Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PDT Sat may 23 2015
Synopsis...a ridge offshore will maintain low level onshore flow
over western Washington through next week. This will keep skies
mostly cloudy and temperatures mild through middle week. Weaker onshore
flow after middle week will allow more afternoon sunshine and a minor
warming trend. Weak low pressure aloft will bring a chance of
showers mainly over the Cascades during the afternoon and evenings.
Short term...the steering flow turned westerly this evening with
thunderstorms over the north Cascades pushing east of the crest. So
the forecast was updated tonight to drop thunderstorms over west
slopes of the Cascades. Meanwhile...low level onshore flow continues
for cloudy skies.
An upper level low over southern b.C. Will bring a chance of showers
to the Cascades Sunday and Memorial Day as it moves into Washington.
The Lowlands will be dry although still mostly cloudy as onshore
flow persists. Temperatures will be a few degrees below normal with highs
in the lower 60s.
The upper level low will slowly shift east to the Washington/Idaho border on
Tuesday. Wrap-around moisture may clip the Cascades for more showers.
However looks like most of the activity will be along the east
Long term...previous discussion...the East Washington upper level low will
finally move east over Idaho Wednesday...allowing an offshore upper level
ridge to slide a little closer to the coast. The approach of the
ridge will allow the air mass to warm...but the greater impact will
be the change to onshore flow. Onshore pressure gradients will
weaken a bit and will shift to a westerly direction which makes it
more difficult to spread stratus across Puget Sound sound. This will
result in more afternoon clearing from Wednesday Onward with maximum
temperatures over the interior warming into the 70s. The air mass over the
Cascades will remain unstable for the Standard afternoon and evening
Aviation...a weak upper level low in southern British Columbia will
move slowly southeast through Sunday. This system will increase
onshore flow and deepen the marine layer. At the surface onshore
flow will decrease tonight then increase Sunday afternoon. The air
mass is moist and weakly unstable but only isolated thunderstorms
are expected near the Cascade crest.
Low level moisture and a deep marine layer will continue resulting
in widespread MVFR ceilings later tonight and Sunday morning.
Ceilings will lift to VFR after midday but breakout is unlikely at
Ksea...ceiling 2500 feet falling to 1-2k feet after midnight. Similar
conditions Sunday with ceiling gradually lifting to 3-4k feet in the
afternoon but breakout is not expected. SW wind 4-8 knots. Chb
Marine...high pressure will remain offshore with lower pressure inland.
Small Craft Advisory strength westerly winds are likely each evening
in the Strait tonight through Monday night. Onshore flow will weaken
Tuesday through Thursday. Chb
Pz...Small Craft Advisory central and east Strait.
You can see an illustrated version of this forecast discussion at