Despite December storms, California's reservoirs are at their lowest levels in years, and experts say it's going to take a lot more rain to replenish the state's water supplies.

But water levels are not likely to come back to normal in 2015, scientists say.

"We are only now entering the normally wettest part of our winter season," Jeanine Jones, deputy drought manager with the California Department of Water Resources, told NBC News. "What happens - or doesn't - in the next six weeks or so will tell us a lot about the likely outcome of the water year."

Last year was the fourth driest year on record in statewide water runoff, Jones said. She said that there was hope that storms could provide snowpack to refill the reservoirs, but often when that happens, flooding occurs.

One-third of California's water is provided by snowpack, but as Sierra Nevada's levels are half of their long-term average, water watchers are looking to January and February storms.

"In the coming weeks we need more snowpack accumulation in the Sierra Nevada and Cascades Ranges to help replenish reservoir storage later in the season," Jones said.

While measurements of the Sierra Nevada snowpack showed more snow than one year ago, it's still not enough to end the drought, according to a report in the Los Angeles Times.

As of the end of December, surveys showed 21.3 inches of snow, which when melted would amount to four inches of water, according to the Department of Water Resources.

Last year at this time, there were 9.3 inches of snow covering the ground, amounting to two to three inches of water, the data shows.

The snowpack is monitored by electronic sensors throughout the Sierras. 

But as California starts another drought year - its fourth - scientists are watching and waiting.

California's low reservoirs in conjunction with low Colorado River flows point to a "critical, critical" water year, said Kevin Werner, director for western services at NOAA's National Climatic Data Center at a press briefing last month.

"The recent storms have been helpful but we've got a long, long way to go," Werner said. "It's going to take a lot of above average precipitation, likely not just this winter but future winters as well."