Mesoscale convective systems over the United States during the 1997-98 El Nino

Large, long-lived mesoscale convective systems (MCSs) over the United States during the 1997-98 El Nino are documented. Two periods of abnormal MCS activity are identified in 1998: from March to mid-April an unusually large number of quasi-linear MCSs were observed in the Midwest; while quasi-circular MCSs in June-August of 1998 were concentrated near 37 degreesN rather than following a seasonal shift similar to that observed in the climatological distribution. Episodic surges of northerly low-level flow were infrequent in March 1998, thereby leading to an unusually high incidence of quasi-linear MCSs and to precipitation anomalies in the central United States.