Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
347 PM akst Wednesday Dec 11 2013
Short term...major winter storm ongoing across Southeast Alaska
this afternoon and will continue through the first half of
Thursday. 985 mb low pressure system in the Gulf and high pressure
over the Yukon will gradually weaken through the short range
period. The combination of these two features is what has
generated the complex weather situation over the Panhandle. Moderate
to heavy snow has fallen today north of a line from Elfin Cove-
Hoonah-Juneau. South of this line precipitation has mainly been
in the form of rain. The tropical moisture feed that originated in
Hawaii and was associated with this frontal system has resulted in
significant snow and rainfall totals. Snowfall reports have been
highly variable due to the marginal temperature profile as well
as wind. Precipitation type in the transition zone between rain
and snow has largely been determined by precipitation rates. The
Mendenhall Valley and out the Road has seen primarily snow with
several inches of accumulation while downtown has seen mainly rain
with little or no accumulation. Snowfall totals in the Haines and
Skagway areas have ranged from 6 to 18 inches with blizzard
conditions reported along The Klondike Highway. 24 hour rainfall
totals across the southern Panhandle have ranged from 2-5 inches
with moderate rain still continuing.
Precipitation intensity is increasing again across the central and
northern Panhandle this afternoon and will continue tonight.
Expect that precipitation rates will be moderate to heavy for much of the
night before gradually decreasing through the day tomorrow. In
general an additional 1 to 2 feet of snow is expected through
tomorrow afternoon. The rain-snow line will gradually shift south
to around Tenakee and Angoon by late tonight as northerly winds
bring low level cold air into those areas. Snow will become a bit
drier overnight as temperatures cool aloft from Juneau northward.
This will act to increase snow ratios allowing for snowfall to
accumulate more quickly. Decided to issue a Winter Storm Warning
for the Yakutat area as a deformation band will waver over the
area through midday Thursday. Snow will be heavy at times with
several inches of accumulation by the time snow diminishes
Strong northerly gradient has resulted in gale force winds across
the northern Inner Channels and gusty outflow winds. While winds
are not making it to the surface in Skagway...reports along The
Klondike Highway indicate 30 to 40 miles per hour winds with considerable
blowing and drifting snow resulting in white out conditions.
Strong low level jet across the southern Panhandle has resulted in
frequent gusts to 40-45 miles per hour today. Lincoln Rock reported gusts up
to 60 kts earlier this morning. Expect northerly gradient will
remain in place through the short term period with only a slight
weakening in the winds Thursday afternoon. Wind gusts across the
south will decrease this evening as low level jet weakens and
shifts southeast of the area.
Used the NAM/GFS for updates to the forecast. Most updates were
local edits but did expand precipitation field to the northwest
into the Yakutat area based on latest models. Forecaster
confidence is above average in continued heavy snowfall across the
northern Panhandle with the rain snow line sinking slowly south
Long term.../ Thursday night to Wednesday / the surface trough as
moved east into British Columbia while a weakening residual low
south of Kenai Peninsula will dissipate by Friday morning. Models
are concurring that there is system to spread out of the central
Pacific Ocean that will be lifting northeast into the Gulf of
Alaska spreading the warm moist air. Cold air has spread back into
portions of the northern Panhandle again that should lead to rainy
or overrunning snow event for the northern half of the Panhandle
late Thursday into Friday. This will also spread a band of gale
force winds across the coastal areas.
This last low will be lifting to the northern Gulf through
Saturday afternoon with the onshore flow pattern that will likely
mean showers for southeast into early of next week. The lower
level temperatures also likely to mean it will be snow showers
headed into next week.
Confidence factor is roughly average into the weekend and below
average for the later time frame. Later solutions from wpc seem to
be relying on an ensemble solution.
public...Winter Weather Advisory from midnight tonight to noon akst
Thursday for akz024.
Winter Storm Warning until 4 PM akst Thursday for akz019-020-025-
Winter Storm Warning until noon akst Thursday for akz017-021-022.
Blizzard Warning until 4 PM akst Thursday for akz018.
Marine...Gale Warning for pkz012-013-021-022-031-032-034-036-043-051.
Small Craft Advisory for pkz011-033-035-052-053.
Small Craft Advisory due to seas for pkz041-042.