Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
234 PM akdt Sat Aug 29 2015
Short term.../ Saturday night and Sunday / the higher amplitude long
wave trough along the coast is shortening to more of a dual closed
low feature with the southern center near northeast Gulf of
Alaska. The southerly return flow that had been moving rains north
over the Panhandle will be pushing inland through the evening
Late Saturday night another short wave in the upper levels
will be moving into the western Gulf. This feature has a solid
jet with speeds of 120-140 knots and with the lift on the exit
region of the jet this will help to rapidly deepen the surface
low overnight that will move into the southern Panhandle Sunday
A fair sized cold pool aloft will be moving southeast to east across
the Gulf of Alaska from Mainland of the state. This combined with
the areas of stronger vertical lift at the middle levels is
suggesting that pockets of isolated thunder may develop through
the night and continue Sunday as the cold air follows the new low
into the southern Panhandle. There was some isolated convection over
the coastal waters early Saturday morning based on satellite
imagery...so there is some support for the models.
The low moving into the southern half of the Panhandle near Cape
Decision around midday Sunday looks as if it could be deepening to
the upper 970s or near 980 mb and as such the resultant pressure
gradients for the southern half of the Panhandle will be supporting
strong gusty winds. I have gone with 35 knots on the outside and 40 knots
in the Clarence Strait area...and gusts hitting the coast of 40 to
50 miles per hour. Will need to watch this through the evening to see if the
new 00z runs are similar...in which case a potential for upgrading
to a wind watch may be warranted.
Rainfall amounts of 1 to 2 inches are expected across the southern
Panhandle and lesser amounts for the northern Panhandle as most of
the precipitation should stay south of Icy Strait.
Long term...a series of upper vorticity lobes are forecast to move southeast
across the Gulf through early next week. The first lobe will be
exiting the southern Panhandle into British Columbia at 06z Monday...followed by the
second sagging across the southeastern Gulf...and the third moving from
interior Alaska early Monday morning across the central/eastern Gulf by Monday
evening. These features will maintain shower development over the
Panhandle...and may augment ascent sufficiently over the Gulf for
isolated thunderstorm development...given the presence of cold
midlevel temperatures...steep lapse rates...and modest MUCAPE
As the third vorticity lobe pivots southeast away from the southern Panhandle Tuesday
afternoon and night...shower coverage/intensity is forecast to
decrease across the region. Surface pressure gradient will
also become northerly...with small craft winds over northern Lynn Canal
probable Monday evening through midday Wednesday. In addition...long fetch
of 20+ knots northwesterly surface flow across the Gulf will maintain seas
at or above 8 feet over the central/southern outside waters Monday and Tuesday.
Surface pressure pattern is then forecast to become weak by Wednesday
night...followed by uncertainty with regard to the next potential
storm system Friday into the weekend. This is evident by deterministic
and ensemble model guidance being out of phase with each other in
terms of surface cyclone development and position. Thus did not
lean heavily toward any particular model-solution during the
second half of the long term period.
Pressure grids were nudged toward the 12z European model (ecmwf) through Wednesday...then
wpc thereafter. Inherited pop grids were blended slightly to the
GFS and European model (ecmwf)...with very little change made to quantitative precipitation forecast.
public...strong wind from Sunday morning through Sunday evening for
Marine...Gale Warning for pkz036-041-042.
Small Craft Advisory for pkz022-033>035-043.
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