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177
FXAK67 PAJK 201421
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
621 AM AKDT Fri Oct 20 2017

.SYNOPSIS...A low in the northern gulf will weaken and move west
of Cape Suckling today as a ridge builds over the Panhandle.
Friday night a gale force front presses north and arrives in the
southeast gulf by Saturday morning.


&&

.SHORT TERM...An impressively strong deformation band associated
with the base of the long-wave trough has moved across much of
the Panhandle and spawned a line of moderate to heavier showers
overnight. Pelican received 0.66 inches in 2 hours time period,
and Juneau Mendenhall Valley up around 0.5 inches between 4 and 6
AM. Another band comes in this afternoon, and thus showers,
scattered to widespread are in the forecast for all zones of
Southeast Alaska today and tonight. As Saturday`s system to deal
with swings east from the central Aleutians, a responding ridge
builds over the eastern gulf and Panhandle today. Thus
southerlies will expand northward today through the Inner
Channels. We still are concerned about the potential for 25 kt
winds through Lynn Canal later today. Elsewhere, high seas will
plague the gulf.

The system well to our west will begin advancing into the gulf of
Alaska region Friday night. Models still spar over the front`s
evolution and track once it enters the eastern gulf. ECMWF and
specific runs of the NAM indicate an impactful wave developing on
the eastern flank of the main low. Should this happen, coastal
locations may experience even gustier winds than we now indicate
in the forecast. Whatever the case, confidence is moderate to high
that Sitka, Prince of Wales Island, and Ketchikan will see gusty
southeast winds up to 50 mph. We have delayed timing a few hours
of the strongest winds. Marine zone 41 winds have been raised to
storm force southeasterly on Saturday morning, and a variety of
southeasterly small craft advisories have been posted for the
southern Inner Channels from Frederick Sound south by 15Z Saturday
morning.

Solutions differ on duration of the low and its track going into
Sunday, therefore, confidence falls quickly. Some solutions take
the low into the northern Panhandle which would extend wind into
the northern Inner channels as well.

We trimmed back mention of snow to the far north and delayed it
generally until Saturday/Saturday night. Flow today will be mainly
onshore, which despite the heavier showers and moderate lapse
rates, snow-melt cooling will be difficult to draw all the way to
the surface. However, by Saturday, northerly flow will help draw
in some drier air which may promote column cooling as
precipitation falls through the column. But at this time, pops
have been diminished as well.  Haines may be the best case given
Chilkat River drainage winds. For now, we keep the snow out of
Gustavus and Juneau in the next few days.

06Z NAM and 00Z ECMWF provided the basis for changes
Saturday/Saturday night. Forecast confidence moderate through
Saturday, but falls sharply into Sunday.


.LONG TERM.../Sunday through Friday/ Models still struggling with
weekend system so basically left Sun and Sun night forecast as is.
For Mon onward, did use latest WPC for the most part. Looking more
likely that another low will move across the area Mon into early
Tue, but models differ on track and strength. WPC seemed like a
good compromise. Later in the week, large system will likely
affect the area for mid to late week. Still some low track details
to work out with this system. This system will most likely have
remnants of Typhoon Lan embedded, which increases potential for a
powerful storm. Overall, a wet long range period is in store for
the area.

System moving in Mon into early Tue may bring some fairly heavy
rain and strong winds with it to the S of the main low track.
Another concern with this system is potential for a snowmelt
cooling event in the deformation zone to N of low track Mon night
into early Tue. Still plenty of time for these issues to be
resolved.

The mid to late week system looks very wet and windy given
infusion of tropical moisture/energy. Evolution of sfc low
development will play a big role in where and when heaviest rain
and strongest wind will occur. Models have been all over the place
on this so confidence in the details remains on the low side.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Gale Warning for PKZ041-042.
     Small Craft Advisory for PKZ033>036-043-052.
     Small Craft Advisory due to Seas for PKZ022-051.

&&

$$

JWA/RWT

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