I reexamined the specimen. The ocellar bristles seem to agree more with S. sabroskyi, but the tibial coloration, anal cell, and anal crossvein seem to indicate S. flavescens. I'm also assuming that the lack of a bristle on the humeral callus is what Steyskal was referring to when he refers to the "humeral bristle", which further indicates S. flavescens.

So I'm content to call this S. flavescens and assume the ocellar bristle placement is more variable than Steyskal was aware of. It's interesting that he mentions that sabroskyi and minimum are more closely related to the darker neotropical species than the sympatric and similarly colored flavescens.

Here's the key. http://www.jstor.org/stable/25083210

Here's a nearly identical specimen from the Chicago area that was tentatively IDed as S. flavescens. http://www.diptera.info/forum/viewthread.php?thread_id=608&pid=2194