A nowcast estimates current conditions similar to how a weather “forecast” looks to the future. The nowcast for beach advisories, presented on this Web site, estimates the probability of E. coli exceeding safe levels in the beach waters right now. 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.

The nowcast is a mathematical model that provides the probability (in percent) that the bathing-water standard for E. coli will be exceeded. (The Ohio single-sample bathing-water standard for E. coli is 235 colony-forming units/100 milliliters). 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 2008, the threshold probability is either 27 or 28 percent, depending on the date. Similarly, for Edgewater, the threshold probability in 2008 ranges from 27 to 32 percent. If the probability is greater than or equal to the threshold, then the beach is posted with an advisory on the nowcast.

How did the nowcast system perform in 2006 and 2007 at Huntington? Nowcasts were provided to the public for 85 days during the recreational season of 2006. The nowcast provided a correct response 80 percent of time. False positive responses were provided 10 percent of time; that means that the nowcast incorrectly predicted that the standard was exceeded on 6 out of 59 days that the standard was actually NOT exceeded. False negative responses were higher – 42 percent.  That means that the nowcast incorrectly predicted that the standard would NOT be exceeded on 11 out of 26 days that the standard was actually exceeded. During 2007, the nowcast provided a correct response 83 percent of the time. False positive and negative results for the nowcast during 2007 were 12 and 47 percent, respectively.

Although the false-negative rate for the nowcast is higher than we would like, the nowcast still provides more accurate information and better estimates of public-health risk than the use of the previous day’s E. coli (the current method used by most beach managers.). During 2006, the previous day’s E. coli provided only 57 percent correct responses. False positives were provided 30 percent and false negatives 72 percent of time. During 2007, the nowcast provided fewer false negatives than using the previous day's E.coli. We will continue to work to improve the predictive ability of the nowcast at Huntington in 2008.