A large (9.0-13.0 cm wingspan) narrow-winged dull grey moth. The forewings each have four fine horizontal black dashes, the uppermost reaching the apex, and a small diffuse black blotch at the wing base. The hindwings are pale grey with two broad black horizontal bands. The abdomen has a series of large alternating black and white spots along the sides. The much commoner S. vashti averages much smaller than chersis (6.5-10.0 cm wingspan), and has sharper black dashes and better defined banding, and usually has a paler forewing costa, especially on the basal half. Other Alberta Sphinx species have either extensive black on the forewings or very different patterns.