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Western Plateau Planning Area Climate

The average annual temperature of the Western Plateau Planning Area (57.9°F) is somewhat cooler than the statewide average (59.5°F).  Average annual precipitation in the planning area is 12.1 inches, the same as the statewide average.  Annual totals vary widely across the area, from 6-9 inches at low elevations (less than 5,000 ft.) and rain shadow stations such as Wahweap, Fredonia, and Beaver Dam, to greater than 20 inches at Williams and Bright Angel Ranger Station in Grand Canyon National Park. On average, the Western Plateau Planning Area exhibits the bi-modal precipitation pattern characteristic of Arizona (see Figure 6.0-8); however, the northwestern part of the planning area, near the borders of Nevada and Utah, exhibits a stronger late winter peak, whereas the eastern and southern part of the area shows a stronger summer peak.

Figure 6.0-8 Average monthly precipitation and temperature from 1930-2002

Figure 6.0-8

Data are from the Western Regional Climate Center. Figure author: CLIMAS

Frontal storm systems moving west-to-east, guided by the jet stream, deliver the area’s winter and spring precipitation.  Summer monsoon thunderstorms arrive later in this part of the state than elsewhere, and August is clearly the peak month, on average, for summer precipitation.  However, year-to-year summer precipitation variability is pronounced, with some years showing July peaks.  The area shows a strong response to the El Niño-Southern Oscillation, with El Niño winters registering wet conditions 52% of the time and dry conditions less than 30% of the time; La Niña winters are dry 54% of the time and wet only 21% of the time.

Average annual temperatures in the Western Plateau Planning Area have been increasing since the 1930s, and especially rapidly since the mid-1970s (see Figure 6.0-9).  The long-term trend is superimposed on decadal variability generated primarily by Pacific Ocean and atmosphere variations. Decadal variations are particularly obvious in the instrumental record of precipitation. Drought conditions are apparent for the decades of the 1940s-early 1970s and since the mid-1990s, whereas the 1930s and mid-1970s through the mid-1990s were relatively wet.

Figure 6.0-9 Average annual temperature and total annual precipitation for the Western Plateau Planning Area from 1930-2002

Figure 6.0-9

Horizontal lines are average temperature (57.9 °F) and precipitation (12.1 inches), respectively. Light lines are yearly values and highlighted lines are 5-year moving average values. Data are from the Western Regional Climate Center. Figure author: CLIMAS.

Winter precipitation records dating to 1000 A.D., estimated from tree-ring reconstructions, show extended periods of above and below average precipitation in every century (Figure 6.0-10).  Notably dry periods include the late 1500s, which feature the driest decade in this part of the state, and the late 1200s. The Western Plateau Planning Area was relatively wet during the late 1400s, early 1600s, and early 1900s.

Figure 6.0-10 Winter (November-April) precipitation departures from average, 1000-1988

Figure 6.0-10

Data are presented as a 20-year moving average to show variability on decadal time scales. Data: Fenbiao Ni, University of Arizona Laboratory of Tree-Ring Research and CLIMAS. Figure author: CLIMAS.


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