Firefighters hike toward the 2017 Brian Head Fire, Utah. Weather plays a crucial role in wildfire behavior. Fire meteorologists need tools such as the newly developed Hot-Dry-Windy Index to predict dangerous con-ditions. These forecasts help protect firefighters and help incident commanders strategically allocate fire-fighting resources.

Hot-Dry-Windy Index Offers Improvement for Fire Prediction

By Dawn Killough

It’s been known since the days of Homer’s Iliad: forest fires are worse on days that are hot, dry, and windy. No one will dispute that fact. The trick is in predicting when those bad days will be and exactly what effect the weather will have on the fire.

For decades, fire crews have been depending on the Haines Index (HI) to determine the worst fire days. Even Don Haines, the index’s inventor, admits that the index is flawed because he did not account for wind in his calculations. And because wind is a major factor in fire spread, the index couldn’t accurately predict bad days. In fact, recent studies have shown that the Haines Index is only slightly more reliable than random guessing at determining the worst fire days. 

Fire crews, whose lives are on the line, need something more reliable than random guessing to help them know when to expect dangerous conditions. In 2013, a team of four scientists started the journey to come up with just such a solution. Brian Potter, Jay Charney, and Dr. Scott Goodrick from the U.S. Forest Service, along with Alan Srock from St. Cloud State University in Minnesota, developed a new tool called the Hot-Dry-Windy Index or HDWI (www.hdwindex.org).

What Is HDWI?

HDWI attempts to predict bad fire days by taking the wind speed and multiplying it by the vapor pressure deficit, which is a function of temperature and moisture. This factor is plotted against 30 years of historical values for the same location. This shows fire teams the long-term trends for the area and helps them determine how likely a bad fire day will happen. 

The index looks at the lowest 500 meters of the atmosphere, which is the area that most affects a fire’s behavior. Evidence shows that days with higher index values are the worst as far as fire behavior. 

The index is based on weather forecast models, specifically the GEFS (Global Ensemble Forecast System). Weather forecasting is an imperfect science, as we all know. However, the index is focused on temperature, moisture content of the air, and wind speed, which are the most accurate parts of a forecast — even out into the future. Depending on how accurate the forecast models are for those characteristics, the index will be accurate too.

Precipitation is one of the most unpredictable pieces of a forecast. Humidity is also difficult to determine, especially as you get closer to the ground. There are terrain variables that can affect the humidity of the air, such as water evaporation and any precipitation that is on the ground. These variables will affect fire behavior and are harder to measure and quantify.

What Does It Mean?

Simply put, the higher the HDWI value, the worse the fire day. Brian Potter, one of the scientists who helped develop the index, admits that they only know generally why fires are affected and how a higher value translates into fire behavior. Because all of their reports have been subjective accounts given after the fact, there isn’t a way to tie higher index numbers to any particular fire behavior. Scientists do know that wind and dry air fan the flames and help dry fuels, but the exact reason HDWI works could be more complex.

In comparison to the Haines Index, however, HDWI outperforms it hands down. When HDWI and the Haines Index were compared on forty forest fires that burned in the U.S. between 2002 and 2011, HDWI was proven better at identifying days when weather contributed to dangerous fire conditions. The HDWI value increased on bad weather days, while the Haines Index did not consistently identify those days. 

“And that is exactly the problem that the Haines Index had with the fire data that he used,” says Potter. “He asked a bunch of fire managers, ‘What were your worst fire days over the last several years?’ And they, subjectively, identified, ‘Well that was a bad day for fighting the fire.’ But we don’t know why. We don’t have that part of his information.”

A remote, automatic weather station collects weather data during a fire. This information is incorporated in near-real time to the models that feed the Hot-Dry-Windy Index. Photo by National Interagency Fire Center

How Can It Help Firefighters in the Field?

Crews should treat the HDWI like a check-engine light in their car, Potter recommends. Firefighters should look for spikes in the index, which indicate potential bad fire days. He says the index shows “the atmospheric potential for how strong or dangerous a fire could be.”

Once a spike is identified, fire crews should consult with the fire meteorologist on-site to see what in the forecast models is causing the spike. A meteorologist can review historical detail and the forecast particulars to see the pattern HDWI has identified and help the crew determine how to address it.

HDWI was used by Joel Curtis, an incident meteorologist who helped test the index during a fire near Libby, Montana. When looking at the predicted values, he noticed a spike coming five days in the future. The forecast models indicated that particular day was going to be one of the worst on record for the area. Because they had this knowledge, the fire team directed their strategies to bring the fire under control before day five. They were successful and saved thousands of dollars, while increasing the safety of the firefighting crew.

Just because the index shows a bad fire day coming, however, it doesn’t necessarily mean that the fire will respond. Ground conditions are not part of the formula that determines the index. Crews need to assess if there is fuel available that will help the fire spread, or if there is already a fire going that may be made worse. 

Hearing from the Field

The HDWI tool has been shaped by several years of input from meteorologists and fire teams, which has helped improve both the tool and the interface on the website. These changes have made the index more approachable and more accessible to fire teams. Overall, firefighters say that the tool is more intuitive and makes more sense than the Haines Index.

The index is always evolving as more data is made available. Firsthand accounts and stories of fire behavior are needed to help the scientists confirm that the index is measuring the correct variables, and to help inform them of how weather conditions actually affect a fire’s behavior.

To access the HDWI, go to www.hdwindex.org. Click on the map to see the predictions for the next few days for a given location. Feedback can be submitted directly to Potter at [email protected], or to Potter or Srock through the website.