Gale Warning 317, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
1050 PM AKST Fri Dec 11 2020

.UPDATE...For some warm aloft spreading northward across Gustavus
and persistent showers associated with a weak boundary lying
across Icy Strait we updated Gustavus with a winter weather
advisory for freezing rain and light snow through this morning. We
also increased snowfall accumulations in Juneau for a total of 2
to 4 inches overnight. We do have high confidence snowfall eases
towards morning but could not ignore the strong banding with
ratios of 18:1 across the Capital City. Thicknesses though should
keep it all snow unlike in Gustavus. Otherwise we did back off the
advent of stronger winds on Saturday, pushing it back a few
hours.

&&

.PREV DISCUSSION......ISSUED AT ...301 PM AKST Fri Dec 11 2020

SHORT TERM...Friday evening through Monday... A passing trough
to the north combined with weak onshore flow has kept snow showers
around the Icy Strait corridor through the afternoon and evening
hours. With temperatures just above freezing, additional snow
accumulations will be around an inch or less. Snow showers will
begin to diminish in coverage and intensity late tonight as flow
aloft takes on a more easterly component, drying out the
atmosphere.

The next big weather system to affect the region moves north out
of the N Pacific and into the southern gulf on Saturday. At the
same time, a strong northerly pressure gradient sets up across the
Panhandle leading to an increase in northerly winds through the
day. Expect small crafts to develop in the Inner Channels from
north to south late Saturday afternoon and continue through Sunday
night. Lynn Canal will see the strongest winds from this outflow
event as gales develop by late Saturday night. Strong wind gusts
to 40 mph are possible in Juneau, Haines, and Skagway during this
time frame as well. This pattern is marginally favorable for
mountain wave development with strong cross barrier flow and an
inversion setting up just above the mountain tops. Looking at
time height cross sections for Downtown Juneau, the critical
level remains quite low for mountain wave development. Guidance
shows the CL around 850 mb, while the optimal location for it is
between 500 mb and 700 mb. Nonetheless, strong wind gusts to 40
mph will still be possible due to the tight pressure gradient even
if a mountain wave doesn't form.

As the low moves NNW through the gulf, an occluded front will
push northward into the Panhandle Sunday evening through Monday.
North of this front, sufficient offshore flow should keep most
areas cold and dry. Flow turns southerly behind the front
allowing for moisture to stream up and over the colder air at the
surface. Overrunning events like this typically are good patterns
for snowfall across the Panhandle. Cold air (below freezing)
across the north half of the area and along the Coast Mountains
is expected to remain in place through at least Monday morning as
a northerly pressure gradient remains at the surface. Currently
thinking that rain will develop along the front in the southern
Panhandle Sunday evening. Rain will change to snow as it moves
northward through the night. QPF amounts look fairly light with
this front with total liquid amounts generally less than a half
inch. As far as snowfall amounts, minor accumulations up to a few
inches possible in the Central Panhandle Sunday night, mainly for
Petersburg and Wrangell. Snowfall will continue into Monday as the
front progresses northward with accumulating snow expected along
the Icy Strait Corridor and Upper Lynn Canal area. A couple inches
to advisory level snow amounts look like the most probable
solution so far for areas across the north. The EC EFI currently
does not highlight any portion of the area for extreme snowfall
for this event. The NBM 50% probabilistic 24 hr snowfall amounts
range from around 3 to 5 inches for the northern half of the
Panhandle 06z Monday to 06z Tuesday. Confidence is good in the
current snowfall forecast for this time frame; however more fine
tuning will be done in the coming days for this event.

LONG TERM...Sun through Fri as of 10 PM Thurs...
The next area of low pressure is expected to swing into the Gulf
of Alaska during the second half of the weekend. The models have
slowed the progression of the low down a bit, so the latest
forecast has the rain moving into the southern regions late Sunday
with the central and northern regions having the moisture arrive
Sunday night. Temperatures on Sunday look to be on the cool side,
with morning lows mostly in the 20s - teens in the far north and a
few 30s in the far south. Afternoon temps are expected to be in
the 20s and 30s.

Coinciding with this will be a northerly outflow event bringing
breezy conditions to much of the area. Winds have been increased
for the northern and central inner channels, especially in Lynn
Canal, Stephen's Passage, and Downtown Juneau with 30 to 40 kt
expected through the northern Inner Channels. We are watching a
possible mountain wave event for the Juneau area, however, not all
the ingredients will be in place. Nonetheless we added gusts to 40
and 45 mph Sunday for Downtown Juneau and Douglas. Winds in the
gulf will also be on the stronger side with easterly gales
expected.

With a little more agreement in the deterministic models heading
into next week, we are starting to piece together an idea of how
next week will start out. As the low moves north late Sunday and
Sunday night, much colder air will move into the panhandle from
the north. This would mean snow in the far northern areas while
still rain in the south. The question to answer over the weekend
will be how far south that cold air will move. That will be the
key to knowing whether rain or snow will be found early next week
in the central panhandle.

As far as the rest of the week goes, model ensembles keep a
generally wet active pattern in place with temperatures on the
cool side expected for five days out, but uncertainty beyond
Monday is high.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind Saturday afternoon for AKZ025.
     Strong Wind Saturday afternoon for AKZ023.
     Winter Weather Advisory until 6 AM AKST Saturday for AKZ020.
     Strong Wind from 9 AM AKST Saturday through Saturday morning for
     AKZ018.
MARINE...Gale Warning for PKZ041-042.
     Small Craft Advisory for PKZ012-013-022-031-033-036-043-051-052.

&&

JWA/CM/GJ

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