Extended Stream Flow Prediction Forecasts Now Available on the Birmingham NWS AHPS Page
Extended Stream Flow Prediction (ESP) Forecasts for river forecast points and reservoirs are now available on the National Weather Service, Birmingham, Advanced Hydrologic Prediction Service (AHPS) web page. These forecasts are issued once a week by the Southeast River Forecast Center in Peachtree City, Georgia, in a user-friendly graphical format.
These probabilistic forecasts are provided to show the weekly chance of exceeding specified levels or flows during a three month period, and also to show the chance of exceeding specified levels or flows anytime during the entire three month period. These forecasts are generated from current model states, calibrated model parameters and historical time series. It is assumed that historical meteorological data are equally likely to occur in the future, and can be used to create many equally likely sequences of future hydrologic conditions, each starting with current conditions. The generated stream flow time series can then be analyzed for products such as peak flows, minimum flows, flow volumes, and river stages, for any period in the future. Graphs of reservoirs indicate inflow into that reservoir and this explains why there are no stage graphics in those segments.
The Weekly Exceedence Plot shows the probability or chance that the maximum stage, flow or volume at a point on a river or reservoir will exceed a particular value in a 90 day period. The vertical axis shows river stage or level (measured in feet), flow (measured in cubic feet per second) or volume (measured in acre-feet) and the horizontal axis shows time. Each vertical bar represents the probabilities for a 7 day period. Color is used to indicate probability levels or chances on the graphic. Shown below are examples of these plots.
Stage - Weekly Chance of Exceeding Levels
Flow – Weekly Chance of Exceeding Levels
Volume – Weekly Chance of Exceeding Levels
The Exceedence Plot for the Entire 90 Day Period shows the chance of the river stage, flow, or volume going above various levels anytime during the forecast period labeled above the graph. Similar plots are usually available for one or more of these variables at this forecast location. The conditional simulation (CS) line indicates chances of the river going above given levels based on current conditions. The historical simulation (HS) line indicates the chances of the river going above given levels based on the total range of past levels.
These long-range forecasts or outlooks allow you to see what computer simulations can tell us about extended periods. Remember that this information is indicative, that is, it only gives you probabilities or chances of possible scenarios. It should be expected that conditions can and will change over such extended periods. These forecasts are updated periodically and should be consulted on a regular basis. Shown below are examples of these graphical plots.
Flow – Chance of Exceeding Flow Levels Entire Period
Stage – Chance of Exceeding Stage Levels Entire Period
Here are some possible scenarios to help you understand this graphic:
More wet than “normal” conditions over the forecast period.
The chances are greater for wet conditions, as indicated by the Conditional Simulation, over the entire range of possible outcomes.
More dry than “normal” conditions over the forecast period.
The chances are greater for dry conditions, as indicated by the Conditional Simulation, over the entire range of possible outcomes.
When the two simulations are very close across the entire range, the chances of the river going over a certain level is similar to the total range of past levels.
As population and water use continues to increase, water managers will be able to use these forecasts for planning and operations. Water managers and other hydrologic partners and users will be able to use these forecast during floods, droughts and other times to increase lead-times for their response.
In Central Alabama, these forecasts will be available for the following sites:
- Coosa River at Weiss Dam
- Coosa River at Gadsden
- Coosa River at Logan Martin Dam
- Coosa River at Childersburg
- Coosa River at Wetumpka
- Tallapoosa River at Wadley
- Tallapoosa River at Milstead
- Tallapoosa River at the Tallapoosa Water Plant
- Catoma Creek at Montgomery
- Alabama River at Montgomery
- Alabama River at R.F. Henry Dam
- Alabama River at Selma
- Cahaba River at Cahaba Heights
- Cahaba River at Centreville
- Cahaba River at Suttle
- Cahaba River at Marion Junction