Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
352 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 showed the cold upper level low was
still over SW b.C. As of 3 PM this afternoon. The air mass over West Washington
remains moist and unstable with widespread showers. There had been
very little lightning up through middle afternoon...but with cell echo
tops still shooting above 16kft in spots...some isolated
thunderstorms are finally developing. There do not appear to be any
more short wave troughs about to traverse the area tonight...so
lacking that source of forcing the only thing left to generate
showers is air mass instability. The chance of showers will continue
tonight...but should begin diminishing gradually late tonight and on
Friday as the low drifts S...and drier north flow develops behind the
low. However if the low takes longer than expected to depart the
chance of showers may need to be extended into Friday morning.
Gradual clearing behind the low still looks good for Friday
The cold air mass from the low will be right overhead tonight so low
temperatures should drop into the lower 40s in the coldest spots.
Friday night should be slightly colder with less cloud cover over
the area. It is possible that some of the coldest spots like over
the SW interior could sneak into the upper 30s.
A weak upper level ridge will move over the area Saturday for a
sunny and warmer day. Maximum temperatures will warm a bit...but only to
slightly below normal.
After the upper level low departs Oregon to the east on Saturday...northwest
flow aloft will develop over the region. The latest model solutions
have stuck with the idea of bringing an upper level shortwave trough
and associated weak cold front down over West Washington on Sunday. This
appears to be the system that was over the western Aleutians near
170w at 3 PM. The 12z GFS timing brings some light rain over the
area late Saturday night then a little rain on Sunday with the weak
front and trailing upper level trough. Kam
Long term...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 and
isolated thunderstorms over the area this afternoon and evening. VFR
conditions over the area with ceilings ranging from 5000 to 6500
feet...mainly broken over the olympic peninsula north into British
Columbia...and partly sunny skies over the central sound southward
into the south interior. Expect showers and the VFR deck to continue
to spread south and east through the south sound and south interior
early this evening. A line of showers extending from kshn through
kawo into the north Cascades is likely the best bet for
thunderstorms in the short term...having already produced a couple
of cells with lightning over Hood Canal and just North East of kawo.
Ceilings will lower with showers...MVFR and possibly lower. Expect
showers and thunderstorms 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 late in the evening.
Ksea...VFR ceilings this afternoon will persist overnight...a band
moving into the Kitsap peninsula at 2230z will move into the metropolitan
area affecting both kbfi and ksea...MVFR ceilings are possible and
this line has been producing isolated thunderstorms/lighting and
gusty winds. Otherwise...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 and isolated
thunderstorms over area waters this evening...with gusty variable
winds possible. Onshore gradients will return Friday and strengthen
during the afternoon and evening. High pressure will build over the
waters Friday into the weekend.
You can see an illustrated version of this discussion at