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Extreme flooding in Arizona during the winter of 1993 resulted from a nearly optimal combination of flood-enhancing factors involving hydroclimatology, hydro-meteorology, and physiography. The floods of January and February 1993 were the result of record precipitation from the passage of an unusually high number of winter storm fronts. These fronts moved across Arizona as part of an exceptionally active storm track that was located unusually far south. The number of individual storms that entered the region and the relative position of each storm track in relation to previous storms was reflected in a complex spatial and temporal distribution of flood peaks. An analysis of the hydroclimatic context of these floods supports a general conclusion that in Arizona, front-generated winter precipitation is most often the cause of extreme floods in large watersheds, even in basins that tend to experience their greatest frequency of flooding from other types of storms. A comparison of the 1993 floods with gauged, historical, and paleoflood data from Arizona indicates that, although many individual flood peaks were quite large, they were within the range of documented extreme flooding over the past 1,000+ yr. The 1993 flood scenario provides a convincing analogue for the climatic and hydrologic processes that must have operated to generate comparably large paleofloods, that is, abnormally high rainfall totals, repeated accumulation and melting of snow, and rain on snow. Such conditions are initiated and perpetuated by a persistent winter circulation anomaly in the North Pacific Ocean that repeatedly steers alternately warm and cold storms into the region along a southerly displaced storm track. This scenario is enhanced by an active subtropical jet stream, common during El Nino-Southern Oscillation periods.

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