File:Heavy Snowpack in California (MODIS).jpg

English: April 26, 2023 April 24, 2022

A boom year for Sierra Nevada snow is expected to create a massive spring “Big Melt”, with potential for widespread flooding.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA's Terra satellite acquired a true-color image of snow atop the Sierra Nevada on April 26, 2023. A second true-color Terra MODIS image, acquired on April 24, 2022, can be seen by clicking on the date below the images. When toggling between the two images, it's easy to see that this year's snowpack is massively larger than a similar date in 2022.

Many parts of central California received 200 percent or more of expected precipitation this water year, which started on October 1. Eleven moderate-strength atmospheric rivers hit the southern Sierra Nevada this year, which is twice the average number. Precipitation from these storms contributed to a breach of the Los Angeles Aqueduct, the re-emergence of Tulare Lake in the San Joaquin River Valley, and heaps of snow on the range.

But it wasn’t only a wet year, it was also an unusually cold year. According to the National Weather Service, parts of the central coast experienced the coldest winter since 1978-1979. The combination of cold and wet added up to an anomalously high snowpack in both the southern Sierra Nevada Mountains and at lower elevations along the range.

As of April 1—which is typically the date of peak snowpack—the Institute of Arctic and Alpine Research (NSTAAR) estimates showed the greatest amount of snow water equivalent since records began. On that date, snow water equivalent (SWE)—a measurement of how much water you would get if all of the snow in a given area melted at once—was measured at four times (439 percent) the average for April 1. Mountains in the central part of the range were at 284 percent of normal, and slopes to the north were 271 percent of normal.

Snow that falls in the Sierra Nevada each winter is a natural reservoir that typically slowly melts and flows down into the river valleys in spring and summer. In a typical year, this snowpack accounts for about 30 percent of California’s water supply.

This year, thanks to rapidly rising temperatures, water is already melting rapidly, raising concerns about flooding from a “big melt”. On April 25, the National Weather Service, San Diego warned of a big warmup over the next several days. Temperatures were expected to reach 90s in some locations, and as high as 105˚F in some desert locations, such as the Coachella Valley in Riverside County.

To prepare for expected flooding, the National Park Service warned that, with the Merced River expected to rise to 2 to 3 feet above flood stage, several locations in Yosemite National Park will likely be closed within the next few days. The Merced is likely to reach flood stage on and off from late April until July.

Both the Tulare Lake Basin and the San Joaquin River Basin are already flooded from heavy winter rains, and are expected to receive a tremendous amount of additional water from snowmelt, which may send a “cascade of water” into the San Joaquin Valley. Based on the heavy snowpack, speculation suggests that increased flows into this region may persist until fall.