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254
FXAK67 PAJK 190046
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
346 PM AKST Wed Jan 18 2017

.SHORT TERM...Very difficult forecast over the next 24 hours.
Therefore, confidence is lower than normal. A gale force low
currently over the eastern gulf will move north and onshore near
Kruzof Island this evening. The low will continue to move north
over the northern panhandle late tonight while rapidly weakening.
The track of this low has set up two different weather regimes
over the area today. From Sitka and Angoon southward, strong
southerly flow has led to temperatures in the 40s with light to
moderate rain falling. North of there, northerly flow has
resulted in temperatures ranging from the upper 20s in Haines to
hovering around freezing along the Icy Strait corridor. Confidence
is high that a significant accumulating snow event will unfold in
Haines and Skagway through late tonight with accumulations from 4
to 9 inches before precip diminishes Thursday morning. The area of
most uncertainty lies to the south along Icy Strait including
Elfin Cove, Pelican, Gustavus, Hoonah, and Juneau. These areas
have a marginal thermal profile to support snow, but concern is
very little low level flow combined with impressive lift and
precip rates could be enough to produce an isothermal layer around
0 C. This would result in a prolonged period of heavy, wet snow
and the potential for significant accumulations. We are already
seeing this occur in Gustavus and confidence is increasing that it
will continue this evening for that area. Confidence is high that
the Juneau airport and south to downtown will see precip fall as
mainly rain with little if any accumulation. Less confidence for
the Mendenhall Valley and north where rates may be enough to
overcome the thermal profile. However, if the rates don`t
materialize, mainly rain will fall and snowfall amounts would be
minimal. A degree or two is going to make the difference between
accumulating snow or rain. Same goes for Elfin Cove, Pelican, and
Hoonah. Refer to the Winter Storm Warning/Winter Weather Advisory
statements for additional information.

As the low tracks inland, winds will shift southwesterly behind
it. Significant cold air advection will occur behind the low on
Thursday. A trough will drop across the northern gulf Thursday
afternoon with snow shower activity increasing across the central
and northern panhandle ahead of it. Expect an additional inch or
two of accumulation.

Used the NAM/GFS for updates to the inherited forecast.

.LONG TERM...Remnants of weak trough moves over the northern and
central panhandle into British Columbia Thursday evening before
building high pressure over the gulf transitions to a dry cold
pattern through the weekend. May be just enough lift and energy
with the wave for a quick hit of snow showers for the Juneau area
and accumulations of 2 to 4 inches possible. However as the
system will be fast moving and dendritic growth zone not
especially deep will need to delve more into slantwise convection
along the coast mountains to get needed accumulation.

With an arctic high also forming over the Yukon and a low over
the NE Pacific the tightening pressure gradient will produce
strong northerly and outflow winds. Getting gradients of 10-13 mb
between PAJN-PAGY and PAJN-PAKT. Nearing max gales for the
northern inner channels and high gusts out of channeled terrain.
Due to colder air keeping mention of freezing spray. Favorable
setup for Taku wind event with well formed inversion and critical
level. Cross barrier flow not as impressive in last model runs
compared to previous events so holing off on gusts much higher
than 50 mph. The pattern shift does drop temps for the northern
areas into the teens, mainly for wind sheltered areas, but
remaining warmer for locations with down slope winds. Model
guidance indicated could get even colder temps, but as 850 mb
mean standard anomaly temps were only a few degrees below average
for a relatively short period did not go as low as models at the
surface, yet.

Through next week a shift back to a warm wet pattern as next low
moves up from the pacific and into the central gulf. This system
is pulling off the to west with latest model runs which may keep
precip over the gulf while still warming the panhandle, thus
limiting chances for an over running snow event. Next deep low
moves south of the Aleutians Monday into Tuesday with the
associated gale force front crossing the gulf and panhandle for
the start of the week.

Nudged winds through the weekend with a blend of NAM/GFS which
increased outflow winds a bit more. Dropped some temps,
especially at higher elevations but otherwise forecast was still
on track through mid range. Lots of model spread for the start of
next week, therefore kept with an ensemble blend using current
WPC. Good forecast confidence initially but then a sharp drop for
next week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory until 6 AM AKST Thursday for AKZ018-025.
     Winter Storm Warning until 6 AM AKST Thursday for AKZ019.
     Winter Weather Advisory until 3 AM AKST Thursday for AKZ020.
MARINE...Small Craft Advisory for PKZ012-013-021-022-031>036-041>043-052.
     Small Craft Advisory due to Seas for PKZ051.
&&

$$

TPS/PRB

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