Small Craft Advisory 409, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
300 PM AKDT FRI JUN 3 2016

.SHORT TERM...A potent low pressure system that looks more like a
fall storm than early June will continue to bring active weather
to the panhandle through tonight and into early Saturday. This is
all being driven by an anomalously strong 130 kt southwesterly
upper level jet. As this system continues to lift into the
northeast gulf and weakens, a ridge of high pressure will rapidly
build northward over the panhandle. This is already evident over
the southern panhandle with surface pressure having risen over 2
mb the past hour.

A 40-50 kt southeasterly low level jet has generated strong wind
gusts up to 45 mph over the southern panhandle this morning. This
low level jet will spread northward reaching the central panhandle
early this evening and the northern panhandle late tonight.
Another area of gale force winds will develop late this afternoon
on the southern periphery of the low. This area of enhanced wind
speeds is expected to reach Sitka by late evening, and aid in a
period of strong wind gusts during the night. The strong pressure
rises mentioned above will spread northward across the area
eventually reaching Skagway by late tonight. This will result in a
period of rapidly increasing winds for the northern Inner
channels. Based upon latest model guidance opted to raise gusts up
to 40-45 mph for Skagway, Haines and Juneau which meets strong
wind criteria. Winds will relax across much of the area on
Saturday but will linger longest in Lynn Canal and into Skagway.
Continued the strong wind headline for Skagway through much of the
day.

A period of light to moderate rainfall is currently spreading over
the central and northern panhandle. 6 hourly rainfall amounts with
this band of precipitation have been a quarter inch or less. A
more significant area of rainfall is currently developing over the
southern panhandle in response to increasing orographic ascent as
the second wrap begins to push inland. Along with increased moisture
flux on the southern side of the low, this will result in an
expanding area of moderate to locally heavy rainfall. The area of
rainfall will spread northward over the central panhandle early
this evening and into the northern panhandle late tonight and
overnight. This continues to be depicted very well by Hi-Res model
guidance. Rainfall totals of up to 1 inch are possible as this
progresses northward. On Saturday, precipitation will diminish
from south to north with a mainly dry afternoon on tap for the
southern half of Southeast Alaska.

Few changes to previous forecast. For what changes were made,
relied on Hi-Res guidance. Forecaster confidence is above average.

.LONG TERM...Another significant system moves into the region for
Sunday followed by a potential dry period for the next week. The
strong upper level jet will have moved east by Sunday with upper
trough moving in through mid week. Typically model agreement is
good for upper level features this time of year, but seeing fair
amount of spread with this trough. At this time the mid level
trough expect to move over the panhandle Sunday with potential
deep low reflected at the surface. This next surface low expected
to track closer to the panhandle coast near Cross Sound while bringing
a long fetch of tropical moisture. Potential heavy rain event for
the central panhandle with 6 hour QPF values upwards of a half an
inch. Another round of stronger winds may accompany this low.
Although models, such as the GFS and ECMWF, have shown fair run to
run consistency have a wider spread between models. 12z GFS/ECMWF
depict a deeper, faster moving low, while the 12z NAM/GEM have a
slower, weaker feature. 18Z NAM is starting to fall in line with
the GFS/ECMWF which continue to be the models of choice. Even
with these differences model QPF values all supported a heavy rain
event. Made little change to the Sunday forecast, but may need to
increase winds for the central panhandle if the ECMWf, depicting
the strongest low, is correct.

As the Sunday system moves inland high pressure ridge builds over
the AK gulf and the moves over the panhandle. Short waves moving
over the ridge keep the northwest panhandle under chance pops
through mid week with drier weather for the rest of the region.
Onshore flow and stable air under the ridge set up for marine
layer formation, so while there may be little in way of measurable
rainfall, a low cloud deck and drizzle are not out of the
question. Later in the week another wave crossing the southern gulf
may hit the southern panhandle for increased precip chances.

Kept with a GFS/ECMWF blend through day 4 then transitioned to
new WPC for more of an ensemble solution. Main change was enough
of an increase to QPF on Sunday for heavy rain. Otherwise only
minimal changes to the forecast. Low confidence due to the
uncertainty on the Sunday system and the differences on upper
level trough analysis mid week.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...STRONG WIND until 7 PM AKDT this evening for AKZ026>028.
     STRONG WIND from 7 PM AKDT this evening through late tonight for
     AKZ025.
     STRONG WIND from 10 PM AKDT this evening through late tonight
     for AKZ023.
     STRONG WIND from 1 AM AKDT Saturday through Saturday morning for
     AKZ019.
     STRONG WIND from late tonight through Saturday afternoon for
     AKZ018.
MARINE...Gale Warning for PKZ036-041-042.
     Small Craft Advisory for PKZ012-013-022-031>035-043-051>053.
&&

TPS/PRB

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