Small Craft Advisory 7, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
301 PM AKST Mon Jan 1 2018

.SHORT TERM...Our weather front that initially brought snow to
the panhandle late last night has made its way through, and now,
an additional weather front can be seen on satellite imagery
moving through the eastern gulf. Most locations reported a few
inches of snow this morning, and some locations even reported
freezing rain later on in the day, with Wrangell reporting
accumulating ice. This was caused by substantial below freezing
air in place near the surface with much warmer air aloft. This
allowed snow to melt while falling through the warm layer of air
and then allowed it to refreeze after coming into contact with the
colder surfaces. Many sites continue to report precipitation but
of the liquid variety as our next front begins to impact the
panhandle, save for our more northern areas, such as Haines and
Skagway, that continue to report snow. We have kept the Winter
Storm Warnings for both the Haines and Klondike Highways to
account for this longer period of snow, as the warmer air has
taken longer to reach here. Eventually, all locations will make a
full transition to rain, as strong warm air advection takes place
ahead of the next front.

This will be a relatively active pattern for the next coming
days, with warmer temperatures and higher precipitation amounts
expected than our previous pattern. At the upper levels of the
atmosphere, a broad, deep trough has developed over the western
gulf and will remain relatively steady here, while a ridge, in
some form or another, remains over the eastern gulf. This broad
trough will contain additional shortwave troughs that will be
advected from the parent circulation and will propagate east,
impacting the panhandle. With our next front moving through
tonight, we expect winds to reach majority Small Craft Advisory
levels throughout the inner channels, with Gales possible through
N Lynn Canal. We also have a Strong Wind headline for the Skagway
and White Pass areas, since we're expecting gusts of 40 mph there
with the front passing through tonight. Marine zones along the
gulf waters will remain near SCA levels through Wednesday due to
the tighter pressure gradient between these two major systems. As
subsequent weather fronts progress east through the panhandle,
winds along the inner channels will float with SCA levels, as each
short burst with the front will increase winds along the inside
waters, albeit briefly. Anomalously high precipitable water
amounts coupled with onshore flow will keep precipitation going
for most areas in the panhandle for the next few days.

We opted not to make many significant changes in the short term,
as model agreement has been lacking, especially past Wednesday.
Any changes we made to the forecast, we used a combination of the
GFS and GEM for any updates.

.LONG TERM.../Wednesday through Sunday night/ Somewhat active and
uncertain long range forecast today. Overall upper level pattern
retains the same idea of a ridge over the western US and trough
over the Bering Sea resulting in general SW flow over the forecast
area. This pattern is only interrupted in the late week period by
a short wave trough moving W to E across the area. Noticeable
differences between operational models on this trough with the GFS
much more progressive moving the trough E then the ECMWF. However
The GFS has better run to run consistency on this feature today,
plus the ensemble means and WPC were supporting this scenario as
well so leaned more in that direction for extended updates.

At the surface, the most noticeable change was for the Wed into
Thu period as current model trends showed the front that was in
the gulf at that time much closer to the panhandle. The result is
a reduction in wind speeds across the gulf (except for near the
panhandle) at that time. Otherwise, that front is still bringing
precip to the area with most areas still warm enough for it to
mostly be rain.

Past Thu, the forecast of where that front goes becomes rather
uncertain due to the differences showing up handling the upper
trough moving through that was mentioned above. The GFS favors a
much more progressive pattern with the front clearing the
panhandle by Fri morning. Some colder air is also being dragged
into the panhandle by the upper trough for Fri into Sat. The ECMWF
on the other hand develops a weak low in the SE gulf that
eventually drifts into the central gulf significantly delaying the
eastward movement of the front and keeping the panhandle much
warmer then the gfs scenario. The differences just keep piling on
from there so favored a more ensemble approach to the forecast
given the differences. Forecast confidence is below average for
Thursday and beyond.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Winter Storm Warning until 6 PM AKST this evening for AKZ019.
     Winter Storm Warning until 6 AM AKST Tuesday for AKZ018.
     Strong Wind from 9 PM AKST this evening through late tonight for
     AKZ018.
     Winter Weather Advisory until midnight AKST tonight for AKZ029.
MARINE...Gale Warning for PKZ012-022-043-051-052.
     Small Craft Advisory for PKZ011-013-021-031>036-041-042-053.

&&

Voveris/EAL

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