Water Quality NowCast/Predictive Modeling Project
Frequently Asked Questions

1. What is a Water Quality Nowcast?

A water quality nowcast provides information similar to a daily weather forecast except the nowcast predicts good or poor water quality for the day at specific beaches. The nowcasts are based on the results of predictive computer models that estimate fecal bacteria levels in the surfzone. Local agencies can compare the computer results to the State’s bacteria health standards for contact with ocean waters to determine if the water is safe for recreational uses such as swimming and surfing. If the model estimates bacteria levels that meet the health standards, the water quality nowcast is “Good”, however, if the model estimates exceed the health standard, the water quality nowcast is “Poor.”

2. Who developed the Predictive Models?

The predictive beach water quality models were developed as part of a study completed by Heal the Bay and Stanford University, and funded by the California State Water Resources Control Board. Experts in beach water quality developed and tested over 700 different beach models using many years of historical data on environmental conditions and bacteria levels from 25 beaches in California. This study represents the most comprehensive study completed to date on using predictive models at marine beaches.

3. Why do we need Predictive Water Quality Models for Beach Water Quality Decisions?

Currently, local health agencies use laboratory analyses of water samples collected at the beach to determine if it is safe for recreational use. Unfortunately, there is a long delay in this approach. In Orange County, it typically takes 24-32 hours to collect the samples, transport them to the lab, and analyze the beach water samples. Meanwhile, water quality can change with environmental conditions between scheduled sampling days. Most California beaches are sampled on a weekly basis, although there are some beaches that are monitored more frequently. Predictive models can provide daily water quality nowcasts based on the most recent environmental conditions at the beach. Local health agencies can then make public notifications of poor water quality in the morning before most people arrive at the beach.

4. What Factors Affect Beach Water Quality?

Fecal bacteria levels can be affected by many factors such as rainfall, tide levels, solar radiation, wind, storm drain flows, and swell conditions. Because the effect of these factors on water quality varies from beach to beach, site-specific models are developed for individual beaches.

5. How Accurate are the Models?

The predictive models generally perform as well as, or better than, the current method that relies on waiting for laboratory analysis of water quality samples to determine if the water is safe for swimming.

6. Are there Shortcomings to Using Nowcasts based on Predictive Models?

Predictive models are developed to capture variations in beach water quality caused by changes in environmental conditions, but are not able to predict unusual events such as a sewage spills.

7. Are Predictive Models being used at other Beaches?

Yes, in the last several years, NowCast models were used at numerous California beaches, from Humboldt to San Diego. Predictive models are also used for daily nowcasts in the summertime at several beaches on the Great Lakes, including beaches in Ohio, Wisconsin, Pennsylvania, New York, and Illinois. At these beaches, local beach agencies successfully use predictive models to determine when the public should be notified of bacteria levels that exceed health standard. Predictive models are also used at Hong Kong coastal beaches where the predictive models accurately predict 30-70% of water quality exceedances.

8. Where will I be able to find the Water Quality NowCasts?

Beach-goers will have easy access to predictive forecasts on Heal the Bay’s Beach Report Card Website and on the Beach Report Card mobile app. Information and links to the forecasts are also on the participating health agencies webpages: Orange County Health Care Agency, Los Angeles County Department of Public Health, and County of Santa Barbara Department of Public Health.


While every effort will be made to ensure accurate information dissemination, the project team (Stanford University, Heal the Bay) and any involved health agencies and departments make no representation about the content for any purpose, and assume no responsibility for anyone’s use of the information.

photo: taking water samples