April 2012 La Niña Drought Tracker

Drought Conditions

Source: U.S. Drought Monitor

La Niña delivered a second consecutive dry winter, with precipitation totaling less than 75 percent of average across most of the Southwest between October 1 and April 9 (top figure). In soggy winters, the westerly winds loop down from the north and combine with damp air wafting from the tropics. During this winter, however, the tropical tap was largely shut off and the westerly winds were pinned to the north, largely the result of the La Niña event. Had it not been for numerous storms in December that delivered more than 200 percent of average rain and snow (Supplemental Figure 1), precipitation deficits would be even more dramatic than they are.

Scant rain and snow in March continued a dry stretch that began in late December, with precipitation amounting to less than 50 percent of average in most of the Southwest (Supplemental Figure 2). Although several storms ferried strong winds to the region in the last month, only one storm delivered precipitation. This has been common this winter, particularly in central Arizona, where rain has fallen only once in the last 90 days (Supplemental Figure 3).

The dry weather in March is intensifying and expanding drought. Moderate or a more severe drought category currently occupies about 94 and 82 percent of Arizona and New Mexico, respectively (bottom figure). Extreme drought developed in central Arizona in the last month and impacts are beginning to emerge around the region. Crop and cattle lands are parched (Supplemental Figure 4). The potential for large fires is above normal for most of Arizona and western New Mexico (Supplemental Figure 5). Water resources are also being adversely affected; irrigation districts in El Paso County in Texas, southern New Mexico, and northern Mexico are receiving only 20 percent of a full water allocation as a result of low flows in the Rio Grande, according to the U.S. Bureau of Reclamation.

The news is not all grim. In early December, extreme or exceptional drought covered about 30 percent of Arizona and 59 percent of New Mexico. Currently, only about 13 and 25 percent, respectively, are classified with these extreme conditions. La Niña is also expected to dissipate into neutral conditions by the end of April, and historical statistics slightly favor the development of El Niño later in the summer. Relief to the dry conditions, however, will likely not arrive until the monsoon begins because April through June is historically the driest period of the year for Arizona and western New Mexico.

Snowpack

Source: National Resources Conservation Service

• The amount of water contained in the snowpack, or snow water equivalent (SWE), was well below average in Arizona and New Mexico on April 5 (above); snowpacks in many basins in both states were less than 25 percent of average.
• Snowpacks at many individual monitoring sites in Arizona completely melted about a month earlier than average. In the Upper Colorado River and Rio Grande basins, snowpacks at many monitoring sites are less than the lowest 5 percent of the historical record (Supplemental Figure 6).
• Streamflow forecasts suggest a 50 percent chance that the April–June flow into Lake Powell will be less than 44 percent of average, which is about 4.5 million acre-feet below average (Supplemental Figure 7).
   

Forecast