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December 2011 La Niña Drought Tracker
December 01, 2011 / Vol. 2 / Issue 1 / Drought Tracker / A Publication by CLIMAS
Arizona and New Mexico remain drought-plagued after a very dry 2010–2011 winter that was influenced heavily by a moderate to strong La Niña event. The monsoon that followed was spottier than normal, alleviating drought in only a few regions, and widespread precipitation deficits remain entrenched. Since October 1, rain and snow generally have been below average (Figure 1). According to the most recent U.S. Drought Monitor, published on November 29, 63 and 31 percent of New Mexico and Arizona, respectively, were classified with extreme or exceptional drought (Figure 2).
Conditions over the next several months will determine whether drought will expand and intensify or improve, particularly because sustained dry conditions make it progressively harder to reduce drought. Forecasts, however, paint a grim picture, as a weak La Niña has returned. La Niña conditions often steer storm tracks north of the Southwest, leaving the region dry. Although the La Niña threatens to drive precipitation deficits farther into the red, there is a silver lining. Winter precipitation likely will not be as scant as last winter if historical statistics bear out and La Niña remains weak. Precipitation deficits often match the strength of the event, so a weak La Niña would not be as dry as a moderate one, and a moderate event would be less dry than a strong event (Supplemental Figures 1–3). Current La Niña forecasts call for more than a 60 percent chance that La Niña will continue through the February–March period, with the expectation that the event will remain either weak or moderate (Supplemental Figure 4).
Within the past week, the jet stream dived from the north, bringing widespread rain and snow (not reflected in the figures above) and frigid temperatures to the region. Many areas, predominantly in central Arizona, received more than a half-inch of rain, and this will likely help improve drought in parts. A relatively wet December is not uncommon during a La Niña. In the 20 winters since 1950 in which La Niña was present, the percent of average precipitation in many areas in Arizona became progressively lower between December and March (Supplemental Figures 5–8). In New Mexico, December and March often have less precipitation deficits than January and February.
Source: National Resources Conservation Service
- Snow water equivalent (SWE) in Arizona and New Mexico snowpacks was largely below average on December 2 (left). However, a winter storm brought snow to the region during December 1–4, boosting snowpacks in most of the region. (Supplemental Figure 9).
- La Niña events often bring dry conditions to Arizona and New Mexico but do not exert such a strong influence in the Upper Colorado River Basin (UCRB).
La Niña events almost always cause spring streamflows in the Verde and Salt rivers to be below average (Supplemental Figures 10–11); the Colorado River, however, often experiences both above- and below-average streamflows (Supplemental Figure 12).
- The precipitation outlook for December–February suggests increased chances for below-average precipitation in nearly all of Arizona and New Mexico (right).
- The December–February temperature outlook calls for equal changes of above-, below-, and near-average conditions in almost all of Arizona and slightly increased odds for above-average conditions in southeastern New Mexico.
- The December precipitation outlook indicates an equal chance of above-, below-, and near-average conditions for most of Arizona, and a 33 percent chance for below-average precipitation in most of New Mexico and southeast Arizona (Supplemental Figure 13).
During the 20 winters in which La Niña conditions were present since 1950, precipitation deficits between 0.4 and 1.2 inches below the December–February average occurred 60–80 percent of the time (Supplemental Figure 14).
- La Niña event will affect the Southwest for the second consecutive winter. La Niñas often push storms further north. They also cause the jet stream to meander more than typical, at times causing the swift air current to loop from the north and deliver frigid temperatures to the region.
- Based largely on the presence of La Niña, the drought forecast calls for drought to persist or intensify in most of Arizona and New Mexico (Supplemental Figure 15).
- Winter storms will occur despite the La Niña. The critical question is how often will conditions favor widespread rain and snow. Historically, weak La Niña events deliver less than 75 percent of the December–March average for most of Arizona and New Mexico (Supplemental Figure 1). Weak events are also not as dry as moderate or strong events (Supplemental Figures 2–3).
- Currently, most of Arizona and New Mexico is classified with at least moderate drought.
It is unlikely that severe drought can be erased in one season, as it often takes longer to climb out of a drought than to develop one. Analyses from previous seasons suggest less than a 5 percent chance that precipitation during November–March will end drought in most of New Mexico, with less than a 10 percent chance for southeast Arizona (Supplemental Figure 16). To end drought in these regions, more than 150 percent of precipitation in this period is needed (Supplemental figure 17)
Precipitation Analysis (National Weather Service Advanced Hydrologic Prediction Service):
Drought Status (National Integrated Drought Information System):
Snowpack Conditions (National Resources Conservation Service):
Precipitation Outlooks (NOAA–Climate Predictions Service):