Forecasting Flash Floods
Can a flash flood be predicted?
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 vegatation 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 llittle 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.
HOW DOES NSSL CONTRIBUTE?
HMT – The NOAA Hydrometeorological Testbed is a concept aimed at accelerating the process of moving new technologies, models, and scientific results developed by the research community into daily forecasting operations of the National Weather Service and the River Forecast Centers (RFC's). The project was first deployed in California . Facilities will be sequentially deployed to other regions on the U.S. to address additional serious hydrometeorology problems that are unique to these locations. The project will run for a few years in each regional demonstration to determine its most useful new tools for improvi8ng precipitation and runoff forecasting methods.
NMQ – NSSL's National Mosaic and Multisensor Quantitative Precipitation Estimation Project – A community-based research and development platform for the creation of new applications, techniques and strategies toward precipitation estimation (QPE), short-range precipitation forecasting (QPF), precipitation classifcation (snow, rain, sleet, etc.,) and severe weather monitoring and prediction.
Q2 – NSSL's next generation of quantitative precipitation estimation focused on high-resolution integration of radar, satellite, model, and surface observations to produce very high resolution precipitation estimates. See QPE-SUMS for more information.
QPE-SUMS – Researchers at NSSL developed a system that computes, analyzes and displays high-resolution radar data and radar-derived products to achieve accurate rainfall and snowfall estimates. The Quantitative Precipitation Estimation and Segregation Using Multiple Sensors (QPE-SUMS) is a suite of real-time algorithms that blends model output with radar, satellite, lightning, and rain gauge data to overcome significant challenges in estimating precipitation type and amount.
Debris Flow forecasting – Two NSSL researchers are serving on an interagency team to create a debris flow, or mudslide warning system for southern California. Debris flows can begin suddenly, accelerate quickly, reach velocities up to 40 mph, and flow down streams or other channels for several miles.
Western Mediterranean flash floods – An NSSL scientist spent five months in 1995-1996 at the University of the Balearic Islands stydying heavy rain events in the Mediterranean to test a mesoscale numerical model as a potential operational tool in support of heavy precipitation forecasts.
The North Carolina Sea Grant Program was a collaboration to bring NSSL's advanced flood warning and monitoring technologies to the coastal Carolinas as part of a demonstration project to improve the accuracy of flood and flash flood warnings. The research project combined expertise in storm surge modeling, hydrologic modeling and quantitative precipitation estimation to build a complete flood forecast system. This technology could help reduce flooding damage that might occur from hurricanes.