Snowmelt-driven flooding in Yosemite Valley and parts of Mariposa County is about to worsen as new thunderstorms packing 50-mile-per-hour winds slam the Sierra Mountains.

Over 90 percent of California and 100 percent Nevada rivers are now rated as “extremely above” their historic daily water levels, according to the latest California Nevada River Forecast Center survey.

With temperatures jumping to the mid-80s in Yosemite National Park and many parts of the Central Sierra Mountains, a rapid snowmelt has unleashed the equivalent of an inch of rainfall each day of the last 7 days. The water rushing down the mountains caused the Merced River to spike to 1.6 feet over its 10-foot flood stage on May 4 through May 6, inundating many river communities.

The good news is that temperatures fell into the 40s on Saturday as a new storm is set to drop 3 to 5 inches of snow in the Sierras above 5,000 feet and bring driving rain to the low elevations. Flooding is subsiding on the Merced as the river is expected to fall to 8 feet by Monday afternoon, according to the River Forecast Center.

But the really bad news is that the 7-foot Central Sierra snowpack at a record 193 percent of its 100-year average for this time of year, and will swell to the water equivalent of about 43 inches, 12 percent higher than its all-time-record, according to the California Department of Resources.

Rain in the lower elevations will push the Merced River back up to within a foot of its flood stage by Thursday. With temperatures predicted to reach the 80s again, the heat could kick-off widespread Sierra snowmelt flooding.

The 100-year average high point for the Sierra snowpack runoff is around July 1. But with more than twice the snowpack this year and an even greater water equivalent, the runoff is expected to last through the entire summer and into the fall. That means the soil around the state’s 1,500 dams, 120 rivers, and thousands of flood control levees will be saturated.

That could set up disastrous flooding next year, with the National Oceanic and Atmospheric Administration predicting the chances of an El Niño hammering West Coast with another year of heavy rainfall currently at 50 percent, and expected to be at over 70 percent by September.

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