Sector chiefs’ weekly Twitter updates point to a mid-March drop in migration in Tucson, Arizona and San Diego, California, the two Border Patrol U.S.-Mexico border sectors that have been encountering the most migrants so far this year.

This is not the usual trend. March—and spring in general—is usually a time of steadily increasing migration, until temperatures get too high. In recent years, though, this has become less predictable, as policy changes, internet-driven rumors, and smuggling patterns have had more effect on the numbers of arriving people.