By Laila Kearney

SAN FRANCISCO (Reuters) - California is coming off of its warmest winter on record, aggravating an enduring drought in the most populous U.S. state, federal weather scientists said Monday.

The state had a average temperature of 48 Fahrenheit (9 Celsius) for December, January and February, an increase from 47.2 F in 1980-81, the last hottest winter, and more than 4 degrees hotter than the 20th-century average in California, the National Oceanic and Atmospheric Administration (NOAA) said in a statement.

Warmer winters could make the already parched state even drier by making it less likely for snow to accumulate in the Sierra Nevada Mountains, NOAA spokesman Brady Phillips said. That snow, melting in the spring and summer and running down through the state's rivers, is vital for providing water in the summer, when the state typically experiences little rain.

"Winter is when states like California amass their main water budget, when snowpack is building," said Phillips, a marine biologist. "If you're starting from a deficit and going into the dry season, it's setting you up for a drier summer."

California is in the grip of a three-year dry spell that threatens to have devastating effects on the state and beyond. Farmers are considering idling a half million acres of cropland, a loss of production that could cause billions of dollars in economic damage, and several small communities are at risk of running out of drinking water.

The state also recorded its driest winter to date by March, despite recent storms, with an average of 4.5 inches of rainfall, compared to 11.7 inches over the previous winter, NOAA said.

Around the West and in the Great Plains, multiple states also experienced warmer temperatures and low rainfall. Arizona had its fourth warmest winter to date and Texas had it lowest reservoir levels in 25 years by March.

Despite regional heavy snow pummeling regions the eastern region of the country, overall rainfall across the United States was far below normal. An average of 5.7 inches of rain fell overall in the United States in the past three months, causing the ninth driest winter on record, NOAA said.

Climatologists and other scientists with NOAA's National Climatic Data Center record a summary of temperatures and rainfall for all 50 states each month. Every three months, the federal agency releases data on spring, summer, fall and winter weather.

The agency is planning to release its spring outlook climate forecast on Thursday.

(Editing by Sharon Bernstein and Lisa Shumaker)