May 2012 La Niña Drought Tracker
May 08, 2012 / Vol. 2 / Issue 6 / Drought Tracker / A Publication by CLIMAS
Source: National Weather Service Advanced Hydrologic Prediction Service
Drought Conditions
Source: U.S. Drought Monitor
La Niña officially dissipated during April but left very dry conditions that began at the end of December. Large swaths of the Southwest have experienced less than 50 percent of average rain and snow since October 1, with the driest regions in eastern New Mexico (top figure). Drier conditions would have been more extreme had it not been for copious precipitation in December—the winter was a one-month wonder (Supplemental Figure 1).
Drought was most expansive and intense across the Southwest in late November, when areas of extreme and exceptional drought covered about 30 and 63 percent of Arizona and New Mexico, respectively. Those values precipitously declined through December and then steadily climbed again as dry conditions settled in after January 1. Between January and March, conditions ranked as the 10th and 13th driest on record for that period in Arizona and New Mexico, respectively. Currently, about 96 percent of Arizona is classified with moderate drought or a more severe drought category, while more than 87 percent of New Mexico is experiencing at least moderate drought (bottom figure). The Southwest is the only region in the West with extreme and exceptional drought. The dry winter, much like last year, has land managers preparing for elevated fire risk (Supplemental Figure 2), which likely will persist until the monsoon begins.
Early monsoon forecasts by the NOAA-Climate Prediction Center (CPC) are a coin-flip—forecasting the monsoon is difficult because many local and global processes, some not well understood, influence the summer rainy season in the Southwest. This year, the evolution of El Niño-Southern Oscillation (ENSO) further complicates monsoon outlooks. The majority of seasonal climate forecast models indicate the persistence of ENSO-neutral conditions through the summer. In past summers when La Niña rapidly transitioned to ENSO-neutral conditions, New Mexico experienced drier-than-average conditions, while parts of southern Arizona saw wetter-than-average weather (Supplemental Figure 3). On the other hand, a transition to El Niño in coming months, which some models are predicting, may cause the opposite pattern to emerge (Supplemental Figure 4). While probabilities for a rapid transition to El Niño are lower than the persistence of ENSO-neutral conditions, six of the 10 back-to-back winter La Niña events that have occurred since 1900 have been followed by an El Niño event. A switch to El Niño in the summer or fall could bring wet conditions this winter, but there is also a possibility that La Niña will reemerge, which happened in four of the 10 back-to-back la Niña winters since 1900.
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 May 3 (above); snowpacks in many basins in both states have completely melted.
- Snowpacks in many monitoring sites in Arizona completely melted about a month earlier than average. In the Upper Colorado River and Rio Grande basins, many snowpacks also completely melted earlier than average (Supplemental Figure 5).
- 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. 4 acre-feet below average (Supplemental Figure 6). There has been no change in this estimate in the last month.
Forecast
Source: NOAA-Climate Prediction Center
- The precipitation outlook for May–July calls for equal chances for above-, below-, or near-average precipitation in all of Arizona and New Mexico (right). This equal chances forecasts reflects the difficulty in projecting early monsoon rainfall.
- Chances for above-average temperatures during the May–July period are greater than 50 percent in most of Arizona and New Mexico (Supplemental Figure 7). Warming trends in recent decades influence this forecast.
- ENSO-neutral conditions are expected to persist through the summer, but some models favor the emergence of an El Niño event during the summer (Supplemental Figure 8).
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The Seasonal Drought Outlook calls for drought to persist or intensify in most of Arizona and New Mexico (Supplemental Figure 9). This forecast is influenced by the historically dry April–June period. In southeastern Arizona and southwestern New Mexico, some drought improvement may occur if the monsoon has a decent start.
Final Word
- La Niña has transitioned to ENSO-neutral conditions, as expected, but it remains in flux. While the majority of models suggest ENSO-neutral conditions will persist through the summer, other models suggest a transition to an El Niño event.
- Expect dry conditions until the monsoon begins in late June or early July. The April–June period historically accounts for less than 12 percent of the annual precipitation in many parts of Arizona and southwestern New Mexico (Supplemental Figure 10).
- Forecasting the monsoon is difficult. This year, it is even more difficult because ENSO conditions may rapidly evolve in the months leading up to the monsoon and influence the timing and strength of summer precipitation. As a result, current monsoon forecasts have high uncertainty.
- Precipitation in the Upper Colorado River Basin since October 1 generally has been below average. Dry conditions have been most severe since January 1 and particularly in the last three months, when rain and snow totals have been less than 50 percent of average in many areas (Supplemental Figure 11).
- Temperatures in the past 30 days have been as much as 10 degrees Fahrenheit above average across parts of the Southwest and Colorado. These conditions and have helped completely melt snowpacks in many locations in the Southwest and the Upper Colorado River Basin (Supplemental Figure 12). Shorter-lived snowpacks help increase risks for wildland fires because the landscape is exposed to a longer dry period before monsoon rains begin.
