climate | weather conditions prevailing in an area over a long period: our agricultural development is constrained by climate.
growing season | number of days from the last recorded 32 degrees in spring to the first recorded 32 degrees in autumn.
frost | 32°F or lower freeze | 28°F or lower.
Gardeners face several challenges in the Flagstaff area: low average precipitation, long periods of drought, high winds, and widely fluctuating temperatures between day and night.
The Flagstaff area’s dry continental air and high elevation above sea level produce a desert mountain climate. Here, the day’s warmth radiates out into the atmosphere quickly after sunset. Any night of the year, the temperature can drop more than 40° degrees Fahrenheit even when summer temperatures reach the 90s during the day. That’s why our growing season averages a brief 103 days.
From November through March, broad frontal storms from the Pacific coast pass over northern Arizona. These storms bring an annual average of 100 inches of snow, but total snowfall varies greatly from year to year. Between storms clear blue skies prevail, sunny areas turning to bare ground as the snow sublimates, its crystals turning directly to water vapor.
Winter is often interrupted with a thaw of several gentle, pleasant days followed by very cold nights. Snowstorms resume through March, which on average is the snowiest month of the year although in some years, very little snow arrives during this month.
Precipitation falls off sharply in April. Bright, sunny days and persistent strong winds begin to dry out the landscape. May and June mean fire season in Flagstaff, with increasingly warm and dry winds and scant precipitation (click to learn about Firewise Landscaping). Nights are still cold; the average last overnight freeze (28 degrees or lower) is May 28; the average last frost (32 degrees or lower) is June 9. This is the most challenging time of the year for plants yet some do thrive, especially if protected from wind.
July brings great relief to landscape and garden as the flow of wind shifts to carry monsoon thunderstorms from the south and southeast. Mornings are usually still clear and bright, but by mid-afternoon, puffy cumulus clouds develop into spectacular storm cells producing thunder, lightning, downbursts, heavy rain, and sometimes hail. These thunderstorms travel an apparently random path, soaking some neighborhoods one day and others the next. Humidity increases, keeping overnight temperatures mostly above 50° and daytime temperatures more consistent across the Flagstaff area. Gardens flourish.
Monsoon thunderstorms usually end in early September. The following days are clear, calm, and bright. Although the average first frost is September 24, protected gardens continue to do well.
Fall is brief in northern Arizona. Like spring, it progresses in fits and starts with alternating periods of fine weather and cold, windy days. The average first freeze is October 5. Winter snowstorms usually begin some time in November.
Click to read the pdf of Richard Hereford’s Climate Variations at Flagstaff, compiled from National Weather Service Data.