A rogue jet stream is responsible for causing severe weather since the fall - including Sunday's devastating tornado in Joplin, Mo. - and could trigger severe flooding in the West, a JPL climatologist warned Monday.

Bill Patzert of the NASA's Jet Propulsion Laboratory in La Ca ada Flintridge said he is "extremely anxious" about the possibility of floods that could arrive in late spring and early summer in the West.

"Especially on the Sacramento and San Joaquin rivers" and the upper Colorado River, he said.

The jet stream in the upper atmosphere, which has seen wild dips since November, drags dry, cold air down from the north, which collides with warm, moist air from the south.

It's the source of recent "heart-breaking" tornado activity in the Midwest and South, Patzert said, as well as the severe storms that have produced record snowpacks in the country's northern tier and historic Mississippi River flooding.

"We've had so much snow in the Rockies and the Sierras, and it has not begun to melt yet," he said. "We might see flooding that we haven't seen in over 25 years here in California."

Thousands of acres of farmland are in the vicinity of the Sacramento and San Joaquin rivers, and many residents have built their homes on flood plains, he said.

State officials say they aren't worried about the Sacramento Valley rivers, since they can handle large quantities of water.

But a rapid snow melt "can be a big cause for concern" for the San Joaquin Valley, since the rivers there are much smaller, said Arthur Hinojosa of the California Department of Water Resources.

The danger is that snow could melt and flow into reservoirs faster than water officials can put it to use on the fields, or faster than the river can safely convey it through the valley, Hinojosa said.

"We're concerned," Hinojosa said.

However, "there is no immediate concern," he added.

"The forecast for the next week or so doesn't give us reason to expect an inordinate amount of snow melt," he said.

What is still uncertain is whether the next few weeks will bring hot sunny weather that "could move things along faster than anyone can handle," Hinojosa said.

The Department of Water Resources is standing by to assist residents in the event of floods, Hinojosa said, whether it be with sandbags, patrolling, or with larger flood works to protect vast areas.

Any potential flooding will probably not affect the state's water supply, he added.

A spokesman for Reclamation District 17, which serves much of San Joaquin County, said he was not particularly concerned about flooding at this time.

"If we get two weeks of good storms coming up from the south, maybe pick up 10 inches of rain in a couple-week period, then I think we'll probably be more anxious. But we don't normally get storms like that this time of year," said Dante Nomellini Sr.

Very weak circulation of air currents in the Arctic has caused the "wild north-south meanders of the jet stream" in the upper atmosphere, Patzert said.

That has led to the severe weather since November, he added.

"These are 30 to 50 year events, where you have such serious flooding on the Mississippi, such heavy snowpack and so many deadly tornadoes," said Patzert.