When is the best time to catch a groundhog
Between 2, and 4 hours after sun up, and right after a rain shower are the best times. Id set up a pop up blind, or other good "hiding place", and wait in the mornings if I were you, unless you can capitalize on a short rain shower to bring them out.
Groundhogs come out after the rain because they like to lick the water off the grass. They usually don't have access to water very often, and get the majority of their moisture from the vegetation they eat. When it rains they come out in droves to take advantage of all the fresh moisture within easy reach of their holes. Groundhogs are also active at night(contrary to poplular belief). They do a good bit of "hole digging" after dark.