Winter Storm and High Wind in Alaska, February 17, 2021

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
309 PM AKST Tue Feb 16 2021

.SHORT TERM...While cats can be a terror, it's the kittens that
sport the sharpest claws. And such is the bite of this terribly
weak kitten like front that flirts with sufficient lift that
could deposit a few inches of light snow across much of the
northern and central Panhandle. But the hunch has always been, let
us lower the QPF, maybe lower the snow ratios, but raise POP. But
a fluffier band this morning cautioned us to avoid too much
change. Accumulation has so far measured one inch in Yakutat and
approaching that in parts of Juneau. We did expand POP early on
the in day to account for more efficient lift in the south such as
Wrangell. However, word from Gustavus has confirmed the fear from
just within the confines of possibility-- that weak fronts beget
shallow columns of precipitation that may indeed host a hidden
part of super-cooled water droplets. Reports on the ground first
indicated a light freezing drizzle, but once reports received of a
bona fide changeover to rain, we posted a winter weather advisory
with less than a quarter inch of ice as word leaks that travel has
become impacted. When to end the advisory was not cut and dry as
no index of ours indeed even hinted at such mischief. A front
though lies just offshore, and an approaching wave into Cape
Fairweather land, may just enhance a convergence zone across the
Icy Strait corridor and thus thickening the moisture profile to
allow precipitation to transition more to snow again by this
evening. But confidence is quite low--like scientifically
explaining what a cat would do, except to cause more mischief in
ways one would least expect. Thus winter weather advisory for
Gustavus is out until only 6 PM.

Given cloud cover tonight, we expect temperatures to fall only
modestly. So we warmed temperatures by 3 to 10 degrees in many
areas. We also warmed White Pass in line with Canadians so that we
are confident that any dangerous wind chills remain in the past--
for now. Snow levels do look low across the Panhandle, such that
snow could fall even in the far south overnight. But a slow
warming trend coincident with greater thicknesses spreads
northward and with it a mix or transition to all rain by Wednesday
night for much of the Panhandle. Like many times, confidence in
Upper Lynn Canal is the hardest. We anticipate a southerly turn
with temperatures rising above temperatures. While this makes
sense with a changeover to rain, we left snow with lower
accumulations for now.

We looked at potential advisory amounts for Yakutat, but so far,
think not. We did raise temperatures a little Wednesday, which
could introduce rain earlier, before 2 to 4 inches of snow falls.
Much of this falls due to an associated front into the northern
gulf with the most energy west of the Panhandle. We did raise pops
for the northern Panhandle, but amounts of snow should be low at
this time. Some of this could be transitioning to rain at this
time as gradients go more parallel. But confidence is not good at
the moment, so very little has changed.

On Thursday, a storm force low begins to pivot the northern gulf
front to the west. This should spread rain and snow for the
Panhandle with the potential of very strong winds for Prince of
Wales Island. While we discussed the possibility of gusts to 60
mph, we do not think evidence supports it as of yet. Thus, we hold
back on issuing a watch, if we even need to issue at all. Outflow
is stronger with this system, and thus the northern Panhandle may
experience more of a legitimate break, even if it's a mere 12
hours long.

.LONG TERM.../Thu through Tue as of 10 pm Mon/ Better model
agreement on a storm force low moving into the AK Gulf from the N
Pacific Thursday. Still uncertainty on timing with NAM being an
initial outlier and while GFS/ECMWF were closer on timing, still
had position difference. Canadian was a good compromise on
position and still had the initial storm force winds. As the low
tracks towards the northeast gulf winds will likely diminish.
Coastal locations expecting at least 40 mph gusts, but if the low
does not fill while tracking north could see gusts closer to 50
to maybe 60 mph. Precip initially starting a snow or rain snow
mix, then transitioning to mostly rain for southern areas with
southerly flow and warmer air moving in. Might see higher QPF
values but getting lots of terrain variation in models. Central
and northern areas start to see increase snow accumulation late
Thursday night into early Friday, but once again for central areas
southerly flow and warming temps looks to transition to more
rain. For Friday as the low weakens in the northeast gulf and
after a short period of zonal onshore flow another low is expected
to track into the central gulf from the west bringing more precip
to the region. High temps will be in the low 30s to low 40s with
lows in the upper 20s to low 30s, with southern regions getting
the warmest temps. However 850 mb temps mostly colder than -3C
which should at least keeping mixed precip in the forecast
initially.

Main changes to the start of the long range were continuing the
trend of increasing gulf winds as models began to favor the storm
force low solution. Used Canadian Thursday due to reasons stated
above, then kept with WPC for remainder of the long range.
Forecast confidence for Thursday is improving but still bit below
average.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory until 6 PM AKST this evening for AKZ020.
MARINE...Gale Warning for PKZ052.
     Small Craft Advisory for PKZ022-041>043-051-053.

&&

JWA/PRB

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