Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 PM PDT Wednesday Sep 17 2014
Synopsis...an upper level trough will produce showers as it moves
across the region through Thursday. A trailing weak upper level
trough will prolong the chance of showers through Friday night. A
strong ridge will build over the region this weekend bringing sunny
skies and warmer weather. Another weak trough will traverse the area
late Monday into Tuesday...followed by a stronger system on
Short term...infrared satellite imagery shows a rather disorganized mass
of middle/high based clouds streaming northward into the region. The upper
low which has fed moisture into the area is becoming cut off but a
portion of this system will get pulled into western Washington tonight. Various
model time-heights show the lower levels slowly becoming saturated
later tonight. A northern stream trough will provide increasing lift with
measurable rainfall likely across most the area.
The 500 mb trough axis shifts eastward by Thursday afternoon. Moist westerly
flow will still produce showers in the mountains. The best chance of
showers across The Lowlands will be early on...then decreasing
during the afternoon. Some mesoscale models and even the gfs40 imply a
convergence zone may develop late in the afternoon into the evening.
The surface ridge west of Puget Sound sound supports this idea with the flow
favoring the north Seattle to Everett corridor. Gradients weaken late
Thursday night and any convergence zone activity should diminish.
Another weak trailing upper trough and associated front will
dissipate as it reaches western Washington Friday. There may still be enough
moisture for some showers...especially coast and mountains in the
morning. The flow turns more northwesterly Friday afternoon as upper heights
start to build. Partial sunshine is expected later in the day with
slightly warmer temperatures.
The warming trend continues into Saturday as high pressure rapidly
amplifies over the Pacific northwest. Thermally induced low pressure will be
near the coast so offshore flow will dry out the air mass and allow
for a warming trend. Some areas away from the water may reach the
Long term...the warming trend continues into Sunday with the ridge
and offshore flow still in place. MOS guidance seems too cool in
this pattern and may be heavily influenced by climatology. Building
upper heights and stronger offshore flow should result in at least a
few degrees warming. High temperatures are trended above MOS...with
widespread middle 80s over the interior.
High pressure aloft begins to shift inland Sunday night and Monday.
Thermally induced low pressure over the interior of western Washington will
shift eastward...ushering in onshore flow Monday. Temperatures will trend
cooler on Monday and more so on Tuesday as a weak system arrives.
This initial trough will fall apart as it encounters the ridge but
there may be enough moisture for some showers.
Long range models are in surprisingly good agreement on the
westerlies arriving by middle week. This pattern change should usher in
wetter and cool conditions just in time for the first week of fall.
Some models indicate it could be rather breezy as well. Mercer
Aviation...an upper level trough is digging offshore this
afternoon...sending middle/high level moisture into western Washington. The
flow aloft is southerly with diffluent flow and weak lift over the
region. Radar shows mainly light showers across western Washington with VFR
conditions expected. Showers will increase tonight through Thursday
morning as the upper level trough shifts inland. 33
Ksea...showers will increase in coverage tonight 06-12z as an upper
level trough shifts inland. Light northerly winds will switch to southerly
Marine...a 998 mb low is spinning off the northern California coast this
afternoon. This low will weaken as it moves east and inland on
Thursday. Pressure gradients and winds will remain light across
western Washington...except west winds rising to 15 to 25 knots through the
Strait of Juan Delaware fuca Thursday evening. A weak cold front will cross the
area on Friday with onshore flow expected. Offshore flow will
develop on Saturday as an inverted surface trough forms along the
coast. This inverted surface trough will weaken on sun. A surface
trough will then cross the coastal waters on Monday. Models show a
stronger system developing over the NE Pacific midweek...however exact
strength and timing is still unclear. 33
You can see an illustrated version of this forecast discussion at