Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
258 PM akdt Monday Aug 31 2015
Short term...upper vorticity lobe positioned over the northern Gulf early
this afternoon is forecast to move to the southeastern Gulf by 12z
Tuesday...and then toward a position immediately east of haida gwaii at
00z Wednesday. As this occurs...showers over the northern Panhandle
/including snow showers at White Pass/ will diminish in
coverage/intensity as large-scale ascent dissipates and drier
offshore flow regime develops.
Farther S...showers will continue over the southern Panhandle tonight
and then begin to dissipate during Tuesday as the upper disturbance
moves S of the region. In the meantime...model guidance shows 500
mb temperatures near -28 c will accompany the southeastward moving
disturbance. This will continue to yield steep lapse rates and
modest MUCAPE values favorable for isolated thunderstorm
development over the southern half of the Gulf...and adjacent southern
Panhandle during Monday night.
Development of offshore flow regime going into Tuesday will be
associated with a tightening surface pressure gradient over Lynn
Canal. As a result...prolonged period of 25+ knots northerly winds will
develop over the canal by 09z Tuesday. Small craft gap winds through
Cross Sound are also anticipated.
No change was made to inherited pressure grids. Changes to winds
were limited to local effects. Pop and quantitative precipitation forecast were updated with a
coarse ensemble blend.
Long term...surface and upper level low will be south of the
forecast area by the start of the extended range forecast period
and remaining shower activity on this side of the Coast Mountains
will clear out Tuesday night. High pressure over the Yukon
combined with an inverted trough extending northwest over the eastern
Gulf will promote dry weather and clearing skies. Strong northerly
winds over the far northern Inner Channels mentioned above will
continue and prolonged nature of these winds will result in seas
over Lynn Canal and northern Chatham Strait higher than what would
normally be associated with 20 to 30 knots winds. The inverted
trough will break down as a surface ridge build in from the
central Gulf on Thursday. This will also lead to a weakening of
the northerly pressure gradient over the far northern channels and
a steady decreasing of winds through Thursday evening. Offshore
winds will also diminish as a result of the proximity of the
higher pressure ridge over the Gulf, so mostly cloudy conditions
will spread inland once again beginning Friday morning. By Friday
afternoon, a low will have moved over Kodiak and the associated
front will be approaching the Panhandle from the west. Model
differences this far out in the future prevent any meaningful
temporal or spatial precision in a precipitation forecast, but it
is starting to look like the Labor Day weekend could be a wet one
for most of Southeast Alaska. Sorry about that.
No changes made to the early portion of the long range forecast.
Updated Thursday and Thursday night with a blend of European model (ecmwf) and GFS
for wind directions over the Gulf. Wind speeds derived from GFS.
Remainder of the forecast heavily weighted towards wpc, which also
resulted in an increase in pop over Southeast Alaska beginning
Saturday morning. Overall forecast confidence is average.
Marine...Small Craft Advisory for pkz012-022-041.
Small Craft Advisory due to seas for pkz042-043.
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