Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
252 am PDT Tuesday Sep 16 2014
Synopsis...onshore flow will bring cooler temperatures to western
Washington today along with areas of morning low clouds. An upper
level trough will bring a chance of showers to the area Wednesday
through Friday night...with showers most likely on Thursday. An
upper level ridge will build over the region this weekend for warmer
and dry weather.
Short term...a SW marine push overnight has brought areas of
stratus inland this morning. Stratus reached Shelton around midnight
and with kshn winds blowing SW 15-25kt should spread the low clouds
across most of Puget Sound sound this morning. The small upper level
trough that has triggered the marine push was centered over Astoria
at 09z/2 am and was headed quickly north. A small area of showers
was along the north Oregon coast along with a solo lightning strike at
0745z. This shower area is small enough to fizzle out this morning
as the upper level trough moves deeper into the upper level ridge
centered over the pacnw...but still bears watching.
The nam12 and arw-west boundary layer relative humidity forecasts seemed a reasonable
indicator for stratus coverage today. Low clouds will probably
blanket Puget Sound sound this morning with a little bit spreading east of
Lake Washington...but should burn off over the sound this afternoon.
However the stratus will be deep enough along the coast and onshore
flow persistent enough so that stratus will probably not burn off
over the lower Chehalis valley today. The rather sparse middle and high
clouds from the upper level trough will not add much to the overall
cloud cover. Maximum temperatures will be a little tricky today...with stronger
cooling expected in the stratus areas...especially along the coast
and over the lower Chehalis.
Maximum temperatures Wednesday were also troublesome. The onshore flow will be
about as strong tonight and Wednesday morning. The marine layer will
be deeper and will spread across most of West Washington...but instability from
the upper level trough approaching from offshore should help the
cloud cover break up a bit Wednesday afternoon. For maximum temperatures this
means that locations today where the stratus is most persistent
could very well be slightly warmer on Wednesday given a better mixed
marine layer. Locations that are stratus free today...may end up
being a little cooler because of the greater coverage of clouds on
Overall the models remain in general agreement with the large upper
level trough currently offshore along 140w ejecting east this week.
Spotty showers will probably spread to the coast Wednesday morning
and then inland Wednesday afternoon. The upper level trough axis
will move across the area Wednesday evening and Thursday morning for
the greatest shower potential and likely probability of precipitation. Thursday should be
the coolest day with the cooler air mass from the trough right
Long term...the upper level trough will move over East Washington Thursday
evening with diminishing showers over West Washington. However another weaker
upper level shortwave trough moving onshore will prolong the chance
of showers into Friday evening. GFS and European model (ecmwf) agree well on building
a strong upper level ridge just offshore Friday night then moving it
inland on Saturday. This will bring another period of east-NE low level
offshore flow...which could push maximum temperatures back up into the 80s this
weekend...particularly on Sunday. Both models have the ridge
shifting east over west Montana Sunday night allowing a weakening front to
approach the coast on Monday. However the front could be weak and
slow enough to eke out a mostly sunny dry Monday as well. Kam
Aviation...low clouds coast and areas of low clouds in Puget Sound sound
this morning. Some of the marine stratus may push east in the Strait
this morning but more likely will hold off until the stronger push
this afternoon. In the areas that do not see morning low
clouds...mainly north of Everett...there will only be some middle and
high clouds. The ridge is moving east and there is SW flow aloft.
Areas of low clouds should develop again tonight and Wednesday morning.
Ksea...morning low clouds are expected...then just some middle and high
clouds. With onshore flow the breeze will remain S to SW.
Marine...onshore flow...and a decent westerly push is likely in the
Strait of Juan Delaware fuca this afternoon and evening. After the marine
push today the onshore flow will be fairly light as weather systems
moving through the area are expected to be rather weak.
Pz...Small Craft Advisory central and eastern Strait of Juan Delaware fuca
from noon today until midnight tonight.
You can see an illustrated version of this forecast discussion at