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.

What we do: 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 being tested in real-time when coastal storms approach North Carolina. NOAA NWS forecasters have access to CI-FLOW during these events to provide feedback on how well the CI-FLOW system estimates total water level. NSSL, with support from the NOAA National Sea Grant leads the unique interdisciplinary team of federal, state, university and private 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.

What we do: NSSL developed and implemented the real-time Multi-Radar Multi-Sensor system in 2004, integrating data from multiple radar networks, surface and upper air observations, lightning detection systems, satellite and numerical weather prediction models. The data is used to estimate and forecast precipitation locations, amounts, and types.

MRMS was transitioned into operations at the National Center for Environmental Prediction in 2014 and provided severe weather and precipitation products for improved decision-making capability within NOAA. The operational MRMS QPE products have high resolution and rapid updating capabilities. The products are also used for verification of satellite rain products and for verification of quantitative rain forecasts from numerical weather prediction models. MRMS serves as a powerful tool for the creation and evaluation of new techniques, strategies and applications to better QPE. As new concepts are developed, they can be tested by easily plugged in and out of MRMS. This process facilitates a rapid science-to-operations transition of new MRMS applications and products for flood and flash flood predictions and water resources management.

The Flooded Locations And Simulated Hydrographs project (FLASH) was launched in early 2012 to improve the accuracy and timing of flash flood warnings. FLASH introduces a new paradigm in flash flood prediction, using MRMS and producing flash flood forecasts with products generated as frequently as every 2 minutes. The primary goal of FLASH is to improve accuracy, timing, specificity, and severity levels of flash flood warnings in the U.S., thus saving lives and protecting infrastructure. The FLASH team is comprised of researchers and students who use an interdisciplinary and collaborative approach to achieve the goal. The FLASH system was transitioned to the National Weather Service in November 2016.

In order to evaluate the forecasting tools, scientists 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 NSSL researchers, in conjunction with partners and colleagues from research, operations and academia, investigate ways to increase flash flood lead times using ensemble stormscale Numerical Weather Prediction products.