Understanding River Forecasts

Current Forecast at Richmond

Thanks to partnerships between the United States Geological Survey (USGS), the National Weather Service (NWS), and local sponsors, we have access to approximately 3,655 real-time forecast points across the Country. These forecasts are vital for residents and communities to respond to flood events to ensure the public’s safety.

NWS Forecast Points

Reading Forecast Graphs

All gauge graphs look similar across the Country and are developed by the area NWS Forecast Office. Along the lower portion of the Brazos River, we rely on the West Gulf River Forecast Center (WGRFC) to produce the forecasts including the gauge point at Richmond. Below is a explanation of the different sections of the forecast graphs.

Reading Gauge

Included in the graphs are the NWS flood stage categories, which are defined as:

Action Stage – The stage which, when reached by a rising stream, lake, or reservoir
represents the level where the NWS or a partner/user needs to take some type of
mitigation action in preparation for possible significant hydrologic activity. The
appropriate action is usually defined in a weather forecast office (WFO) hydrologic
services manual.
Flood Stage – The stage which minimal or no property damage, but possibly some public threat.
Moderate Flood Stage – The stage which some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations.
Major Flood Stage – The stage which extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.

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The biggest questions related to the graphs are about the data included in the forecasts and how the “Stage” relates to elevation.

Forecast Data

At Richmond the forecast includes three critical components.

The actual rainfall observed in across the watershed.
The next 12 hours of predicted rainfall across the watershed. During extreme events, such as Hurricane Harvey, the NWS can choose to include more than the typical 12 hours.
The upstream reservoir discharges.

‘Gage’ Stage Versus True Elevation

The most misunderstood part of the graphs includes the Stage. Due to the age of the gauge point at Richmond, the USGS still uses an adjusted value for water surface readings. It is not the depth of water nor, in many cases, is it the true elevation at the gauge location. In the lower left corner of the graph, you will see what adjustment is needed, if applicable. As examples, here are the adjustments for the Richmond and San Felipe Gauges.

Richmond Gauge Adjustment
- In Richmond, we use an adjustment of 27.02′ to convert from the gauge reading to true ground elevation (NAVD 88). For instance, during Hurricane Harvey the Brazos River crested at 55.19 (Gage) with a true elevation of 82.21′ (NAVD).
San Felipe Gauge Adjustment
- Since the Gauge at San Felipe is relatively new, the Stage is reported as the true elevation (NAVD 88) and does not require an adjustment.