Arizona receives the majority of annual precipitation during two seasons: summer and winter

Monsoon thunderstorm activity accounts for roughly half of the annual precipitation in central and northern Arizona, and two-thirds to three-fourths of the annual precipitation in southern Arizona. The short-lived, intense monsoon thunderstorms can often cause flash flooding in areas with steep terrain, low-lying roads, or normally dry washes. Lightning, hail, dust storms and strong winds are common during the monsoon season. 

Winter precipitation comes from cold fronts and low pressure systems. These systems move south from the Pacific Northwest, often sweeping across Nevada or southern California before bringing cold air and rain or snow into Arizona. The northern half of the state as well as higher elevations across the state typically receive snowfall in the winter. Northern Arizona receives just over half their annual precipitation in the winter.