Synopsis: A transition to ENSO-neutral is expected to occur by February 2017, with ENSO-neutral then continuing through the first half of 2017.
La Niña continued during December, with negative sea surface temperature (SST) anomalies continuing across the central and eastern equatorial Pacific. The weekly Niño index values fluctuated during the last month, with the Niño-3 and Niño-3.4 regions hovering near and slightly warmer than -0.5°C. The upper-ocean heat content anomaly was near zero when averaged across the eastern Pacific, though near-to-below average subsurface temperatures were evident closer to the surface. Atmospheric convection remained suppressed over the central tropical Pacific and enhanced over Indonesia. The low-level easterly winds were slightly enhanced over the western Pacific, and upper-level westerly anomalies were observed across the eastern Pacific. Overall, the ocean and atmosphere system remained consistent with a weak La Niña.
The multi-model averages favor an imminent transition to ENSO-neutral (3-month average Niño-3.4 index between -0.5°C and 0.5°C), with ENSO-neutral lasting through August-October (ASO) 2017. Along with the model forecasts, the decay of the subsurface temperature anomalies and marginally cool conditions at and near the ocean surface portends the return of ENSO-neutral over the next month. In summary, a transition to ENSO-neutral is expected to occur by February 2017, with ENSO-neutral then continuing through the first half of 2017.
Even as the tropical Pacific Ocean returns to ENSO-neutral conditions, the atmospheric impacts from La Niña could persist during the upcoming months. The current seasonal outlook for JFM 2017 favors above-average temperatures and below-median precipitation across much of the southern tier of the U.S., and below-average temperatures and above-median precipitation in portions of the northern tier of the U.S.
The western plains and northwest of should be warmer than normal and the rest of the country should be cooler than normal and the Southeast considerably cooler than normal while Maine should be warmer than normal.
Most of Washington, Oregon and California will have near normal to slightly below normal precipitation except southern California that should be moderately drier than normal and the northern half of Nevada should be wetter than normal. The Great Plains should be wetter than normal. The Southwest and across the Gulf states to Florida and up the Mississippi valley should be slightly to moderately drier than normal as well as the Great Lakes region. The eastern seaboard should be near normal to slightly drier than normal.