Heavy Snow in Alaska, January 6 to 7, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
641 AM AKST Mon Jan 6 2020

.SHORT TERM...A front pressing into the northern Panhandle has
been responsible for a few hours of heavy snowfall from the coast
all the way to Juneau. Given the timing, we have heard very little
in the way of ground truth. However, a quick 1.5 inches fell in
Juneau's Mendenhall Valley with potential higher amounts to the
west, as the system has weakened as it moved ashore.

What looks concerning on satellite is a potent gale force low that
will commence a period of overrunning moisture for portions of
the northern Panhandle. Although not a classic overrunning event
as there is some instability involved, there is just enough warmer
air ushered by the front to promote some lifting over the cold
air dome across the Panhandle. A consensus of models pushed QPF
numbers higher than forecast through the Icy Strait Corridor, and
leaving snow ratios alone at anywhere between 12:1 to 15:1, we
upgraded the winter weather advisory for Juneau to a warning for 5
to 10 inches of snowfall from noon to 6 AM Tuesday morning. We
also posted warnings for Gustavus for 4 to 8 inches and Pelican
for 5 to 9 inches. Not to be completely outdone, we have
advisories for Yakutat (which may get warning rates this morning)
and Hoonah. Plenty of southwest flow in the low to mid- levels
along with good orographics both in Pelican and Juneau favor
heavier snow amounts. And many of the models push higher omega
amounts into portions of the snow-growth zone. Some models, namely
the high res and NAM pushed snowfall potential well higher than
forecast, but as there could be a narrow band of heavy shower
training, we felt irresponsible to go that high, but clearly, the
potential for someone in this region receiving a foot or more is
not completely out of the question, depending on where this
presumed band sets up.

In the wake of the first front moving into BC this morning,
ridging will help dry things for the southern Panhandle tonight
into Tuesday as the action clearly is directed across the north.
The question for the north, is after the deeper system moves into
the northern Panhandle tonight, northern gulf low pressure may
keep sending snow showers into the northern Panhandle for possibly
two or three days beyond. Currently warnings go through through 6
AM Tuesday, but extensions are possible.

Winds have been relegated to second-fiddle today. We have
increased westerlies this afternoon for Cape Spencer to small-
craft levels and enhanced the winds through Stephens Passage, in
line with the system approaching from the shore. We also backed
off some of the earlier stronger winds through Lynn Canal.

We used ensembles for changes.  Forecast confidence is average.

.LONG TERM.../Wednesday through Sunday/ As of 10pm Sunday. An
upper level trough and assoc vort max moves southward out of
interior Alaska and crosses Yakutat Wednesday morning, then over
the central panhandle Wednesday evening. This will result in a new
low center tracking SE along the outer coast and another round of
snow showers for the region. Placement of any particular band of
snow is uncertain at this time, but starting to come into enough
agreement to increase POP and snowfall amounts. Greatest snowfall
out of this will likely be around Icy Strait-Juneau, but how
quickly it moves through and how fast the dry air on the north
side of the low clears things out will determine if a winter
weather statement is needed. As the low tracks SE, the southern
panhandle will also see accumulating snow as well.

NW flow behind the low gives a break in the weather for Thursday,
but also northerly outflow winds increasing and temperatures
falling. As a result some freezing spray is likely over the
northern inside waters and cold wind chills of 20 to 30 below.
The break in precip will be short-lived for the southern panhandle
as a stronger weather system tracks across the southern Gulf of
Alaska on Thursday. There remain some timing differences of when
it affects the region and how far north-south the low center and
frontal band tracks. Latest models are leaning toward a more
southern track, which keeps Juneau and the northern panhandle
dry. The far southern panhandle could see another round of
advisory level snowfall from this low, but also a good chance of
rain to be mixed in, which would limit accumulations near sea
level.

An arctic front quickly moves in behind the low Friday night with
continued dry northerly winds through the inner channels. Partly
cloudy to mostly clear skies and CAA will keep us below normal for
temperatures with many places seeing the teens to single digits.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Warning from noon today to 6 AM AKST Tuesday for
     AKZ020-022-025.
     Winter Weather Advisory from noon today to 6 AM AKST Tuesday for
     AKZ021.
     Winter Weather Advisory until 6 PM AKST this evening for AKZ017.
MARINE...Gale Warning for PKZ051.
     Small Craft Advisory for PKZ022-031-041>043-052-053.

&&

JWA/Ferrin

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