Winter Storm Warning 9, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
424 AM AKST Wed Dec 4 2019

.SHORT TERM...Main forecast concerns in the short term continue to
revolve around snow shower activity for the northern Panhandle.

Weak closed upper low continues to spin off the coast of Baranof
Island early this morning as evidenced by GOES 17 WV imagery. This
feature and its attendant surface low will track slowly eastward
across the central Panhandle this afternoon. To the east of this
feature, shower activity continues as cold air aloft and onshore
flow of 20 to 25kt leads to good orographic ascent in a region of
modest PV advection. To the north, a weak frontal boundary marking
the intrusion of true continental air has hung up just south of
the Haines/Skagway areas. Snow has been reported off and on
through the night in Skagway and Haines as some forcing for ascent
from weak mid level deformation and overrunning of the arctic
boundary continues. Areas south of the boundary such as Juneau,
Gustavus, and Hoonah remain in a modified polar airmass with
boundary layer temperatures remaining marginal for snow. Snow
levels look to be between 500 and 1000 ft per obs at elevation and
snow has been reported at many sites when low level mixing has
been inhibited. As the surface low slides south of the Icy Strait
corridor this afternoon, think the surface gradient will finally
relax and thus heavier snowfall rates in the weak deformation
north of the mid level low will give us the best chance for
accumulations near sea level as melting drives the snow level to
the surface. Elected to extend the Winter Weather Advisory for the
Juneau area to account for any accumulations this afternoon.

A brief break this evening into the overnight hours before the
next, potentially more significant system approaches from the SW.
Vigorous shortwave currently over the western Gulf will ingest
moisture from a weak southern stream wave SW of Kodiak and track
into the eastern Gulf by late tonight. Low level SW flow
strengthens in response to this system through Thursday evening.
Should still be in a convective environment with low to mid level
lapse rates near 7 and 8 C/km. Forecast profiles show strong
ascent within the dendritic growth zone and given the amount of
synoptic and orographic forcing, expect some fairly intense
snowfall rates in the showers. Unfortunately it is once again a
question of will the boundary layer be cool enough in the lowest
500 to 1000ft to support accumulating snow? Some modest cooling is
indicated by most guidance across the northern Panhandle in the
wake of todays system, but not much. The true continental air
remains over the far north, in the Haines/Skagway area. With no
true airmass change, think the warm boundary layer will once again
keep the true potential of this system from being realized in the
Juneau, Gustavus and Hoonah areas. Elected to issue a Winter
Storm Watch for the far northern Panhandle including Haines and
Skagway where the low level wind will either be light northerly or
decoupled. Still some timing and track differences in the
guidance which will need to be worked out, but it appears that
this area will have the best shot at 4 to 8 inches of snow with
this system. Should the track shift a bit further south or should
the precipitation rates be able to overcome the warm boundary
layer, winter headlines may be needed for the Icy Strait corridor
as well.

As is usual, snow, especially showery snow remains one of the
harder things to forecast in SE Alaska and thus confidence is not
as high as one would like.

.LONG TERM...Friday through Wednesday as of 10 PM Tuesday/ Friday
begins with the remnants of dissipating low pressure system
providing a chance of snow over the northern and central inner
channels. However, this will mark the beginning of a slow warming
pattern for southeast Alaska through the early part of next week.
The chance of a frontal passage this weekend still exists but
remains uncertain. It ultimately relies on how the ridging pattern
in place over the Gulf evolves this weekend. Thus a chance of
precipitation remains in the the forecast through the weekend with
increased probability along the northeast Gulf coast.
Unfortunately, forecast confidence is low in the extended as large
ensemble variability still exists and run to run consistency
remains poor; thus, little change was made to inherited forecast.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory until 6 PM AKST this evening for AKZ025.
     Winter Storm Watch from late tonight through Thursday evening
     for AKZ018-019.
MARINE...Small Craft Advisory for PKZ022-041>043-051-052.

&&

DEL/JB

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