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

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