TRUCKEE, Calif. — Snowmelt runoff into Sierra rivers and streams has peaked far earlier than normal this year, putting an important emphasis on rain this spring.

Peaking on April 30, this year marks the third quickest peak for runoff since the early 1970s, when such records were kept, said U.S. District Court Water Master Chad Blanchard. The only faster peak years were 1988 and 1977.

“We could come back up from rain, but the snowmelt peak has passed,” he said.

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The National Oceanic and Atmospheric Administration is predicting a chance of showers and thunderstorms for the greater Lake Tahoe Area until Friday night, before a return to drier conditions and above normal temperatures for the weekend.

The minimum water level required to open Tahoe reservoirs to flow into the Truckee River is below 500 cubic feet per second, according to a previous report. This is referred to as the Floriston Rate — a long-standing federal rule that stipulates that as long as the flow is high through Floriston in the Truckee River, the U.S. District Court Water Masters Office can’t release extra water from Lake Tahoe.

Blanchard said rainstorms would be particularly beneficial to the rise of Tahoe’s lake level, since the water would fall directly onto the lake’s surface, rather than be absorbed by the dry ground and plants as it would with other watering holes.

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In November and December 2012, Lake Tahoe had a great rise, he said, but it has since been “dismal” due to January through March being the driest on record in more than a century, according to previous report.

Less snowmelt runoff creates a demand on regional reservoir storage, Blanchard said, in terms of having to tap into less stored water sooner.

Already, some stored water is being released into the Truckee River for fish, he said. As for Truckee River flow for the coming summer, it should be fairly normal due to releases from Lake Tahoe at Tahoe City and the river’s system of reservoirs.

The minimum water level required to open Tahoe reservoirs to flow into the Truckee River is below 500 cubic feet per second, according to a previous report. This is referred to as the Floriston Rate — a long-standing federal rule that stipulates that as long as the flow is high through Floriston in the Truckee River, the U.S. District Court Water Masters Office can’t release extra water from Lake Tahoe.

As of Monday afternoon, Blanchard said Lake Tahoe is 3.14 feet above its natural rim of 6,223 feet above sea level, a majority of which will be lost when water evaporation from warm temperatures exceeds inflow.

Blanchard predicts Tahoe’s lake level will start dropping earlier and could reach its rim in the fall.

The situation not only puts emphasis on rain this spring, Blanchard said, but also on the need for a strong winter in 2013-14, considering the past two winters saw lower-than-normal snowfall levels at Tahoe and Truckee.

“We really need a good year next year or we’re going to have a problem meeting flows,” Blanchard said.