Winter Storm in Alaska, December 6 to 8, 2016

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
537 AM AKST Tue Dec 6 2016

.SHORT TERM...A period of snow for the northern panhandle is the
main weather headline for the next 24-30 hours. A weak area of
low pressure over by Kodiak Island is spreading several bands of
showers across the gulf and northern gulf coast at the moment
(Yakutat started snowing around 4 am.) Those bands will continue
to spread across the gulf and into the northern panhandle today
with snow showers continuing through Tuesday night. Meanwhile the
southern panhandle east of Chatham Strait and south of Frederick
Sound will see little or no precip out of this as shifting upper
level flow will stall the eastward progress of the precip Tuesday
night

Snow accumulations for the northern panhandle are rather tricky
to pin down as the more convective nature of the precipitation
will cause locally higher and lower amounts. Overrunning of the
cold air that is entrenched in the panhandle will also become a
factor enhancing snow totals even more especially just to the
north of the area where the developing southerly winds run into
the weakened northerly outflow. Forecast right now has that
convergence zone most likely developing from around Elfin Cove
and Pelican east northeast to Gustavus and Berners Bay Tuesday
night resulting in a band of heavier snow expected there. Decided
to issue some winter storm warnings for Gustavus and Pelican for 7
to 15 inches of snow starting this afternoon and continuing
through early Wednesday morning with the heaviest snow rates
expected Tues night. However, if this band decides to develop any
further south or north, areas not currently in the warning could
receive more snow than what is forecast. Speaking of other areas,
most of the rest of the northern panhandle is under winter weather
advisories for snow accumulations of around 3 to 6 inches Tuesday
night.

Winds will not be as big of a factor as the snow through tonight.
Northerly outflow winds have diminished to the point that there
is not even a small craft headline for northern Lynn Canal any
more and gusts out of other passes and inlets have diminished as
well. In fact, the only marine hazards due to wind are in the gulf
as the southerly flow creates a weak barrier jet along the coast
this afternoon and evening. Expected winds are only going to be
around 30 kt at most.

Models of choice focused on the higher res Nam model to try and
get good placement on the heavier snow rates from the convergence
zone. Agreement between the various models was sound on this
feature though they differed on if the band would influence the
Haines area or not. Current thinking has most of the Haines
Highway in the shadow of the Chilkat Mountains with the highest
accumulation possible around Haines and Mud Bay.

.LONG TERM...Transitions back to dry cold weather pattern as snow
bands pull back to the west. Low pressure system in the western
gulf continues to weaken to an open wave and track to the south
and west through Late Wednesday / early Thursday. Next potential
systems for the weekend are now tracking further to the south,
near Vancouver island, limiting precip impacts on the panhandle.
There will still be an increase pressure gradients with potential
Gales or higher for inside channels. High pressure remains over
the Yukon and British Columbia keeping cold air in place.

As the gulf low pulls back west, mid level short wave extends
over the central panhandle pushing north. This will keep shower
activity lasting into Wednesday morning before a sharp decrease in
pops. Snow for Yakutat will last through the day Wednesday before
the switch to the expected dry weather pattern for the rest of
the week. There is a mid level wave extending down from a arctic
low, crossing the Yukon then panhandle Saturday into Sunday. Not
getting much surface reflection from this feature yet just cloud
cover, but something to watch. Still have model variation on QPF
but for now the tail end of the Wednesday system should only
produce a few more inches of snow.

As the week progresses setting up for more outflow wind events and
increasing inner channel marine winds due to northerly pressure
gradient. Once again overnight temperatures dropping into the
single digits later in the week. Potential to remain in this cold
dry pattern for an extended period if current model trends
continue.

Besides QPF early on main model differences are later in the
forecast with depiction of outflow winds over the gulf. GFS most
aggressive on wind speeds. Used a blend of 12z NAM/GFS then
GFS/ECMWF for pressure and winds. Forecast confidence is average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from 6 PM this evening to 5 AM AKST
     Wednesday for AKZ025.
     Winter Storm Warning from noon today to 9 AM AKST Wednesday for
     AKZ020-022.
     Winter Weather Advisory from 9 PM this evening to 9 AM AKST
     Wednesday for AKZ019.
     Winter Weather Advisory from 3 PM this afternoon to 9 AM AKST
     Wednesday for AKZ021.
     Winter Weather Advisory until 9 PM AKST this evening for AKZ017.
     Wind Chill Advisory until 9 AM AKST this morning for AKZ018.
MARINE...Small Craft Advisory for PKZ021-022-041>043-051-052.
&&

EAL/PRB

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