The 1 PM Sunday visible satellite imagery shows the incipient storm, nearly due west of the CA/OR border. Can you see the swirl of clouds around the low and the very unstable air (with popcorn-looking clouds) to the storm's west?
The latest WRF run (forced by the US GFS model) shows the forecast sea level pressures (solid lines) at 4 PM Sunday. A 995 hPa central pressure.
Here are the wind gusts forecast by the ultra-high resolution (1.3 km grid spacing) UW WRF system.
By 6 AM Monday morning, strong winds have reached the coast (50-60 knots) and over portions of NW Washington.
By 1 PM, the low center is over Tatoosh Island and winds over Puget Sound are revving, with gusts to 50 knots over south Seattle and more around the San Juans.
The action continues through 3 PM, with the winds starting to back off along the southern WA coast.
But what about other models and ensemble (many forecast) products? The NWS SREF ensemble system's forecast of sustained winds at Sea Tac shows that most runs indicate windy conditions, with an average sustained wind of about 20 knots, which would imply gusts to around 25-30 knots. There is considerable uncertainty, with some of the runs with much stronger winds.
The vaunted European Center model is taking the low farther offshore with the gusts over Puget Sound reaching 40-45 knots, with more over the coast and San Juans (see graphics of pressure and wind gusts below for 4 PM Monday).
A forecast of gusts over Seattle of 30-50 mph seems reasonable. Add 20 mph for the San Juans and the coasts. Considering that we are early in the season and lots of leaves are still on the trees, one should expect that thousands of folks will lose power tomorrow.