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About the Tool

Extreme Value Theory (EVT) is a statistical methodology used for describing rare events. There are several ways to apply EVT to weather variables including fitting a Generalized Extreme Value distribution (GEV) over Block Maxima (annual maximum value) and the Peaks-Over-Threshold approach where probability distribution of exceedances over a pre-defined threshold are modeled using a generalized Pareto distribution. This tool explores extreme events in California using a Block Maxima approach.

Annual Maximum values of the climate variable from a 21 day window around the day of interest are extracted from a 30 year daily timeseries for the Baseline Period (1991–2020). A GEV distribution for Temperature and an inverted Weibull distribution for Wind Speed is applied to this time series. Shape and scale parameters for the distribution are estimated using the Maximum Likelihood method. Exceedance Probabilities for different threshold values (return levels) are estimated from the fitted model with 95% confidence intervals.

User Advisory: The Extreme Weather Tool is designed to broadly inform estimated probabilities of extreme weather events across a wide range of environments and climate zones in California. On a local scale, different statistical assumptions (i.e. fitting techniques for distribution parameters, choice of extreme value distribution) may be more appropriate. We encourage users to ensure the empirical fit of the applied distribution is acceptable to their end use before using estimates produced from this tool for planning purposes.

Data Sources

The following list of datasets were used to create this tool. Download data visualized in the charts by clicking the Download Chart button. For more download options follow the links below.

Hourly Observed Historical Data
Met Office Hadley Centre

The Hourly Observed Historical Data product consists of 39 stations across the state, each with an observation period of greater than 47 years (1973 to present) from the HadISD global record. Stations identified for use in this data product were chosen based on being considered high quality for temperature. Due to observing techniques, instrumentation used, and similarities in QA/QC protocols, it’s likely that data for dew point and mean sea level pressure will be of similar quality, however this has not been assessed fully.

Note: Only 38 of the 39 stations from the Hourly Observed Historical Data (Doherty 2020) are presented in this tool. The Monterey NAF station was archived by HadISD on 12/31/2020.

Data Access:


  • Explanatory Guide to Hourly Observed Historical Data on Cal-Adapt (link)
  • Doherty, Owen and Amato Evan. 2020. Weather and Climate Informatics for the Electricity Sector. California Energy Commission. Publication Number: CEC-500-2020-039. (link)
Near-Term Weather Forecast
National Weather Service, NOAA

The National Weather Service (NWS) is an agency of the United States federal government that provides weather, water, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas. The Near-Term forecast provided by NWS focuses on large-scale temperature and precipitation patterns for the next 7 days.

Data Access:

National Centers for Environmental Information, NOAA

The National Centers for Environmental Information (NCEI) is an agency of the United States federal government that manages one of the world's largest archives of atmospheric data. The Global Historical Climatology Network - Daily (GHCN-Daily) dataset integrates daily land surface observations from around the world. If observed, the station dataset includes max and minimum temperatures, total precipitation, snowfall, and depth of snow on ground.

Data Access: