Heavy Snow in Alaska, March 1, 2021

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
650 AM AKST Mon Mar 1 2021

.SHORT TERM...A vicious weather pattern full of imposing
decisions continues to stand before us. Temperatures in the mid
30s across most of the region, but with small, though not minute,
dewpoint depressions threaten to make mountains of snow out of
what should be slushy conditions at worst. MOS guidance is also
flummoxed with disconnects between guidance fields related to
temperature and predicted snow amounts not adding up. In unison
they call for highs on this Monday in the high to mid 30s for much
of the region, but pushing 8 inches-plus of snow in 24 hours in
Yakutat, and even 4 to 6 inches in Juneau.

We decided to act in Yakutat's case as hourly MOS numbers came in
somewhat lower than actual highs/lows, and the expectation of
easterly winds ahead of the approaching front perhaps offering
some chance of a cool air wedge forming to promote some efficient
overrunning. Confidence is low as sounding values have come in
with a warm layer below 1K feet. But again saturation increases
from frontal precipitation should aid snow-melt cooling and will
likely closes the gap later this morning, offering a real chance
of significant accumulations.

Farther, east, rain and snow is expected to increase. This
morning, precipitation should be more showery. As the front
approaches from the west, steadier precipitation takes over. Sitka
reported various periods with dustings of snow overnight, but
then a rain changeover cleans it up. Given relatively cool 850 mb
temperatures between -6 and -7.5 C, we expect no real skill level
at indicating time trends of when more rain should fall than more
snow or the opposite, so we have largely gone mixed in the
forecast well into the next 36 hours. These temperatures should
guarantee snow with rain, if not all snow at times, despite above
freezing temperatures. As rates increase late this afternoon and
the evening, some light snow accumulations will likely occur in
spots even as temperatures are marginally above freezing. Should
temperatures be off by even a couple of degrees, real snowfall
amounts could differ markedly from forecasted amounts. Thus, our
forecasted snow amounts will be even more error-prone than normal
during the short-term forecast period (through Tuesday night).

As a gale force front approaches the Panhandle this afternoon,
ridging across the Panhandle strengthens and thus we do expect
strong winds developing through Upper Lynn Canal. In Skagway they
already have. With temperatures in the mid 20s and up to 7 inches
of accumulation through the next 24 hours at White Pass, we
expect blowing snow and drifting to become impactful. Thus, we
have posted a winter weather advisory through 6 AM Tuesday for
White Pass. We end it here, because actual snowfall lessens on
Tuesday. But winds could still support blowing snow well into
Tuesday.

By Tuesday, winds ease through the Inner Channels and colder air
associated with a long-wave trough across the gulf enhances
instability even further. Some of this colder air sneaks into the
northeast gulf Monday night and adds the threat of thunderstorms
from Yakutat to Pelican. While instability grows on Tuesday, we
suspect the thunderstorm threat to be lower as winds aloft will
not add to instability of the sharp lapse rates of 8 C/km plus.
Given these deeply sharp lapse rates though, the threat of rain
and snow should continue across the Panhandle, and not just the
north. Some areas could see light accumulations even far to the
south, but most will see little at all, given a marine moderated
air mass.

Trends through this afternoon in measured precipitation and
temperature should give stronger clues if more significant
snowfall accumulations are in any of our futures. Confidence in
the winter storm in Yakutat is low to moderate, lower than we
typically want for a warning. Confidence in the rain/snow and
light accumulations is moderate and for the winter weather
advisory and strong winds for the South Klondike Highway are
moderate to higher.

.LONG TERM.../Wednesday through Monday night/..."Variety's the
very spice of life, that gives it all its flavor," according to
William Cowper's "The Task" (1785), and the long term looks to
deliver plenty of variety as the progressive pattern continues
with more than a few dashes of uncertainty to keep the forecasters
on their toes. As has been the case recently, there will be some
rain, snow, wind, waves, with a break possible at the very end of
this timeframe. Of note, temperatures do not appear to warm or
cool substantially during the long term. Temps will add their
spice to the forecast by just hanging uncomfortably close enough
to freezing to make snow a consideration in at least some portion
of the forecast area.

Models are in good agreement Wednesday and Thursday with a digging
upper level trough with the trough axis bisecting the Gulf of AK
Wednesday night. At its maximum amplitude, the base of the trough
extends all the way to 35N latitude in the central Pacific. At
the surface models are also in decent agreement with a broad low
in the gulf, and disturbances rotating around the system bringing
periods of precipitation and wind to the panhandle. Precipitation
type and the timing and strength of the impulses will be the
forecast challenges. There appears to be one rather weak impulse
that will impact the coast Wednesday afternoon bringing showers
and possible small craft advisory winds for some of the channels.
Deterministic and ensemble models are showing a stronger surface
low approaching the panhandle from the southeast and riding up the
coast throughout the day on Friday, but there are differences in
both timing and track. Model trends will need to be watched on
this system as it could bring more impactful winds, possibly
gales, and precipitation, including snow, depending on the exact
track. After the Friday system passes, yet another weak system is
modeled to form in the gulf Saturday and approach the southern
panhandle, this time from the west, and exits quickly to the
southeast. Sunday and Monday look be the best chance for a break
from the active weather, but with model disagreement present as
early as Friday, confidence is low for the extended period.

To conclude on the subject of variety, an array of models were
used to construct the long term forecast. For the first two days,
a blend of the GFS, Canadian, European, and NAM were used due to
the very good agreement. For Friday and Saturday, the European
appeared to have the best handle on the Friday system so a blend
favoring the European model was used. Beyond that the operational
blends were leveraged (WPC, NBM).

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind from 3 PM AKST this afternoon through this evening
     for AKZ019.
     Winter Weather Advisory from 9 AM this morning to 6 AM AKST
     Tuesday for AKZ018.
     Winter Storm Warning until 9 PM AKST this evening for AKZ017.
MARINE...Gale Warning for PKZ012-043-051-052.
     Small Craft Advisory for PKZ011-013-021-022-031>036-041-042-053.

&&

JWA/BFL

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