On September 19, the Climate Prediction Center released its winter forecast, highlighting that areas shaded in green indicate above-average rainfall, white represents near-average conditions, and orange signifies below-average precipitation. The latest monthly update suggests that the likelihood of La Niña phenomena occurring this fall in California is decreasing. Historically, it is anticipated that the Bay Area will see rainfall close to average, while Southern California may experience drier conditions than usual.

According to a report from the San Francisco Chronicle, the Climate Prediction Center estimates a 60% chance of La Niña developing this fall (September to November), down from 71% reported last month. Michelle L’Heureux, a climate scientist at the center, noted that there is now a 40% chance that La Niña could not materialize this season.

La Niña refers to the cooling of sea surface temperatures in the central and eastern equatorial Pacific, which influences the jet stream’s northward shift. This shift could lead to drier winter conditions in Southern California. Currently, forecasters lean toward a weak La Niña formation, though other weather and climate patterns may play a more significant role in dictating California’s winter weather.

Currently, sea temperatures in the equatorial central and eastern Pacific are around normal levels but are starting to cool down. The Climate Prediction Center predicts that the chances of La Niña developing will peak at 75% between November and January. However, this event is expected to be short-lived, lasting only from January to March. L’Heureux pointed out that this probability lower than in previous months, as mathematical models suggest warmer ocean temperatures. One reason for this change could be fluctuations in the trade winds, which blow from east to west along the equator and stir cold water from deeper ocean layers.

During La Niña events, trade winds typically strengthen, bringing more cold water to the surface. However, this September, the Eastern Pacific has not developed as expected, leading to less cold water upwelling than predicted, indicating that this will likely be a weaker La Niña event. So, what specific impacts might this shift have on California’s weather this fall and winter?

On average, during weak La Niña events, northern California’s rainfall hovers around normal levels. In some southern locales, conditions may reflect a slightly drier trend, as indicated by areas shown in light orange on the map. However, L’Heureux cautioned against generalizations, emphasizing that each La Niña or El Niño event behaves differently and can lead to varied outcomes.