Sierra Snowpack

By Emily Benson and Natalie Jacewicz
Staff writers
Posted:    02/29/2016 – 02:56:00 PM PST
Updated: 02/29/2016 – 04:55:23 PM PST

After a dismally dry February, drought-weary Californians are hoping a series of storms predicted to roll through in early March blanket the Sierra Nevada with a much-needed additional layer of snow, building up the state’s vital snowpack that all but disappeared last year.

Starting Friday, forecasters are predicting a chance of rain or snow for 10 consecutive days.

How much water those storms might eventually supply to the state’s reservoirs depends partly on where they come from — warmer, wetter storms sweeping across the Pacific generally bring more snow to the mountains than colder, dryer tempests barreling down from the Gulf of Alaska. Cold air generally doesn’t hold as much moisture as warm air.

Before Mother Nature turned off the tap in February — leaving many to question what happened to El Niño’s drought-busting potential — the Sierra benefited from colder storms in December and warmer ones in January. What’s in store for March?

“Whether they’re particularly warm or cold storms remains to be seen,” said Daniel Swain, a Stanford University doctoral student who runs the California Weather Blog. “They’re certainly storms that would add to the water supply.”

Despite the February stall, the Sierra is having a better snow year than at any point since 2011. Statewide, as of Monday, the snowpack was about 85 percent of normal for this time of year, compared with 19 percent last year, the lowest number on record.

On Tuesday morning, Frank Gehrke, the chief of California’s snow survey program, will measure the depth of the snow piled up at Echo Summit near Lake Tahoe, a monthly winter ritual that helps the state keep tabs on its precious frozen reservoir.

The state takes similar manual readings at more than 200 sites throughout the Sierra. The snowpack supplies about a third of California’s water in a typical year. To monitor the frosty resource, the state also deploys surveys, sensors, satellites and aerial flybys.

Things looked much better last month. The Feb. 1 statewide snowpack was 114 percent of normal. Then the rain all but stopped: Most Bay Area cities recorded less than 1 inch in February, including San Francisco (0.98 inch, average 4.46), Oakland (0.30 inch, average 3.95) and San Jose (0.31 inch, average 3.32), the city’s ninth driest February since 1893, according to data from the National Weather Service.

In February, El Niño’s unusual position created a high pressure ridge that shunted California’s much-needed precipitation north to Seattle, said meteorologist Jan Null of Golden Gate Weather Services in Saratoga. This El Niño’s warmest water is much farther west than usual.

This warm patch of water is causing air to suddenly rise — triggering a strong loop of circulating atmosphere, called a “Hadley cell,” according to Null and Swain. As the air in that loop falls back to earth, it creates a high pressure ridge — right off our coast. Few storms can get through.

“Ridges and troughs happen all the time — and clearly there were troughs in late December and off-and-on in January,” Null said.

The storms that came in December arrived before the ridge did, Swain said. “Those were fairly cold storms, and they dumped a lot of snow in the mountains,” he added.

The precipitation pattern the state experienced in January — a relatively warm and wet storm about every other day over several weeks — was an ideal situation, according to Null. Those squalls came from across the Pacific, he added. They weren’t the wet, hot “atmospheric rivers” — sometimes called Pineapple Express storms — that can sweep in from the tropics. Pauses between periods of precipitation allow water to run off or soak into soil more slowly, preventing flooding.

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