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SW Climate Outlook - La Niña Tracker - Dec 2017

Friday, December 22, 2017

After a relatively late start, La Niña has ramped up over the past 30 days in terms of observed conditions and projected intensity, with sea-surface temperatures (SSTs) demonstrating a more consistent La Niña pattern (Figs. 1-2). Current forecasts and outlooks suggest a weak-to-moderate La Niña event lasting through the winter. On Dec. 11, the Japanese Meteorological Agency (JMA) identified ongoing La Niña conditions and called for a 60-percent chance of these conditions persisting until spring 2018. On Dec. 14, the NOAA Climate Prediction Center (CPC) extended its La Niña advisory, identifying an 80-percent chance of La Niña conditions lasting through the winter, with a likely transition to ENSO-neutral in the spring. The CPC forecast consensus identified La Niña conditions in the sea-surface and sub-surface temperatures as well as in atmospheric patterns. On Dec. 19, the Australian Bureau of Meteorology updated its ENSO tracker to reflect the emergence of La Niña conditions, but noted that this event was “expected to be short-lived.” On Dec. 19, the International Research Institute (IRI) issued its December ENSO quick look, calling for La Niña to last into the spring (Fig. 3), most likely as a weak event, but with the possibility of increasing to moderate strength. The North American Multi-Model Ensemble (NMME) is consistently indicative of a La Niña event of weak to moderate intensity this winter (Fig. 4). 

Summary: The seasonal outlooks converged on a forecast for La Niña to last through the winter based on consistent La Niña patterns observed in both oceanic and atmospheric indicators. The intensity of the event is still under consideration, with a weak event the most likely scenario, but with increasing possibility of a moderate event mainly due to models and forecasts nudging towards the moderate threshold in the past month. Given the warmer- and drier-than-average winter conditions associated with La Niña in the Southwest, its presence may heighten ongoing concerns regarding winter precipitation and persistent drought. Southwestern winters are already relatively dry, however, so the emergence of a La Niña doesn’t necessarily ensure an exceptionally dry winter, it just takes wetter-than-average winters off the table based on past La Niña events. If the La Niña strengthens to moderate intensity, the likelihood of an even drier Southwest winter increases (see following page for a few examples).

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SW Climate Outlook - La Niña Winter Precip in the SW

Friday, December 22, 2017

Looking more closely at winter Dec-Feb (DJF) precipitation, most weak La Niña events (ENSO Index Value between -0.5 and -1.0) recorded below-average precipitation, although a few years (1968, 1985) are notable outliers (Figs. 5-6). Looking at the monthly breakdown of weak, moderate, and strong La Niña events reveals that while the DJF totals for Tucson, AZ and Las Cruces, NM are mostly below average (Figs. 7-8), there have been some individual months that recorded precipitation above the monthly average (represented by black lines on the plots). The most likely outcome is below-average precipitation totals for the winter season, but the way that these events unfold will have an impact on how residents of the Southwest perceive and experience this La Niña event.

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SW Climate Outlook Dec 2017 - Climate Summary

Thursday, December 21, 2017

Precipitation & Temperature: November precipitation was below average across most of Arizona, with record-dry conditions in the western third of the state (Fig. 1a). In New Mexico, precipitation was average to much-below average, with small pockets of record-dry conditions in the central part of the state (Fig. 1a). November temperatures broke record highs across nearly all of Arizona and New Mexico (Fig. 1b). Thus far, December has continued the trend of above-average to near-record temperatures and very dry conditions (Fig. 2), although at the time of this writing (Dec. 20), a series of storms had brought welcome precipitation to the Southwest (Fig. 3). Year-to-date precipitation ranges widely from much-below average in southeastern Arizona to much-above average in northeastern New Mexico (Fig. 4a). Year-to-date temperatures have been consistently warmer than average, with most of Arizona and New Mexico recording either much-above average or record-warmest conditions (Fig. 4b).

Snowpack & Water Supply: Snowpack and snow water equivalent (SWE) are below average across the Southwest, California, and the Pacific Northwest, with a mix of above- and below-average conditions in the Intermountain West (Fig. 5). Most of the Southwest has experienced above-average temperatures and below-average precipitation for much of the fall season (Sept-Oct-Nov), which is a primary factor in the below-average snowpack and concerns about water supply for 2017-2018. The ongoing La Niña event – and its associated warmer and drier conditions in the Southwest – has potential implications for drought and water resource management over the winter season (see Arizona and New Mexico reservoir volumes).

Drought: Above-average temperatures and below-normal precipitation are reflected in the expanding areas of drought designation, with both Arizona and New Mexico seeing increases in extent and intensity of drought. On the Dec. 19 U.S. Drought Monitor (Fig. 6), nearly all of Arizona is classified as moderate drought (D1), with a pocket of severe drought (D2) on the U.S.-Mexico border. New Mexico has likewise seen a widespread expansion of drought conditions, with most of the state now classified as abnormally dry (D0) and intensifying to moderate drought (D1) along the western edge. These classifications are primarily the result of below-average precipitation across much of the region over the last few months, but also include the effect of long-term persistent drought.

ENSO & La Niña: After a relatively late start, La Niña has ramped up in terms of observed conditions and projected intensity, and current forecasts suggest a weak-to-moderate La Niña event lasting through winter 2018. Weak La Niña events tend to produce drier-than-average winters, but moderate events have resulted in more consistently dry conditions over the winter season (see La Niña Tracker and DJF La Niña Precip in the SW for details), Thus, this projected increase in strength is worth watching over the next few months to see how the Southwest fares in terms of winter precipitation, drought, and water resource management.

Precipitation & Temperature Forecast: The three-month outlook for January through March calls for increased chances of below-average precipitation for all of Arizona and New Mexico (Fig. 7, top), and increased chances of above-average temperatures for the entire southwestern United States (Fig. 7, bottom).

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