FXAK67 PAJK 202242
Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
242 PM AKDT Tue Mar 20 2018
.SHORT TERM...Satellite imagery this afternoon showing a low
pressure system in the NE AK Gulf tracking to the south. The front
associated with this low produced mix of rain and snow overnight
for a majority of Panhandle. Precip transitioned to showers as
cold air aloft moved in creating unstable conditions. Model
sounding lapse rates of 5 c/km were confirmed by the PAYA 12z
sounding. As the low center moves south shower bands were expected
to increase under deformation zone for the northern central
panhandle. However difficult to pin point exact location of
banding. Due to warm boundary layer for most locations (expect the
far norther regions) snow fall accumulation at lower elevations
limited to less than 1 inch, with 1 to 2 inches for the northern
regions. As cold air continues to move in behind the low over
night temps will drop below freezing, which will aid in snow
accumulation for remaining scattered showers. Tuesday night into
Wednesday will switch to a drying period as the low center tracks
south. Still expect a healthy diurnal temperatures range as
increased March daylight will help warm temps.
Main change in weather will be building high pressure over the
Yukon and tightening northerly pressure gradient between the
current Gulf low, which will have moved just off the Haida Gwaii
coast. Gale force outflow winds over the coastal waters pick up
late tonight and last through Thursday. Expecting at least max
northerly gales for the northern inner channels and 40 to 50 mph
gusts out of interior passes. Long wind fetch should increase
waves heights down Lynn Canal and into N Chatham Strait. While
Downtown Juneau and Douglas will be windy at this time not
expecting a true Taku wind event and for now have gusts at 50 mph.
While getting good cross barrier flow and ridge top temp inversion,
not getting the necessary critical level at 500 mb, where wind
speed/direction shift, which promotes mountain wave development,
since the vertically stacked low is very deep.
Southern regions will likely be back into a wet pattern as the
Haida Gwaii surface low tracks east moving in more precip bands.
Not expecting these bands to move much further north than Sumner
Strait and as of now no significant QPF. Did start to warm surface
temperatures for those areas due to the more SE'ly flow so likely
see more rain than snow.
Initially models still mostly in line with inherited forecast,
did a blend of NAM/Canadian for pop field as the low was slightly
slower in moving to the south this evening. Timing of winds
transitioning from west and south to north and east may be delayed
a few hours due to the slower moving low. Some NAMNest blended for
local wind effects on Wed night and Thu. Used more GFS/ECMWF for
Thu to depict the Haida Gwaii low and increased pop moving over
the south. Forecast confidence is average.
.LONG TERM.../Thursday through Tuesday/...as of 10 pm Monday.
Little change in the overall upper level pattern through the long
range today. The nearly stationary upper low over the NE Pacific
along with the upstream ridge over western Alaska all remain in
relatively the same place as yesterday. Guidance has also not
moved much on when the upstream ridge breaks down (still forecast
for Saturday) and we transition to more zonal flow by next week.
It is past that pattern change that guidance is still having
troubles. Timing of individual short waves for early next week is
low confidence at best at the moment, but the general trend is for
a W to SW zonal flow.
At the surface, a cold air mass dropping S from the Yukon
dominates the late week period with outflow winds up to gale
force blowing out of the usual outflow areas. Highest winds appear
to be Thu as one last trough from the departing upper low and the
building high in the Yukon combine to strengthen pressure
gradients across the panhandle. Main changes here were for local
effects to strengthen winds in some areas. There is also the
possibility of some Taku winds developing for downtown Juneau and
Douglas on Thu through the lack of a critical level will likely
limit observed wind speeds.
Into the weekend and early next week, the pattern shift aloft
results in a more active and wet pattern for the panhandle.
Generally favored ECMWF and WPC for guidance out here as the 18z
GFS had a much further south storm track (and a drier forecast as
a result) then either the ECMWF, WPC, or the ensemble means were
showing. The new 0z GFS offered more support to the wetter
solutions of the ECMWF and WPC by trending further north with the
storm tracks for next week. Decided to keep (and in some cases
raise) the chance and likely pops that are present past Sunday.
PUBLIC...Strong Wind from Wednesday morning through Wednesday afternoon
MARINE...Gale Warning for PKZ012-042-043-051-052.
Small Craft Advisory for PKZ011-013-022-031-041-053.
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