08/23/2025
“Spreads like wildfire" is a common saying for something that spreads quickly. But how quickly can wildfires spread and how does that impact response?
Certain weather and fuel conditions can cause wildfires to spread out of control very quickly. High winds, hot temperatures, low humidity and little to no recent precipitation can create the perfect storm for rapid fire growth.
The Initial Spread Index (ISI) is a numerical value that estimates how fast a fire will spread immediately after it starts. It reflects how volatile the environment is, how dry the fine fuels are and how strong the winds are.
When the ISI is high, fires can grow so fast that by the time crews arrive, the fire is already beyond the reach of initial attack (which is the critical first response aimed at stopping a fire before it gets too big).
In B.C., most fires are suppressed by initial attack crews. So far in 2025, 94 per cent of wildfires have been kept to four hectares or less thanks to their hard work, but understanding the ISI helps us better prepare for difficult days when conditions will challenge their success.
Learn more about the Initial Spread Index and other Fire Weather Indices: https://www2.gov.bc.ca/gov/content/safety/wildfire-status/prepare/weather-fire-danger/fire-weather/fire-weather-index-system
