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New index improves fire weather forecasting

OREGON—Predicting the weather is notoriously complicated, which can be a challenge for fire managers. Weather plays a major role in how a wildfire behaves and whether it might become erratic or endanger firefighters. For 30 years, fire weather forecasters used the “Haines Index” to assess how weather might intensify wildfire and drive its spread.

Forecasters knew that the index had shortcomings. But very few studies had ever evaluated its performance, and no peer-reviewed studies had ever quantitatively examined the performance of the index for multiple days of multiple, individual fires. Brian Potter (Pacific Northwest Research Station), Scott Goodrick, (Southern Research Station), Joseph J. Charney (Northern Research Station), and their colleagues at Michigan State, Texas Tech and St. Cloud State universities sought to address this gap, conducting a study that evaluated the accuracy of the Haines Index for 47 fires that burned in the United States from 2004 to 2017.

The study revealed fundamental flaws in the Haines Index, so Potter and his colleagues developed a new system. This new fire weather index, called the Hot-Dry-Windy Index, is based on physics (rather than statistics), and appears to have predictive skill at identifying days when weather processes could contribute to especially dangerous fire behavior.

It is also intuitive. Ask a wildland fire expert about the types of weather that create difficult fire conditions and they will likely reply with some combination of “hot, dry and windy.” This new fire-weather prediction tool is based on the key atmospheric variables that affect wildland fire: temperature, moisture and wind. It works with the same weather models that are used every day in fire weather forecasts and can be applied anywhere in the world.

The National Weather Service has recommended that fire weather forecasters begin evaluating the Hot-Dry-Windy Index as a potential fire weather tool.

Dustin Smith noting down field observations.
Fire effects monitor Dustin Smith taking field weather observations. National Interagency Fire Center photo by Kari Greer.