Small Craft Advisory 342, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
254 PM AKDT Sat Jun 17 2017

.SYNOPSIS....An upper level low over northern British Columbia will
continue moving east as an upper ridge builds from the south over
the eastern gulf, the panhandle, and western Yukon. The ridge will
shift east late Sunday as another upper low moves northeast into
the central gulf by Monday afternoon. The associated surface front
will arrive over the southern panhandle late Sunday night.

&&

.SHORT TERM.../Sunday through Monday night/ Although the upper
low that produced heavy showers yesterday and last night has moved
northeast into Canada, clouds and showers continue to wrap around
the low center and are streaming south over the northern
panhandle and northeast gulf coast as of this afternoon. This
shower activity will end later this evening and high pressure, at
the surface and aloft, will build in over southeast Alaska. This
will make for a very pleasant Father's Day Sunday, with dry
weather, warmer temps, and decreasing clouds. The break will only
last one day, unfortunately, as the next front moves into the
southeastern gulf Sunday night. This front will spread rain from
south to north through the day and then transition to showers by
Monday night. Low level warm air advection will accompany this
front, so the warmer conditions in store for Sunday will remain
even after the rain returns.

Better consensus exists now on the strength of this next front and
have increased winds over the eastern gulf to 30 kts for Sunday
night through Monday morning. No significant winds are expected
over the inner channels in response to this front apart from small
craft easterlies out of Cross Sound. More immediately, the strong
southerly pressure gradient that has existed over the far
northern inner channels for the past few days has begun to relax
and the strong wind headline for Skagway has been dropped.
Likewise for the small craft advisory for northern Lynn Canal. A
weak tip jet persists vicinity Cape Spencer and northwesterly winds
there are close to small craft intensity, but are also expected
to relax tonight.

used an ensemble approach for updates to pressure. Used GFS22 as a
foundation for winds speeds over the gulf with minor changes to
speed and intensity on the inside. Temps adjusted per GFS MOS. PoP
a blend of official with the latest SREF. QPF from RFC and NAM12.
Overall forecast confidence is average.

.LONG TERM...Despite high confidence that a low will move into the
central gulf and send a front northeast into the eastern gulf by
Monday morning, there is still significant spread in where these
features track. Thus changes were difficult to undertake with
confidence. However, models have both strengthened the system for
Monday and boosted winds ahead of it. Without a lot of reliable
guidance for change, we upped the winds across the southern and
central inner channels to 15 kt, but we foresee small crafts
possible Monday in some locations, particularly in the south.

While the wind forecast is more difficult, confidence is very high
that rain will spread across the Panhandle Monday. Therefore we
have elevated pops to 100% for much of the southern, central and
coastal Panhandle. A front pushes into Canada by late Monday
afternoon with cooler air aloft, increased instability and onshore
flow in its wake. With the low appearing to very slowly push
eastward just offshore, we feel showers will persist into mid-week
with a slight potential continuing into Thursday. In short, pops
decline slowly at first through mid week, and then faster towards
the end.

Signatures of drying toward the end of the week have been a theme
the last couple of days. But operational models are struggling
with the transition. And even ensembles indicate some weakness in
the ridge over the Panhandle and eastern gulf. However, if
anything persists in the gulf, it should be very weak. Thus, we
have introduced a warming trend late next week, while still
hedging a bit for pops.

Little change was made for Monday/Monday night given model
spread, however, we did try to improve local winds given the mean
synoptic forecast. Changes consisted of GFS/ECMWF/WPC
Tuesday/Wednesday which led to a nudge to new WPC toward late next
week. Confidence is average.

&&

.MARINE...Cross Sound and Lynn Canal remain at small craft levels
through this morning. However, marine conditions become more
favorable through the day becoming light in many areas both over
the inner channels and gulf as a broad area of high pressure
builds. Tomorrow expect winds to gradually increase over the gulf
as the next weather front moves north.

&&

.HYDROLOGY...With shower activity winding down, area creeks,
streams, and rivers are generally falling. Exceptions are the
Situk River near Yakutat and area lakes. Still, all are well below
bankfull. And then there is Suicide Basin, which is continuing to
fill based on the latest webcam imagery. Next release date TBD.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ041.

&&

Fritsch/JWA

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