It's hard to process yesterday's deaths of 19 firefighters in Arizona. The tragedy is so stark an outlier that most states haven't seen that many deaths of firefighters due to wildfire in their combined histories. But there is one worrisome trend: fires are getting bigger and often deadlier.

The National Interagency Fire Center tracks wildfire incidents, scale, and damage for the United States. Included in that data is a list of firefighter fatalities through 2011, broken down by cause and county. No state has seen more firefighter deaths than California, which had 324 through that year. It is one of 11 states that has had more deaths than Arizona saw yesterday — with one of those states being Arizona. That largely tracks with this interesting map of fires by location. Most fires occur in the Southwest; many in California.

Over time, the number of firefighter deaths has been consistently low. Only two incidents on the NIFC's list were more deadly than yesterday's: a 1910 fire in Idaho and a 1933 blaze in Los Angeles. (This is only wildfires, of course. September 11th was the deadliest day for firefighters overall, and total firefighter deaths are declining.) But, as the graph below shows, there's been a slight uptick in deaths per year over the past few decades.

That uptick tracks with an increase in fire size. The NIFC has data on acreage burned and the number of fires going back 50 years, though data prior to 1983 uses different records. Here's the number of fires and acres burned during that time period.

Over the past decade, there's been a noticeable trend: fires have gotten bigger. The number of acres burned per fire reached a new high last year.

One reason for last year's devastation was last year's massive drought. The Southwest has been consistently dry for several years, but last year — and into this year — the drought reached levels not seen since the Dust Bowl era.

The wildfires seen so far in 2013 largely track with the drought. Below, the number of acres burned by state. The image that follows shows how the drought has shifted since January. Darker colors indicate a more severe state of drought. It has moved south and west.

All of this, of course, is largely academic. For the state of Arizona, the broader trends in fighting wildfires pale in comparison to the devastation of losing homes and the people who signed up to protect them. As New York City's Firemen's Memorial says, the lost firefighters are "soldiers in a war that never ends." Yesterday's loss is a reminder that even with our modern advantages, the war is still a struggle.

Photo: Still from a timelapse video of the Yarnell fire by Matt Oss on YouTube.