April 2018 SW Climate Outlook - La Niña Tracker

From the April edition of the CLIMAS Southwest Climate Outlook

Oceanic and atmospheric conditions over the last month remained generally consistent with a La Niña event (Figs. 1-2), but given the rapid decline of these conditions and the imminent seasonal transition, it is only a matter of time before ENSO-neutral conditions return. The current ENSO forecasts reflect this steady weakening, with most indicating a likely transition to ENSO-neutral conditions over the spring, and others having already declared an end to this La Niña event. On April 10, the Japanese Meteorological Agency (JMA) identified ongoing La Niña conditions but called for a 90-percent chance that this event will end in spring. On April 10, the Australian Bureau of Meteorology maintained their ENSO Outlook at “inactive,” stating “there is little sign of El Niño or La Niña developing in the coming months.” On April 12, the NOAA Climate Prediction Center (CPC) continued its La Niña advisory but expected a transition to ENSO-neutral conditions by May and forecast a greater-than-50-percent chance of ENSO-neutral lasting through summer. On April 19, the International Research Institute (IRI) issued its ENSO Quick Look, which still identified weak La Niña conditions present but called for a rapid transition to ENSO-neutral conditions over spring (Fig. 3). The North American Multi-Model Ensemble (NMME) is consistently indicative of a return to ENSO-neutral conditions, but with greater uncertainty over what the latter half of 2018 might hold (Fig. 4).

Summary: In the Southwest, the effects of La Niña events can be difficult to distinguish from normal winter conditions. La Niñas are associated with warmer- and drier-than-average winters, but as Southwest winters are relatively dry to begin with, La Niña does not always reflect a radical departure from normal. That said, the above-average temperatures and below-average precipitation of this winter and spring have been characteristic of a La Niña year. The resulting reduction in snowpack and snow water equivalent across the western United States raises concerns about water resource management since the region relies on snowmelt and streamflow to provide a steady supply of water for the region. Thus the impacts of a seasonal event like La Niña, especially in a drought-sensitive area such as the Southwest, can extend far beyond the duration of that event to affect long-term strategies to manage demand, use, and supply in a resource-constrained environment. Water systems in the Southwest are designed with arid conditions in mind, but accumulated precipitation deficits associated with long-term drought, seasonal climate phenomena like La Niña, and the variability of weather all threaten the stability of these systems and the communities and sectors that depend on these water resources.