Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
938 PM PDT sun Sep 21 2014
Corrected typo in the second paragraph of the short term segment.
Synopsis...a weak low pressure system offshore will continue
heading our way overnight. This system will move across the area
on Monday. A stronger system will impact the region Tuesday and
Wednesday for wet and unsettled conditions or typical Fall
a weak short wave trough aloft and associated weakening cold or
occluded front will continue heading our way overnight. There has
been some drizzle occurring on the coast well ahead of the front.
This drizzle was more associated with a rapidly deepening marine
layer. Elsewhere...expect increasing clouds overnight.
Will need to keep an eye on the convection over the Oregon
Cascades overnight. The showers and thunderstorms were moving north at
this time. Present thinking is that this activity will end up
staying just east of the crest as it moves north up the Washington
Cascades late tonight or Monday morning but cannot completely rule
out a risk west of the crest Monday morning.
The weak upper trough will move across the area on Monday. The front
will likely dissipate before reaching the coast. Anticipate
increasing diffluent flow aloft Monday night ahead of a strong low
pressure system...resulting in scattered showers over the County Warning Area.
A strong low pressure system will impact the region Tuesday and Wednesday
for wet weather...especially on Wednesday. Wednesday also appeared to be a
breezy day for the coast and parts of the northwest interior. In
other words...expect typical Fall weather over western Washington. Autumn
begins Monday at 729 PM PDT.
Long term from the previous discussion...
an upper trough will bring a few showers to the area Thursday and Friday
as the bulk of the upper trough digs in south of the area into
calif and Nevada. For the weekend an upper ridge will nose into the
area. It may be a few degrees below normal for temperatures Thursday and Friday
and then recover over the weekend. The 12z European model (ecmwf) looked
unusual...creating a closed upper level low over calif and Nevada for
the weekend with perhaps wrap around precipitation over the Oregon
Cascades and into eastern Washington...but the Euro was dry over western Washington and
showed 500mb heights rising to around 580 dam over Washington. The GFS
kept the trough open and progressive with no threat of wrap around
moisture from a closed low. That said...the GFS did show a weak
system dissipating into the ridge as it reached Vancouver Island
Saturday night. For now a dry weekend looks like a fair bet.
Aviation...an upper level trough over the eastern Pacific combined
with a ridge of high pressure well inland will give increasing
southwest flow aloft. At the surface...a small-scale ridge of high
pressure over the coastal waters combined with thermally induced low
pressure inland that is shifting eastward will push marine air inland
overnight. The air mass is stable with increasing low level
Satellite imagery shows stratus pushing inland through the lower
Chehalis valley into the southern portion of the Hood Canal at 04z/9
PM. To the north stratus starting to move down the Strait with the
stratus still west of Port Angeles. Stratus bases along the Coast
Range from 300 to 800 feet. Over the interior the bases at Shelton
at 04z are 2000 feet.
Stratus will continue to surge inland with all but the kbli terminal
getting the stratus before 12z. Ceilings in the 1000-1500 foot range
expected with the higher terminals...ksea and kpae...possibly juts
below 1000 feet. Ceilings improving after 16z Monday morning with
ceilings lifting back up into VFR Monday afternoon. Felton
Ksea...stratus arriving around 08z with ceilings near 1000 feet.
Ceilings lifting to above 2000 feet late Monday morning and to above
3000 feet Monday afternoon. Southwest wind 8 to 12 knots easing late
Monday morning. Felton
Marine...a skinny ridge of high pressure is sitting over the coastal
waters with a thermally induced trough of low pressure moving east of
the Cascades tonight. Marine air pushing inland overnight with gale
force winds in the central and eastern Strait until the early
morning hours on Monday.
A weak frontal system will move into the coastal waters Monday then
stall and gradually dissipate during the afternoon and evening
hours. Small Craft Advisory south to southeast winds over the
coastal waters as the front approaches on Monday. The winds will
ease over the coastal waters Monday evening as the front dissipates.
A stronger frontal system...associated with a developing surface low
well offshore...will move into the offshore waters Tuesday and into the
coastal waters on Wednesday. At this time it appears that S to southeast winds
ahead of this front will reach around 30 knots...but gale force winds
are possible. Small Craft Advisory southeast winds are likely in
northwest-southeast channeled waters such as the northern inland waters...the East
Entrance to the Strait of Juan Delaware fuca...and Admiralty Inlet.
Winds will only slowly relax later Wednesday through the end of the week
as a mature 980 mb low near 46n 135w Wednesday slowly fills as it remains
nearly stationary. Felton/albrecht
Pz...Gale Warning in effect until 3 am Monday for the central and
eastern Strait of Juan Delaware fuca.
Small Craft Advisory in effect from 6 am until 5 PM Monday for
the coastal waters.
You can see an illustrated version of this forecast discussion at