February 2012 La Niña Drought Tracker

Drought Conditions

Source: U.S. Drought Monitor

After a wet December, more typical, dry La Niña conditions   returned in January. Across Arizona and New Mexico precipitation generally was less than 50 percent of average, with large swaths of both states experiencing less than 25 percent of average (top figure). Most of the West also experienced scant rain and snow, including the mountains of the Upper Colorado River Basin, where about 70 percent of the water in the Colorado River originates. In many La Niña winters, the impacts of dry conditions are minimized by average or above-average snow in these mountains, which was the case last winter. This year, however, storms have been pushed farther north than typical by a dome of high-pressure off the northwestern coast. The Pacific Northwest, for example, which typically bares the brunt of winter storms during La Niña, was exceptionally dry for most of December and January.

Warm conditions also accompanied January’s scarce precipitation. January temperatures were between 2 and 6 degrees F above average (Supplemental Figure 1), which helped drive a precipitous decline in mountain snow. Most of the country also experienced unseasonably mild temperatures, and many scientists point to the Arctic Oscillation (AO) as part of the cause. The AO describes changes in surface pressure in and around the Arctic (Supplemental Figure 2) that intensify or slacken the winds circulating the polar regions. In the positive phase of the AO, fierce winds prevent the frigid air from flowing south, while the reverse occurs during the negative phase. Up until mid-January this winter, the AO was positive (Supplemental Figure 3). Historically, the confluence of a positive AO and La Niña tends to bring warmer conditions to the Southwest (Supplemental Figure 4), jiving with temperatures in the region in the past month. The AO recently switched to negative and may help bring colder conditions in coming weeks; the AO was negative during February 2011, when several cold snaps froze the region.

Drought conditions are still widespread and extend into Mexico (Supplement Figure 5). The soggy December spurred only minimal drought improvements because wet conditions did not persist. With a recent return of dry weather, moderate drought expanded in Arizona by about 13 percent since January 3, most notably in central Arizona (bottom figure). Abnormally dry conditions or a more severe drought category currently cover more than 92 percent of both Arizona and New Mexico. Forecasts also suggest La Niña will continue through the February–April period (Supplemental Figure 6), likely bringing more dry weather.

Snowpack

Source: National Resources Conservation Service

• The amount of water contained in the snowpack, or snow water equivalent (SWE), was largely below average in Arizona and New Mexico on February 6 (left); SWE in southern mountains dropped by more than 50 percent from one month ago.
• Winter storms were few and far between in the Upper Colorado and Rio Grande basins in January. As of February 8, SWE in these basins were reporting less than 80 percent of average (Supplemental Figure 7).
• Early streamflow forecasts suggest only a 50 percent chance that the April–June flow into Lake Powell will be above 64 percent of average (Supplemental Figure 8); streamflow forecasts progressively become more accurate as the winter advances.

Forecast