Got a side view when she landed on some grass seed. Good thing because the white hairs helped the id. The experts who helped me identify her said the field marks were tibiae partially reddish, mystax black, (lighter on some) ventral surface of abdomen has long white hairs; female has long blade-like ovipositor. This species does not seem to have a common name other than the generic robber fly.
Female Robber Fly (Efferia aestuans)