The Denver Cyclone and tornadoes 25 years later: the continued challenge of predicting non-supercell tornadoes

The year 2006 marks the 25th anniversary of the "discovery" of the Denver Cyclone. The installation of a mesonetwork of automated stations in 1981 by NOAA/GSD (then NOAA/PROFS) provided sufficient resolution to observe the Denver Cyclone (also known as the Denver Covergence Vorticity Zone or DCVZ), although speculation had existed for a number of years based on METARS that some type of boundary occurred under ambient southeast low-level flow. It was quickly recognized that many tornadoes in Northeastern Colorado were associated with the DCVZ, and further studies led to the non-supercell or landspout tornado mechanism. This area, then, represents a location where landspouts and their associated low-level boundary have been studied longer than perhaps any other region in the United States. This is a consequence not only of a number of research facilities in the Boulder area, but also the consistent nature of the DCVZ boundary. With all of the studies of the DCVZ and non-supercell tornadogenesis in this region, and with considerable information having been shared with the local WFO (Weather Forecast Office), there is probably as much experience in predicting non-supercell tornadoes in this region as there is anywhere.

So what have we learned over these 25 years? Are there still challenges in issuing warnings for non-supercell tornadoes? This paper will give a brief overview of the last 25 years of DCVZ/landspout work, and then use an impressive case that occurred near the Denver International Airport (DIA) during the time of the last Severe Storms Conference to illustrate some of the issues that remain in trying to predict non-supercell tornadoes. The case occurred on 4 October 2004, when in the course of about 30 minutes ten non-supercell tornadoes were reported along a strong DCVZ in a small area near and north of DIA. The tornadic area was well-observed by the NWS 88-D radar located near DIA, allowing us to examine what types of signals were present under observing circumstances that are likely far better than for most events. We will also explore whether experimental observing technologies, such as infrasound detection, provided any additional capabilities to predict the event.