Winter Storm in Alaska, December 10 to 11, 2018

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
301 PM AKST Sun Dec 9 2018

.SHORT TERM...Very active weather continues for SE. We have one
system pushing through this afternoon with moderate to heavy
showers and gusty winds occurring. Late tonight another front will
cross the gulf with winds increasing to 40-50kt over the outside
waters and heavy rain. A wave developing along the front will
work to enhance it as it moves onshore Monday morning. Have
widespread strong winds (40-50mph gusts) expected for majority of
the panhandle as the front passes through the day. This front
again looks warm, so near sea level will see all rain. However,
White Pass should remain just cold enough for another round of
heavy snow. Have 12-20 inches over a 24hr period for there,
encompassing Monday.

Once this front passes, winds will shift to the SW with cold air
advection behind it. Expect a transition back to showers with the
CAA and even some snow showers. Ocean effect rain and snow
showers will continue through Tuesday night with some minor
accumulations possible.

Adjusted wind speeds up using primarily the NAMnest. Some
uncertainty remains with timing and strength of waves along the
front, any adjustment to these would mainly impact placement of
strongest winds. Confidence in categorical rain or showers is
above average, going to be a wet couple of days with negligible
breaks.

.LONG TERM.../Tuesday through Sunday night/...Very active long
range forecast period. Extended range forecast begins with a
large and fairly broad negatively tilted upper trough oriented
northwest to southeast across the Gulf of Alaska. Models in very
good agreement on this and all significant jet energy begins this
period well to the south. A progressive upper ridge will displace
the trough to the east and come to lie over the eastern gulf by
Wednesday morning. This feature is then also displaced to the east
as an upper short wave moves into the gulf on Wednesday night. A
short lived upper low will form, but this will get ejected to the
north or become absorbed by another, more sharply defined upper
trough Thursday afternoon and continues to the east into Canada by
Friday afternoon. Once more upper ridge moves over on Saturday and
by Sunday, it looks as if a large upper low will set up over the
gulf and remain in place into next week.

At the surface, model agreement not nearly as good. All agree at
the start that there will be a large, complex area of low pressure
than spans from the western interior, east into the Yukon, and to
the southern Gulf of Alaska. The difficulty is that numerous
surface circulations are embedded within the larger complex and no
two models are in agreement on precisely where those surface
circulations are at the start or where they will go/how they
evolve over the course of the upcoming week. This is no trivial
problem either, as the various model solutions have yet another
storm force low crossing the southeastern gulf on Tuesday. Beyond
that, at least three more high end gale force fronts on Wednesday
evening, Saturday, and Sunday will affect the eastern gulf and
Southeast Alaska.

Very gradual cooling of the low levels is also forecast for the
upcoming week with overnight lows getting down to freezing or
below for nearly all populated locations by Thursday night. Thus,
there is the possibility of some snow at the lower elevations.
This is not a certainty, since the pattern, with fronts coming in
from the west, is not a reliable snow scenario. Time will tell.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 3 PM AKST this afternoon for AKZ026.
     Strong Wind from Monday morning through Monday afternoon for
     AKZ021-023-025>027.
     Strong Wind from Monday morning through Monday afternoon for
     AKZ017-024-028.
     Strong Wind from Monday morning through Monday afternoon for
     AKZ022.
     Winter Storm Warning from 3 AM Monday to 5 AM AKST Tuesday for
     AKZ018.
MARINE...Storm Warning for PKZ041-042.
     Gale Warning for PKZ021-022-031>036-043-051-052.
     Small Craft Advisory for PKZ011>013-053.

&&

Ferrin/Fritsch

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