Scientific Forecaster Discussion
Southeast Alaska forecast discussion
National Weather Service Juneau Alaska
415 PM akdt sun Sep 21 2014
Short term...rapidly evolving weather situation this afternoon
with storm force low in the eastern Gulf moving north. As expected
there has a quick switch to gusty southerly winds across the
southern Panhandle with frequent gusts to 40 miles per hour. Rain continues
to fall over much of the area with the heaviest precipitation
rates over the northern Panhandle. This will continue to be the
focus of the heavier rainfall for this evening.
As the low continues to move north these winds will spread over
the central and northern areas. A couple of things will aid these
stronger winds...strong pressure rises as the low pressure system
jumps inland later this evening, 55-65 knots low level jet moving
overhead, and cold air advection which will help to destabilize
the atmosphere and Transfer those winds aloft to the surface.
Therefore have small craft to gale winds across many of the Inner
Channels with strong wind headlines for several land zones this
evening. Winds will gradually diminish overnight...lingering
longest across the far northern Panhandle including Lynn Canal and
the Skagway area.
Significant cold air advection aloft behind the front with 850 mb
temperatures falling as low as 0 c overnight. This will
transition to a more unstable environment with showers becoming
the main precipitation type instead of steady stratiform rain.
Some of these showers may be heavy at times especially in the
vicinity of Yakutat. Have included isolated thunderstorms tonight
and Monday in the Gulf with cape values around 500 j/kg and lifted
indices around -2 to -3 c. Will need to monitor whether this
thunderstorm potential could spread into the outer coastal areas.
Made few changes to the previous forecast as it remained
consistent with forecaster thinking. Confidence is above average
in widespread wind event this evening with diminishing winds and
showers persisting through Monday afternoon.
Long term...by Monday night the current storm in the eastern Gulf will
be dissipating and jumping inland into the Yukon. European model (ecmwf) and GFS
trend toward a brief dry period middle week...especially northern
Panhandle. New upper level low center approaches southern
Panhandle about Wednesday for slowly increasing chance of rain again
over southern zones. Forecast confidence in the overall trend is high.
Main effort today was focused on transitioning from very strong
storm causing high winds and seas to a drier and calm pattern by
Tuesday. An upper level trough moves east across the interior of Alaska
Monday and Tuesday and passes to our north. Expect north winds to
increase northern Inner Channels after this feature passes
through. There is a possibility of localized strong gap winds
starting about Tuesday night as well but I did not add this detail in
the gridded forecasts yet.
Wednesday into Thursday the ec and GFS indicate the northern Panhandle will
be situated between the upper trough to our north-NE and a closed low
centered west of Vancouver Island. This pattern usually results in
dry weather and very light winds. As a result we will have to
watch out for chilly temperatures and perhaps areas of frost Tuesday
morning and then the possibility of another fog episode Wednesday-Thursday
time frame as the North-East winds diminish. The closed low to our south is
predicted to edge its way north from Thursday through the
weekend...eventually reaching west of haida gwaii...gradually wrapping
more clouds and light precipitation into mainly the southern half of the
Panhandle late in the week.
Hydrology...moderate to heavy rain over the northern Panhandle
has resulted in rises on area creeks and rivers. Water levels are
only expected to reach bankfull but the situation will continue
to be monitored. A Special Weather Statement has been issued with
public...strong wind until 7 PM akdt this evening for akz023-026>028.
Strong wind until 1 am akdt Monday for akz025.
Strong wind until 10 PM akdt this evening for akz022.
Strong wind from 10 PM akdt this evening through late tonight for
Strong wind from 10 PM akdt this evening through Monday
morning for akz018.
Marine...Gale Warning for pkz012-013-021-022-031-033-035-036-041>043-051.
Small Craft Advisory for pkz011-032-034-052-053.
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