The Wildland Fire Potential Index (WFPI) from the USGS forecasts the risk for large fire activity across the U.S.
It is a unit-less number that ranges from 0 to 150 and relates to vegetation flammability. It describes the ratio of live to dead fuel and includes variables for wind speed, dry bulb temperature, and rainfall.
The wildfires in the L.A. area are not unexpected. The WFPI values around L.A. are mostly in the red zone. The Santa Ana winds spread the fire.