Climatology of summer midtropospheric perturbations in the U.S. northern plains. Part I: influence on northwest flow severe weather outbreaks

Northwest flow severe weather outbreaks (NWF outbreaks) describe a type of summer convective storm that occurs in areas of mid-level NWF in the central United States. Convective storms associated with NWF outbreaks often travel a long distance systematically along a northwest-southeast oriented track across the northern plains. Previous studies have observed that these migrating convective storms are frequently coupled with subsynoptic-scale midtropospheric perturbations (MPs) initiated over the Rocky Mountains. This study traces MPs for the decade of 1997-2006 using the North American Regional Reanalysis to examine their climatology and possible influence on NWF outbreaks. MPs are characterized by a well organized divergent circulation with persistent ascending motion at the leading edge promoting convection. The divergent circulation is further enhanced by low-level convergence along the northern terminus of the Great Plains low-level jet. The downstream propagation of MPs assists in forming the progressive feature of the associated convective storms. MPs have a maximum frequency in July, consistent with NWF outbreaks. In July and August, the fully developed North American anticyclone produces prevailing NWF over the northern plains, where up to 60% of rainfall and storm reports are linked to MPs. The movement, timing and rainfall distribution of MPs remarkably resemble those of NWF outbreaks. When encountering strong low-level jets, ascending motion and convergence of water vapor flux associated with MPs intensify considerably and precipitation is greatly enhanced. It is likely that NWF outbreaks are generated whenever MPs occur in association with strong low-level jets.