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Climate of the Eastern Plateau Planning Area

The Eastern Plateau Planning Area is a semi-arid, relatively high elevation region with cooler average temperatures than in other parts of Arizona.  Average annual maximum temperatures in the planning area range from 61˚ F at Greer to 82˚F at Cameron.  Annual average temperature is 50.8°F, compared to the state-wide average of 59.9°F.  Eastern Plateau temperatures display a long-term warming trend (Figure 2.0-7), as in other parts of Arizona.

Figure 2.0-7 Average Monthly Precipitation and Temperature from 1930-2002

Figure 2.0-7

Data are from selected Western Regional Climate Center cooperative

weather observation stations located south of the Little Colorado River.

(http://www.wrcc.dri.edu/summary/climsmaz.html).

Figure author: CLIMAS.

Parts of the Eastern Plateau downwind of the Central Highlands Planning Area receive diminished precipitation due to the “rain shadow effect.”  As moisture-laden air flows over topographic features such as mountain ranges, the air is lifted and cooled, resulting in greater precipitation on the windward side of the mountain.  In contrast, the leeward side of mountain ranges receives much less precipitation as the air sinks, warms, and dries, creating a “rain shadow.”

Precipitation in the Eastern Plateau is characterized by a multi-peaked distribution similar to much of Arizona (Figure 2.0-8).  Precipitation is highest during July and August when the area receives over 43% of yearly precipitation, while the driest months on average are April, May, and June.  Average annual precipitation ranges from about 4 inches at Monument Valley in the far northeastern part of the planning area to 36 inches in the White Mountains, Mogollon Rim and San Francisco Peak areas.  Most of the Navajo and Hopi Reservation lands receive less than 10 inches of rainfall a year.  The highest precipitation on the Navajo Reservation is in the Chuska Mountains with an average annual precipitation of 25 inches (Navajo Nation, 2001).

Figure 2.0-8 Average Temperature and Total Precipitation in the Eastern

Plateau Planning Area from 1930-2002

Figure 2.0-8

Horizontal lines are average temperature (50.8 °F) and precipitation (13.0 inches), respectively.

Light lines are yearly values and highlighted lines are 5-year moving average values. 

Data are from selected Western Regional Climate Center cooperative weather observation stations

located south of the Little Colorado River. (http://www.wrcc.dri.edu/summary/climsmaz.html).

Figure author: CLIMAS

Much of the state’s snowfall occurs along the Mogollon Rim and White Mountains in the Eastern Plateau and Central Highlands Planning Areas. Snowfall is an important water source and is often defined in terms of snow-water equivalent (SWE).  SWE is dependent on snow density and describes the amount of liquid water present in a melted sample of snow; light, powdery snow yields less water than dense wet snow.  Observations recorded March 1st from 1983 to 2006 at Mt. Baldy in the southeastern portion of the region show SWE variations from 1983 to the present (Figure 2.0-9).  The Mt. Baldy record shows relatively high snow pack during the 1980s and early-to-mid 1990s, followed by substantially lower snow pack since 1999.

Figure 2.0-9 Mt. Baldy Snow-Water Equivalent (SWE) for 1983-2006

Figure 2.0-9

Observations were recorded March 1st for each year except 2006,where February 15 was used.

The horizontal, bold line is average SWE from 1983-2006 and highest SWE Observations were recorded

March 1st for each year except 2006, where February 15 was used. The horizontal, bold line is

average SWE from 1983-2006 and highest SWE years (1993) and lowest SWE years (1999 and 2006) are highlighted.

Figure author: CLIMAS

Two important features of precipitation in this region are variability between individual years, and shifts between wetter and drier than average periods on longer, 10-20 year (decadal) time scales (Figure 2.0-7 and Figure 2.0-10).  Winter precipitation records dating from 1000 A.D., estimated from tree ring reconstructions for Arizona climate divisions, show extended periods of above and below average precipitation in every century. A climate division is a region within a state that is generally climatically homogeneous. Arizona has been divided into seven climate divisions. Climate Division 2 includes the entire Eastern Plateau Planning Area and extends west and south.

There have been multiple extended periods of above and below-average winter precipitation in the planning area during every century since 1000 A.D. (Figure 2.0-10).  The 1200s, 1500s, and 1700s were notably dry; in contrast, the mid-1000s, early 1300s, and early 1900s were notably wet.  More recently, the 1950s were relatively dry, whereas the 1980s received above-average precipitation (Figure 2.0-7).  These decadal shifts are related to circulation changes in the Pacific Ocean.  On time scales of 2-7 years, the well-known El Niño-Southern Oscillation (ENSO) in the Pacific Ocean, with its phases of El Niño and La Niña, is associated with precipitation variations in the region, most notably during winter months (November-April).  During El Niño episodes, there is a greater likelihood of increased precipitation; nevertheless El Niño winters can produce below-average precipitation.  Generally, La Niña conditions are associated with drought in the region.

Figure 2.0-10  Arizona NOAA Climate Division 2 (Northeastern Arizona;

Coconino, Navajo, and Apache Counties) winter (November-April) precipitation

departures from average, 1000-1988, reconstructed from tree rings.

Figure 2.0-10

Data are presented as a 20-year moving average to show variability on decadal time scales.

The average winter precipitation for 1000-1988 is 6.1 inches.

Data: Fenbiao Ni, University of Arizona Laboratory of Tree-Ring Research and CLIMAS.

Figure author: CLIMAS.

 

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Download pdf of entire Eastern Plateau Planning Area Download pdf of the Eastern Plateau Planning Area Overview References and Supplemental Reading for the Eastern Plateau Planning Area Overview
Colorado River Eastern Plateau Planning Area Arizona Water Atlas Volume 2 Download Little Colorado River Plateau Basin pdf References and Suggested Reading