In a three dimensional simulation of an observed derecho-producing squall line, Schmidt (1991) diagnosed an upward-directed pressure gradient force(PGF), due to an induced low formed by a rotating updraft or a mesocyclone. This served to bring stable air up from the surface, which is negatively buoyant to begin with. Once the stable air is displaced from the region of uplift then it will descend rapidly back towards the surface with melting and evaporation of precipitation enhancing its negative buoyancy. This “up-down” downdraft (Knupp 1987) is thought to be a major component of nocturnal derechos where there is a plethora of stable air beneath the system. Bernardet and Cotton (1998) simulated a long-lived derecho-producing MCS in both its daytime and nighttime phases. The source of the downdraft air is ahead of the storm in both phases, but during the day it comes from the surface air lifted by the gust front and upward-directed PGF enhanced by a mesocyclone, while at night it is from the surface stable layer which is lifted solely by the mesocyclone-driven upward-directed PGF. The rear-inflow-jet and its manifestation of a bow-echo in this concept is not directly involved in downdraft formation, but it responds to the elevated low pressure that develops in the storm. The same low pressure region that drives the rear-inflow-jet forms the vertical PGF, which drives the up-down downdraft circulation.

In this talk we examine observational evidence supporting the role of mesoscale vortices and mesocyclones in derecho formation. A total of five derechos were selected from the warm seasons of 1999 and 2000. The synoptic scale features in the prestorm environments were analyzed and compared to previous derecho climatologies. Doppler radar data were also examined to gain insight into the structure of the derechos, specifically the circulations that are near to wind reports as these may provide insights into significant mechanisms for forming and enhancing downdrafts.

It was found that:

• A large number of wind reports were not associated with a bow echo or bowing segment. This signals a slightly new way of thinking about severe wind events. Fifty-five percent of the wind reports within the coverage area of the radar sites were associated with a long-lived circulation or mesocyclone.

• Using circulations to diagnose surface winds has its problems. Many of the wind reports were not associated with a long-lived circulation or detected by the radar algorithm. The maximum rotational velocity of the circulations showed large numbers of false alarms as a forecasting tool, which may be reduced by coupling it with its height. This way, strong velocities above the expected downdraft formation levels would not be included, resulting in a better prognosis of surface gusts.