Every time it rains there is a potential for flooding to occur. This is especially true after major storms. It’s also common in the spring when snow melts at higher elevations, causing mountainous streams to surge. As water from rain storms and snow melt collects in larger rivers, it raises the water level. The greater the amount of water that enters a river, the higher the level can become, and the greater the risk of a major flood event.
Each year, it is estimated that on average 133 lives are lost and over $4 billion in damages to homes, businesses, bridges and other infrastructure occurs due to inland flooding from rivers. (Coastal flooding from storm surge causes even more damage.) To help save lives and minimize damage, the National Weather Service regularly forecasts the potential for inland flooding around the country. The National Weather Services’ Advanced Hydrologic Prediction Service (AHPS) analyzes data from over 5,000 stream gauges (many of which are operated by the USGS), and combines that data with numerical models to issue daily as well as long-range forecasts of the potential for flooding to occur.
The main page of the AHPS website displays a map of all of the active stream gages in the country. Each point is colored based on the current water level. Oranges and reds signify rivers with high water levels where flooding may be occurring, while greens indicate rivers that are currently at normal levels.
The AHPS forecast map (see above) displays a subset of stations where forecasts are available. On this map, dots are colored based on the potential for flooding to occur during the next 48 hours. You can click on each point to see a graph of the river’s water level over the last 2 days, as well as the forecasted level over the next two days. Lines on each graph show the water levels where, based on past observations, minor or major flooding is likely to occur and cause damage. These graphs are updated regularly. During a major storm event, you will often see the forecast lines change over time as new data comes in and more accurate predictions can be made.
Educator Tip: The AHPS river forecast website provides great way to introduce students to flood monitoring and forecasting. Students can investigate the real-time conditions at rivers in their local area or across the country to see where floods might occur. The national maps of real-time and forecasted flooding provides a great overview of conditions across the country, and students can easily compare these maps with weather radar data to investigate the correlation of rainfall with potential flooding.
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