Heavy Snow in Alaska, January 2 to 3, 2020

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
1053 PM AKST Wed Jan 1 2020

.UPDATE...As of 9pm Sitka observer has reported near 2 inches of
snow since the late afternoon. Therefore the winter weather
advisory appears to be working out. However, trends inland have
caused us to back off on snow totals for Juneau for tonight. New
overnight amounts of around 1 inch are expected.

While monitoring the current dynamic weather pattern, conditions
have warranted an update in the near-term forecast. The main area
of focus has been on the Friday morning to Friday night timeframe.
During this time frame a low pressure center will track towards
the southern panhandle from the south. Heavy precip will be
associated with this system leading to an increase in QPF values
across the southern panhandle including Ketchikan, Hyder, and
Klawock. Temperatures were lowered across the board during this
time frame as guidance continues to support the cooling trend.
Along with cooling temperatures, snow levels will also fall to sea
level allowing for precip to fall as snow. With this update,
there still is a good chance for significant snowfall for the
southern panhandle including Ketchikan this weekend. With current
thinking, 4-6 inches are possible Friday into Friday night with
lower amounts for Wrangell (3-5 in) and Prince of Wales Island
(2-4 in).

Winds were also held back as we have slowed the progression of
the northward movement of low. As the low tracks north, pressure
gradients will tighten across the Panhandle leading to the
strongest winds to occur during the Friday timeframe. North to
northeast flow will increase across the northern panhandle. With
this increased offshore flow, sky cover was decreased further.

&&

.PREV DISCUSSION......ISSUED AT ...300 PM AKST Wed Jan 1 2020

SHORT TERM...The New Year brings with it a new chance of snow for
SE Alaska. A cold front has been pushing through the area
throughout the day, beginning the long-awaited cool down as the
cold air moves into the area from the west. The warmer waters out
over the gulf will try to moderate temperatures some, however
observations from many of the buoys in the gulf point towards air
temperatures at or below freezing continuing to stream steadily
towards the panhandle, and some impressive cloud streets are visible
in satellite imagery, extending several hundred miles long.

The precipitation which began as rain has already begun to mix with
or change to snow as the cooler air has started to move in. The
nature of banded snow showers means that forecasting precise QPF
amounts and snow totals will prove to be difficult as both will
depend on how many and how strong the showers are that impact the
area. For now, better snow totals will be on the western upslope in
the panhandle, and winter weather advisories will be in effect for
areas ranging from Yakutat to Pelican and Sitka during the evening
hours. Elsewhere across the panhandle, expect many areas to be in
the two or one to three inch forecast totals for the overnight
hours. Given the continuing cold air advection, these snow showers
will likely continue to impact SE Alaska through tomorrow as well,
before an outflow event beginning Thursday night cuts off the snow
chances for many parts of the Panhandle.

Aside from some gusty winds during the evening hours tonight, the
outflow event beginning Thursday evening onwards will be the
primary wind concern for the area. A combination of cold air, a
high pressure over the Yukon, and a low pressure system stationed
in the gulf will encourage the development of strong winds in the
inner channels. With winds reaching small craft throughout most
channels and interior passageways, and gale force in Northern Lynn
Canal. These winds are currently progged to continue through the
remainder of the forecast period.

Late Thursday models still have not finalized the position and
strength of a low moving out of the Pacific towards Haida Gwaii.
Guidance is split on whether or not the low will push far enough
north to impact the southern panhandle, or whether or not it will
remain over, or even south of, Haida Gwaii. This will do a couple of
things, change the flow in the southern panhandle more NE-E
(offshore) which would limit showers, or bring a stratiform
precipitation event into the southern/southeastern panhandle area.

Regarding convection, there continues to remain one small area
that might see some lightning northeast gulf Thursday morning, but
the lack of warm air in the gulf to to help sustain any
substantial thunderstorm activity should keep any strikes which do
occur few and far between. Overall forecast confidence is average.

LONG TERM...In the spirit of what New Year's Day means to many
out there--football. We elect to emphasize an action verb,
commonly associated with the sport regarding the extended forecast.
WE PUNT!

Guidance has consistently suggested an impactful storm forming in
the region of strong baroclinicity along the jet just to our south
that would induce strong northerlies through much of the Inner
Channels and produce impactful snow for much of the south early
this weekend. At this writing, consensus has actually diminished
on any track or timing that this storm takes. But yet, the idea is
not lost. Therefore, our forecast for these events remains
essentially the same. Look for possibly heavy snow for Hyder a
period period of moderate snow for Ketchikan and the southern
Inner Channels sometime Friday. For Ketchikan and the southern
Inner Channels the trends to watch will be temperatures, as
currently we forecast a mix or changeover to rain Friday
afternoon, but depending on timing and the stability of the colder
air mass, we could see a later changeover, or none at all.
Likewise, Petersburg and especially Wrangell may need to be alert
for potential heavier snow than forecast as isentropic upglide
tends towards more efficient snow production that models
indicate. And still there is a chance that snow reaches as far
north as Juneau, but impactful snow is not likely at this
juncture.

Where this low goes we do not have a great handle on. Some
solutions take it into the BC Coast, leaving most of Southeast
Alaska out of the snow threat except Hyder. But others take it
offshore of Prince of Wales Island, which would expand the snow
threat as far north as Juneau. Whatever the case on Friday, it
would appear another system may spread moisture atop our cool air
mass Saturday that could bring a possible second (or first, should
the first fizzle) round of impactful snow for much of the
southern and central Panhandle again, but again guidance is
conflicted. Why even look beyond, when the mid- range is so
muddled? What we can say is this: it looks cool and active which
would favor more chances of snow. Happy New Year!

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory until 6 AM AKST Thursday for AKZ017-022-
     023.
MARINE...Small Craft Advisory for PKZ022-041>043-051>053.

&&

CC/GFS/JWA

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