The nowcast is a mathematical system that uses easily measured environmental and water-quality “variables,” such as turbidity and rainfall, to estimate levels of E. coli. Mathematical models are developed from several years of measurements taken at a particular site, and all models used in the Ohio Nowcast are beach or site specific.

The nowcast for Lake Erie beaches provides the probability (in percent) that the established Ohio standard for E. coli will be exceeded. (The single-sample bathing-water standard for E. coli is 235). So on any given morning, there could be from a 1- to 100- percent probability that the standard would be exceeded. How does one know when the probability presents too great a risk to go swimming? Would you go swimming if there was an 80-percent probability that the standard would be exceeded? What about a 25 percent chance? To help out, beach managers establish a beach-specific threshold probability based on historical data. For Huntington in 2009, the threshold probability is either 23 or 26 percent, depending on the date. Similarly, for Edgewater, the threshold probability in 2009 ranges from 22 to 33 percent. If the probability is greater than or equal to the threshold, then the beach is posted with an advisory on the nowcast.

The nowcast for the Cuyahoga River at Jaite provides a predicted E. coli concentration. Daily information on whether the predicted concentration exceeds or does not exceed the Ohio standard for E. coli will be posted online. (The single-sample primary-contact standard for E. coli is 298 colony-forming units/100 milliliters). For the Cuyahoga River, the daily predicted E. coli concentration will be for information only. The park managers will not issue advisories based on these estimates.

We can measure the nowcast variables each morning, enter them into a computer program, and obtain the probability of exceeding the bathing-water standard (Lake Erie) or the estimated E. coli concentration (Cuyahoga River). Because the nowcast assessments of recreational water quality are ready within an hour, they are better than the old methods that take 18 hours to complete. The nowcast results are used to report beach monitoring sample results for Edgewater and Huntington through the Ohio Department of Health’s beach monitoring program. 

The nowcast for beach advisories, presented on this website, are estimates of the probability of E. coli exceeding safe levels in the beach waters right now. Similarly, the nowcast for the Cuyahoga River is an estimate of the current E. coli concentration. Obviously, there is some uncertainty with a nowcast, just as there is uncertainty with a weather forecast; but past tests have shown that nowcasting does fairly well in determining whether the beach is safe for swimming.