Gale Warning 272, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
320 PM AKST Sat Nov 25 2017

.SHORT TERM...Tonight through Monday...According to water vapor
satellite imagery, a weakening upper level trough extends over the
eastern gulf, and a deepening ridge extends over the western gulf,
with a strengthening shortwave digging east through the Aleutian
Islands. A strong jet can also be seen wrapping around the trough
over the eastern gulf, advecting moisture aloft from south of the
Haida Gwaii area. In addition, weak surface lows can be seen
rotating around each other in the associated upper level low, and
a strong high remains over British Columbia and extends into the
Yukon Territories. Due to this synoptic setup, outflow winds will
persist throughout most of the northern inner channels. We
upgraded Lynn Canal to Storm Warning this afternoon, as current
observations indicated as such. We expect the winds to remain this
strong through this evening and diminish through tonight. We also
have some lingering showers being reported across the panhandle.
This is due to the TROWAL associated with one of the lingering
surface lows lifting north that is supported by divergence aloft
from the left exit region of the jet that was mentioned
previously.

The main story for tonight and continuing into tomorrow is the
winds, as precipitation should be drying out through most
locations tonight. A shortwave will swing northeast into the
southern panhandle beginning tonight and lift northeast into
British Columbia tomorrow. This feature will help to tighten the
pressure gradient between the high that's already over Canada.
We've been monitoring high resolution model guidance, in addition
to current observations, for our wind analysis. These models have
been struggling with wind values through the inner channels, so
most of the local effects for wind throughout the inside waters
have been added manually. The Storm Warning will remain for Lynn
Canal through early this evening, but expect winds to diminish
overnight. High res models and mountain wave guidance also hinted
at a strong wind event for the downtown Juneau and Douglas area.
Forecast soundings for Juneau, mainly the GFS, also indicated
possible favorable conditions, so we opted to follow HRRR guidance
for timing and strength of the wind downtown; we're expecting
winds out of the northeast at 25 to 30 mph tonight and continuing
into tomorrow morning, with wind gusts to 45 mph.

On Monday, a weather front associated with a surface low
developing over the western gulf will lift northeast, increasing
winds along the offshore and eastern gulf marine zones. PoPs will
also increase along this front and bring more snow to the northern
two-thirds of the panhandle and rain over the far south once it
lifts north early Monday.

We opted to use a blend of GFS and EC for pressure adjustments and
opted for high res models and hand analysis for the winds over the
gulf and throughout the inner channels. A combination of the NAM,
GFS, and SREF was used for PoP updates.

Overall, forecast confidence in the short range is average,
trending below average due to model disagreement.

.LONG TERM...Rather different weather pattern for the Gulf of
Alaska and SE Alaska for the long range period compared to what we
have seen over the last few weeks. Mean 500 mb trough axis is
forecast to shift westward to be more over western Alaska and the
Bering Sea opening the door for warmer and wetter SW flow through
much of next week. Several shortwaves are embedded in the SW flow
pattern and promise to bring rather unsettled conditions through
late week at least.

Several new trends are apparent in the long range forecast today.
First off, the scenario of a weak low that was forecast yesterday
to advance on Cross Sound from the SW on Tue has been abandon in
favor of a stronger low crossing over Haida Gwaii late Mon night.
This has stabilized wind forecasts for the NE gulf and gulf coast
Tue with more general SW winds expected now, but has dropped
confidence on wind forecasts for the extreme southern panhandle
Mon night (due to some indications that guidance wants to put the
low even further SE and a wide spread on low strength). Used Nam
and some ECMWF to represent it as both had a decent track and
strength.

Second forecast trend is for a general increase in winds in many
areas Tue night into Wed. Several models are now suggesting a
rather vigorous front will travel east across the gulf at that
time with 20 kt (Canadian) to 35 kt (GFS) winds accompanying it.
Ended up increasing wind in the forecast with a wide area of 25 to
30 kt winds across the gulf. Winds also increased in the inner
channels to 15 to 25 kt especially for the southern panhandle.

Final trend is the general southerly flow and warm air advection
that will affect just about the entire panhandle. Warmest
temperatures appear mid week with many areas threatening to rise
above freezing by Wed. With these warmer temps snow level will be
rising above sea level and snow will be changing to rain for many
areas.

Main changes to the forecast were from Mon night through Wed with
changes in low position and strengthening fronts changing around
the wind forecast a bit. Main guidance was Nam with some ECMWF
followed by WPC guidance for later periods.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind from 9 PM AKST this evening through Sunday morning
     for AKZ018-025.
MARINE...Heavy Freezing Spray Warning for PKZ012.
     Storm Warning for PKZ012-013.
     Gale Warning for PKZ022-032-035-043-051-052.
     Small Craft Advisory for PKZ011-021-031-034-041-042-053.

&&

Voveris/PRB

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