Winter Storm Warning 16, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
105 PM AKST Sat Feb 22 2020

.SHORT TERM...Afternoon GOES 17 imagery shows a polar front draped
across the eastern Gulf roughly from Yakutat south-southeastward
along and east of 140W. A developing baroclinic wave is seen along
this front and is tracking north-northeastward towards the
eastern Gulf coast. This wave and its eventual strength and track
will determine how much snow will fall over the northern Panhandle
tonight into Sunday afternoon.

Guidance is still in disagreement on the exact track of the wave,
with the NAM a western and stronger outlier. Leaned more towards
the EC/Canadian consensus for this forecast as those models have
been a bit more consistent as of late and less prone to rapid
shifts.

All models show an area of enhanced low to mid level
frontogenesis lifting northward this evening and tonight to a
position along and just north of Icy Strait. The strength and
depth of the baroclinic wave will be the key as to how much
precipitation is able to fall to the east of the track of the
wave. A deeper, and stronger system will lead to stronger easterly
flow in the lowest 5kft which would dry out a lot of the area
outside of the immediate Gulf coast. A weaker and shallower system
would allow for the strong frontogenetical forcing for ascent to
overcome weak easterly flow below and lead to periods of moderate
to heavy snowfall along the Icy Strait corridor including Juneau.
This forecaster has been burned before in this scenario,
especially when guidance struggles with liquid qpf amounts in
situations where the forcing is driven by low to mid level
frontogenesis. For now went with a Winter Weather Advisory for the
entire eastern Gulf coast and for the northern Panhandle areas
from Gustavus to Juneau and northward to Haines where a general 3
to 7 inches of snow is expected through Sunday afternoon. As the
wave weakens and translates eastward Sunday morning, any easterly
flow should diminish and allow for more substantial snowfall
further inland for the Juneau and Haines area. Needless to say
later shifts will need to monitor observations and forecast trends
for any potential upgrade to warnings.

After Sunday evening, cold westerly flow will continue as a chilly
airmass advects into the Panhandle following the passage of the
polar front mentioned above. Some weak snow shower activity
remains likely along west facing slopes through Sunday night. The
next strong front looks to lift north into the southern Gulf
Monday night with strong easterly and southeasterly gales in the
the Gulf and overrunning precipitation spreading over the rest of
the Panhandle. At this point it looks like the precipitation will
start out as snow across a large portion of the area but exact
amounts still remain to be determined.

.LONG TERM.../Monday through Saturday as of 10 pm Friday/ Warm and
wet is the main theme of the long range today as the jet stream
will be aimed right at the panhandle from the SW through most of
the period. At least two strong systems will advance on the
panhandle. The first will come mid week and take the form of a
gale force front coming up from the south on Tue followed by the
main low moving N through the Central gulf on Wed. System number
two will be hot on the first's heels moving NE through the gulf
early Thursday.

System number one was the one giving guidance fits a few days ago.
The forecast has appeared to stabilize for this system now with
only timing issues remaining for the most part. Main change today
was to generally speed up the front so that it arrives in the
panhandle late Mon night/early Tue bringing precip and wind with
it. As with most fronts this time of year, we have the issue of
how fast various places will warm up and change any snow over to
rain, and how much snow these places will get before that happens.
In this case, the east/west orientation of the front as it comes
up leans more toward there being more northerly/easterly flow
ahead of the front bringing more colder air to support snow.
However, QPF is not forecast to be that substantial with this
front and the front is moving through rather quickly. Heaviest
snow is expected in the northern panhandle and may accumulate up
to 5 inches in places before snow starts mixing with rain Tue
afternoon. Forecast confidence is moderate as timing of the front
could still change and result in more or less precip making it
into various areas before it is warm enough to change to rain, or
the colder air ahead of the front may turn out to be warmer then
expected and result in more rain instead.

After the front, general SW flow resumes with first the parent low
of the front pushing NE through the gulf on Wed followed by a
second fast moving low from the SW on Thu. Onshore flow continues
through both of these systems keeping the panhandle damp through
the week. In addition, 850 mb temps remain in the -3 to -7C range
through the period, so the south is likely to remain rain through
this period, but the north may switch back and forth between rain
and snow during this time with some accumulation occurring at
times. There are some indications of a small cold air outbreak for
early next weekend, but the confidence on that is very low at the
moment. WPC and NBM used where needed for updates.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from 3 AM to 3 PM AKST Sunday for AKZ019-
     025.
     Winter Weather Advisory from 9 PM this evening to noon AKST
     Sunday for AKZ020>023.
MARINE...Small Craft Advisory for PKZ021-022-041>043-051>053.

&&

DEL/EAL

Visit us at http://www.weather.gov/Juneau