The flash flood warning will be in effect Thursday from 11 a.m. to midnight. The National Weather Service has issued a flash flood warning for a large portion of central and northern Colorado on Thursday. The alert is in effect from 11 a.m. through midnight for various cities, including Denver, Arvada, Lakewood, Littleton, Castle Rock, Golden, and Boulder. The weather alert says, “Increasing moisture and instability today will lead to widespread thunderstorm development by the early afternoon.