Where will record snowfall in California melt into floods? It is complicated

After months of heavy rain across California and record snowfall in the southern Sierra Nevada, state officials are warning that melting snow could turn into flooding as the state’s skies get brighter in the spring and summer.

The basins of Lake Tulare and the San Joaquin River, which have already experienced storm flooding this year, are of particular concern. But specific forecasts remain elusive despite the new forecast data, as water managers continue to crunch the numbers.

What is clear is that runoff from melting snow could rise to dangerous levels with higher-than-expected temperatures, particularly in Southern California. And if the residents of the Golden State predicted anything, it’s the weather that deviates from the historical averages.

“For any data that results from a forecast, there is uncertainty in future conditions,” said David Rizzardo, director of hydrology for the Department of Water Resources. “Over the last 10 years, we’ve seen a lot of extremes… forecasting measures are trying to adapt to that.”

Northern California rivers, including the American and Sacramento rivers, are projected for a typical wet year with continued high flows based on more than 100% of the historical average runoff. The region simply has more water infrastructure — dams and reservoirs that capture most of the heavy rainfall during the wet season, said Jeffrey Mount, a senior fellow at the California Water Policy Center’s Public Policy Institute.

It’s the waterways of the San Joaquin Valley in the south of the state that are now in uncharted territory, and runoff is projected to reach more than 400% of historical averages along the Kern River. Runoff flows can be nearly as high in the Lake Tulare basin, according to runoff forecast data from the Department of Water Resources.

Before making model predictions about exactly where melting ice could turn into floods, officials said they need more data on reservoir operations and agricultural water needs across the state’s tightly managed water system. But Mount cautions against trusting those predictions.

“What we do know is that there will be widespread flooding. It will be very large and destructive, but it will unfold slowly and hopefully won’t kill anyone,” Mount said. “But since they were on the edge of our models, they wouldn’t be very good… When we have extremes, whether it’s too wet or too dry, we often discover weaknesses in our models.”

As of Tuesday, the National Weather Service’s River Forecast Center shows the San Joaquin River in Modesto, Vernales, Patterson, Newman, and Stephenson above the watch phase, indicating the possibility of approaching flood conditions.

Rapid melting of snow is one cause of potential flooding, but water released from dams can also double already-high river flows. The town of Hanford south of Fresno, for example, is under a flood warning notice due to a flood gate release.

Managers of California’s complex network of reservoirs and canals are now trying to strike a balance between moving enough water and not too much. The goal is to make way for the snow to melt while maintaining fullness for future droughts.

Water resource management data will be given to dam operators, including the Federal Army Corps of Engineers, who will compare it to historical patterns to determine when and where water is released from full reservoirs.

These decisions will also be driven by information on water demand from cities and agriculture, said Jenny Fromm, the Corps’ chief of water management. She said she expects release from dams in the San Joaquin and Tulare watersheds throughout the spring, and even into the summer, to create space in the reservoirs in anticipation of the snowmelt.

“It’s the rock and a hard place with the reservoirs. Does it release that water and create a little bit more room for the possibility of more snow and rain?” said Jeremy Arich, director of DWR’s flood management division. “But if we release the water and don’t fill the tank, we’re going to be under a lot of pressure on the other side.”

After reservoir and irrigation district managers make their estimates, DWR said, the agency can begin forecasting and preparing for flooding in certain parts of the state and making preparations to protect public safety and property.

“Once we understand a good portrayal of what is at risk, overlaying that with some of these thresholds and triggers will help us get to the point where we know where to focus resources,” said Arish.

State officials have recommended that residents of the San Joaquin Valley and Tulare Basin pay attention to local emergency responders and plan evacuation routes in the event of flooding. State climatologist Michael Anderson said the weather is the final arbiter.

It’s the long periods of high temperatures in May and June that will melt record snow at peak rates. After a warmer week, mild temperatures are expected for the next several days in Sacramento and across central California, according to the National Weather Service.

“That’s the pattern we’ll see in April, you get these warm periods and then some cooler as we move out of the wet season and move into the dry season,” said state climatologist Michael Anderson.

“How we play this year is going to depend on the weather and how quickly we warm up and how fast the sun can get into this pack until the pack is ready to melt.”

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