Originally Published: December 2, 2009 6:02 a.m.
Meteorologists are in general agreement that surface water temperatures along the equatorial Pacific Ocean heavily influence winter weather patterns in Arizona.
When those waters are 1-3 degrees Celsius cooler than normal, the odds favor below average precipitation. When their temperatures are 1-3 degrees above average, the chances increase for a wetter than average winter.
Known as El Niño/La Niña effect, it is a phenomenon based on statistical probabilities and prior observations.
But weather prognostication is far more complex than simply taking the temperature of the Pacific Ocean.
"El Niño effect is just one piece of a complex global weather pattern. We can't just say that because of an El Niño, or a lack of an El Niño, this is where our weather is going to be," said Nick Petro, science operations officer for the National Weather Service in Flagstaff.
The truth behind Petro's statement has been borne out in some recent El Niños and El Niñas to emerge from the ocean depths.
In the winter 2007-08, Arizona experienced the wettest La Niña winter in its history. In spite of below average surface water temperatures in the equatorial Pacific, the Verde River watershed received 10 percent above average precipitation. The Salt River was 20 percent above normal.
Conversely, the last two El Niño winters, 2002-03 and 2006-07, produced snowfalls at the Flagstaff Airport that were significantly below average.
Peruvian and Chilean fisherman bestowed the name El Niño, which means "the little boy" and specifically refers to the Christ child, on the warmer surface water phenomenon. They observed that when, and if, the waters warmed, it was always around Christmas time.
This year, the South American fishermen and the National Weather Service, have noted there is a moderate El Niño already shaping up in the Pacific.
But, according to Petro, a moderate El Niño is no guarantee of a wet winter.
But a strong El Niño is another thing.
"The only time you can see a strong correlation between El Niño and above-average precipitation, particularly along the Mogollon Rim, is when there is a strong El Niño," Petro says.
Strong El Niños have occurred four times in the last 50 years. In 1972-73 Flagstaff received 201 percent of normal snowfall. In 1982-83, it was 134 percent. In 1991-92, 150 percent. And in 1997-98, 128 percent.
The weather service defines a strong El Niño as a 2 to 3.5 degree above normal rise in surface water temperatures, on a line along the equator from the International Dateline to the west coast of South America.
Most computer models show the water temperatures will not reach that level this winter. But a couple of models do. The water is already 1 to 2 degrees above normal, qualifying this year as moderate, and is predicted to continue rising into the start of 2010.
But, even if it doesn't reach the 2 degree Celsius above normal threshold, there is hope.
The weather service classified the winter of 1992-93, when the Verde Valley was nearly washed away, as a moderate El Niño.