A preliminary investigation of derecho-producing MCSs in environments of very low dewpoints

Because of the damaging winds and other severe weather threats with which they are associated, derecho-producing MCSs pose an important challenge to forecasters. In late spring and summer, derechos typically occur in environments of substantial convective instability, with very moist boundary layer inflow. During the cool season, these events are more commonly associated with amplifying disturbances in the westerlies. While derechos occasionally occur with only a modest degree of convective instability, the boundary layer inflow associated with derecho-producing MCSs is typically moist for the season. Occasionally, however, derecho MCSs occur with very dry boundary layer conditions (surface dewpoints at or below 10 C). Because low dewpoint derechos develop in environments not commonly associated with widespread damaging thunderstorms, these events sometimes catch forecasters by surprise. This paper examines several low dew point derecho cases that have occurred in recent years, with emphasis on the synoptic and mesoscale factors that appear to be important in their development.