Rapid Environmental Changes Observed by Remote Sensing Systems in the Local Vicinity of an Unusual Colorado Tornado
The combined Wind Profiler and VLAPS datasets indicated that the storm-relative helicity in the 0–1 km AGL layer jumped from 50 to 350 m2 s-2 in the two hours prior to tornado touchdown. This extremely rapid and enormous change was associated with deepening and descent of a lower tropospheric layer of strong vertical wind shear. The radiometer showed a gradual increase of SBCAPE (Surface-Based Convective Available Potential Energy) throughout the morning hours, despite thick cloud cover, to 3100 J/kg at 1815 UTC, just prior to passage of a pronounced dryline attendant the explosive development of the Windsor supercell storm. Even more interesting was a sudden increase in potential instability in the very low levels of the atmosphere (below 200 m AGL) in concert with the aforementioned rapid increases in vertical wind shear and storm-relative helicity. This could not be attributed to near-surface warming/moistening, but rather to the appearance of a region of cooling in the 700 – 800 hPa layer in association with the deepening of the shear layer. The origin of this cooling was traced to air that had been adiabatically cooled as it was advected and lifted over the Palmer Lake Divide, a mesoscale region of ~1.0 km higher terrain just south of Denver and Windsor.
Another interesting finding from this study concerns the likely lifting mechanism that created a long-lasting supercell from this very moist, unstable air with exceedingly high vertical shear. One factor may have been the orographic generation of elongated streamers of isentropic potential vorticity seen at 500–700 hPa in the VLAPS analyses. These streamers were aligned parallel to the south-southeasterly flow regime existing across the region, and as the most intense streamer propagated slowly eastward past the Front Range to the immediate west of Windsor, strong ascent was diagnosed. This process was preceded by ~90 min by a rapid increase in low-level frontogenesis in the Windsor region that correlated even better in time with observed convection initiation. Frontogenesis developed along a northward bulge in the warm front associated with flow around a pronounced low-level “Denver cyclone” as a dryline advancing northward toward this region from the Palmer Lake Divide merged with the front. The confluence of all these factors in a very short period of time and juxtaposed over the proximate Windsor region was likely critical in explaining how this rare tornadic event could have happened so suddenly and precisely where it did.