And Wikipedia needs an update.
Snow is a very rare occurrence for the city of Phoenix; snowfall was first officially recorded in 1898, and since then, accumulations of 0.1 inches or greater have occurred only eight times.
The heaviest snowstorm on record dates to Jan. 21, 1937 – Jan. 22, 1937, when 1 to 4 inches fell in parts of the city and did not melt entirely for three days. Before that, 1 inch had fallen on Jan. 20, 1933. On Feb. 2, 1939, 0.5 inches fell. Snow also fell on March 12, 1917 and on Nov. 28, 1919.
The most recent snow of significance fell on, Dec. 6, 1998 across the northwest portions of the valley that are below 2,000 feet. During the 1998 event, Sky Harbor reported a dusting of snow. The last measurable snowfall was recorded when 0.1 inches fell in central Phoenix on Dec. 11, 1985.
On Dec. 30, 2010, graupel (soft hail or snow pellets) fell, although it was widely believed to be snow.
And now today, Feb. 20, 2012! Most are saying it's graupel, but I believe there was some snow.