Area Forecast Discussion
Issued by NWS
FXAK67 PAJK 282319
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
319 PM AKDT WED SEP 28 2016
.SHORT TERM...Relatively quiet weather over the next 24 hours. Low
pressure over the far SE gulf will continue to move away to the
south and take lingering showers and thunderstorm risk currently over
the southern gulf with it this evening. Cloud cover has decreased
quickly from north to south today and cool/dry air is settling
in. Clearing skies will lead to temperatures falling quickly after
sunset. Expect many places to have lows in the 30s and below
freezing for wind sheltered places across the northern panhandle.
This will likely be the first frost of the season for the
Mendenhall Valley, Gustavus, and Hoonah. Tomorrow ample sunshine
should allow for temperatures similar to today, however we will be
starting off quite a bit cooler than we did today. For this reason
and CAA from the north, decided to go cooler than MOS guidance
would have suggested.
High pressure will build over the Yukon through the night and
tomorrow. This will increase the north winds through the inside
waters. Have Lynn Canal increasing to small craft level Thursday
morning then those 25kt winds will progress south through Chatham
Strait and Stephens Passage in the afternoon. Expect Haines,
Skagway and Downtown Juneau to start seeing some gusty north winds
through the afternoon as well but not up to strong wind criteria
Used mainly the hi-res models to increase outflow winds tomorrow
afternoon. Otherwise changes were limited to local effects.
Forecast confidence is above average.
.LONG TERM...Conditions still look favorable for windy conditions
across the northern panhandle starting thu night and continuing
through Sat. One change from previous forecasts is the inclusion
of a wind max late thu night into fri as an upper level short
wave drops southward over the panhandle from the Yukon. Model
pressure gradients have also shifted to be the strongest over
that time period. The result is an increase in winds to gale in
Lynn Canal with higher winds in the other northern inner channels
late Thu night and continuing into Fri. Maintained wind speeds
from Fri afternoon into Sat as cold outflow remains present with
strong pressure gradients lingering.
A Taku wind event for downtown Juneau and Douglas still looks
likely. However, how strong the winds will be is still a little
uncertain. Cross barrier flow is topping out in several models at
around 45 to 55 kt late Thu night and Fri morning so there is no
shortage of that. There is even an inversion present with the cold
air outflow. However it is a weak one as the air mass spilling out
of Canada is not as cold as it could be if it was mid winter. The
critical level will likely be the deciding factor on how strong
winds get and currently it is not looking that impressive on model
soundings. The best formation of the CL is just before the cross
barrier flow picks up Thu evening. After that period though there
is little to no directional shear during the period of high cross
barrier flow and weak speed shear. At this time i think that
strong wind gusts of up to 55 mph in downtown Juneau and Douglas
are the most likely outcome. However, the chance is there for the
strong cross barrier flow to induce its own critical level
bringing higher winds to the surface. I just think that at this
time that those gusts will be infrequent and be more isolated in
nature over the course of the event if they happen at all. So
elected to not put out a high wind watch at this time but still
have strong gusts up to 55 mph for the taku wind area.
For the rest of the extended, outflow winds slacken off late Sat
as the cold air mass that was fueling it moves off to the east.
Overall flow still remains out of the north into early next week
as what remains of an upper ridge remains to our west and another
trough drops south through the Yukon. Next chance of rain does
not show up until mid next week (or earlier for the southern
panhandle), but exactly when is up for debate. Extended models are
having a terrible time dealing with a piece of energy that breaks
off from a Bering Sea low and heads into the gulf so forecast
confidence beyond the weekend is poor at the moment.
MARINE...Small Craft Advisory for PKZ012-013-031>033.
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