Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
1000 PM PDT Thursday Sep 3 2015
Synopsis...an upper level low will move slowly south across western
Washington tonight...then over Oregon on Friday. Scattered showers
will continue in the cool unstable air mass along with isolated
evening thunderstorms. As the upper low moves south of the area on
Friday the air mass will gradually dry and warm...leading to a sunny
Saturday. A weak frontal system will bring a chance of rain late
Saturday night and Sunday...then a weak upper level ridge will
produce a partly sunny Labor Day. Weak systems may bring a chance of
showers Monday night through Tuesday night. An upper level ridge
may build over the region Wednesday.
Short term...satellite imagery shows upper level low over east
central Vancouver Island at 04z. Doppler radar shows shower activity
decreasing this evening with most of the shower activity now north
of Seattle. Temperatures at 9 PM were in the 50s and lower 60s.
00z models continuing with the idea of the upper level low dropping
south ending up over western Oregon by 00z Saturday. This does not
look to be far enough south to keep a chance of showers out of at
least the southern portion of the area Friday afternoon. Will make a
quick update and add some probability of precipitation south of Seattle. By 06z Saturday low
well to the south near Klamath Falls with drying northerly flow
aloft beginning over the area.
Saturday looks to be a dry day with the low kicking off to the east
and the next system moving down from the northwest just reaching the
northern tip of Vancouver Island bu 00z Sunday. Rain out ahead of
the front spreading into most of western Washington Saturday night
with the front moving through on Sunday morning. Trailing upper
level trough moving into the area Sunday afternoon keeping a chance
of precipitation in the forecast.
Drying trend once again Sunday night with the trough east of the
area and northwesterly flow aloft over the area. High temperatures
will remain below normal through the period...mostly in the 60s.
Long term...previous discussion follows. The GFS European model (ecmwf) and Canadian
are still having a tough time with early next week. Consistency has
been elusive so confidence is still low. The 18z GFS now shows a
healthy upper level ridge building over West Washington on Labor Day for what
could be a mostly sunny day. The 12z European model (ecmwf) has a much weaker ridge
and brings some precipitation almost to the coast. In general the models
still show some weak troughs embedded in the northwest flow...which at this
Point May or may not bring precipitation to West Washington on any of the days.
Earlier runs had a broad upper level ridge building over the area
around mid-week...but now that has evolved to west-northwest flow aloft with
somewhat higher 500 mb heights. So for now the low confidence
forecast is unchanged... chance of showers Monday night through
Tuesday night and then dry and warmer Wednesday and Thursday. Kam
Aviation...an upper level low slowly digging south into Oregon
continues to support enough instability for scattered showers this
evening. VFR conditions over the area with ceilings ranging from
5000 to 6500 feet...mainly broken from Snohomish County northward.
Ceilings will lower with showers...MVFR and possibly lower. Expect
showers to taper off late this evening with ceilings remaining broken
but generally VFR through Friday morning. Light northerly flow
aloft. The moist and somewhat unstable air mass this evening will
become stable overnight.
Ksea...VFR ceilings this afternoon will persist overnight...ceilings
will remain broken but VFR through the night as showers taper off.
Light southerly winds will become northerly late in the evening.
Marine...a weak surface low will work south along the outer coastal
waters this evening...bringing light offshore flow late tonight into
Friday morning. The upper level trough moving over western
Washington will continue to support scattered showers this evening.
Onshore gradients will return Friday and strengthen during the
afternoon and evening. High pressure will build over the waters
Friday into the weekend.
Sew watches/warnings/advisories... Washington...none.
You can see an illustrated version of this discussion at