The hydrograph is computed by fetching rainfall estimates for each of 46 sub-watersheds of Brays Bayou for the last 60 hours, feeding them into an Army Corps of Engineers HEC1 model for the bayou, and graphing the results over the 24 hour period centered on the timestamp of the last rainfall received. Whenever the scale of the graph changes to accommodate a rising or falling flow, the background color of the predicted flow changes as well in order to call attention to the change in scale (part of the reason for this is that the movie Jude used for his defense suffered from a sudden visual effect of a "drop in flow" because the scale used to render the graph jumped from 0 to 4,000 CFS to 0 to 30,000 CFS - this was quite disconcerting. The change in color is meant to reduce this confusion).
Automated alerts are issued to key security personnel when predicted flow rises above thresholds that are indicated by changes in color to yellow, orange, or red.

The red line indicates observed data at HCOEM gage 419 where Main Street crosses the Bayou. This is drawn from floodstage observations pulled from the HCOEM website and converted to flow via a standard rating curve for the gage.

Graph fetched at 2008/08/20-09:33CDT