Synopsis: A strong El Niño is expected to gradually weaken through spring 2016, and to transition to ENSO-neutral during late spring or early summer.
A strong El Niño continued during December, with well above-average sea surface temperatures (SSTs) across the central and eastern equatorial Pacific Ocean. All weekly Niño indices decreased slightly from the previous month. The subsurface temperatures in the central and eastern Pacific, while still well above average, weakened due to an upwelling equatorial oceanic Kelvin wave. Significant low-level westerly wind anomalies and upper-level easterly wind anomalies continued over much of the tropical Pacific. During the last week, another westerly wind burst occurred in the east-central Pacific. The traditional and equatorial Southern Oscillation Index (SOI) values remained strongly negative. Also, convection remained strong over the central and east-central tropical Pacific, and suppressed over Indonesia. Collectively, these atmospheric and oceanic anomalies reflect the continuation of a strong El Niño episode.
Most models indicate that a strong El Niño will weaken with a transition to ENSO-neutral during the late spring or early summer. The forecasters are in agreement with the model consensus, though the exact timing of the transition is difficult to predict. A strong El Niño is expected to gradually weaken through spring 2016, and to transition to ENSO-neutral during late spring or early summer.
El Niño has already produced significant global impacts and is expected to affect temperature and precipitation patterns across the United States during the upcoming months. The seasonal outlooks for January - March indicate an increased likelihood of above-median precipitation across the southern tier of the United States, and below-median precipitation over the northern tier of the United States. Above-average temperatures are favored in the West and northern half of the country with below-average temperatures favored in the southern Plains and along the Gulf Coast.
Considerably warmer than normal across nearly the entire four state region with only coastal California slightly to moderately cooler than normal.
Near normal to slightly cooler across nearly the entire four state region with only southern California and far southern Nevada slightly warmer than normal.
California, Nevada and Oregon should be near normal to slightly drier than normal. Most of Washington will be slightly drier than normal except the mountainous areas west of Seattle to the border with Canada that should be slightly wetter than normal.
In California, most of the state should be near normal to slightly wetter than normal in the south and slightly drier than normal in the Northern Sierra and east of Sacramento up to the border with Oregon. Nevada should be slightly drier in the northern half and near normal to the south. Oregon should be near normal to slightly wetter than normal in the central part of the state. For Washington almost all the state should be near normal to slightly wetter than normal.