Gale Warning 285, NWS Juneau, AK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
307 PM AKST Sun Nov 29 2020

.SHORT TERM...This afternoon we sit on the edge of our seats
with eyes locked on an approaching storm force low that enters
the gulf Monday afternoon. This storm will blanket the panhandle
with a variety of hazards Monday through Thursday. But before
getting carried away, lets talk about this evening through Monday
morning. Satellite imagery shows a surface low located SE of
Kodiak Island streaming onshore showers across the northern and
central panhandle. This low will move into Prince William Sound
through the night allowing for shower activity to become scattered
across the north and central panhandle. An associated frontal
band has blanketed over the southern panhandle. Areas from Sitka
north have been seeing widespread showers through the afternoon
while areas to the south have been seeing more organized precip
with moderate to heavy rain at times. Furthermore, this evening, a
low will develop and surge north along the frontal boundary
impacting the southern panhandle. This will allow for precip rates
to stay elevated dropping an additional 1 to 2 inches of rain.
With continued rainfall, rivers have been rising across the south.
An areal Flood Advisory has been issued for the southern Inner
Channels until 9 PM tonight. Minor flooding is possible especially
for areas around Thorne River and Ward Lake. By late tonight,
precip will become more scattered as the low becomes displaced to
the east into BC.

Monday morning, weak surface ridging builds through the inner
channels while onshore flow keeps isolated to scattered showers
moving through the panhandle. These conditions persist ahead of
the next large system. By Monday evening, a storm force front will
have entered the central Gulf and will be heading towards the
panhandle. Winds will begin to increase through the inner channels
and over land Monday evening ahead of the frontal band. Storm
force winds along coastal marine areas along with high end gales
are expected through inner channels Monday night into Tuesday
morning. Over land, sustained winds 20 to 30 mph with gusts up to
55 mph are expected. For Yakutat and Sitka gusts to 60 mph are
possible prompting a High Wind Watch for the surrounding areas.

Not only will this system cause high winds but it will also
bring a atmospheric river producing widespread heavy rain with 3
hr rainfall rates anywhere between 0.50 to 1.00 inches. This has
historical significants has some areas will likely reach near
record rainfall amounts. With consistent heavy rainfall rates
flooding becomes a concern. A Flood Watch has been issued for the
central and northern panhandle. This storm will have deep moisture
support with a large fetch from the southern Gulf. Moisture
support will remain through Wednesday night allowing for
impressive 48 hr rainfall amounts between 5 to 10 inches across
the north central panhandle with 2 to 4 inches across the south.

With December fast approaching, lets not forget about the winter
weather potential. Although temperatures will not be conducive
for snow due to strong warm air advection with advancing frontal
band, areas across the far north will hold on to colder temps the
longest. Building high pressure in the Yukon will keep
temperatures near Haines Boarder Customs as well as White Pass
below freezing ahead of the front. With heavy rain rates producing
large QPF values combined with subfreezing temps, a Winter Storm
Watch has been issued from Monday night through Tuesday night for
the highways near Haines Boarder Customs and White Pass. The city
of Haines and Skagway are expected to see all rain as temperatures
surge above freezing.

Overall, a very busy week approaches. Analysis will continue as
this storm continues to approach the panhandle. Confidence is
moderate with the wind and heavy rain aspects of the forecast
with lower confidence on winter weather along the highways.
Ultimately, there remains room for forecast improvements as
better guidance and understanding of local dynamics develop.

.LONG TERM.../Tuesday through Sunday night/ Tuesday starts with
the atmospheric river in place across the northern panhandle and
the weather front running southward towards Hawaii. A developing
wave on the front will move along it into the northern panhandle
Wednesday enhancing the heavy precipitation for another 18 hours
or so into early Thursday morning. The front then moves into
Western Canada by Thursday midday. Rain will be letting up or
even briefly stopping by that afternoon. Totals from the event
are looking very impressive based on the climate stats, and are in
ranges that flooding and other related heavy rain issues become
much more likely. This will be looked at closely over the next
couple of shifts to monitor.

Warm air being driven north ahead of the front will have warmed
most highs in the 40s to near 50. Behind the front expect some
minor cooling, however most freezing levels will still be between
1000 and 2500 feet for the northern panhandle.

Depending on the intensity of the warming, and the snow level, the
higher elevations of the Klondike Highway could see a significant
event near the summit with accumulations 1 to 3 feet, thats only
if the warming doesn't completely rise above the 3000 foot level
there.

After that front, the confidence in the forecast shifts
drastically as cross model agreement has flown out the window.
They agree that there will be a low in the gulf Thursday, but
strength has differences of 15 to 20 mb and placement is roughly
500 nm. The panhandle should get some more rain, and potentially
some wind that would result in at least small craft being issued
for the marines. Ensemble guidance have a general idea the
weather more than a typical fall/early winter systems may impact
the panhandle.

At the tail end of the timeframe another low for the eastern gulf
moves out of the Pacific for Sunday, while a much more
significant very deep low (near 950 mb) moves east south of the
central Aleutians.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Areal Flood Advisory until 9 PM AKST this evening for AKZ028.
MARINE...Small Craft Advisory for PKZ022-036-041>043-051>053.

&&

CC/Bezenek

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

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