Conditions in and above the Atlantic Ocean continue to suggest that this year’s hurricane season will be an above-average one, a government scientist said Wednesday.Matthew Rosencrans, of the National Oceanic and Atmospheric Administration, said that an updated forecast suggested that there would be 15 to 21 named storms, including seven to 10 hurricanes, by the end of the season on Nov. 30. Three to five of the hurricanes could be major ones of Category 3 or higher, with sustained winds above 110 miles an hour.

The updated numbers are only slightly changed from NOAA’s preseason forecast in May.

“There’s now a 65 percent chance for an above-average season,” Mr. Rosencrans said. An average year has 14 named storms, seven of which are hurricanes, including three major ones.

Mr. Rosencrans said one reason for the slightly changed forecast, and for the continued prediction of above-average activity, is that NOAA forecasters now say the climate pattern called La Niña may develop later this year.

In a La Niña, sea surface temperatures drop in the equatorial Pacific Ocean, and that can affect weather elsewhere. In the Atlantic, that often means less wind shear, changes in wind speed and direction that can affect the structure of storms. Less wind shear means storms are more likely to form and be stronger.

Hurricane season begins on June 1, although every year since 2015, storms have developed before June. This year, Tropical Storm Ana formed in late May.

Ana was the first of five named storms so far. The fifth, and first hurricane, Elsa, formed on July 1, making 2021 the fastest to reach five storms, ahead of 2020.

As a Category 1 hurricane, with top wind speeds of about 85 miles an hour, Elsa caused flooding and other damage in parts of the Caribbean before briefly entering the Gulf of Mexico, crossing northern Florida and traveling up the East Coast. Downgraded to a tropical storm, Elsa contributed to flooding in and around New York City on July 8.

Since Elsa, the season has been quiet. But mid-August through October tends to be the most active period, in part because the ocean has warmed through the summer, providing more energy for the rise of large rotating, or cyclonic, storm systems. During those months wind shear also tends to be reduced, even without a La Niña.

The National Hurricane Center is currently tracking three areas of low-pressure air in the Atlantic Ocean, two off West Africa and one closer to the east coast of South America. These kinds of atmospheric disturbances in the tropical Atlantic can lead to tropical storms or hurricanes. But the hurricane center said the likelihood of these becoming storms was currently low.

Researchers have documented that global warming has affected cyclonic storms, although there is debate about some of the ways they may be linked.

Climate change is producing stronger storms, and they produce more rainfall, in part because there is more moisture in warmer air, and in part because they tend to slow down. Rising seas and slower-moving storms can make for more destructive storm surges

Facing an acute and growing drought, California will reduce the amount of water that farmers in the state’s agricultural heartland are allowed to draw from its largest rivers, officials announced this week. It is the most severe measure taken by the state since a drought emergency was declared for most of California in May.

The unanimous vote by the State Water Resources Control Board on Tuesday will come into force in about two weeks, when thousands of farmers in the watershed of the Sacramento and San Joaquin Rivers — the lifeblood of the agricultural Central Valley — would be subject to drawing restrictions. Depending on a farmer’s rights and status, the amount of water that can be drawn could be reduced or cut altogether.

A separate curtailment order was passed for the Russian River north of San Francisco.

It is the fourth time in recent decades that California has curtailed water rights for farmers, and experts say climate change is likely to make similar restrictions more regular.

California, by far the largest agricultural producer in the United States, may see declines this year in lower-value crops like corn or alfalfa because of the drought, said Jay Lund, an expert on California’s water system at the University of California, Davis. But many farmers will still be able to draw on ground water and other stored water for their needs, he said.