Heavy winter rains have left the Missouri and Mississippi River basins, from Iowa to Louisiana, at an elevated risk of moderate flooding through June, U.S. government forecasters said on Thursday.

The risk extends to eastern Texas and the southeastern Coastal Plain, the National Oceanic and Atmospheric Administration (NOAA) said in issuing its spring outlook.

Early spring storms fueled by El Nino have already drenched areas of Louisiana, eastern Texas, Mississippi and Arkansas with up to 20 inches (50.8 cm) of rain, causing widespread flooding, said Tom Graziano, acting director of NOAA's National Water Center.

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"The good news is that once the ongoing river flooding recedes, the risk for additional widespread major flooding is low across the country for the remainder of the spring," Graziano said in a conference call with reporters.

"Because of a mild winter and little snow accumulation, the western half of the country, the Upper Midwest, the Middle Atlantic and Northeast all have a low risk of river flooding, which is typically enhanced by snow melt," Graziano said.

Warm trend to persist

Following a record-warm winter in the contiguous United States, above-normal temperatures should persist in most of the country through June, except for the southern Plains.

"There might be some cool outbreaks from time to time but the general spring outlook is for a warm-weather pattern," said Brad Rippey, a U.S. Department of Agriculture meteorologist on the call.

However, he cautioned farmers and gardeners against planting before typical last-frost dates, saying, "you can still have these episodic freezes or cold outbreaks within an overall mild regime."

NOAA called for above-normal precipitation through June for the southern half of the United States and an equal chance of above- or below-normal rainfall in the northern Plains, Midwest and Northeast.

Mixed picture for California

In California, which has endured a multi-year drought, conditions are improving in the northern half of the state, particularly after storms in the last two weeks.

The state's water resources "are more favorable than they have been since 2011," said Rob Hartman, a hydrologist with NOAA's California Nevada River Forecast Center, on the call.

But Southern California is still mired in drought that is expected to persist through June. Ending California's drought statewide will take multiple years, Hartman said.

"It's going to take a while. Over the last four years prior to this year, we were missing between one and two years of rainfall. That has to be made up in some fashion," Hartman said.