Southwest Climate Outlook December 2008

December Climate Summary
Drought – Short-term drought conditions reported for October in Arizona worsened
in many regions, most notably in the Little Colorado River watershed where
abnormally dry conditions in September were downgraded to a moderate drought
classification; long-term drought status remains the same. In New Mexico, November
drought conditions did not change from last month.
Temperature – During the past 30 days, temperatures in Arizona and western
New Mexico have been 2 to 6 degrees F warmer than average. The recent storm
system that passed through the Southwest on December 16–18 brought colderthan-
average temperatures to most of Arizona and northern New Mexico.
Precipitation – Until recently, most of the Southwest had received little precipitation
since the beginning of the water year on October 1. However, storms on
Thanksgiving and between December 16 and 18 finally brought wetter conditions
and delivered record snowfalls in some areas. In the past 30 days, most of Arizona
and northern New Mexico have had 100 to 1,200 percent of average precipitation,
due entirely to those two storms.
ENSO – The International Research Institute for Climate and Society (IRI) reports
that the equatorial Pacific Ocean is presently on the borderline of ENSO-neutral
and weak La Niña conditions. The IRI also states there is a 50–55 percent probability
of ENSO-neutral conditions persisting over the coming season, and a 45–50
percent chance for weak La Niña conditions.
Snow – The water contained in the snow (snow water equivalent, or SWE) in most
watersheds in the Upper Colorado River Basin on December 18 is generally between
70 and 90 percent of the 1971–2000 average. In the headwaters of the Rio
Grande, SWE is about 70 percent of the average. Recent storms have elevated SWE
in Arizona, with values ranging between 150 and 225 percent near Flagstaff and the
central Mogollon Rim.
The Bottom Line – The first winter storms of the 2009 water year finally arrived,
providing much needed precipitation that may help alleviate short-term drought
conditions in some regions. These storms also brought cooler temperatures and
record snowfalls. As winter continues, attention will be on the snowpack in the Upper
Colorado River Basin (UCRB) which provides about 70 percent of the river’s
yearly flow; presently, the snow water equivalent in the UCRB is slightly less than
the historical average.