California's snowpack was already meager. Now comes an extraordinary heat wave

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The warm winter has left very little snow in California’s Sierra Nevada, and now an extreme heat wave is accelerating the rapid melt in the mountains.The Sierra snowpack measures 48% of average for this time of year, according to state data, down from 73% of average in late February.When water expert Newsha Ajami went skiing near Lake Tahoe in early March, she saw snow from the last round of storms had rapidly disappeared from the slopes, and many ski lifts were closed.“There was a lot of bare land, bare mountain with no snow,” she said.“Almost all of it was gone.

It was kind of scary.” California Satellite images show the Sierra Nevada before and after a major heat wave melted significant portions of California’s snow.California relies on the Sierra snowpack for about 30% of its water, on average.But the extraordinary warmth across the West this winter, which broke records in many areas, brought more precipitation falling as rain instead of snow.Scientific research has shown that human-caused climate change is pushing average snow lines higher in the mountains and changing the timing of runoff.Warming driven by the use of fossil fuels and rising levels of greenhouse gases also is bringing longer and more extreme heat waves.California’s snowpack typically reaches its peak around April 1.

But this year, state measurements from across the Sierra Nevada show that the snowpack has been shrinking since Feb.25, and the rapid loss of snow is set to continue this week as the West bakes in a heat wave that is forecast to break records in many areas.The National Weather Service said the “rare summerlike heat” this week will bring high temperatures 15 to 30 degrees above normal across much of the Southwest.

Areas where the heat is expected to set records include Los Angeles, Palm Springs, Fresno and Phoenix.The National Weather Service warned that the heat wave, in addit...