For the first time in the 15-year history of the National Weather Service Drought Monitor, 100 percent of California is judged to be in moderate to exceptional drought.
It’s a dramatic-sounding milestone, although the state has been lingering at the 99.8 percent mark since March 18.
Though it’s a first for California to be completely in drought, other states have been there before, including New Mexico in June 2013, and Texas from September through November in 2011.
You’d need to return to a time before the Drought Monitor existed to see California in such a dire situation.
“It’s too bad the Drought Monitor doesn’t go back to 1976 and 1977,” says Richard Heim, meteorologist with the NOAA National Climatic Data Center. “The drought over that winter had the same kind of jet-stream pattern we’ve been seeing for the last year to year and half.”
Here’s what California’s drought map looks like this week:
A Brief History of Drought Tracking
In 1965, meteorologist Wayne Palmer came up with the Palmer Drought Index, which used precipitation, temperature and soil moisture numbers to quantify the state of drought at any given location. Where the number came out negative there was drought, and where the number was positive there was not.
While groundbreaking, the Palmer Index has several problems. For one, it didn’t factor in snowpack, which is a key component of natural water storage in California.
A host of other tools popped up over the next few decades, and each used a different suite of measurements to quantify drought.
“We had all these gazillion different indices, and some were saying one thing and some were saying other things,” Heim says. “A group decided at the end of the 1990s to coordinate all this stuff and make sense of it.”
The Drought Monitor was born as a partnership between the National Drought Mitigation Center at the University of Nebraska-Lincoln, the U.S. Department of Agriculture and the National Oceanographic and Atmospheric Administration.
A small team works each week to create a map and report on the state of drought across the United States. Their composite drought indicator uses the Palmer Index along with a suite of other tools to apply a drought severity scale to geographic regions.
Today it is the most widely used gauge of drought.
Decoding the Scale
The Drought Monitor scale was built to be similar to those used to track tornadoes and hurricanes. Each of these categories are based on historical percentiles if you look at 100-year records.
D0 – Abnormally Dry
D1 – Moderate Drought (11-20% of the time)
D2 – Severe Drought (6%-10% of the time)
D3 – Extreme Drought (3%-5% of the time)
D4 – Exceptional Drought (2% of the time or less)
About a quarter of California is currently classified as D4. Droughts in this category are thought of as “50-year droughts,” and they are exceptionally rare.