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Will California’s ‘big melt’ cause catastrophic floods? Here are four scenarios.

An employee from the California Department of Water Resources inserts a snow depth survey pole into the snow during the final snow survey of the season at Phillips Station in El Dorado County, Calif., on May 1. (Kenneth James/AP)
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California’s “big melt” is underway, and if forecasts bear out, much of the water being held in mountain snow will flow downhill in May and June. But at the moment, the state’s snowpack remains huge — about three times its normal size for this time of year — and depending on coming conditions, the snow can either dissipate slowly or quickly cause trouble.

Snowmelt often accelerates in May with warmer weather, longer days and a higher sun angle.

“May is typically one of, if not the, month with the most melt,” said Andrew Schwartz, lead scientist at the University of California at Berkeley’s Central Sierra Snow Lab. “There is definitely going to be a whole lot of melt coming our way.”

Although it started out at a higher point, California’s snowpack is already melting faster than it did in 1983, a year of historic flooding in the San Joaquin Valley, thanks to a dry April and a heat wave late in the month.

Unseasonably cool weather over the past week helped to slow the melt and even added to the snow totals in the mountains.

“But you’ve put off until tomorrow what could have happened today,” said Jeffrey Mount, a senior fellow with the Public Policy Institute of California’s Water Policy Center. “It’s all going to run off at some point.”

With the weather driving how quickly snow will melt, here are four scenarios that could determine flood severity this spring and summer.

Extreme heat that leads to severe flooding

Experts’ biggest worry is an extreme heat wave, especially if it happens later in May, when melting and river flows are forecast to peak.

During an intense or prolonged heat wave, temperatures could soar into the 80s at high elevations.

This scenario could produce severe flooding, with quickly rising waters that could damage roads, flood homes and even cause fatalities. Such a dramatic escalation is most likely on the San Joaquin River as it runs northwest through the valley, where many levees are vulnerable.

“That wave [of melt], if it’s too big, is going to overwhelm the levees on that river and you will have flooding on that flood plain,” Mount said. “Levee failure is not a certainty, but likely if things really heat up.”

An atmospheric river: Worrisome, but unlikely

An atmospheric river, or other type of heavy rainstorm, would pour water into streams and reservoirs already running high. It could also warm snowpack and prime it for faster melt. Flooding impacts would depend on the strength and temperature of that atmospheric river, and where it made landfall.

“We … want to avoid any warm events with heavy rain,” said Dan McEvoy, an associate research professor and climatologist at the Desert Research Institute in Reno, Nev. “That is fairly uncommon in spring but still something to keep an eye on.”

The Climate Prediction Center’s latest outlooks favor rain chances, along with warm weather, for California into the third week of May.

Warm spells that drive faster melt with possible flooding

This weekend, California will be on the southern end of a much more intense heat wave that will impact the Pacific Northwest and Canada. For now, the state isn’t expecting anything extreme.

Forecasts indicate that temperatures could range from the upper 50s to mid-60s at higher elevations, Schwartz said.

“It does accelerate the melt but not to the point where we’re worried about catastrophic melting,” Schwartz said.

During these warmer conditions, some rivers could exceed flood stage but that’s not a given, he said, similar to the brief heat wave in late April.

The best scenario: Cool into summer

Continued cool weather with no major heat waves would spread runoff over many weeks, without the big pulses that could escalate flood dangers. It would also keep California’s high elevation snowpack intact through the summer, allowing runoff to refill reservoirs as they are being drawn down by high water demand during the dry season.

Regardless of weather, Tulare basin will flood

With an extended warmup on the horizon, a continuously cool spring and early summer doesn’t appear to be in the cards.

“Going forward the best thing we could hope for is an absence of extreme heat over the next month or so,” McEvoy of the Desert Research Institute said. However, flooding is inevitable in places such as the Tulare basin “simply due to the amount of snow still remaining in the mountains,” he said.

The basin’s vast expanse could guard against rapid water rises, allowing time for evacuations. Still, evacuating facilities such as the California State Prison in Corcoran will be a difficult and complicated task, Mount said.

The water flowing into the bowl-shaped lake bed has no real outlet. Some of the incoming flows can be siphoned off to the San Joaquin River, Mount said, but only about 10 to 20 percent.

“The rest has to go into Tulare basin,” he said. “The water will inevitably rise and all we’re arguing about at this point is how fast.”