Monday, October 24, 2011

Why is Flagstaff so snowy?

It's October and my friends on the Front Range are already getting their first big snow storm, which makes me especially antzy for the winter season to arrive here in Flagstaff.   People are often very surprised to hear how diverse the climate of Arizona can be, so I thought I'd write a blog post about why Flagstaff is so snowy. 

Flagstaff, situated an elevation of 7,000 one of the snowiest cities in the United States, averaging over 100 inches of snow annually.   For a comparison, Denver and Salt Lake City average around 60 inches, Minneapolis averages near 54 inches, New York City averages near 23 inches, and my hometown in Medford, Oregon averages near 5 inches each winter.

But being at such a low latitude, it is surprising that Flagstaff can compete with some of our snowy northern neighbors.  However, Flagstaff has two great topographical features working in its favor to produce the significant snows, not to mention the already very high altitude.   The first, and primary feature is called the Mogollon Rim.  This rim is the demarcation of the edge of the Colorado plateau, essentially where the high altitude that characterizes much of Utah and Colorado meets the lower altitudes of the Sonoran desert.   This zone is characterized by a sharp increase in altitude and change in vegetation from what is seen in the lower deserts. 
Incoming air moving eastward with the prevailing wind, is forced to rise over this topographic barrier.  As air rises - it cools and condenses.  This is why the higher terrain is so much wetter than rest of the state, as shown in the graphic below.
I also put together a graphic depicting what happens as air rises over the topography across northern Arizona.  As you can see, air coming in from the warm and drier lower elevations meets the edge of the Mogollon rim and is forced to rise...cooling and moistening as it does so.

The second feature that amplifies snowfall for Flagstaff is the San Fransisco Peaks.  These extinct volcanic peaks, exceed 12,000 feet in elevation.  These peaks act as a secondary forcing mechanism forcing the nearby air up from the already high 7,000 foot elevation to even higher as air is lifted over the peaks.  So Flagstaff benefits from the upward lift of air rising over the Mogollon rim ,and as it rises over the nearby peaks.

The result of these two topographic features working together is the high frequency of heavy snowfalls whenever a storm system progresses through the southwest.  While only 150 miles away is the dry and warm Phoenix Metro, a storm passing through may result in mere sprinkles over Phoenix while Flagstaff may be seeing several inches of accumulating snow.  It's a pretty fascinating climate.  I attached a few photographs of the past winter season, and some highlights from years past when winter really unloaded on the city of Flagstaff.

 February 2010, measuring snow outside my office
 San Fransisco Peaks, February 2011
 Heavy Snow, December 2009 (Not my picture)
 Snow drifts blowing into the office breezeway (Dec 2010)
Bellemont Neighborhoods, December 2009

Being as far south as Flagstaff is, we typically do have to wait until late November or December before winter really sets in and the jet stream digs this far south.   Time will only tell what this winter will bring, but I'm hoping for lots of snow despite the lingering La Nina which tends to dry us out.  We will see!

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