Originally Published: October 4, 2010 10:32 p.m.
Prescott faced an unusual monsoon this year, characterized by a late start and dry end.
That led UofA scientist Zack Guido to appropriately characterize it as a "one-month wonder" in the University of Arizona's Climate Assessment for the Southwest monthly climate outlook report. Most of the rain fell in mid-July through mid-August.
"The rapid transition from El Niño to La Niña helped set the stage," Guido wrote in the September outlook report. "Rain had higher variability from place to place than is typical. Humidity was high even in periods without rain. Storms were glued to the mountains. Nighttime temperatures soared.
"And most of the rain came in a four-week period, leaving many forecasters and climatologists calling this season a one-month wonder."
Ironically, Prescott already has experienced several strong storms this month, just a few days after a dry September marked the official end of the monsoon. Heavy rain and wind gusts hit Monday afternoon in Prescott, with the airport reporting gusts as strong as 47 mph and Seligman reporting large hail Sunday and Monday.
By Monday evening the Weather Service was forecasting a 70-percent chance for more heavy rain and wind gusts in Prescott Tuesday.
The months of June through September brought only 4.95 inches of rain to Prescott's Sundog weather station on the northeast side of the city, with zero rain in June, 1.51 inches in July, 3.34 inches in August and 0.10 inches in September.
That's only 60 percent of the 112-year average for June-September rainfall of 8.2 inches.
Since the monsoon is one of the two wet seasons for this region, the dry monsoon leaves a 3.2-inch precipitation deficit in a city that gets only 18.92 inches of precip each year. The average monsoon contributes 43 percent of Prescott's annual precipitation.
The month of September was the biggest culprit, producing only 6 percent of the precipitation average.
September also was hotter than average; the average high was 86 degrees compared to the 112-year average of 81.8 degrees. The average monthly low was 52.3 degrees versus the 112-year average of 48.6 degrees. Prescott set a record high temperature of 92 degrees for Sept. 29, beating the previous record of 91 set in 1933.
Much of southeastern Arizona and southern New Mexico fared far better in their total monsoon precipitation, although they also were warmer than average.
Weather Service forecasters expect the drier-than-average and hotter-than-average trends to continue into this winter for Prescott as well as the rest of Arizona and New Mexico.
That's because a strong La Niña is setting up, perhaps the strongest since the winter of 1988-89, according to Erik Pytlak, science and operations officer for the National Weather Service in Tucson.
Winter forecasters also are recognizing a warming trend has been occurring here in recent decades because of climate change.
"The chances of a wet winter now are very low," Pytlak said in a written monsoon review.
La Niña, a cooling trend in the eastern equatorial Pacific Ocean, has far-reaching impacts across the entire country. It tends to bring drier winters to the southern one-third of the country and wetter winters to the northern one-third.