Abstract

The case of a tornadic thunderstorm on 26 July 1985 in northeastern Colorado is described from the synoptic to the thunderstorm scale utilizing a number of datasets some of which will become operational in the 1990s. The available data included profilers, Doppler radar, surface mesonet, satellite, and special soundings. Although the synoptic environment did not favor tornadic thunderstorms, strong thunderstorms formed in localized area during a 2-h period in the afternoon and produced an 18-min tornado. A number of events took place to produce the stronger then anticipated development, including interaction among mesoscale outflow and stationary boundaries. Of particular importance was the change in the local environment along a stationary boundary known as the Denver Convergence-Vorticity Zone. Special soundings taken near the stationary boundary revealed a deepening moist layer over time in association with the convergent wind field. Additional forcing from the collision of this boundary with other outflow boundaries was required, to release the increasing convective potential. Of further importance was the coincidence of sunshine over a portion of the boundary where the eventual collision would occur. In association with experiments under way at the time, three groups having various access to the datasets issued probability forecasts in real time. The short-range forecasts and warnings of thunderstorm initiation and severe weather provide a useful evaluation of the problems of predicting the events of this day. Two major problems encountered on this day are commonly faced by forecasters of summertime convection: evaluating the importance and timing of an approaching weak upper-1evel feature, and monitoring low-level mesoscale boundaries. Use of the datasets in real time to diagnose these problems is emphasized. The forecasters using the new datasets demonstrated the ability to focus on the most likely area of thunderstorm initiation, although the timing and intensity of the development still presented forecast problems. Analyses of the special soundings revealed the above-surface temperature and moisture changes resulting from the two features and their importance for this case. Means of addressing such changes operationally with the new datasets are discussed.

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