Gale Warning 282, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
249 PM AKST Fri Dec 29 2017

.SHORT TERM...According to water vapor satellite imagery, an
upper level ridge of high pressure currently extends north of
British Columbia, Canada, while a weakening shortwave low over
the southeastern gulf pushes east. This set up exemplifies a Wrex
Block pattern, with a quintessential "high over low," which has
contributed to our days of cooler temperatures and lack of
precipitation. This pattern has also provided us with the offshore
flow, where dry and cold continental Canadian air has been the
source for our air mass. At the surface, a low associated with the
shortwave over the gulf has helped in tightening the gradient
between it and a very strong surface high over Canada.
Subsequently, this has provided very strong winds in certain
areas, mainly through Lynn Canal and for the Skagway and Juneau
areas, as my poor Christmas lights on Douglas Island can attest
to. It must be mother nature letting me know the holidays are
over.

As the shortwave continues to transition east tonight, the
gradient should begin to loosen, and we'll start to see winds
diminish across our area and continue Saturday. Generally,
Saturday will be a quiet weather day, save for possible snow
showers in and around Yakutat Bay. Beginning Sunday and
continuing into Sunday night, a weather front associated with a
developing low over the western gulf, will lift northeast across
the eastern gulf, increasing winds along eastern gulf waters. In
addition, most of the operational models continue to show a weak
surface low developing out of the southeastern gulf Sunday
afternoon into early Monday morning and moving north-northeast
along the panhandle coastline. With enough cold air already in
place, this feature may help to condition the environment for an
overrunning event, with the possibility of some, dare I say, snow
for New Year's Day. However, models currently have a large spread
regarding this feature, so there is some uncertainty with this
forecast.

Generally, we didn't make any significant changes to the forecast
for tonight through Saturday night, as its been holding
relatively well. The operational models still have some spread
going into Sunday and Sunday night, so we opted to make updates
using a combination of GFS, ECMWF, and GEM for pressures and
winds and a combination of NAM and GEM for precip chances, QPF,
and some snow elements.

.LONG TERM.../Sunday through Friday as of 10 pm Thursday/ Overall
pattern for the first few days of 2018 features the return of the
US west coast upper ridge with a trough over the Bering Sea. For
the Panhandle this means the return of warmer more active weather
as general SW flow establishes itself. This pattern is not
expected to change much through the rest of the long range period.
The exception is possibly for next Friday as there are
suggestions that a weak trough may move through at that time
bringing some cooler air in its wake.

Trickiest part of the forecast will be the Sunday night into
Tuesday period as two weather fronts advance on the panhandle.
Several things complicate the forecast. The first is that mid
range models are not handling the wave structure on the first
front very well (some runs are developing another weak low along
the front Sunday night). This is causing some models to bring
more precip and wind into the panhandle east of Cape Fairweather
then others. Second is how fast various areas warm up (many areas
are expected to be above freezing by Tuesday at the latest). The
result is a somewhat uncertain forecast for Sunday night into
Tuesday regarding winds and precip amounts. Current forecast has
most of the higher winds staying offshore. The northern panhandle
also has the potential for at least a few inches of snow before
temps warm up enough to change it to rain, but the possibility is
there for lower or higher amounts.

Mid to late week period remains active as another front moves
across the gulf. Timing and structure of the system remains
inconsistent between models and model runs at this time so a more
ensemble approach to the forecast with WPC and ECMWF were used
where needed.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 3 PM AKST this afternoon for AKZ018.
     Wind Chill Advisory until 9 AM AKST Saturday for AKZ018.
MARINE...Gale Warning for PKZ012.
     Small Craft Advisory for PKZ013-021-022-031-035.

&&

Voveris/EAL

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