The Scarlet Tanager has been a sporadic local visitor beginning in 2008. I have never seen a male, and I have just recently (7/20/2014) decided that this bird is not a Summer Tanager.

These are the females.

The obvious contrast here is in the size of the bill, and secondly in the coloration. The Scarlet Tanager has the smaller bill and has a more green cast to its feathers. The Summer Tanager has the noticeably larger bill and a more yellowish coloration.

"Female [Scarlet Tanager] has uniformly olive head, back, and rump, whitish wing linings; bill smaller and stubbier than Summer Tanager" (Jon L. Dunn & Jonathan Alderfer. Field Guide to the Birds of North America, p. 488)

"Some [Summer Tanager] females, especially of eastern rubra, show overall reddish wash; most have a mustard tone, lack olive of female Scarlet Tanager; bill larger. Western birds (cooperi) are larger; longer billed, and paler; females generally grayer above" (Jon L. Dunn & Jonathan Alderfer. Field Guide to the Birds of North America, p. 488)