Gale Warning 8, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
255 PM AKST Tue Dec 31 2019

.SHORT TERM...Change is in the air, and it is looking likely that
SE Alaska will finally experience a taste of winter in the days
ahead. By Wednesday afternoon a colder air mass is expected to
replace the relatively warm air that has been in place over much
of Southeast AK recently. Satellite imagery indicates that
Northerly flow is driving cold arctic air across the Aleutians and
into the western Gulf. A strong upper wave has developed into a
mid level low pressure system near the AK Peninsula and is
generating rain through much of SE Alaska, while on Wednesday a
southerly stream wave will lift towards Haida Gwaii.

Southwesterly flow will turn more southeasterly and moderate
through the day which will likely result in decreasing rain
showers especially across parts of the central and southern inner
channels this afternoon. During the overnight hours, the
diminishing winds and lack of rain may result in some patchy fog
across parts of the central and southern panhandle, though don't
expect it to last for very long as the next front targets SE
Alaska in the morning. The front will push across the E gulf and
works its way into the panhandle through the morning hours on
Wednesday. There is some moisture associated with this wave, and
so think that moderate to heavy rainfall rates will be possible as
the wave pushes through on Wednesday. Behind this wave front a
westerly push of colder air will begin moving across the entire
panhandle Wednesday afternoon. As the cold air moves across the
panhandle, rain showers will begin changing over to snow showers
Wednesday afternoon and persist overnight and through Thursday.
Surface temperatures will continue to drop allowing for better
chance of snow accumulation late. Snowfall amounts will likely
vary widely across the area due to the nature of showers. Bands of
snow showers will result in variable accumulations, with
accumulations ranging from 1 to 6 inches possible depending on
where the bands of snow line up.

On Thursday afternoon, models are in fairly good agreement that a
wave will ride up the south stream of the jet and that, should it
reach far enough north, impact parts of the southern panhandle
through the latter half of Thursday and much of Friday. Although
it is too soon to tell for certain, it does currently look like
this has the potential to give some additional snow to parts of
the panhandle, including Ketchikan. Accumulations will largely
depend on the track that this low ends up taking. The GFS
brings the center of the low right through Ketchikan, while the
Euro takes the center through Haida Gwaii, and the Canadian keeps
the system south of the area entirely. Will need to get better
consensus before saying anything with confidence, but for the time
being, am leaning more towards the GFS as it seems to have a solid
grasp of the current synoptic setup. Overall forecast confidence
is average.

.LONG TERM...Thursday onshore flow will be weakening as ridging
begins to nose into the Panhandle ahead of a developing system
well to the south. Thus with far less forcing, we lowered QPF
amounts across Southeast Thursday, and thus snow totals. And given
the showery nature of the precipitation, we feel limiting amounts
to a trace to a couple of inches of snow works well for this
period. We also lowered snow ratios to around 80 percent of
modeled values to account for less forcing.

Now with such a cold air mass settled across the gulf, the zone
of deepest baroclinicity along with the strongest jet will be
squarely in latitudes south of us. So to monitor storm develop
that is the direction we turn. We have seen hints of a storm
developing in certain model runs over the last couple of days,
but agreements and ensembles have warmed to the idea sufficiently
to make bigger changes from Thursday night into the early weekend.
What do we see? This storm will spark an outflow offshore wind
"event" and a heightened possibility of significant snow for
portions of the south.

For starters, we nudged upward northerly winds through the Inner
Channels up to small craft through the central areas and touching
gale in Lynn Canal. We also touched upwards winds through the
communities most inclined to be responsive to such outflow, namely
Downtown Juneau/Douglas, Skagway, and Haines. Should outflow be
strong enough, we may need to bump up the winds in other areas
such as Tenakee Springs, Klawock, Gustavus, and Kake, among
others.

Then the subject turns to snow. Temperatures will be cold enough
to support snow at times, even in the First City of Ketchikan.
What seems most clear is that Hyder looks to see yet another
significant snow event as until now winter has only resided in
Hyder, Haines, and Skagway. We are looking at possibly a foot of
snow from Thursday night into Friday as the storm to the south
approaches Dixon Entrance. Perhaps even more interesting to watch
will be guidance trends for temperatures in Ketchikan and Prince
of Wales Island. While we warm them to mid to high 30s on Friday,
should outflow be strong enough, it could allow for a potentially
significant overrunning snow event for the southern Inner
Channels. Currently we favor 1 to 4 inches for Prince of Wales
with maybe 3 to 5 for the Ketchikan area, even with a rain
changeover Friday. Even farther north, the cold air will
undoubtedly keep the precipitation mainly snow, so Petersburg and
Wrangell should monitor changing trends in the forecast. Our
experiences respect the efficiencies of overrunning precipitation
events, and the 2 to 4 inches of snow Friday may need to be
raised. Fresh model runs from both the 00Z ECMWF and the 00Z GFS
are taking the storm to the south on a track east of the old runs.
This would favor less QPF particularly for the coastal, the
northern, and central Panhandle. But trends have not been as
consistent as we would prefer.

Snow showers look to stick around into the early weekend, after
which consistency falls apart. However, outlooks still point to
cooler temperatures than normal with little direction on
precipitation. With the baroclinic zone not too far away to our
south and southwest, we suspect all areas of Southeast Alaska are
ripe for at least one or two overrunning events good for at least
a modest snow event. But in this winter of 2019-2020, modest
means significant.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Areal Flood Advisory until 4 PM AKST this afternoon for AKZ028.
MARINE...Gale Warning for PKZ041-042.
     Small Craft Advisory for PKZ011-012-021-022-035-043-051>053.

&&

GFS/JWA

Visit us at http://www.weather.gov/Juneau

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE JUNEAU AK
149 PM AKST TUE DEC 31 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 AM     FLOOD            1 NNE WARD COVE         55.42N 131.71W
12/30/2019                                      AK   PARK/FOREST SRVC

            LOW LYING PORTIONS OF THE TRAIL SYSTEM ALONG WARD LAKE
            RECREATION AREA WAS REPORTED FLOODED.

0800 AM     FLOOD            1 NNE WARD COVE         55.42N 131.71W
12/30/2019                                      AK   PARK/FOREST SRVC

            LOW LYING CAMPGROUNDS AND SIGNAL CREEK ARE FLOODED IN THE
            WARD LAKE RECREATION AREA.

0800 AM     FLOOD            1 NNE WARD COVE         55.42N 131.71W
12/30/2019                                      AK   PARK/FOREST SRVC

            THE ROADSIDE DAY USE AREA IN THE WARD LAKE RECREATION
            AREA IS FLOODED.

&&

ABJ

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