A launch from Vandenberg, seen from Seattle
From Seattle, Vandenberg SFB is about 1438 km away to the S (compass bearing 174°). A night or twilight launch is unlikely to see; a daytime launch is unlikely to see. Face S shortly after liftoff and watch low on the horizon — the rocket climbs as it heads downrange.
You're not watching the rocket itself — beyond a few km the vehicle is far too small to see. What's visible is the engine plume at 65–100+ km altitude: at night a brilliant moving flame (routinely seen 300+ km away), at dawn/dusk a sunlit "jellyfish" cloud that has been documented 800+ km away, in daytime only a white contrail with much shorter range. Cloud cover, local horizon and trajectory all matter.
Nearby cities
For exact launch times and live trajectories, check the provider's stream. How this estimate works.