Don’t low-pressure systems usually keep moving?

Dear Tom,
On your forecast maps, you often show (during the warm season at least) a stationary low-pressure area over the California-Arizona state line. I thought low-pressure systems usually keep moving.
— William Newell, Libertyville
Dear William,
You are referring to a thermal low-pressure area (or heat low) that frequently develops in the border area of California and Arizona during the warm season. It is stationary. Thermal lows are not associated with warm and cold fronts the way moving low-pressure systems usually are.
They occur over continents during the warm season as a result of intense heating compared with their surrounding environment. Because hot air is less dense than colder air, their air pressure is less. In the United States, a favored area of such development is the California-Arizona border area, a low-elevation region surrounded by much higher, colder elevations.