| Summary: | This is an attempt to measure the frequency of summer Dry Spells over the Canadian Prairies and to discover their possible causes. Statistical and synoptic climatological techniques are used as tools in the analysis and discussion of the occurrence and causes of Dry Spells and, in addition, make possible the delineation of areas characterized by low or high frequency of Dry Spells. The analysis reveals a higher frequency of Dry Spells and a greater variability of precipitation in the summer months in a zone extending along the Saskatchewan-Alberta boundary. A very significant factor related to Dry Spells is the position of an upper level stationary or quasi-stationary ridge (north-south axis) over the Prairies. The position of the upper level ridge affects the dry conditions in two ways: firstly, because of the blocking action, the upper level ridge dislocates the jet stream, tracks of cyclones and associated moist air masses, so that they lie north of the Prairies; secondly, the anticyclonic circulation under this ridge of high pressure induces divergence and subsidence of air accompanied by inversion of temperature at lower levels. Other significant factors which contribute to Dry Spells are associated with surface synoptic conditions. These latter include: an apparent eastward migration of high and low pressure systems (Aleutian Low, North Pacific High, Hudson Bay Low and North Atlantic High); and the position of the migratory anticyclones over the southern and southeastern Prairies. The dry mPs air mass originating from the Pacific inversion layer also has some influence on the occurrence of Dry Spells. The upper level and surface atmospheric flow patterns (i.e., from surface to 500 millibar levels) are classified into five synoptic Weather Types and is demonstrated that Weather Type WA (ridge dominant over British Columbia), Weather Type WB (ridge dominant over central Prairies) and Weather Type We (ridge dominant over eastern Prairies) correlate with a high probability of dry conditions over the Prairies. When the preceding flow patterns are persistent with a quasi-stationary upper level ridge (north-south axis) over the Prairies, Dry Spells occur. These synoptic and dynamic atmospheric conditions are summarized in three models explaining possible relationships pertaining to the probability of dry conditions and Dry Spells over the Prairies.
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