Winter Storm in Alaska, February 1 to 2, 2021

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
306 PM AKST Sun Jan 31 2021

.SHORT TERM.../Monday through Tuesday/ Dry weather Sunday evening
won't stick around for long as active weather moves in on Monday.
A plethora of Winter Weather Advisories and Warnings have been
issued across Southeast Alaska ahead of the next winter storm to
impact the region.

The next weather maker drops down from the arctic tonight and
moves SE along the outer coast through Tuesday. Aloft, an upper
low near the arctic coast moves southward over the region while a
strong 120+ kt jet streak pushes into the the northern gulf.
Upper level divergence in the left exit region of the jet streak
and lee side troughing will induce surface cyclogenesis in the
northern gulf late tonight. This new surface low will be pulled SE
through the eastern gulf over the next 24 to 48 hours. High
pressure already in place over NW Canada will keep its influence
over the Panhandle during this time frame as a strong northerly
pressure gradient develops keeping cold air trapped at the
surface.

As a stacked low develops and moves SE along the coast, winds turn
southerly then southeasterly aloft pumping moisture into the
Panhandle. Expect widespread snow showers to develop first along
the NE Gulf Coast near Yakutat then spread SE through the
Panhandle Monday through Tuesday morning. Models continue to
hint at a convergence zone setting up across the Central
Panhandle as a piece of energy from a system south of Haida Gwaii
pushes northward and interacts with the low developing in the
Gulf. Guidance has been consistent with showing a heavy snow band
setting up across the Central Panhandle along this convergence
zone, becoming a quasi- deformation band as the gulf low moves SE
near POW Island. Expect a N/S band to develop late Monday morning
then slowly pivot and become E/W oriented early Tuesday.
Orographic lifting along the eastern slopes of Baranof Island may
help sustain this band through the day as well as increase
snowfall rates across the Central Panhandle. Guidance indicates
that this band will be relatively thin. Low confidence exists
regarding the exact placement of this heavy snow band, as a small
deviation in its location and movement can significantly affect
snowfall amounts for any given area. Current thinking is that an
area from Southern Admiralty Island to Kake, to Little Port Walter
to Prince of Wales Island will receive heavy snowfall from this
band.

Model soundings across the Panhandle show a stable layer right at
the surface with northerly flow keeping cold air locked in place.
Above the stable layer and through nearly the entire column,
elevated instability exists with lapse rates above 8C/Km. A
saturated DGZ along with elevated instability indicates the
potential for heavy snow and high snowfall rates. Expect the
highest snowfall amounts to be along the NE Gulf Coast and under
the heavy snow band in the Central Panhandle. Lighter snowfall
amounts expected further inland near the Coast Mountains, including
Juneau, as downsloping winds are expected to dry out the lower
atmosphere enough to inhibit heavy snow development there. A
Winter Weather Advisory is out for Yakutat, Gustavus, Hoonah,
Angoon, Tenakee Springs, and Haines for 3 to 6 inches of snow. A
Winter Storm Warning is in effect for Pelican and Kake for 6 to 10
inches of snow. Currently have lower snow amounts for
Petersburg/Wrangell as the best forcing and heavy snow band should
remain west of there. A Winter Weather Advisory is out for Prince
of Wales Island as the heavy snow band is expected affect this
region. Higher snowfall amounts expected along the POW road system
with lower amounts at sea level.

Northerly winds dramatically increase as this low passes by as the
gradient tightens. Expect storm force winds to develop in northern
Lynn Canal Monday night into Tuesday with small crafts developing
in the Central Inner Channels. Gale force winds expected coming
out of Interior passes and into the Coastal Waters Monday night
and Tuesday. Outflow begins to relax across the region by Tuesday
night with the exception of northern Lynn Canal.

Confidence in the overall forecast is at or below average. Areas
of uncertainty include the exact placement of the heavy snow band
in the Central Panhandle and how much downsloping will limit
snowfall amounts along the Coast Mountains.

.LONG TERM.../Tuesday through Sunday night/...The extended range
forecast begins with a sharply defined upper level ridge extending
from the north Pacific northward across the Peninsula and clear up
to the Chukchi Sea. To the east lies an upper trough centered
over the Yukon. Between them, a 135 kt upper jet will be screaming
south from the Arctic Ocean, across the interior of Alaska, and
southeast across the Gulf. The upper ridge will become broad and
fat and generally shift east as the next upstream trough digs
south and impinges upon it. This will result in very strong
northwesterly upper flow over the Panhandle by Tuesday afternoon.
In terms of the conditions aloft, this pretty much sums up the
upper level pattern for the coming week. Variations include the
orientation of the jet stream overhead and how sharply defined the
various upper troughs and ridges will be. In summary, we are in
store for an active weather pattern.

At the surface, the long range forecast begins with a low situated
off the southern outer coast and an intensifying northerly
pressure gradient as high pressure shifts southeast out of the
interior, across the Yukon, and into British Columbia. A brief and
weak surface ridge will eject the surface low to the
southeast...only to be displaced itself by a low and associated
frontal system by Wednesday afternoon. Then another transient
ridge, and another front by Friday afternoon. The Friday front
looks like it will be reinforced by a developing wave Friday
night. There should be a lot of precipitation across Southeastern
Alaska Friday afternoon through Sunday night.

Precipitation through mid week should be mostly in the form of
snow as the northerly gradient pushes south and colder air from
the Arctic front presses against the coast mountains. Exception
will be the far southern Panhandle. By the time the friday front
arrives - from the southwest - significantly warmer low level air
arrive and drive the rain/snow line east and north such that all
locations should transition to rain by saturday night except the
far northern inner channels. Still way too far off to say with any
confidence, but this could be a significant snow event for at
least Haines, Skagway, and the highways. That said, we'll keep an
eye on it for you. -Fritsch

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Warning from 9 AM Monday to 9 AM AKST Tuesday for
     AKZ026.
     Winter Weather Advisory from 6 PM Monday to 3 PM AKST Tuesday
     for AKZ027-028.
     Winter Weather Advisory from 6 AM Monday to 6 AM AKST Tuesday
     for AKZ020-021-024.
     Winter Storm Warning from 3 AM Monday to midnight AKST Monday
     night for AKZ022.
     Winter Weather Advisory from 3 AM to 6 PM AKST Monday for AKZ019.
     Strong Wind through Monday afternoon for AKZ018.
     Winter Weather Advisory from 9 PM this evening to 3 PM AKST
     Monday for AKZ017.
MARINE...Gale Warning for PKZ012-013-022-043-051.
     Small Craft Advisory for PKZ011-021-031-032-041-042-052-053.

&&

CM/Fritsch

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