co 737-800 las-cle on 2/20, pilot indicated on ground that it was a good day to fly except for the usual bumps over the rockies. after the rocky bumps the flight was bumpy the rest of the way (FL37). kept reassuring us that atc was saying smooth sailing ahead but it never happened. Can the conditions change that rapidly? appeared to be jet stream as there wasn't a cloud in the sky until last part of flight. not much fun being strapped in for almost 4 hours. was light to moderate but annoying.