Synopsis: La Niña is favored to develop (~70% chance) during the Northern Hemisphere fall 2016 and slightly favored to persist (~55% chance) during winter 2016-17.
ENSO-Neutral conditions were observed during September, with negative sea surface temperatures (SSTs) anomalies expanding across the eastern equatorial Pacific Ocean by early October. All of the Niño regions cooled considerably during late September and early October, with the latest weekly value of Niño-3.4 index at -0.9°C. Subsurface temperature anomalies also decreased toward the end of the month, reflecting the strengthening of below-average temperatures at depth in the east-central equatorial Pacific. Atmospheric anomalies across the equatorial Pacific edged toward La Niña during September, with a stronger tendency toward La Niña late in the month. The traditional Southern Oscillation index and the equatorial Southern Oscillation index were positive. The lower-level winds were near average across most of the basin during the month, but enhanced easterlies were becoming more persistent west of the International Date Line. Upper-level winds were anomalously westerly near and just east of the International Date Line. Convection was weakly suppressed over the central tropical Pacific and was more enhanced over Indonesia compared to last month. Overall, the combined ocean and atmosphere system reflects ENSO-Neutral during September, but are more clearly trending toward La Niña conditions.
The multi-model averages favor borderline Neutral-La Niña conditions (3-month average Niño-3.4 index less than or equal to -0.5°C) persisting during the Northern Hemisphere fall and continuing into the winter. Because of the recent cooling in the Niño-3.4 region and signs of renewed atmospheric coupling, the forecaster consensus now favors the formation of a weak La Niña in the near term, becoming less confident that La Niña will persist through the winter. In summary, La Niña is favored to develop (~70% chance) during the Northern Hemisphere fall 2016 and slightly favored to persist (~55%% chance) during winter 2016-17.
Most of the four state region should be slightly to moderately warmer than normal with the exception of coastal California from San Francisco south to San Diego.
The intermountain region should be warmer than normal and Texas through to Florida and Georgia should be moderately warmer than normal. The Great Lakes and through to the Northeast should be slightly warmer than normal.
Most of Washington and Oregon should be slightly drier than normal while most of California and Nevada should be slightly wetter than normal with the exception of far southern California and Nevada that should be drier than normal.
The northern plains should be slightly to moderately drier than normal along with the Great Lakes and Upper Mississippi Valley while the Southwest should be moderately drier than normal. The Great Plans should be slightly to moderately wetter than normal while the eastern seaboard should be near normal.