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# WFPI-based Fire Spread Probability (WFSP)

WFSP (Figure 1) is the probability that an existing one-acre fire will spread and burn more than 500 acres.

Predictions for Spread Probability are based on the historic average probabilities adjusted, up or down, by the WFPI value for that day. Historic averages are specific for each location and day-in-year. Fire occurrence data from the period 2001-2015 were used to evaluate average probabilities and WFPI adjustments.

Historically, higher numbers of ignitions have spread to become large fire (>500 acres) at the highest levels of WFSP (Table 1).

Table 1: Observed proportion of ignitions becoming large fires large fires (>500 acres) per voxel (km2-day) at each predicted WFSP group level.

Predicted WFSP

(percent)

Observed Proportion1

(percent)

0.0 – 0.5 0.19
0.5 – 1.0       0.83
1.0 – 2.0 1.35
2.0 – 5.0 3.56
5.0 – 10 6.95
10 – 20 13.86
20 – 30 20.63
> 30 34.19

1 Observed proportion based on the historic period 2001-2015

The accuracy of the predictions is demonstrated by the reliability diagram (Figure 2.), where observed values are plotted against predicted values.  Here, observed values were evaluated by counting the fraction of one-acre fires per year, in each WFSP category, that spread to burn more than 500 acres. Calculations of observed fractions were done for each year (2001-2015) separately using cross validation. Specifically, the predicted probabilities for each voxel were evaluated using data from all other years excluding the year in question. Figure 2. Reliability diagram for the WFSP model. On average the risk of spread was observed to be ~10 times greater when the predicted risk was extreme (risk category red; predicted class >=30) than when the predicted risk was moderate (risk category turquoise; predicted class 2-5). Between year variability is demonstrated by the length of the boxes and the extended whiskers. The horizontal line, at ~3%, is the overall observed historic average.