As wildfires become more intense and wildfire season becomes longer, meteorologists and fire managers are turning to more advanced tools to forecast extreme fire behavior.

One of the most important of these is the Hot-Dry-Windy Index (HDWI). This is a newer, research-backed metric designed to identify days when the atmosphere is primed for dangerous wildfire conditions.

The HDWI blends three key ingredients that drive explosive fire growth: high temperatures, low humidity and strong winds. This better predicts how intense and fast-moving a fire could become.

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For years, fire managers relied on tools like the Haines Index, which primarily measured atmospheric stability and dryness. But that index lacked wind information, which is a major gap, since wind is one of the most critical factors in fire spread. The HDWI fills that gap by incorporating wind speed into its calculation, offering a more complete picture of wildfire potential.

The index is especially useful during multi-day fire weather events, helping crews anticipate periods of rapid fire growth and plan resources accordingly. In research studies, high HDW values have closely correlated with large fire outbreaks across the western U.S. and other wildfire-prone regions.

A “high HDW day” doesn’t mean fire will definitely start; it means if a fire ignites, it’s more likely to become severe and spread quickly. Fire weather forecasters often look at HDW maps alongside red flag warnings and drought indicators to assess wildfire danger.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.