Flash floods represent different forecast and detection challenges because they are not always caused by meteorological phenomena. Flash floods result when favorable meteorological and hydrological conditions exist together. Although heavy rainfall is necessary, a given amount and duration of rainfall may or may not result in a flash flood, depending on the hydrologic characteristics of the watershed where it is raining. Variables include knowing how much water runs off (as well as where it runs to), how strong the stream is flowing, how wide an area is getting rain, how hard and fast it is raining, how long it has been raining in a particular drainage basin, where the storm is located and how it fast or slow it is moving, how porous the soil is and how much water it already holds, the amount of vegetation covering the soil, how much surface is paved, whether there are storm drains or closely space buildings, and the general geography and slope of the land.
Although flash floods can be caused or enhanced by many different factors, they do have a few things in common. We know that flash floods seem to occur when there is deep moist convection, a deep moist layer of air, a low level jet feeding moisture into a boundary or front that is not really moving, and conditions that encourage cells or storms to mature or move in a sequence over the same general region.
How do hydrologists measure flood potential?
Hydrologists - people who study the effects of water on the earth's surface and in the atmosphere - use gauges to measure the water levels in streams, rivers and lakes. They also measure the water content of snow using snow gauges. They take into account recent precipitation amounts (because soil moisture affects how much rain will soak in and how much will run off), and how much more precipitation forecasters expect. The data are sent to a river forecast center where computers analyze the information to predict river and stream levels in their area. When local forecasters receive the data they compare it with charts for their area and issue a flood warning if necessary.
Forecasters can usually tell in advance when conditions are right for flash floods to occur, but there is often little lead-time for an actual warning. (By contrast, flooding on large rivers can sometimes be predicted days ahead.)
Scientists are working to understand the types of storms that have high precipitation rates and long duration, and to determine what factors can be used in forecast models and in forecast operations to help forecast floods.
FLASH FLOOD or FLOOD WATCH: flash flooding or flooding is possible within the designated watch area - be alert.
FLASH FLOOD or FLOOD WARNING: flash flooding or flooding has been reported or is imminent - take necessary precautions at once!
URBAN and SMALL STREAM ADVISORY: flooding of small streams, streets and low-lying areas, such as railroad underpasses and urban storm drains is occurring.
FLASH FLOOD or FLOOD STATEMENT: follow-up information regarding a flash flood/flood event.
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