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Since dry air is necessary to produce a surface mesohigh , this finding is consistent with Evans ( 1998 ) , who found that long-lived bow echoes are associated with comparatively strong system-relative flow near the surface . Although the lower tropospheric kinematic field appeared too weak to support " classical " supercell evolution , considerable vertically-oriented vorticity was in fact present in the vicinity of the merging cold front and dry line by the afternoon of the 27th ( compare , for example , post-frontal and pre-dry line surface winds near Waco at 1200 and 1700 UTC ) . At RUL , our preservation architecture is based on open source software products that will enable maximum flexibility in dealing with an evolving infrastructure . The concept is one most veterinarians are already familiar with ; veterinarians around the country use generic drugs to control costs for the owners of their animal patients . When hundreds gather for the traditional four-hour service on Sunday mornings , it is almost like being in Egypt . Although the vector approach is generally reserved for use in situations involving large summertime MCSs over the central and eastern United States , the concept is also useful in determining the behavior and motion of other types of convective systems .