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174
FXAK67 PAJK 182321
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
221 PM AKST Sat Nov 18 2017

.SHORT TERM...Tonight through Monday...As of 1300 this afternoon,
we can see from water vapor satellite imagery a weakening upper
level trough positioned over the eastern gulf and SE Alaskan
panhandle, with a strong upper level ridge over the far western
gulf that extends into mainland Alaska. We`ve seen a few
remaining snow showers in some areas today due to the TROWAL
wrapping into the circulation head of a weakening surface low
This surface low is associated with the weakening trough aloft
mentioned previously. Due to the placement of the low and a
strong high pressure over northern Canada, the pressure gradient
will be tight over the panhandle, leading to the continuation of
strong outflow winds for the remainder of today and into tonight.
The snow showers will continue to diminish from north to south
throughout this evening and into tonight as the low pushes further
into Canada. This should then allow for some weak ridging to
build in along the eastern gulf coast briefly on Sunday and allow
for a possible brief decrease in the outflow winds before the next
system moves in.

Beginning Sunday night, a shortwave aloft will dig southeast
through western portions of mainland Alaska and break off from
the main upper level flow, helping to create another surface low
over the gulf. This cutoff low will continue to strengthen and
linger over the gulf on Monday and then slowly swing south on
Tuesday. Light snow accumulations may be possible along coastal
locations depending on the placement of the low. However, some
uncertainty as to the placement of the low remains, so precip
amounts, as well as types, across the panhandle are still
debatable. Winds will also be a concern with this system, as gale
force winds are expected along the associated front that will set
up and stall over offshore and eastern gulf waters. In addition,
winds will increase throughout the inner channels as the gradient
tightens between these two systems and offshore flow persists once
again.

Generally, there is decent agreement among the models in the
short-term. However, after Tuesday, models tend to disagree as to
the position of the low that develops over the gulf. We trended
more towards the GFS for any significant updates to the forecast,
using more higher resolution models for QPF and temperature
adjustments. The GFS has had a little more run-to-run consistency,
and the recent ECMWF runs have trended more towards the GFS as of
late. The NAM continues to remain more of the outlier for this
set up.

.LONG TERM.../Tuesday through Saturday/It is the time of year
where only short reprieves between low pressure systems are
common, this week will be just that way. By Tuesday, a low center
will be in the south-central gulf/North Pacific, then by early
Wednesday morning the low will cover much of the eastern gulf.
Confidence is good through this time and then starts to falter a
bit with more differences between the models. Used the EC to do a
small blend to the current forecast, an ensemble approach would
continue to be prudent.

By this time temperatures will have warmed over most of the
southern half of the Panhandle to bring in rain for the south and
a mix of rain and snow in the central during the day. Temperatures
will still be cold enough to change to all snow in the
overnight/early morning hours for these areas. The warmer air
should advance up to the Elfin Cove/Gustavus/Hoonah/Juneau
corridor on Thursday. This is where some confidence slips as to
whether the precipitation in the central Panhandle, as far north
as Elfin Cove/Juneau will be snow, rain or a mix of both. Outer
coastal areas will be a more certain rain for the precipitation
type. By the weekend Skagway and Haines will be waffling between
rain and snow for precipitation type, for the near sea level
locations and remaining snow for the highways.

There are reasonably healthy QPF amounts for the southern
Panhandle which could give several inches of rain and/or snow
Tuesday night/Wednesday. Then spread into the central Panhandle
Wednesday. With temperatures rising above freezing during the day
for most locations and then falling below at night amounts will be
tricky. Downsloping winds will likely keep some local areas in
the north relatively drier.

Brisk winds continue around the northern Panhandle with the
northerly wind. In the south the winds will be getting the
influence of the low in the gulf leading to a southeasterly wind.
Wind speeds for much of the Panhandle will range from 15 to 25
mph with higher gusts. A lot of SCA, with some Gales on Monday.
Winds will be on a slow diminishing trend through the week, but
stay mindful for the strong outflow winds in the north through mid
week. These outflow winds will linger between Yakutat and Dry Bay
through Thursday.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind through Sunday morning for AKZ018.
MARINE...Gale Warning for PKZ012-013-022-052-053.
     Small Craft Advisory for PKZ011-021-031-043-051.
     Small Craft Advisory due to Seas for PKZ041-042.

&&

$$

Voveris/KV

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