|Solitary underwing. Catocala sp.|
Before I sent it in to BugGuide, I spent a while scanning their photos to see if it was there already. And I found a match; the Betrothed Underwing, Catocala innubens. Except that its range is 'way over on the East coast.
I gave up and submitted my photo. A few minutes later, I had an answer; it was possibly either the Once-married, unijuga, or the semirelicta, which translates as "Half-widowed", both very similar, but found on this coast.
Researching these, I found mentions of other look-alike underwings: the Old-wife, palaeogama, the Little Wife, the Connubial, the Mother, the Sweetheart (amatrix), the Bride (neogama), the Widow, the Divorced (repudiata), the Cheater (adultera), and even the Girlfriend.
Some are named after interesting women from history and legend: Delilah, Magdalen, Sappho, Scarlett, Aholibah, Helen (of Troy?), Andromache, Desdemona.
Or they are named after their mood: Dejected, Tearful, Sad, Mourning, Penitent, Inconsolable. I didn't see any happy underwings, although one is called Serene.
The common names given to species of Catocala are often fanciful and arbitrary. (BugGuide, Unijuga page)
For a more complete list, look at this Moth Photographers' Group page, or the Wikipedia list of Palearctic species.