Small Craft Advisory 548, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
311 PM AKST Sun Nov 5 2017

.SYNOPSIS...A storm force front will move into the eastern gulf
Sunday night and remain through Monday night. Strong high pressure
will persist across northwest Canada. Colder air trapped ahead of
the system may result in a narrow band of very light snow
accumulation in the central Panhandle and near Hyder, some of the
first lowland snow of the season.

&&

.SHORT TERM...Storm force front inducing 50 kt winds across the
gulf and in northern Lynn Canal still looks reasonable. Great
model agreement and a solid forecast have allowed us to
concentrate on future questions that have not been fully resolved
yet Monday night into Tuesday night, most notably rain/snow line.

Looking at the structure of the storm, we feel strongly that the
tight gradient and warm air aloft will keep temperatures from
falling too much on Monday night. We have warmed lows during this
time and drawn the rain/snow line northward from previous. As we
have discussed for several shifts, our consensus is that the best
chances for frozen precipitation are where we are least confident
that precipitation falls at all. However, we do acknowledge that
there may be a band of snow on the northern extent of the
precipitation where the colder air is too entrenched to scour out
too quickly. Tenakee looks to be one such spot where an inch could
fall, but potentially Angoon and somewhere along Frederick Sound,
mainly Monday night. Given stronger outflow winds at Hyder, we
feel that it will be the big winner with a good 3 inches Monday
night once the highest lift enters the area.

Early changes include limiting wind acceleration in the north
until 6 AM, and we therefore lowered temperatures overnight
tonight by 5 degrees or so. While we understand the advancing
cloud cover, the very low drops on Sunday morning into the teens
in some places, demanded that we consider mid to upper 20s in some
areas, thus honoring the power of the current air mass.

Strong wind headlines were extended beyond Monday afternoon, where
confidence was high that we will experience them beyond. Given a
few non-events, or over-forecasted ones, we interrogated Downtown
Juneau winds a little longer. Given 925 mb northeasterlies do not
rise until Monday night in the least, we do not think mountain
wave/outflow winds make a significant appearance until Monday
night/Tuesday. So we do not have strong wind headlines for zone
25. Also, mountain wave ingredients come together only on Tuesday
morning.

NAM/GFS was used for any edits and MOS tonight for wind delays.
Forecast confidence moderate.

.LONG TERM... Tuesday through Sunday... A progressive high
amplitude flow pattern aloft will bring a variety of weather
conditions to SEAK this coming week. After the incoming strong
storm system moves south of the Panhandle by mid week we can
expect another round of cool dry weather and locally strong N
winds Wed-Thu. Global models indicate the next chance of rain
will arrive about Fri or Fri night.

The northern branch of the polar jetstream on Tues is forecast to
outline broad upper troughs west of AK and and near Hudson Bay
Canada. In between these troughs a sharp upper level ridge axis
will extend N-S through the interior of AK and W Canada. The
strong low pressure surface and aloft trying to move S past SEAK
Tues is essentially cut off from the steering flow aloft and will
be slow to move S well off the BC coast around 145W. This leaves
SEAK in a N-S pressure gradient pattern which will push more cool
dry air S through the inner channels mid week. Min temps could
fall to at or below 20F Wed and Thurs mornings from roughly Icy
Strait and Juneau north to the border including Yakutat. N-S
aligned water ways like Lynn Canal and Glacier Bay may experience
40 kt winds mid week. Blowing snow may affect driving conds over
White Pass. Temps and winds will moderate farther south.

Longer range forecast models indicate the upper ridging could
persist over W AK and the Bering late this week. This would result
in NW flow aloft across the interior, over the N gulf and into
SEAK again. A new upper level short wave and associated surface
weather front is predicted to ride along this NW flow into the E
Gulf AK Fri and Fri night. The upper trough would cause winds to
change from the dry northerly direction to a somewhat more moist
west direction. The extended forecast for Fri was updated using
the GFS model to show more cloud cover and increasing chance of
rain/mtn snow starting Fri. The latest models also indicate the
track of this system on Fri may stay far enough offshore in the E
Gulf so as to not produce significant precip.

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-033-041-042-051-052.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from Monday afternoon through late Monday night for
     AKZ026-028.
     Strong Wind from Monday afternoon through Tuesday afternoon for
     AKZ027.
     Strong Wind from Monday afternoon through Tuesday morning for
     AKZ022.
     Strong Wind from Monday afternoon through Tuesday morning for
     AKZ023.
     Strong Wind from Monday afternoon through Wednesday morning for
     AKZ018.
MARINE...Storm Warning for PKZ041>043.
     Gale Warning for PKZ012-022-033-035-036-051-052.
     Small Craft Advisory for PKZ011-013-021-031-032-034-053.

&&

JWA/TA

Visit us at http://www.weather.gov/Juneau