Gale Warning 121, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
226 PM AKDT Sun Mar 25 2018

.SHORT TERM...Complex early spring storm will affect SE AK over
the next 24 to 36 hours. Arctic boundary marking the dividing line
between cool and very dry continental air to the north and a more
moist marine airmass to the south lies draped across the southern
Panhandle early this afternoon. Strong shortwave trough along the
Aleutian chain will track NE towards the northern Gulf by late
Monday. A northern stream shortwave will continue sinking south
into the Yukon tonight and will act to suppress the Gulf system
Monday and Monday night. Occluded front will move from S to N
through the Gulf Monday before stalling somewhere over the
northern Panhandle Monday evening as the weakening shortwave over
the Gulf shears out to the east. This front and the interaction of
the northern and southern stream shortwaves will be the key in
determining precip types and amounts across especially the
northern one third of the Panhandle through early Tuesday.

As the front lifts north and merges with the weak Arctic boundary
over the southern Panhandle Monday morning, moist Gulf air will
begin overrunning the cool, dry continental airmass over the
northern channels. Enough dry air will be in place for the
precipitation to begin as snow from roughly Petersburg to Port
Alexander and points north. With the cold stable air near the
surface and the tendency for high pressure to be maintained
somewhat over NW Canada, expect the rain/snow line to have a hard
time creeping north. A period of moderate snowfall is likely
Monday evening especially for areas from Pelican to Juneau. Models
are in good agreement showing strong overrunning with lift aided
by some upper jet dynamics from a departing jet entrance region
over the Yukon. The tricky part of the forecast comes into play
Monday night. As the upper wave over the NE Gulf shears out to the
east, a surface trough will lie east/west across some portion of
the northern channels. Continued pieces of weak mid level energy
will run along parallel to this surface boundary. Models all
indicate decent lapse rates of around 7 C/Km in the dendritic
growth zone which coupled with dry air above 500 mb may set the
stage for a training snow shower event along and just north of the
weakening surface trough. The question is, where does this trough
set up? The Canadian model is bullish on keeping it just south of
the Icy Strait corridor, placing the Juneau area under the gun
for likely warning amounts of snow while the GFS and EC continue
slowly moving the surface trough north into the central Lynn Canal
area. For now, went with a Winter Weather Advisory for widespread
3 to 6 inches for Juneau northward which would be a top 10 late
March snow for many areas. Later shifts will have to monitor the
positioning of the surface trough to determine if any upgrade
might be necessary later on.

Gale force easterly winds associated with the front will develop
over the eastern gulf by monday morning with outflow gales out of
Cross Sound. Post frontal wind shift to the southwest spreading
from southwest to northeast across the eastern gulf through the
day Monday with a reduction small craft intensity. On the inside,
northerly winds will shift southerly with the progression from
south to north, beginning with Clarence Strait. Transition to
southerly winds will be complete with northern Lynn Canal by
Tuesday morning as the front stalls over the northern panhandle.
Some small craft winds on the inside, except for gales over Lynn
Canal and Cross Sound. With the low pressure trough associated
with the front and high pressure over northwest Canada, small
craft northerlies over the Canal will increase to gale force by
tomorrow morning over the northern half. Currently, only have 30
kt northerlies over the southern half, but higher winds may be
necessary. After all...its Lynn Canal. Northern inner channels
gales should should diminish to small craft intensity by Monday
evening. Southwest flow over the eastern gulf and southerly winds
over the inner channels should remain through the end of the short
range forecast period.

.LONG TERM...Tuesday through Saturday as of 9 PM Saturday...Model
solutions continue to diverge and ensemble spreads remain high
beginning Tues. There is fair agreement in the synoptic pattern
being highly zonal through mid next week but even 500mb spreads
increase significantly heading into next weekend. Models indicate
another low will move over the gulf for the latter half of the
week but differ significantly on track and timing. Elected to
trend inherited forecast towards WPC solution after coordinating
with neighboring office. This solutions tracks a closed low
across the gulf Thu and is in the middle of the EC/GEM solutions.
Adjusted mass fields towards 24/12Z WPC to represent this updated
track with continued low confidence in the exact solution and
track.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Weather Advisory from 1 PM Monday to 7 AM AKDT Tuesday
     for AKZ017-019>022-025.
     Strong Wind from Monday morning through Monday afternoon for
     AKZ018.
MARINE...Gale Warning for PKZ012-022-043-051-052.
     Small Craft Advisory for PKZ013-021-031>033-036-041-042.

&&

DEL//Fritsch/BC

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