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Monthly Archive | CLIMAS

Monthly Archive

Southwest Climate Outlook December 2018 - Climate Summary

Tuesday, December 18, 2018

November Precipitation and Temperature: After a wetter- and cooler than-average October, November was closer to long-term averages. Precipitation ranged from average to below-average in Arizona and New Mexico (Fig. 1a). Temperatures were mostly within the range of average, with some below-average observations in eastern New Mexico and the Four Corners region, and with a region of above-average temperatures on the Arizona/California border (Fig. 1b).

Seasonal & Annual Precipitation and Temperature: Autumn (Sept-Nov) precipitation was average to much-above average, save for a small pocket of below-average rainfall in northwestern New Mexico (Fig. 2a), while temperatures for the same period were average to above average throughout Arizona and New Mexico (Fig. 2b). Year-to-date precipitation ranges from record driest to much-above average, with the marked precipitation deficit continuing in the Four Corners region (Fig. 3a). Year-to-date temperature maps reduce the effect of monthly variability and reveal much-above-average temperatures across most of the Southwest, including some local areas of record-warmest conditions (Fig. 3b).

Drought: The Dec. 11 U.S. Drought Monitor (USDM) highlights the presence of drought across the entire Southwest, with persistent and severe drought conditions in the Four Corners region (Fig. 4). Drought in the Southwest poses a challenge in mapping different timescales and intensities of drought on a weekly basis. In a region already characterized by dry conditions, where accumulated precipitation deficits build over seasons and years, these drought characterizations can struggle to capture all of these inputs. The 36-month standardized precipitation index (SPI) for the Southwest (Fig. 5) highlights differential patterns of drought and precipitation deficit.

Snowpack & Water Supply: Snow water equivalent (SWE) has fluctuated considerably in southern Arizona and New Mexico this fall, with current values generally near or below average as of Dec. 15 (Fig. 6). Reservoir storage remains a persistent concern, as water levels have been impacted by long-term drought and accumulated precipitation deficit. Most of the reservoirs are at or below their long-term averages, and a few of the Rio Grande reservoirs are especially low (see Arizona and New Mexico reservoir storage).

El Niño Tracker: An El Niño event appears imminent, with oceanic indicators now well into El Niño territory while atmospheric indicators continue to lag behind. Most forecasts noted a lack of coupling between ocean and atmosphere but remain confident of an El Niño event during winter 2018-2019, with forecast probabilities hovering around an 80- to 90-percent likelihood (see El Niño Tracker).

Precipitation and Temperature Forecast: The three-month outlook for December through February calls for increased chances of above-normal precipitation in most of Arizona, New Mexico, and northern Mexico (Fig. 7, top), and mostly equal chances of above- or below-average temperatures for the southwestern United States and northern Mexico (Fig. 7, bottom).

Online Resources

  • Figures 1-3 - National Centers for Environmental Information -
  • Figure 4 - U.S. Drought Monitor -
  • Figures 5-6 - Western Regional Climate Center -
  • Figure 7 - International Research Institute for Climate and Society -

Southwest Climate Outlook El Niño Tracker - December 2018

Tuesday, December 18, 2018

Sea-surface temperatures (SSTs) are above-average across the equatorial Pacific (Figs. 1-2). The coupling between oceanic and atmospheric conditions that typically characterizes an El Nino event continues to be lacking, however, as atmospheric conditions lag behind. Forecasts and outlooks remain bullish on the emergence of an El Niño, and project atmospheric conditions to catch up with oceanic conditions. On Dec. 4, the Australian Bureau of Meteorology highlighted persistent above-average oceanic temperatures, but noted “the atmosphere has yet to show a consistent El Niño signal,” with a lack of coupling between oceanic and atmospheric conditions. The agency maintained its ENSO Outlook at an “El Niño Alert,” with a 70-percent chance of its formation in 2018—three times the normal likelihood.  On Dec. 8, the International Research Institute (IRI) issued an ENSO Quick Look that reflected the warmer-than-average oceanic temperatures and lagging atmospheric conditions, but still maintained a greater-than-80-percent chance of an El Niño event by the end of 2018 (Fig. 3). On Dec. 10, the Japanese Meteorological Agency (JMA) maintained its assertion of the presence of El Niño conditions in the equatorial Pacific and called for an 80-percent chance of these conditions lasting through spring 2019. This was despite the general absence of atmospheric conditions consistent with El Niño, but which saw SSTs and sub-surface temperatures above normal across the equatorial Pacific. On Dec. 13, the NOAA Climate Prediction Center (CPC) continued its El Niño watch, stating “despite the above-average ocean temperatures, the overall coupled ocean-atmosphere system remained ENSO-neutral.” CPC’s outlook calls for a 90-percent chance of an El Niño event developing this winter, and a 60-percent chance of it lasting through spring. The North American Multi-Model Ensemble (NMME) continues to point toward a weak-to-moderate El Niño at present, lasting through spring 2018 (Fig. 4).

Summary: Equatorial SSTs (and sub-surface temperatures) are well within the range of an El Niño event, but the ongoing delay in the development of atmospheric conditions is the main factor preventing a more definitive declaration. Based on a limited number of past events from which to compare, this is a late start for an El Niño event. Despite the delay, most forecasts have us on the cusp of an El Niño event that is expected to last through spring 2019. Cool-season precipitation totals (Oct – March) in the Southwest during previous El Niño events reveal considerable variability under weak events, including some drier-than-average seasonal totals. However, under moderate-intensity events, drier-than-average cool seasons have been rare, and it is not difficult to understand why there is eager anticipation for anything that might increase our chances of more winter rain.

Online Resources

  • Figure 1 - Australian Bureau of Meteorology -
  • Figure 2 - NOAA - Climate Prediction Center -
  • Figure 3 - International Research Institute for Climate and Society -
  • Figure 4 - NOAA - Climate Prediction Center -