High Wind Watch 7, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
318 PM AKDT Tue Oct 23 2018

.SHORT TERM...This has been a fairly difficult weather setup to
forecast for through our short term period. The low center of our
most recent system which gave us storm force winds early this
morning was near Middleton Island as of this afternoon, as the
SLP at the Middleton Island observation site indicated 965 mb. It
has since moved on to be nearly over Cook Inlet, and IR satellite
imagery continues to show this low occluding and maturing,
indicating that it's weakening. Despite the weakening, this low
continues to be a very dynamic system. The associated front
has stalled just along the Alaskan panhandle, causing some winds
to pickup on land. Lightning has also been detected along the NE
Gulf Coast, and the potential for thunderstorms will persist
through tonight for coastal areas along the eastern gulf. CAPE
around 300 J/kg and lapse rates in excess of 7 degrees C/km should
help provide enough instability for stronger cells to develop as
the cooler air wraps in aloft. WV satellite imagery as of this
afternoon is already showing signs of cellular development. Due to
the convective nature, precipitation will continue, albeit in the
form of showers, but precip amounts should less as we further
lose moisture transport. Winds are expected to lighten somewhat
tonight as the front weakens, but this will be short-lived as
another, stronger low rears its head.

Beginning Wednesday morning, a stronger low will develop over the
southeast gulf and push north through Thursday. This low appears
to have a much sharper baroclinic zone to work with, as compared
to the low that impacted us yesterday. This will lead to a rapid
strengthening of the low Wednesday afternoon into the overnight
hours. Winds will definitely be the biggest concern with this
feature. Storm force winds are expected along the eastern gulf
marine zones beginning tomorrow afternoon, and strong gales will
move into the inner channels, mainly Clarence Strait and Chatham
Strait, through tomorrow night. Strong SCA level winds can be
expected for much of the remaining marine zones within the inner
channels. As for land sites, the highest winds are expected to
occur for Zones 23, which includes Sitka, and 27, which includes
Hydaburg, where gusts to 60 mph may be possible for locations
within these zones. We do currently have a High Wind Watch out for
both of these zones. Many other locations can expect Strong Wind
conditions with gusts to around 40 mph. Another interesting aspect
with this weather setup will be the opportunity for Juneau to
receive a one-two punch with the winds from this next low. Due to
a strong ridge positioned over western Canada and the placement of
this low tomorrow afternoon, downtown Juneau and Douglas can
expect gap winds up to 25 mph in addition to the strong winds that
will come up through the channels with the low Thursday
afternoon.

Precipitation chances will also increase again as this low lifts
up to the NE Gulf Coast through Thursday, and showers should
transition to more stable stratiform rain by tomorrow. Once again,
thunderstorms will be a concern by early morning Thursday. The
possibility looks to continue through the day for southern areas
and eastern gulf marine zones as the cooler air wraps in aloft,
giving us the needed instability.

This forecast has been difficult in that even though
deterministic models for the most part agree to the development of
this low, tracking the low center remains elusive. Where the
center develops will determine where the higher winds set up shop.
This is why we've issued only a watch at this point and for these
locations. We also decided to use the GFS/EC with the forecast
today, as these both had better agreement, including with the
Canadian. These models moved the low center slightly west but
didn't deplete its strength. The NAM run from this morning when
work began was the outlier and continued to bring the low more
east than the others. As more model runs come in tonight, we
should have a better idea on the placement of things. For this
reason, forecast confidence remains above average regarding windy
conditions but average to below average on just how high winds
will get and who will see the strongest winds.

.LONG TERM.../ Thursday to Tuesday / Our attention continues to be
drawn to the most intense storm in a series of A-list stars due to
impact us Wednesday night/Thursday. As last week we endured an
atmospheric river meandering through the northern and central
Panhandle, this week's story details a barrage of giant Pacific
cyclones hurling northward into the Gulf of Alaska. Quick heavy
douses of rain but more importantly periods of high winds along
the outer coast.

Guidance for the storm Wednesday night Thursday continues to
center around the low 960s, with the NAM climaxing at 959 mb.
Today, we extended its stay in the gulf and focused much of our
attention on Thursday, as we think the Inner Channels will
continue to breezy from the south. High wind watches are probable
for this system along the outer coast including Sitka after
Tuesday's front passes and the winds abate in Yakutat, again along
the coast. Models seem sure of the possibility, flaunting us
meteorologists with 80 kt 925 mb winds. Inland from the coast,
winds will even gust between 40 to 50 mph including Juneau and
Ketchikan. While we did not add any thunderstorms to the forecast
on Thursday, we acknowledge some unstable air with decent lapse
rates with shortwave support approaching the southern Panhandle.

Looking forward, the low exits westward which opens the door to a
brief period of ridging ahead of an increasingly likely weekend
system. However, do not expect much of a break from the rain as
showers will remain in the forecast. The weekend system looks to
be in the form an east-west type front to move northward. This
would favor precipitation across the south, and not so much in the
north, welcome news to those most affected by the drought. But
winds appear very strong with the system as well, so Prince of
Wales Island will potentially see more wind this weekend.

We used an ensemble of global models and the NAM for the 960 mb
low Thursday, and moved toward WPC by Friday/Saturday, with
letting the forecast remain idle Sunday onward. Finally,
low temperatures were raised to account for an active, breezy, and
wet pattern.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...High Wind Watch from Wednesday morning through late Wednesday
     night for AKZ023-027.
     Strong Wind Wednesday afternoon for AKZ025.
     Strong Wind Wednesday afternoon for AKZ022-028.
MARINE...Storm Warning for PKZ041>043.
     Gale Warning for PKZ033-035-036-051-052.
     Small Craft Advisory for PKZ013-021-022-031-032-034-053.

&&

Voveris/JWA

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