Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
330 am PDT Thursday Sep 3 2015
Synopsis...an upper level low over southwest British Columbia is
expected to sag south over western Washington through today. The
associated cool unstable air mass should maintain some showers along
with a threat of thunderstorms. The upper low is forecast to
continue south into Oregon Friday offering a gradual drying and
warming trend for western Washington Friday afternoon and Saturday.
A weak weather system moving into western Canada may bring some
showers on Sunday with another threat by Tuesday.
Short term...there is a chance of showers and thunderstorms across
western Washington today as an upper level low moves overhead. This low is
currently spinning over southern b.C. Where most of the showers
activity is located. But this will all shift south today as the low
moves down. With the cold upper low overhead temperatures will remain in
the 60s today...below normal. Snow levels are down to 5500-6000 feet
with a few inches of snow possible in the mountains. Showers will
diminish tonight as the low moves farther south. On Friday the upper
level low will spin over western Oregon with just a chance of
showers in the Olympics and Cascades. Temperatures will remain cool and
mostly in the 60s once again. Expect drier and warmer weather on
Saturday as the upper low exits east and an upper level ridge nudges
inland from the west. Temperatures will rebound with highs back into the
upper 60s to lower 70s.
Sunday will be a little cooler and wetter as a weak cold front moves
through western Washington. The trailing upper level trough will cross
overhead Sunday night for a few more showers. There will be a break
in the weather on Labor Day as we wait for the next system. Look for
partly sunny skies with temperatures a few degrees below normal. 33
Long term...an upper level short wave trough may clip western Washington on
Tuesday for a chance of showers. However this system is looking
weaker in the latest solutions. Beyond then...the models start to
diverge. The European model (ecmwf) tries to build another ridge over the Pacific northwest for
warmer and drier weather. Meanwhile...the GFS brings another through
through the region. Thus confidence remains low at this point. 33
Aviation...an upper level low will continue to slowly move south
along the Washington coast today. There will be enough instability
and moisture that in addition to a chance of showers...isolated
thunderstorms are possible today. VFR conditions over most areas
this morning with more clouds in the northern areas. Local lower
ceilings and visibilities in spots this morning...some IFR conditions are
possible. Ceilings should improve this afternoon...although lower
ceilings in any showers.
Ksea...VFR ceilings for a while...but abundant low level moisture
should bring ceilings lower to MVFR levels early this morning...
with some IFR conditions possible. Winds will be light then
southerly early this afternoon and will turn northerly late this
afternoon or early evening.
Marine...low level onshore flow will weaken somewhat today...and
again tonight into Friday as a weak surface low moves through the
offshore waters. There is still a chance of thunderstorms
today...mainly over the coastal waters this morning and then over
all areas this afternoon. High pressure will build over the waters
Friday into the weekend.
You can see an illustrated version of this discussion at