Marine Heat Waves

Summary of CBEFS Marine Heat Wave Forecasting

Marine heat waves occur when the water temperature is elevated for a prolonged period of time and can be detrimental to marine life, with the frequency of marine heat waves in the Bay increasing over time. Elevated water temperatures are forecast in CBEFS using two different methods. The first is based on the definition of a marine heat wave and the second alternate method is based simply on the water temperature being high. The below bullets describe the two calculations. The alternate forecast is provided in a dropdown menu near the bottom of the page.

1. Marine Heat Wave: Using this definition a marine heat wave is forecast if the surface water temperature is forecast to be above the 90% occurrence temperature for 5 consecutive days. The 90% occurrence temperature is the temperature where 90% of the historical daily-averaged water temperatures were less than this value on a given day. That is, 90% of the years between 1985 and 2023 had a daily-averaged water temperature less than this value on a given day of the year. The 90% occurrence temperature varies by day of the year (cooler in winter and warmer in summer) and based on location in the Bay. This definition can result in marine heat waves occurring during the winter because it is based on seasonally-varying water temperatures. The 90% temperature was determined using long-term model simulations suitable for determining the climatological (average) conditions throughout the Bay.
2. Alternate High-Temperature Marine Heat Wave: Using this definition a high temperature alert is forecast if the surface water temperature is forecast to be above 82.5 degrees Fahrenheit for 5 consecutive days. Using this method a high temperature alert will only be forecast in the summer. Please note that we are still evaluating the specific temperature threshold to use for forecasting a high temperature alert and the 82.5 Fahrenheit temperature could change in the future.

For CBEFS to forecast a marine heat wave, the area of water experiencing a marine heat wave must be above 1000 acres, approximately 0.04% of the Bay surface area. This allows the forecasts to focus on more significant marine heat wave events and not small-scale localized elevated water temperatures.

Forecast Using Alternate High-Temperature Definition

This section provides the forecast of water temperature being higher than 82.5 degrees Fahrenheit. Maps show current locations of forecast high temperatures (if any) and a line plot shows the forecast area of the Bay through time. Please note high temperature forecasting began in March 2025, so forecast areas are not available prior to that.