A Synoptic Climatology of Northwest-Flow Severe Weather Outbreaks. Part II: Meteorological Parameters and Synoptic Patterns

Abstract

A climatology of meteorological parameters and synoptic patterns associated with severe weather outbreaks occurring in arm where the mid-tropospheric flow has a north of west component is presented. This climatology utilizes data and criteria previously described by Johns. A comparison of the northwest flow parameters and those associated with general severe weather is given. The importance of conditional instability and low-level warm advection in northwest flow situations is discussed. An explanation is offered for the location of the axes of highest frequency of northwest flow outbreaks. Furthermore, the varying nature of wind shear associated with severe weather is discussed and the importance of the directional contribution of wind shear to northwest flow severe weather is demonstrated.