The space agency said sea surface height data from its Jason-1 and -2 satellites show the milder repeat of last year's strong La Nina has recently intensified.
This is the second consecutive year the satellites have measured lower-than-normal sea surface heights in the equatorial Pacific and unusually high sea surface heights in the western Pacific, a NASA release said Wednesday.
The height of the sea surface in a given area is an indicator of ocean temperature and other factors that influence climate, it said.
"Conditions are ripe for a stormy, wet winter in the Pacific Northwest and a dry, relatively rainless winter in Southern California, the Southwest and the southern tier of the United States," climatologist Bill Patzert of NASA's Jet Propulsion Laboratory in Pasadena, Calif., said.
"This La Nina could deepen the drought in the already parched Southwest and could also worsen conditions that have fueled recent deadly wildfires," he said.
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