Maybe the dry 2009 monsoon season will be the start of an unusually dry pattern of years.
It probably won't happen, because measurement factors vary greatly from year to year, according to Glen Sampson, chief meteorologist of the U.S. Weather Service's southeastern Arizona office the past 12 years. Average yearly rainfall here is 12-13 inches.
"Each season can be – and most often is – quite different, depending on five key factors," Sampson said. "This year, warmer El Niño sea surface temperatures near the equator and a high pressure area, usually in the Four Corners area, slid further south to give us an unusually dry monsoon thunderstorm season."
The Foothills area "probably got a little more moisture" since it's on the edge of the Santa Catalina Mountains.
Other years, the other three determining factors – land surface conditions, convergence of tropical zones and moisture transportation mechanisms – can affect the June 15-Sept. 30 season.
So how pathetic was this year's season? Only an official 2.86 inches at the Tucson International Airport – less than 50 percent of the historic 6.06-inch average amount. That's the 11th driest monsoon season in Tucson history dating back to 1897.
To explain greater variability of monsoon seasons, Sampson offers two recent extreme examples. "In 2004, we received only 2.42 inches of monsoonal rain (fourth driest on record), but two years later the Santa Cruz River was nearly over its banks in a 10.2-inch year." That remains the sixth-wettest season ever.
The driest and wettest monsoon records in the 112 years of record keeping: 1.59 inches in 1924 with 1.15 coming in July, and 13.84 inches in 1964.
"We were in great shape in July with the high pressure area perfectly positioned where Arizona, New Mexico, Utah and Colorado meet," said Erik Pytlak, science and operations officer, "but then it weakened after moving to southeast New Mexico in August."
This 2009 dry season occurred because "no predominate patterns were present," adds Sampson, who received his bachelor's and master's degrees in meteorology from University of Utah in the early 1980s. "There was no upper-level 'organization' (which can cause large rainfall)."
Climate forecasts, he continues, foresee a moderate El Niño forecast going into the winter months. That means a near-normal prediction, which means slightly above or below the classic 3-4 inches of moisture for December, January and February.
The 23-person staff develops forecasts for virtually every weather situation in Pima, Santa Cruz, Cochise, Graham, Greenlee and part of Pinal counties. Nestled within the University of Arizona campus, the office operates on a 24-hour basis year round.
"Forecasts have only been with us about 30 years," adds Sampson. "Remember, the first satellites went into space in the 1960s and the forecasts weren't fully used until the 1990s."
He reminds the world is one big weather system with nearby influences making big differences. "With us (in southern Arizona), the Pacific Ocean can greatly affect our changing conditions."
Predicting types of future monsoon seasons is very difficult, but Sampson reminds, "El Niño and La Niña are interrelated — and cause everything."
This year's 2.86 inches of monsoonal rain failed to dent the 10 driest years since recordkeeping began in 1897, but it came close. Here are the 10 driest years with rainfall amounts:
1.59 inches – 1924.
2.33 inches – 1973.
2.4 inches – 1989.
2.42 inches – 2004.
2.45 inches – 1900.
2.5 inches – 1902.
2.58 inches – 1994.
2.75 inches – 1913.
2.79 inches – 1918.
2.81 inches – 2001.
The top 10 wettest years in order:
13.84 inches – 1964.
13.08 inches – 1955.
11.04 inches – 1921.
10.5 inches – 1983.
10.21 inches – 1919.
10.2 inches – 2006.
9.94 inches – 1984.
9.85 inches – 1990.
9.43 inches – 1965.
8.54 inches – 1954.
Northwest moisture totals for the 2009 monsoon, June 15 through Oct. 5:
Oracle – 4.96 inches.
Canada del Oro near SaddleBrooke – 4.65
SaddleBrooke -- 5.04
Mount Lemmon – 8.27
CDO at Golder Ranch Road — 3.62
Catalina State Park — 6.34
Big Wash at Rancho Vistoso — 5.35
CDO at Big Wash — 4.17
Moore Road at La Cholla – 3.98
Calle Concordia at Calle El Milagro – 5.47
CDO at Ina Road — 4.69
Santa Cruz River at Ina Road — 4.17
Picture Rocks Community Center — 3.66