Portions of the Pacific Ocean cooled considerably in July, affirming that a La Nina weather system likely will form this fall, the National Oceanic and Atmospheric Administration said Aug. 12.

NOAA’s Climate Prediction Center pegged the odds of a La Nina prevailing in November, December and January at 69%.

Most likely, the La Nina will be weak, according to NOAA, though the agency estimated the chances of a moderate La Nina at 1-in-3 and a strong La Nina at 1-in-10.

La Ninas occur when cooling sea-surface temperatures trigger atmospheric changes that shift incoming jet streams northward.

La Nina winters are often cooler and wetter in Washington and much of Oregon and Idaho, while California and other southern-tier states are warmer and drier. La Nina could mean a good snowpack for Northwest irrigators, but worsen the drought in the Southwest.

The U.S. Drought Monitor on Aug. 12 reported that 95% of Arizona, California, Idaho, Montana, Nevada, New Mexico, Oregon, Utah and Washington were in drought.

One-quarter of the West was in an “exceptional drought,” the worst classification. California and Utah are the hardest hit states.

An El Nino shifts jet streams southward, intensifying winter storms in the southern tier of the U.S. and leaving the Northwest warm and dry. NOAA sees almost no chance that an El Nino will form this winter.

An La Nina prevailed last winter, as Washington built up a good snowpack that’s helping irrigators weather a dry spring and hot summer.

The La Nina faded last spring and since then ocean temperatures have been normal. NOAA expects the neutral conditions to prevail for the rest of the summer.