Winter Storm Warning 10, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
305 PM AKST Sun Mar 5 2017

.SHORT TERM...Windy day across parts of the northern panhandle
today as places like downtown Juneau and Douglas, and Skagway
have seen gusts as high as 35 to 45 mph. Like wise some marine
areas in the northern panhandle have also been windy with gale
force winds noted in Cross Sound, Lynn Canal, and Taku Inlet
today.

These winds are only expected to increase tonight and tomorrow as
a new stronger shot of cold air flows into the area tonight right
behind an upper trough that will be dropping southward. The cold
surface high in the Yukon will be strengthening to 1035 mb
tonight as the cold air moves in increasing pressure gradients
across the northern panhandle. Gale warning and heavy freezing
spray warnings remain up for many northern inner channel areas
with gales and small craft advisories up for several other marine
areas. Strong wind headlines also remain up for zones 17, 18, 19,
and 25 for gusts to 40 to 50 mph tonight and tomorrow.

Meanwhile, there is a marked difference between the northern and
southern panhandle as far as cloud cover. The northern panhandle
is mostly clear due to all the outflow. The southern panhandle
however had a layer of mid level stratus form in the relatively
calm flow under an inverted trough. Some of these clouds have
also been thick enough to produce some snow showers this
afternoon with light accumulations. These clouds should mostly
clear out later tonight as the upper trough moves through the
area.

The clearing skies will not last long for the south. Clouds and
snow showers will move back into the southern panhandle late
tomorrow afternoon as a low, that is the surface reflection of the
upper trough, develops in the eastern gulf and then moves toward
Dixon Entrance. Marked difference between forecast models on when
the main mass of snow showers will move into the southern
panhandle (12z nam was as early as 1 pm Mon while others are as
late as 9 pm Mon night). With this uncertainly, decided to not
change the start time (around 4 pm Mon afternoon) of the snow
shower all that much from the previous forecast to await what
future model runs say about this.

Main changes from the previous forecast focused on local wind
edits. Extended some of the stronger winds farther down Chatham
Strait and Stephens Passage per satellite wind observations. Also
switched wind directions for the extreme southern panhandle to SE
and increased chances for precip there for tomorrow afternoon as
the new low begins to affect that area. Otherwise, other changes
were minimal.

.LONG TERM...Upper level trough moving into the AK gulf will
remain in place though the end of the week with a blocking high
to the west. The developing surface low in the SE gulf expected to
remain centered over the southern region with shower bands not
moving much further than Sumner Strait. The low should weaken and
track to the South by late Tuesday with snow showers diminishing
by Tuesday afternoon. Still fair amount of uncertainty among
models on the exact track of the system but enough consensus to
issue a Winter Storm Watch for z27 and 28 with total accumulation
of 7 to 14 inches possible. Do not think Z23, 26 and 29 will get
much more than scattered showers and only a couple of inches of
accumulation on the borders. At most some increasing clouds for
the central and northern panhandle with this system then a return
to clear skies.

Strong northerly and outflow winds for Monday night through
Tuesday night between the cold arctic high over the Yukon and the
low to the south. Storm force winds over N Lynn canal are likely
with max gales for most other central and northern inner channels.
Gusts out of marine channels could get up to 60 kt. With strong
inversion, critical level and some cross barrier flow potential
for Taku wind event over Downtown Juneau and Douglas. Kept gusts
just at 50 mph as while cross barrier flow is strong was not
getting a model consensus on the strength. However Taku gusts to
60 mph are not out of the question.

Due to model spread with latest runs decided to keep press and
wind fields mostly as is. ECMWF/Canadian move precip further
north but split on timing. GFS keeps majority of precip to the
south and moves out earlier. NAM close to GFS overall but
initially moves north faster. For rest of the time frame kept with
a WPC solution which is still in line with previous forecast so
little change in the extended range. Forecast confidence is
average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Watch from Monday afternoon through Tuesday
     afternoon for AKZ027-028.
     Strong Wind through late Monday night for AKZ025.
     Strong Wind from midnight AKST tonight through late Monday night
     for AKZ019.
     Strong Wind from 9 PM AKST this evening through late Monday
     night for AKZ018.
     Wind Chill Advisory until 3 PM AKST Tuesday for AKZ018.
     Strong Wind from 9 PM AKST this evening through Monday afternoon
     for AKZ017.
MARINE...Heavy Freezing Spray Warning for PKZ011-013-031.
     Heavy Freezing Spray Warning for PKZ012.
     Gale Warning for PKZ012-013-021-022-031-032-043-051.
     Small Craft Advisory for PKZ011-033>035-041-042-052-053.
&&

EAL/PRB

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