A flash flood watch is in effect for Mercer, Somerset, Hunterdon, Middlesex, Monmouth and Burlington counties until 2 a.m. Thursday. A flash flood watch is issued by the National Weather Service when conditions are favorable for flash flooding in flood-prone areas, usually when grounds are already saturated from recent rains, or when upcoming rains will have the potential to cause a flash flood. Road beds may be washed out under flood waters. Never drive through flooded roadways – you do not know the condition of the road under the water. Turn around, don’t drown.