A flash flood is sudden flooding that is caused by heavy, sustained rainfall on already saturated soil. This is common when an area is hit by a sequence of thunderstorms one right after the other, or when there is an extremely slow moving multicell storm that keeps generating new cells that mature over the same place. Hard ground, frozen ground and metropolitan areas (due to all the asphalt and concrete) are especially susceptible to this kind of flooding because water can't be absorbed into the soil and runoff can become easily conjested or blocked. A regular "flood", on the other hand, is caused by overflowing rivers and other normal bodies of water. While the onslaught of flash flooding tends to be sudden, a regular river flood usually takes a good deal of time and often provides sufficient warning before damage begins to occur. Flash floods are very dangerous, especially when people attempt to drive their automobiles through a flooded area. Waters are deeper than they appear and many people have died from driving through deep water thinking it was fairly shallow. Flash flooding is common during severe weather as thunderstorms empty the latent moisture in the atmosphere on the ground in a short period of time - low level jets and moisture advection can increase moisture levels which further enhances the probability of flash flooding. Flash floods are also helped by downbursts (microbursts or macrobursts, depending on its size) from thunderstorms, which are characterized by violent outflow of rain and wind as the top of the storm literally "dumps" its water toward the ground. If runoff and absorbtion into the soil cannot occur quickly enough, flash flooding results.