Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
334 PM PDT sun Apr 19 2015
Corrected minor typo in the synopsis.
Synopsis...high pressure aloft will prevail over the Pacific
northwest through Monday. The high will move east of the region on
Tuesday and this will allow a low pressure system to approach from
the Pacific. Look for what is left of the cold front to move across
the area Tuesday afternoon. The low pressure system aloft will move
across the region on Wednesday.
a gradually weakening upper ridge over the Pacific northwest will
move over the intermountain west on Tuesday. This will allow an upper
trough over the northeast Pacific to approach the region. This...in
turn...will serve to initiate a marine push. It looks like the
push will begin Monday afternoon but will strengthen on Tuesday.
Consequently...anticipate cooler and cloudier conditions on Tuesday.
What is left of a cold front will move across the County Warning Area Tuesday
afternoon for a threat of showers. It looks like the risk of thunderstorms
will stay east of the Cascades...so the possibility was removed
from the forecast.
Concerning maximum temperatures for Monday and Tuesday...confidence in the forecast
for Monday was not very high. The weakening upper ridge combined with
increased low level onshore flow would support temperatures being closer
to the European model (ecmwf) MOS guidance. Since the European model (ecmwf) tends to handle split
flow regimes better...decided to lean toward this model. With that
said...Tuesday should be noticeably cooler across much of the County Warning Area.
The NAM and GFS appeared too warm...especially when factoring in
the strength of the onshore pressure gradient. Thus...leaned
toward the European model (ecmwf) guidance. Although...there is a chance that the
forecast maximum temperatures for Tuesday could be 3-5 degrees too warm.
The European model (ecmwf) indicated that the trough in the northern branch will
approach the area Tuesday night and then move across the County Warning Area on Wednesday.
Conditions should be cool and a bit on the cloudy side. There will
also be a threat of showers. Maximum temperatures across most lowland locales
on Wednesday should only be in the middle to upper 50s.
the extended period still appeared cool and unsettled...however
confidence in the details were not high due to lack of consistency
and agreement in the medium range solutions. Anticipate near or
slightly below normal daytime temperatures during this period.
Aviation...an upper level ridge is over the pacnw. Fair skies with
a dry stable air mass will remain in place. The flow aloft is northwest
with just a little cirrus.
Ksea...a little cirrus...northerly breeze.
Marine...thermally induced low pressure over calif extends north
along the coast. Onshore flow should develop Monday evening and then
increase Tuesday as a front reaches the area. Onshore flow will
continue through the end of the week.
You can see an illustrated version of this forecast discussion at