Could be
White Evening Primrose (Oenothera pallida), but I can't really see enough of it to be sure.
There are many species of white-flowered Oenotheras growing in the American West, but most of them are difficult to grow outside their native range. I've tried growing
Tufted Evening Primrose (Oenothera cespitosa), for example, which is native to my state (although at the other end), but have never been able to keep them alive for very long. We'll get a stretch of hot humid weather in the summer and that's the end of them. They are so well adapted at preventing moisture loss through their leaves that, when confronted with high humidity, they just melt.
Oenothera pallida is one of the exceptions and can occasionally be seen in gardens in the East.
