The alert from Hackensack’s fire department shortly before 8 p.m. Tuesday was clear — stay off the roads. Even sheltering in a Jersey diner is better than getting stuck in serious flash flooding, which had overtaken about half of the city and roadways across the state during Tuesday night. “Nearly half the city of Hackensack is flooded,” the fire department posted on X. “Please avoid Hackensack if possible. If you’re out on the road find a diner somewhere to eat or a dry spot and just wait till the water recedes.” It was one of many dire warnings up and down the Garden State, as the first of t…