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I I I I I I I I I I I I I I I I I I I was instrumental in identifying the weather patterns which most favored the occurrence of wintertime precipitation over the Arizona mountains. Medina found that there were four prominent weather patterns which occur most frequently and account for a substantial amount of the wintertime Arizona mountain precipitation. These are described in detail in the Bureau's report and are briefly summarized herein. The first type, indicated as A-1, is characterized by the initial presence of a cold air mass over the British Columbia region of Canada which is advected southward by a strong strength- ening low-pressure wave. As the air mass plunges southward a closed circulation develops aloft which frequently slows its progressive movement. Usually a cold front strengthens over the Nevada-Utah area, which ultimately moves across Arizona. Precipitation, which can be moderate to heavy, commences and continues intermittently until after the front passes and drier air advects into the region. The key features of this weather type are the plunging cold air mass, the slow-moving circulation and the availability of ample Pacific Ocean moisture. A similar type, indicated as A-2, is where the southward plunge of the cold air mass occurs farther east and the circulation does not close in time to substantially impact Arizona. Sometimes the upper circulation does not close at all or closes well off the coast and then opens to a trough as the system moves eastward over land. Key differences of this type from A-1 are that it does not advect moisture as well and also moves faster. The consequence of this is that it generally produces less precipitation over Arizona. 3-4