How Much Precipitation is Needed to End the Drought?

The nation is still sharply divided, as it has been for weeks, in terms of precipitation. In the East, one storm after another, combined with unusually low temperatures, has left piles of snow on the ground — and so much on roofs that many are in danger of collapse. And with a thaw going on this week, you can add the risk of flooding to the mix.

Out West, though, it’s another story: in California, especially, the worst drought in more than a generation threatens to devastate one of America’s most crucial agricultural economies and puts drinking-water supplies at risk. The state is so deep in the hole in terms of water supply that a series of powerful storms over the past week or two has barely put a dent in the precipitation deficit.

The graphic above shows just how much rain or rain-equivalent in snowfall California would need over the next month to end the drought (as measured by the Palmer Drought Severity Index). These amounts are so drastically above normal in most places that the chance of it happening are 1-in-1000.

The snowpack is especially crucial: melting snow is a major source of water in Western states during the otherwise dry summer. But snowpack in the West is dramatically lower than it was last year, raising the risk of inadequate water this summer and dangerous wildfires as a result. Big fires have been increasing since 1970, as temperatures have risen. And because drought in already dry places is something scientists expect with global warming, it's plausible that the situation in California could be exacerbated by climate change. Either way, a warming planet will make all droughts worse because they will be hotter, drying soils more thoroughly and evaporating water supplies more rapidly.