My son Dallin found this caterpillar on one of our ash trees. It is a large (about 3 inches) green caterpillar with a black and white horizontal stripe across the top part of it’s body. It also has light blue dots all over it. Has two yellow “eyes” and “horns” that come out occasionally. Thought it was a hornworm, but doesn’t look like the pictures I have seen. (South Jordan, Utah)
The caterpillar Dallin found was a swallowtail butterfly caterpillar. There are several swallowtails that have caterpillars that look like it. Because of where you live and that it was feeding on ash it is probably the two-tailed swallowtail but could be the western tiger swallowtail, the eastern tiger swallowtail or the Pale tiger swallowtail.
Each of these caterpillars feed on various tree foliage but ash is actually listed for the two-tailed. The caterpillars look like bird droppings in the early stages then change to the green caterpillar like yours, then when the caterpillar is full grown and getting ready to pupate it turns brown.