Heavy Snow in Alaska, March 10 to 11, 2021

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
323 PM AKST Tue Mar 9 2021

.SHORT TERM...Not too much has changed in the last 12 hours
regarding the next winter storm as we edge closer to spring.
Most of Southeast experienced another sunny, but seasonably chilly
day with highs in the mid to upper 30s across the north and in the
low 40s over the south. Some residual cloudiness from a stubborn
boundary lying across the southern Inner Channels have supported
cloudiness over Petersburg, Wrangell, and portions of the Prince
of Wales. Also, some higher level marine stratus has entered the
northeast gulf and threatens Pelican, Elfin Cove, and even Sitka
may envelop portions of the coast tonight. Some models indicate
some sprinkles or drizzle or flurries with it, but impacts should
be low, and this was not our focus.

Wednesday, moisture profiles begin to increase from the west and
we fully expect snow to begin in Yakutat at least by Wednesday
afternoon. But temperatures initially should be above freezing by
a few degrees. Snowmelt cooling should lower temperatures fairly
quickly as the front interacts with our dry, polar air mass. These
processes should repeat as moisture spreads eastward into the
northern Panhandle Wednesday night. In March, timing is
everything, and a front carries more weight with snowfall
nocturnally than during the day without the surface. Thicknesses
over the north will be more than sufficient for precipitation to
be all snow Wednesday night. Temperatures at 850, depending on the
model, come in from -7 C to -11 C with the coldest being the NAM.
What caused us to expand the winter storm watch is the MOS numbers
suggesting 6 to 8 inches plus for Yakutat. Haines admittedly is a
stretch but, the possibility is still there such that we included
them in the new advisory, too. For other areas of the north, 1 to
4 inches will do. Gustavus does suffer from a wide spread though.
As mentioned 12 hours earlier, thicknesses come in around 1290 m
across the Panhandle, so we feel fairly confident that snow will
at least mix in with precipitation everywhere. Even Petersburg may
result with a couple of inches.

Thursday, wave track differences prevented much in the way of
improvement to the existing forecast.

.LONG TERM...Thursday through Tuesday as of 10 pm Monday/ A cold
upper trough moving through the area will be the main driver of
the weather through the long range period. The coldest weather is
expected over the weekend as 850 mb temps drop into the negative teens
C across the panhandle (-10C at 850 mb will be seen as far south
as 45N latitude across the NE Pacific with this trough). This cold
trough will not last however and will be pushed off to the east by
an upper ridge come early next week.

At the surface, the long range starts out with the lingering
precip from Wed night's front. Temperatures, at the surface and
aloft, will still be cold enough for snow for most areas through
any snow accumulations will be starting to taper off as
precipitation becomes more convective in nature rather then
stratiform. Timing of the front has not chanced all that much
from yesterday so confidence is higher on this feature than it has
been the last few days.

As for the next system coming in for Fri, guidance is still in
flux. The actual low itself is now aimed more toward the southern
panhandle as the increasingly strong upper trough keeps pushing it
farther and farther E and S. Most of its precipitation and wind
is now aimed at the BC coast rather then the southern panhandle,
though some will still hit the southern panhandle (will need to
watch the Hyder area as depending on temperature they could get a
significant amount of snow or they could get mostly rain).
However, there is still some guidance that is suggesting that
some of the heavier precip may get pulled farther north ahead of
the low and pass through the Central Panhandle as a result.
Confidence is not great on this as it is only a few models that
are even suggesting it. Overall, the trend with this system is
lower precip amounts (and thus lower snow amounts) across the
central panhandle, and lower winds across the southern half of the
panhandle as well.

Into the late weekend and early next week, indications are for
another weak to moderate outflow event with high pressure again
building into the southern Yukon. Winds in the northern inner
channels increased for Sun as a result (to 25 kt in Lynn Canal).
The panhandle will also be drying out as offshore flow increases.
Timing is not yet set in stone however so forecast confidence for
this feature is not that great yet.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch from Wednesday afternoon through Thursday
     morning for AKZ017-019-025.
MARINE...Gale Warning for PKZ052.
     Small Craft Advisory for PKZ041-043-051.

&&

JWA/EAL

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