La Niña Tracker - Mar 2018


La Niña Tracker

La Niña conditions continued for another month as indicated by atmospheric and oceanic conditions (Figs. 1-2). Most forecasts saw some signs of weakening and identified a likely transition to ENSO-neutral conditions over the spring, although the exact timing varies. On March 8, the NOAA Climate Prediction Center (CPC) continued its La Niña advisory but predicted a 55-percent chance of the weakening La Niña event to transition to ENSO-neutral in the spring. On March 8, the International Research Institute’s (IRI) ENSO Quick Look called for La Niña to last into early spring (Fig. 3) and a return to neutral conditions by mid-spring. On March 9, the Japanese Meteorological Agency (JMA) identified ongoing La Niña conditions and called for a 70-percent chance that this event will end this summer, with ENSO-neutral conditions likely over the summer. On March 13, the Australian Bureau of Meteorology declared the La Niña event had ended, identifying that ENSO indicators had “eased back to neutral over the past several weeks.” The North American Multi-Model Ensemble (NMME) is consistently indicative of a weak La Niña event returning to neutral conditions over spring (Fig. 4).

Summary: Warmer- and drier-than-average winter conditions are associated with La Niña in the Southwest. However, Southwest winters are dry to begin with, so La Niña does not necessarily reflect a radical departure from normal. What is clear from looking at past events is that La Niña winters are rarely wetter than average at a seasonal timescale. But wetter-than-average months have occurred during past La Niña events, and February 2018 demonstrated what that looks like, at least in southern Arizona and southwestern New Mexico. Those regions recorded above-average precipitation in February, albeit with relatively few days of rain, while most of the rest of the region was stuck with warm and dry conditions. These storms were a welcome change of pace but did little to alter long-term drought or reverse the accumulated precipitation deficits. Warm temperatures also meant some precipitation fell as rain instead of snow at much higher elevations than might typically be expected in February. Persistent warm temperatures and below-average winter precipitation also reduced snowpack and snow water equivalent across the western United States, raising concerns about water resource management going into spring and summer, when water banked as snow provides a steady supply of water for the region in the form of snowmelt and streamflow. February 2018 may have been an outlier compared to a normal La Niña February, but the Dec-Feb precipitation totals and average temperatures were generally in line with typical La Niña events in the Southwest.