Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
506 PM akst Tuesday Dec 10 2013
Short term...major winter storm to impact Southeast Alaska
beginning tonight and into Wednesday. 987 mb low pressure system
is tracking northeast into the central Gulf this afternoon. A
large stream of tropical moisture originating in Hawaii and
stretching into the central Gulf is accompanying the front with
precipitable water values over 1 inch. An upper level trough is
dropping southeast into the Yukon and will act to strengthen the
high pressure over that area this evening and into Wednesday.
This will set the stage for a significant snow and wind event
across Southeast Alaska.
The tropical moisture will move into the Panhandle this evening
then spread north through the night. Rain will become heavy at
times over the southern Panhandle with gusty winds as a 50 to
60 knots low level jet moves over the area. Snow will increase in
intensity across the Haines and Skagway areas overnight. The main
question continues to be exactly how far south the rain snow line
will get tonight. Thinking remains that it will be just to the
north of the Gustavus and the Juneau Airport. Places like the
Mendenhall Valley and farther north in the Juneau area will
likely hold in just enough cold air to see wet snow overnight but
feel accumulations will be limited due to the nature of the
snowfall. As low level cold air filters south Wednesday morning
surface temperatures will cool below freezing in the Icy Strait
corridor with precipitation mixing with and changing to snow by
late morning. Latest model runs have come in slightly faster with
this low level cold air and therefore a quicker changeover to
snow tomorrow morning. Snow accumulation through the afternoon
will likely be in the 2 to 4 inch range. Convergence zone will
set up across the northern Panhandle on Wednesday as warm
southerly flow aloft meets the cold northerly flow. This will
enhance lift and snowfall rates with up to a foot of snow
possible by Wednesday afternoon in Haines and Skagway. The other
area of concern is Hyder where heavy snow starts this evening and
will persist through Wednesday. Hyder will hold in enough cold
air as offshore gradient and light winds allow temperatures to
stay just below freezing. Snowfall totals through Wednesday
afternoon will likely reach up to 10 to 20 inches.
After coordinating with the long range desk decided to upgrade
the Winter Storm Watch to a Blizzard Warning for the Skagway zone
and a Winter Storm Warning for the Haines area. Also have issued
a Winter Storm Warning for Hyder. Blizzard conditions are
expected near White Pass with gusts to at least 40 miles per hour and heavy
snow resulting in prolonged visibility of less than one quarter
of a mile. Additional watches for the north central Panhandle
have been issued in the long term period beginning Wednesday
Used a blend of the NAM/ec for updates for the forecast today.
Blended in the local WRF model for winds to better capture
outflow winds in the Gulf late tonight and Wednesday. Confidence
is average over most of the forecast but some uncertainty remains
in the exact rain snow line tonight. Confidence increases in the
rain snow line pushing south through the day on Wednesday.
Long term...a very active pattern for most of the long term
forecast period. The major winter storm discussed above in the
short term discussion will continue through Thursday afternoon.
From Thursday night into Friday, there will be a break before the
next storm system impact Southeast Alaska for the weekend. The
active pattern will continue through early next week.
A very favorable flow pattern is in place for Southeast Alaska. A
high amplitude bocking is set up over the international dateline.
A strong upper level trough over north central Pacific is drawing
subtropical moisture into the eastern Gulf of Alaska. Meanwhile, a
potent upper level ridge centered over the Bering Sea is sending
upper level disturbances southeast towards the Panhandle and
British Columbia. An Arctic cold front over central Yukon, enhanced by
an upper level disturbance, is pushing southward towards the
Panhandle. This Arctic cold front will enhance vertical motion and
snowfall rate, thus this weather feature will mark the area with
the heaviest snowfall.
At the start of the long term forecast period on Wednesday
evening, the Arctic front will become stationary over
Juneau/southern Lynn/Icy Strait area. North of the cold front, in
the Lynn Canal, Haines, and Skagway area, moderate to heavy snow
will continue through the night. Strong northerly wind will keep
temperatures around 20 degrees or colder. With saturated dendritic
growth zone, and air around -5 to -15 c below the dendritic growth
zone, do expect snow ratio of around 20 to 1 in Haines and Skagway
area. 6 to 12 inches expected for Wednesday night, and then
another 4 to 9 inches for Thursday. With northerly wind gusting to
40 mph, significantly reducing visibility along The Klondike and
Haines Highway. Heavy freezing spray will be a concern for the
northern Inner Channels with the northerly gale force winds in the
area. After coordination with the short term forecaster, decided
that the Blizzard Warning for Skagway and the Winter Storm Warning
for Haines will be valid through Thursday afternoon.
As the Arctic cold front straddle across north central Panhandle,
has more uncertainty in the snowfall amounts. Thus, Winter Storm
Watch is issued for central Panhandle including Juneau, Hoonah,
Gustavus, and Elfin Cove for Wednesday afternoon through Thursday
afternoon. Current thinking is that these areas have potential to
see significant snowfall amounts, if sufficient low-level cold air
can sneak southward into the area. The stronger the low-level cold
air, the higher the snow ratio. Strong vertical air motion,
abundant moisture, and sufficiently cold air are all favorable
factors leading to heavy snowfall rate. The details of this winter
storm for these areas will be ironed out in the next forecast
This major winter storm is expected to conclude on Thursday
afternoon. A brief break is expected overnight Thursday into
Friday before the next storm system impact Southeast Alaska.
Currently, thinking this next system will be much warmer than the
Wednesday/Thursday storm. The biggest snow threat with the next
system will be along The Klondike and Haines Highway, with
rain/snow mix expected across central Panhandle. Strong winds is
also a concern with the next system. Onshore flow will persist,
thus the wet conditions will last through the end of the long term
forecast period early next week.
Forecast confidence is about average. Less confidence due to
complexity of snow physics. Snow accumulation vary significantly
due to temperature profile. Highest snow to liquid ratio will be
confined along the highways, while coastal areas will see much
lower snow to liquid ratios. Model of choice is NAM, as it is
better in detecting the mesoscale feature such as the shallow
Arctic cold air coming down The Channel. For the second half of
the long range, uses guidance from wpc.
public...strong wind from midnight akst tonight through Wednesday morning
Winter Storm Watch from Wednesday afternoon through Thursday
afternoon for akz020>022-025.
Strong wind from 3 am akst Wednesday through Wednesday morning
Winter Storm Warning until 9 PM akst Wednesday for akz029.
Winter Storm Warning from 5 am Wednesday to 4 PM akst Thursday
Blizzard Warning from 5 am Wednesday to 4 PM akst Thursday for
Marine...heavy freezing spray warning for pkz012.
Gale Warning for pkz012-013-021-022-031-036-041-043-051.
Small Craft Advisory for pkz011-032>035-042-052-053.