The Willow Flycatcher is a commonly sighted migratory bird throughout North America, enjoying wet shrubby habitat from Maine to Southern California. They winter from southern Mexico to northern South America, and tend to come north in early May. We're starting to get reports now in California. If you want to see how they enter the United States, you can see it clearly in the Cornell Lab of Ornithology GIF below:

As far as where we are now, check out the eBird map below showing sightings so far this month. Note how the birds show up first in California and the Pacific Flyway: