| description abstract | On the afternoon of 3 June 1981 a severe thunderstorm spawned two tornadoes which moved across a portion of metropolitan Denver. The tornadoes were classified as strong F2 intensity, and caused damage totaling over $1 5 million. The synoptic-scale setting for this event was similar to that associated with many other occurrences of severe convection in eastern Colorado, with post-frontal moist southeasterly upslope flow at low levels and southwesterly flow aloft in advance of an approaching trough. We chose to study this event in part because of its occurrence within the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork. Emphasis is placed on mesoscale evolution culminating in the formation of the tornadic storm. A zone of surface convergence and cyclonic vorticity developed during the early daylight hours over and north of Denver between southeasterly flow over the plains and a region of lighter, generally northerly flow just east of the foothills. The tornadic storm formed from a complex interaction of older thunderstorm cells, and subsequently intensified at the southern end of the convergence-vorticity zone and moved to the north-northeast. Other severe convection also occurred along this zone later in the afternoon. The development of this convergence-vorticity zone under conditions of ambient southeasterly flow appears to be topographically forced by a ridge of higher terrain which extends eastward from south of Denver, but the specific processes involved are unclear. The zone occurs frequently and, for the years 1981 and 1982 for which the PROFS mesonet data were available, was associated with a disproportionate number of severe weather events, especially tornadoes. | |
