This wasp was found nectaring on narrow-leaf milkweed (Asclepias fascicularis) along with some pepsines. When initially seen in the field, I guessed it was also some sort of pompiliid.

Then, after seeing the full-size photos...and the form of the petiole attachment to the metasoma...I figured it was a Podalonia. When I attempted to key it in Murray(1940) and Fernald(1927), I ended up with P. morrisoni and P. nicholi, respectively...which Bohart & Menke(1)(1976) list as synonyms under P. argentipilus. But the descriptions in both Murray(1940) and Fernald(1927) didn't fit with the individual here (e.g. having long, dense, silvery-white hair almost uniformly distributed throughout; and black on various portions of the abdomen).

So I reconsidered the genus, and with careful scrutiny of the 2nd image here was able to see the 2nd recurrent vein ends in the 3rd submarginal cell...which led to tribe Sphecini in Bohart & Menke(1976)(1), and eventually (via 2nd submarginal cell broader than high) led to genus Sphex. Checking images on BugGuide led to Robyn's nice posts of a male and female for species S. ashmeadi:

       

This is a male (from the 13 antennal segments and 7 visible metasomal segments), and the black femora are consistent with Robyn's post and the remarks on leg color on the Sphex ashmeadi info page.

Hoping to double-check the ID here, I looked for references on the Sphex info page, but was unable to access Bohart & Menke(1963) online. So, instead, I consulted the key in Fernald(1927), which is mentioned in Bohart & Menke(1)(1976) as being outdated but "of some use in the New World". There I ended up there at Chlorion (Proterosphex) ashmeadi, which is now Sphex ashmeadi.

Crucial for arriving at S. ashmeadi via couplet 17 in Fernald's key is the presence of a "stigmal groove" on the metapleura (cf. "d6" in Fernald's Fig. 1, or better, what Bohart & Menke equivalently refer to as the "spiracular groove" in Fig. 29D here). Since I'm not sure I actually see this "groove" (perhaps blocked from view by the mid femur here), I'm posting this to Aculeata and hoping John Ascher or others can confirm the ID.