The Denver Cyclone and Tornadoes 26 Years Later: The Continued Challenge of Predicting Nonsupercell Tornadoes

Abstract

The year 2006 marks the 25th anniversary of the discovery of the Denver Cyclone, made possible after the installation of a mesonetwork of automated weather stations in 1981 by then NOAA/PROFS (Program for Regional Observing and Forecasting Services). The “PROFS mesonet” provided sufficient resolution to observe the Denver Cyclone, a zone of low-level convergence and cyclonic vorticity, formed under conditions of ambient low-level south or southeast flow (as illustrated in the schematic in Figure 1). The history of the Denver Cyclone and associated research on this feature, along with its connection to nonsupercell tornadoes, is reviewed in Section 2. The Denver Cyclone's association with tornadogenesis has been vigorously investigated. Knowledge from those studies has been shared with forecasters through workshops and other means at the local National Weather Service (NWS) Weather Forecast Office (WFO), now collocated in Boulder with NOAA/Earth System Research Laboratory (ESRL). With this knowledge, and the close proximity of the Denver Cyclone to the Denver radar (located near DEN in Fig. 1), the Boulder WFO may be one of the most experienced WFOs with respect to nonsupercell tornadoes. Nonetheless, a number of challenges remain when it comes to issuing warnings with non-negligible lead-time for these types of tornadoes. A recent case of ten reported nonsupercell tornadoes in the vicinity of the Denver International Airport (DIA) and northwards, which happened to occur during the last Severe Local Storms Conference (October of 2004), will be used to illustrate some of the issues that still can occur even when the phenomenon that produces the tornadoes is well understood.