Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PDT Wednesday Jul 30 2014
Synopsis...high pressure aloft will bring warm sunny days to
western Washington through much of next week. Weak onshore flow will
result in some night and morning low clouds...especially along the
coast. Weak instability aloft Friday through Sunday could result in
isolated afternoon and evening thunderstorms over the mountains.
Short term...a strong upper ridge extends northward along the Rocky
Mountains from a 5950 high pressure center near the border of Mexico
and New Mexico. Southwest flow aloft continues over western
Washington. Low level onshore flow is still generally weak...but the
evening seabreeze is again resulting in westerlies gales in the
Strait of Juan Delaware fuca. The forecast area has mainly clear skies...
and there is little in the way of coastal stratus this evening even
over the waters.
As we have been advertising for days now...the large scale pattern
will change little during the short term -- that is tonight through
Some stratus and patchy fog could form over the coastal waters and
in the Strait of Juan Delaware fuca tonight and Thursday morning. But the
absence of coastal low clouds this evening strongly suggests that
there will not be much over the western Washington interior.
Thursday should be another sunny day and warmer than today...with
highs in the 70s at the coast and mostly 80s to lower 90s inland.
Coastal stratus probably will be present Thursday evening...and it
will likely push weakly inland Thursday night -- through the Strait
of Juan Delaware fuca to Whidbey Island and the San Juans...and through
the Chehalis gap to the south Puget Sound sound region and parts of the
southwest interior. The stratus that does move inland should burn
off middle to late Friday morning...and Friday should be another
generally sunny day with highs in the lower 70s at the coast and middle
70s to middle 80s inland. Friday night and Saturday should repeat the
Monsoon moisture rotating northward along the west side of the upper
high is producing in isolated thunderstorms over the Oregon Cascades
again this evening. There is a good chance that a very weak
disturbance moving into British Columbia will spread the convection
to the Washington Cascades crest Friday and Saturday during the
afternoon and evening hours. Mcdonnal
Long term...here is the long term section from the afternoon
forecast discussion -- the weakness in the ridge will move off into
b.C. On Sunday. A few isolated thunderstorms could still pop
over the northern Cascades but the air mass over the mountains should
begin to stabilize. Both the GFS/European model (ecmwf) agree on a period of ridging
Monday through at least Tuesday. This should maintain the warm and
dry weather pattern with less morning clouds. By middle week some
models start to show stronger westerly or SW flow aloft over western Washington
with low level onshore flow. This is still probably a dry pattern
but could produce renewed marine clouds and backing off of high
temperatures. Highs in the 80s early to middle week may struggle to
reach the upper 70s...or near average...for this time of year. Mercer
Aviation...an upper ridge is over the western U.S. With light southwest
flow aloft over Washington. At the surface there is high pressure just
offshore with thermally induced low pressure east of the Cascades.
Areas of late night and morning fog and low clouds should be mostly
along the coast but possibly into the Chehalis gap and Strait of
Juan Delaware fuca by daybreak. Elsewhere clear skies continue.
Ksea...clear with northerly breeze.
Marine...Race Rocks had west 35kts at 8pm and a Gale Warning is up
for the central Strait...but buoy88 at the westernmost Point of the
East Entrance zone only had west 21g25kt and a Small Craft Advisory
is up there. Destruction Island has been right around nw27kt all
evening and a Small Craft Advisory is in effect for the nearshore
coastal zone that includes Destruction Island and the zone south of
that...otherwise no changes to the forecast this evening.
There is high pressure over the offshore waters with thermally
induced low pressure east of the Cascades and there will be little
change in the weather pattern into the weekend.
Pz...Gale Warning central Strait...Small Craft Advisory eastern
entrance to the Strait...and Small Craft Advisory James Island to
Cape Shoalwater and out 10 nm.
You can see an illustrated version of the forecast discussion at