The University of Arizona

La Niña Throws a Curveball in December

January 11, 2011
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December turned out to be surprisingly wet for most of the state after an ominous dry spell set up through much of November. Exceptionally warm and dry conditions, associated with a strong and possibly La Niña-fueled ridge of high pressure, dominated the weather scene across Arizona for the first half of December. This ridge was beaten down by a series of strong and cold low-pressure systems that traveled down from the Bering Sea along the West coast and eventually through the Southwest. Several storms were also able to tap into a deep subtropical plume of moisture originating from the tropical equatorial Pacific Ocean (i.e. 'Pineapple Express'). The combination of strong low pressure and plentiful moisture made several of these storms epic precipitation producers across southern California up through northwestern Arizona. Several day rainfall totals in excess of 10 inches caused localized flooding and damage in portions of Mohave County, Arizona. Overall, storms between December 17 and December 31 brought several inches of rain to low desert areas in central Arizona and several feet of snow to high country areas in northern Arizona, along the Mogollon Rim, and to Sky Island ranges in the southern part of the state. Extreme southeastern Arizona did miss out on much of the action with these storms, though. Less than an inch of precipitation was observed across much of Cochise County where December totals were less than 50% of average.

Tropical Pacific La Niña events and the Jet Stream can both influence winter precipitation across the Southwest. Credit: NASA.


The precipitation in December broke a dry spell that looked like the beginning of a classic dry and warm La Niña winter. But how much relief did the precipitation bring? Do local short or long-term drought conditions still exist in your area? Please visit Arizona DroughtWatch ( and let us know how conditions have changed and what types of drought impacts you are still observing. La Niña is still in full swing and is expected to persist through the remainder of the winter season. This means that warm and dry conditions could quickly return and dominate Arizona's weather into the late spring. This could cause short-term drought conditions to quickly return and gains from recent precipitation could literally evaporate. Your reports on Arizona DroughtWatch are invaluable as the Governor's Drought Task Force and National Drought Monitor work to continually update drought status maps for Arizona. Note that even if you observed no impacts in your location, submitting a blank drought impact survey is helpful to signal that drought conditions have improved in your area.

Please contact Mike Crimmins ( with questions or comments.