Severe Weather 101

Flood Forecasting

Flash floods represent different forecast and detection challenges because they are not always caused by meteorological phenomena. Flash floods result when favorable meteorologic and hydrologic 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 spaced buildings
  • the general geography and slope of the land

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 meteorologists 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.

Coastal and Inland Flooding Observation and Warning (CI-FLOW) is a collaborative prototype real-time system that predicts total water level in North Carolina. CI-FLOW captures the complex interaction between rainfall, river flows, waves, tides, and storm surge, and how they will impact ocean and water levels. CI-FLOW is currently being tested in real-time when coastal storms approach North Carolina. NOAA NWS forecasters have access to CI-FLOW during these events to help them make more accurate flood and flash flood forecasts and warnings. NSSL, with support from the NOAA National Sea Grant leads the unique interdisciplinary team of federal, state and university partners.

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. NSSL created a powerful research and development tool for the creation of new techniques, strategies, and applications to better estimate and forecast precipitation amounts, locations, and types. The tool is called NMQ, the National Mosaic and Multi-sensor Quantitative Precipitation Estimation. NMQ uses a combination of observing systems ranging from radars to satellites on a national scale to produce precipitation forecasts. As new concepts are developed, they can be tested by easily plugging them in and out of NMQ. Current projects include integration of polarimetric radar data since it improves the accuracy of precipitation estimates. Special techniques are also being employed to reduce effects of radar beam blockage, especially troublesome in the West but fairly common elsewhere.

NMQ's prototype Quantitative Precipitation Estimation products are also known as “Q2” – next-generation products combining the most effective multi-sensor techniques to estimate precipitation. Q2 products have been used in various meteorological, aviation, and hydrological applications. NSSL also works with NWS forecasters to improve science applications and to transfer these new ideas to operations

The Flooded Locations And Simulated Hydrographs project (FLASH) was launched in early 2012 to improve the accuracy and timing of flash flood warnings. FLASH uses real-time high-resolution, accurate rainfall observations from the NMQ/Q2 project to produce flash flood forecasts at 1-km/5-min resolution. FLASH project development continues to be an active collaboration between members of NSSL's Stormscale Hydrometeorology and Hydromodeling Groups, and the HyDROS Lab at the University of Oklahoma.

In order to evaluate the forecasting tools, we need observations of flash flooding. We've assembled flash flood observations from USGS automated discharge measurements, trained spotter reports from the NWS, and from NSSL's Severe Hazards Analysis and Verification Experiment (SHAVE). This database is available for community research purposes.

During the NOAA Hazardous Weather Testbed spring experiment we investigate ways to increase flash flood lead times using ensemble stormscale Numerical Weather Prediction products.