Landspout tornados and severe thunderstorms impacted southeast Colorado and southwest Kansas during the afternoon of 21 May 2020 (spc reports). GOES-East satellite imagery was used by NWS PUB forecasters to analyze the cumulus cloud field leading up to convective initiation, and gauge convective evolution as storms matured.
Analyzing 1-min visible imagery during the two hours between 1859 and 2059 UTC, the cu field across southeast Colorado from just north of Holly to Campo became increasingly agitated, with the greatest clumping and vertical growth occurring between Lamar and Holly (Fig 1). These cumulus cloud trends were signs of imminent convective initiation. Additionally, forecasters diagnosed significant low-level cyclonic vorticty given the evolution and character of the broad cu field in the imagery across southeast Colorado into southwest Kansas. Finally, the movement of cu clouds to the west with towering cu tops and orphan anvils drifting to the east highlighted the presence of substantial low-mid-level shear.
The insight gleaned from satellite imagery along with additional knowledge about the environment gave forecasters confidence that a landspout tornado threat existed with any storm that might develop in the area, in addition to the large hail threat. The trends in satellite imagery also helped forecasters to message this risk to core partners and the public. Two tweets were sent during this period, highlighting the threat for strong to severe thunderstorms (Fig 2), and landspout tornados (Fig 3).
Visible imagery during the following two hours (2059 – 2258 UTC) showed continued upscale growth of cumulus clouds across the region, including convective initiation and eventual development of strong-severe thunderstorms (Fig 4). Multiple instances of landspout tornados were confirmed during this period near the CO/KS border in association with the growing cumulus clouds.
In addition to the landspout threat, forecasters were motioning the development of severe thunderstorms. As convection continued to grow upscale and mature from Springfield to north of Holly/Lamar, forecasters paid close attention to IR-Window imagery (Fig 5). The IR imagery showed where the most rapid convective initiation was occurring, and was used to help forecasters determine which cell(s) would become most dominant. The most impressive and persistent cold cloud top temperatures and overshooting top developed within the broader region of convection with a storm between Lamar and Holly. This storm also quickly produced an above anvil cirrus plume, indicating a particularly strong updraft and severe potential. Severe thunderstorm and tornado warnings were issued with this region of development, which produced confirmed tornados and baseball sized hail.
Combining the VIS and IR into a sandwich image combo reveals the development of a dominant cell quite well with this case through the blending of texture and brightness temperature information (Fig 6). Details of the overshooting top and above anvil cirrus plume become obvious.
Viewing an animation covering the full duration of this event, the evolution of the cu field in the context of RAP analyzed sfc vorticty and 0-3 km MLCAPE (good fields for assessing landspout potential) is observed (Fig 7). The large values of sfc vorticty from the model generally agree with what is implied from the satellite imagery. Values of 0-3 km MLCAPE increased over and east of Campo during the afternoon. The region of overlap between the two fields near the CO/KS border is where landspout tornados were observed.
Bill Line (NESDIS and CIRA) and Klint Skelly (NWS PUB)