LARGE-SCALE ANTECEDENT CONDITIONS ASSOCIATED WITH THE 12-14 MARCH 1993 CYCLONE (SUPERSTORM-93) OVER EASTERN NORTH-AMERICA

The results of a multiscale analysis of the 12-14 March 1993 superstor m (SS93) over eastern North America are presented. A time sequence of overlapping 10-day time-mean 500-hPa geopotential height and anomaly c omposites shows that the Northern Hemisphere (NH) flow pattern from 18 February to 15 March 1993 is characterized by 1) three persistent tro ughs situated over eastern Asia and the northwestern Pacific, over eas tern North America, and over northwestern Africa and southwestern Euro pe eastward to central Russia; and 2) a massive blocking anticyclone l ocated over the central and eastern Atlantic. Beginning 8-9 March 1993 the planetary-scale flow amplifies substantially. The explosive SS93 cyclogenesis and the transport of cold air to very low latitudes occur s a few days later as the NH available potential energy content, after peaking on 9 March 1993, decreases by about 6%-7%. A dynamical tropop ause analysis is used to track coherent transient potential vorticity (PV) anomalies and show their qualitative interaction with the planeta ry-scale flow. SS93 is attributed to the interaction and eventual merg er of strong PV anomalies embedded in the northern and southern branch es of the westerlies in a background confluent northwesterly flow asso ciated with an amplifying positive-phase Pacific-North American flow p attern. The northern PV anomaly originates in southwestern Canada on 1 8 February and circumnavigates the NH at relatively high latitudes, a track that permits it to maintain arctic characteristics prior to merg er. The southern PV anomalies, tracked from Europe and western Asia ea stward across the Pacific, reach North America by 11 March 1993 where they become associated with widespread convection over southern Texas and the northwestern Gulf of Mexico beginning 12 March 1993, The uniqu e aspects of SS93 are attributed to 1) the near simultaneous amplifica tion of the planetary-scale flow and the lateral and vertical interact ion of individual PV anomalies east of the Rockies during the merger p rocess, and 2) the lag of the northern PV anomaly relative to the sout hern anomaly so that a baroclinic zone containing lower-tropospheric a ir of significant conditional instability is allowed to remain in plac e over southern Texas and the northwest Gulf of Mexico in the cyclogen etic environment ahead of the northern PV anomaly.