River Forecasting

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River Forecasting
Darin Figurskey, Meteorologist-in-Charge NOAAs
National Weather Service, Raleigh 28 October 2008
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Why Do We Need River Forecasts?
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How Are River Forecasts Developed?

• Customer need
• Mostly flood mitigation, but occasionally other
  needs
• Must have observations
• Gauge sites developed through desire of and
  funding by communities, not normally the NWS
• Cost, 22K to install, 14K/year for maintenance
  and communication, mostly via satellite or phone
• Takes a big effort for river forecast centers to
  develop forecasting capability for a site
• Unit hydrograph needs to be developed
• Streamflow-to-stage relationship for the point as
  well as for routing
• Ongoing process to improve

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What is a Unit Hydrograph?

• A unit hydrograph is
• The hydrograph that results from one inch of
  excess precipitation (or runoff) spread uniformly
  in space and time over a watershed for a given
  duration.
• Basically, how the river responds to an inch of
  excess rain.
• The technique results in predicted flood peaks
  within 25 of observed values, which is typically
  deemed initially sufficient for planning.
• May take a while to get enough rain (flooding
  rain) to figure out a good relationship.

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Hydrograph Individual Responses
Dependent on soil, geography, steepness,
width/depth of channel, urbanization, upstream
water, etc. If these change, modelling must
change.
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The Southeast River Forecast Center Makes the
Forecast WFO Issues Warnings and Coordinates
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Need to know the relationship of water at a point
as well as what may come from upstream
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Note how many large basins were on the last
slide, and how many small basins make up the
larger basins!
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USGS-maintained gauges except for Moncure Flood
stages determined in coordination with customers
(damage, protect life)
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Resources - AHPS

• Advanced Hydrologic Prediction Service (AHPS)
• Web-based, Graphical Output Easy Access to
• Inundation Maps
• Historical Crest Data
• Specific Flood Impacts
• Probability Graphs of Exceeding River Stages
• Locally Produced Products for planning
• Tools for Emergency Economic Managers
• Can be Customized for Specific Content
• weather.gov/rah, then look for AHPS on the left
  side under Hydrology submenu

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Stages and Impacts Example - Clayton
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Weather resources
The NWS on the web
http//weather.gov/rah
Your home on the Internet for forecasts and
warnings, direct from the NWS.