrow.

Overnight lows and daytime highs for tomorrow were another vexing
part of the forecast today. MOS guidance is showing significantly
cooler temperatures for tomorrow, but this signature was not found
in either 850mb or 925mb temperature charts, nor low level
temperature advection charts. In the end, opted to remain pretty
close to the existing max temp forecast for Monday - which is
significantly cooler for some locations that today. Yakutat and
Skagway hit 76 degrees as of early this afternoon and Juneau was
at 71 degrees. Do not expect to see such warm conditions tomorrow,
but low to mid 60s will still count as seasonably warm highs for
late August. Overnight lows will be complicated by the variable
cloud cover. The larger the clear area and the longer it lasts
will determine how cool it gets tonight and values range from the
mid 40 to the mid 50s. The situation in downtown Juneau last night
with daytime type temperatures lasting all through the night may
repeat again tonight. Did not go as warm as last night, but
downtown temps are forecast to remain significantly warmer than
out in the Mendenhall Valley.

.LONG TERM...Broad upper trough over western Canada and the
panhandle with an embedded upper low southwest of Haida Gwaii
will continue to influence the weather through Thu. This results
in generally offshore flow aloft for the northern panhandle and
parallel to somewhat onshore flow for the southern panhandle. This
pattern changes Thu into Fri as an upstream ridge over the N
Pacific and Aleutian Islands breaks down allowing a strong 70 to
80 kt jet streak to push E. This results in a much more
progressive pattern as the jet slams into the pacific northwest
with several waves getting pushed through the gulf. Several model
differences regarding the details creep into the forecast here
with differences in strength of the jet streak as well as
differences in timing of the individual waves. So forecast
confidence suffers from Friday onward.

At the surface, the surface reflection of the upper low will
continue to send various short waves into the panhandle bringing
more showers to the area. General flow is from the S to SE in the
southern panhandle supporting showers continuing for that area
through Thu or Fri. Farther north overall flow turns more offshore
due to the pressure gradient between the low near Haida Gwaii and
the area of high pressure in Canada promoting drier conditions. As
a result lowered pops in the northern inner channels mid to late
week. Strongest offshore flow is expected on Mon night into Tue
before the Canadian high weakens mid week. Northerly winds will be
at there strongest at this time with small craft winds expected in
Lynn Canal and some stronger winds out of Cross Sound and the
passes east of Yakutat.

Forecast becomes a little more wet and uncertain into the weekend
as the upper level pattern becomes more progressive. The first
front of what looks like a few will be approaching the panhandle
during the late weekend. However, where these fronts are going and
when is up for debate. Some models are diving the front to the
south keeping most of the panhandle dry while others are bringing
it north and bringing rain to most of the panhandle. As a result
kept chance of rain and mostly cloudy to clouds skies in the
forecast.

&&

.AVIATION...Some low level wind shear and mod turbulence below
the flight levels across the panhandle through the next 24 hours
Large areas of mostly clear skies will give way to increasing
cloud cover with sucker holes. VMC conditions under broken to
overcast skies for the northern half. VMC trending towards MVMC
for CIGs over the southern half. Mostly clear skies west of Cape
Fairweather through the next 24 hours, but outflow winds along the
coast east of Yakutat will make for some particularly bumpy
flying.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-036-041-042-051.
&&

Fritsch/EAL

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