Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PDT Tuesday Sep 30 2014
Synopsis...a weak ridge will gradually build into the area tonight
giving a decreasing chance of showers. Another trough of low
pressure aloft is expected to pass to the north of the area...
leaving western Washington dry through Thursday. An upper ridge will
help limit rain to the north coast and mountains on Friday. By the
weekend...a front may bring light rain to some areas. The chance of
light rain extends into early next week as a front lingers.
Short term...northwesterly flow aloft with low level onshore flow
continues this evening. Showers have been decreasing with the loss
of insolation and are generally limited to the west slopes of the
Cascades. Still...there is abundant low level moisture across
the interior and over the mountains with the onshore flow and some
lingering convergence around the Olympics and mountains of Vancouver
Island. Shower activity should become quite isolated overnight as a
weak short wave ridge that extends from around Jasper Alberta SW to
45n 135w drops southeast into the area.
Partly to mostly sunny conditions can be expected Wednesday as northwest flow
decreases and the short wave upper ridge passes. Only a small
chance of showers is expected over the mountains and adjacent
foothills zones on Wednesday.
The next short wave trough can be seen from southeastern Yukon
across the Panhandle of Alaska. This trough will move southeast across
British Columbia and into Alberta Wednesday evening. The 00z nam12 shows
the energy with this trough remaining NE of the forecast area and
500 mb heights remaining around 570 dam as it passes. The nam12
shows some brief middle and high level clouds accompanying the trough
passage over the area Wednesday afternoon and evening...followed by some
stratus later Wednesday night into Thursday morning. Some high level warm air
advection will give a rather thick high clouds to northern portions
of the area Thursday afternoon.
Forecasts were updated this evening to decrease clouds somewhat
over the coastal zones and away from the convergence zones in the
interior. Patchy fog was also introduced late tonight into Wednesday
morning for the typically foggy southwest interior and Chehalis gap
zones as well as along the coast. The remainder of the forecast
looks fine. Albrecht
Long term...from the previous long term discussion...the forecast
for the weekend and early next week remains in flux. While models
are in better agreement today on the large scale features...the
location of the stalled frontal band is still up in the air.
Notably...the European model (ecmwf) has trended toward a dry solution keeping all
the rain confined north of the border until early next week. The GFS
drags a decaying front southward into western Washington on Saturday which could
produce light rain at times. Most models are dry on Sunday before
indicating some threat of light rain returning early next week. The
extended was adjusted accordingly with a high degree of
Aviation...an upper ridge will remain over the region for continued
strongly northerly flow aloft. Low level onshore flow will persist.
Anticipate localized fog /vsbys at or below 2sm range/ and ceilings at or below 500 feet
to develop overnight but mainly over the coast and SW interior...
or where skies were nearly cloud free.
Ksea...VFR conditions should persist during much of the night.
Anticipate ceilings in the 1-2k feet range to develop about 1200 UTC.
Winds will be variable near 5 knots.
Marine...a 1027 mb high over the offshore waters with lower pressure
inland will result in onshore or westerly flow through Wednesday. The flow
will become northerly Wednesday night and continue into Thursday due to high
pressure over British Columbia and lower pressure over Oregon.
Pz...Small Craft Advisory in effect until midnight tonight for
the central and eastern Strait of Juan Delaware fuca.
You can see an illustrated version of the forecast discussion at